- Email: [email protected]
Vis reflectance
Accepted Manuscript
Lunar Mare TiO2 Abundances Estimated from UV/Vis Reflectance Hiroyuki Sato, Mark S. Robinson, Samuel J. Lawrence, Brett W. Denevi, Bruce Hapke, Bradley L. Jolliff, Harald Hiesinger PII: DOI: Reference:
S0019-1035(16)30659-5 10.1016/j.icarus.2017.06.013 YICAR 12497
To appear in:
Icarus
Received date: Revised date: Accepted date:
11 October 2016 4 June 2017 7 June 2017
Please cite this article as: Hiroyuki Sato, Mark S. Robinson, Samuel J. Lawrence, Brett W. Denevi, Bruce Hapke, Bradley L. Jolliff, Harald Hiesinger, Lunar Mare TiO2 Abundances Estimated from UV/Vis Reflectance, Icarus (2017), doi: 10.1016/j.icarus.2017.06.013
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Highlights • We constructed a new TiO2 map from LROC WAC near-global mosaic.
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• TiO2 contents of lunar soil samples and 321/415 nm ratio are linearly correlated.
• Clementine-based TiO2 estimates are higher relative to WAC TiO2.
• Lunar Prospector GRS-based TiO2 values are comparable with WAC TiO2 .
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• Trend of mare volcanism may have shifted at 2.6 Ga.
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Hiroyuki Satoa,∗, Mark S. Robinsona , Samuel J. Lawrenceb , Brett W. Denevic , Bruce Hapked , Bradley L. Jolliffe , Harald Hiesingerf
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Lunar Mare TiO2 Abundances Estimated from UV/Vis Reflectance
a School
of Earth and Space Exploration, Arizona State University, 1100 S. Cady Mall, INTDS A, Tempe, AZ 85287-3603, USA. b Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, USA. c Applied Physics Laboratory, Johns Hopkins University, 11100 John Hopkins Rd, Laurel, MD 20723-6005, USA. d Department of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara Street, Pittsburgh, PA 15260, USA. e Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, St Louis, Missouri 63130, USA. f Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany.
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Abstract
The visible (VIS; 400-700 nm) and near-infrared (NIR; 700-2800 nm) re-
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flectance of the lunar regolith is dominantly controlled by variations in the
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abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g.,
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ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced
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rims on soil grains, and Fe-metal). The same materials control reflectance into
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the near-UV (250-400 nm) with varying degrees of importance. A key differ-
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ence is that while ilmenite is spectrally neutral in the VIS and NIR, it exhibits
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a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than
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about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC)
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filters were specifically designed to take advantage of this spectral feature to
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enable more accurate mapping of ilmenite within mare soils than previously
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possible. Using the reflectance measured at 321 and 415 nm during 62 months
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of repeated near-global WAC observations, first we found a linear correlation
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between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of
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each sample return site. We then used the coefficients from the linear regression
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and the near-global WAC multispectral mosaic to derive a new TiO2 map. The
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∗ Corresponding author at: Arizona State University, 1100 S. Cady Mall, INTDS A, Tempe, AZ 85287-3603, USA. Email address: [email protected] (Hiroyuki Sato). Phone Preprint submitted to Icarus June number: 8, 2017 480-727-0099
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average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 val-
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ues were found in Mare Tranquillitatis (∼12.6 wt%) and Oceanus Procellarum
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(∼11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the
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low TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium)
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exhibit very low TiO2 values (<2 wt%). We find that the Clementine visi-
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ble to near-infrared based TiO2 maps (Lucey et al., 2000) have systematically
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higher values relative to the WAC estimates. The Lunar Prospector Gamma-
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Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map,
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although there are local offsets possibly due to the different depth sensitivities
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and large pixel scale of the GRS relative to the WAC. We find a wide variation
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of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga),
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whereas only medium- to high-TiO2 values (average = 6.8 wt%, minimum =
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4.5 wt%) are found for younger mare units (<2.6 Ga).
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Keywords: Moon, Volcanism, Thermal histories, Image processing,
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Spectroscopy
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1. Introduction
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From lunar meteorites (Korotev et al., 2003) and Apollo and Luna returned
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samples (McKay et al., 1991), we have a preliminary understanding, albeit in-
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complete, of the major lunar rock types, particularly mare basalts. Since basalts
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form through partial melting of the lunar mantle, they provide our best view of
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the composition of the lunar interior; understanding the compositional range of
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mare basalts is, therefore, critical to understanding the geology and the interior
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of the Moon and thus the other terrestrial planets (Lunar Exploration Analysis
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Group, 2016).
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Apollo-era petrologic investigations of returned samples relied on titanium
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content as a primary tool for the first-order classification of mare basalt types,
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owing to the large range of titanium (0 to >10 wt% TiO2 ) among the basalt
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samples (Shearer et al., 2006). The Apollo mare samples have a bimodal distri-
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bution in the range of titanium concentrations, with relatively larger numbers
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of samples below 5 wt% TiO2 and higher than 9 wt% TiO2 (Papike et al., 1998;
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Taylor et al., 1991). Historically, lunar basalts were classified as either very
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low-Ti (VLT) basalts with <1 wt% TiO2 , low-Ti basalts with 1-5 wt%, and
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high-Ti basalts (9-14 wt%), with only a few samples falling into the range of 5-9
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wt% (Papike et al., 1976; Papike and Vaniman, 1978; Taylor et al., 1991; Papike
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et al., 1998); alternative simplified classification schemes also exist (Neal and
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Taylor, 1992). The recent Chang’e-3 mission measured basalts with 5.2 wt%
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TiO2 in northern Mare Imbrium (Ling et al., 2015).
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Earth-based telescopic observations combined with knowledge of sample
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chemistry were used to predict the TiO2 content of mare materials (Whitaker,
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1972; McCord et al., 1972; Charette and McCord, 1974; Johnson et al., 1977;
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Pieters, 1978; Johnson et al., 1991). These early studies extended estimates of
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TiO2 abundance to unvisited mare surfaces across the nearside. Later, space-
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craft multispectral observations were utilized to extend TiO2 abundance esti-
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mates to farside areas, such as Mariner 10 (Robinson et al., 1992), Galileo Solid
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State Imaging experiment (Lucey et al., 1998; Giguere et al., 2000), and Clemen-
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tine Ultraviolet/Visible (UVVIS) camera (Lucey et al., 1995, 1998; Blewett
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et al., 1997; Jolliff, 1999; Lucey et al., 2000; Giguere et al., 2000; Gillis et al.,
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2003). Clementine UVVIS images (415-1000 nm) provided the means for Lucey
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and coworkers to estimate FeO and TiO2 content of the surface at a pixel scale
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of 100 m (Lucey et al., 1995; Blewett et al., 1997; Lucey et al., 1998, 2000).
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These products were the first global and uniformly processed maps that were
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broadly used to investigate the composition of the lunar crust. Giguere et al.
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(2000) demonstrated that the TiO2 content in mare basalts has a continuous
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distribution ranging from 0-14 wt% with a mode near ∼2 wt%, not the bimodal
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distribution as suggested by the lunar samples. Recent spectral reflectance mea-
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surements from Chang’e-1 Interference Imaging Spectrometer (IIM) (Wu, 2012;
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Wu et al., 2012), and the SELENE Multiband Imager (Otake et al., 2012) in-
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creased the spatial resolution of TiO2 estimates up to 20 m/pixel. These series
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of works estimated TiO2 abundance from the visible to NIR spectral slopes
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acquired from Earth-based observations (e.g., 300-1100 nm in Whitaker, 1972; 4
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400-560 nm in McCord et al., 1972; Charette and McCord, 1974; Johnson et al.,
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1977; Pieters, 1978; Johnson et al., 1991), flyby observations (e.g., 480-580 nm
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by Mariner 10; 415-750 nm by Galileo), and orbital measurements (Clementine,
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Chang’e-1, and SELENE), based on the correlation with the TiO2 content of re-
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turned lunar samples. Since the sensitivity depth of reflectance spectra in these
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observations is the upper few millimeters (Hapke, 2012), these TiO2 estimates
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represent only the uppermost surface materials.
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The Lunar Prospector Gamma-Ray Spectrometer (GRS) and Neutron Spec-
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trometer (NS) (Elphic et al., 2000, 1998, 2002; Prettyman et al., 2006) provided
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an independent measure of titanium at coarser scales (data were binned at 2 and
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5 degrees per pixel). The NS/GRS senses into the regolith ∼30 cm (Lawrence
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et al., 2002), thus its TiO2 estimates are deeper relative to the spectral re-
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flectance methods. In some areas, it is reported that the TiO2 estimates based
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on visible (400-750 nm) reflectance including Clementine UVVIS and Chang’e-1
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IIM (Lucey et al., 2000; Wu et al., 2012) are high relative to Lunar Prospector
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NS/GRS based TiO2 estimates (Elphic et al., 2002; Prettyman et al., 2006).
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The cause of this discrepancy is still not fully understood, but is in some cases
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may be related to the different sampling depths.
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Visible to near-infrared (400-2800 nm) spectral reflectance of the lunar re-
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golith is mainly controlled by variations in abundance of four components: pla-
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gioclase, iron-bearing silicate minerals, opaque minerals, and maturation prod-
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ucts (e.g., agglutinate glass, Fe metal, and amorphous radiation-damaged min-
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eral grains) (Hapke et al., 1975; Pieters et al., 2000; Hapke, 2001). Additionally,
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it has long been known that the mare exhibit distinct color contrasts into the
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UV relative to visible wavelengths (Whitaker, 1972; McCord et al., 1972; Wells
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and Hapke, 1977; Wagner et al., 1987; Rava and Hapke, 1987; Robinson et al.,
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2007; Cloutis et al., 2008). While the spectral reflectance of most major lu-
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nar rock-forming minerals decreases toward shorter wavelengths (Fig. 1), the
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spectrum of ilmenite is flat through the visible and increases with decreasing
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wavelength below ∼450 nm (Clark et al., 1993; Cloutis et al., 2008). A strong
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linear correlation was demonstrated between the visible-UV ratio (502/250 nm) 5
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of Apollo 17 sample stations as imaged by the Hubble Space Telescope and
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the TiO2 contents of the corresponding returned soil samples (Robinson et al.,
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2007). Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC)
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near-global observations (±70° latitude) provide the first opportunity to map
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all mare deposits in the near-ultraviolet to visible range (321 and 360 nm in
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UV bands; 415, 566, 604, 643, and 689 nm in visible bands) with uniform
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resolution (∼400 m in UV, ∼100 m in visible) (Robinson et al., 2010; Mahanti
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et al., 2016). An original science goal of the WAC was to investigate TiO2
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abundance in the maria and the 321 nm and 415 nm bands were designed for this
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purpose (Robinson et al., 2010) based on promising results from Hubble Space
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Telescope observations of the Taurus Littrow valley (Apollo 17 landing site)
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(Robinson et al., 2007). Denevi et al. (2014) examined the WAC 321/415 ratio
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variations of the Moon in relation to the effects of space weathering, reporting
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that the fresh craters have low ratio values in the mare and in the moderate-iron
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highlands. However, they also found that the space weathering “saturates” in
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the UV relatively quickly (up to IS /FeO ∼40) compared to the visible and the
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near-infrared (Lucey et al., 2000) (up to IS /FeO ∼60), suggesting that the UV
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is less dependent on the maturity variations. From the ratio of the 321 nm
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band to the 415 nm band, a new TiO2 abundance map is presented here and
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compared with two maps derived from Clementine near-infrared spectral images
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(Lucey et al., 2000; Gillis-Davis et al., 2006) and global estimates derived from
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the Lunar Prospector GRS measurements (Elphic et al., 2002; Prettyman et al.,
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2006). We also discuss the distribution of ilmenite (the main carrier of TiO2 )
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and its geologic implications within each mare. Finally, we discuss the variation
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of TiO2 abundance as a function of age to understand the evolution of lunar
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mare volcanism.
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2. Derivation of WAC TiO2 Map For the lunar mare, we assume that variations in the 321/415 nm ratio
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are dominantly controlled by ilmenite (and thus TiO2 ) abundance, based on
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laboratory and remote-sensing studies (Charette and McCord, 1974; Robinson
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et al., 2007; Cloutis et al., 2008; Coman et al., 2016). In fact, the spectra of
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the returned regolith samples are known to show a clear correlation between
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the UV over visible wavelength ratio and the ilmenite-sourced TiO2 (Charette
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and McCord, 1974; Coman et al., 2016). First, we determined a correlation
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between TiO2 contents of returned sample soils and the WAC 321/415 ratios
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from appropriate sampling sites (Table 1). From the derived coefficients, we then
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constructed a new TiO2 abundance map from a near-global WAC multispectral
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mosaic.
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2.1. WAC Observations at Sampling Sites
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The areas from which returned samples were collected do not necessarily
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fully represent the surface observed from orbit, mostly due to gross differences
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in sampling size; remote sensing pixels measure areas hundreds to thousands
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of meters in size, whereas the returned samples are from areas typically much
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less than a square meter. Therefore, we only utilized samples acquired from
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sites deemed to be relatively geologically uniform (based on the albedo and
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morphological variations detected by the LROC Narrow Angle Camera (NAC)
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images with a pixel scale of ∼2 m/pixel) at the pixel scale of the WAC 321 nm
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band (typically 400 m) (Table 1). Sites with complicated geology at the scale of WAC pixels, such as Apollo 17-LRV2 on a narrow lobe of the landslide from
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South Massif, Apollo 15-9A at the edge of Hadley Rille, and Apollo 17 Station
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5 on Camelot crater ejecta, were all excluded (Table 2). Lunar soils with less
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than ∼2 wt% TiO2 do not show a strong correlation between their 321/415 nm
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ratio and TiO2 content (Gillis-Davis et al., 2006; Coman et al., 2016), therefore
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Apollo 16 and Luna 20 samples (highland materials with no or very low ilmenite
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content) were also excluded. In total, 11 sample sites were used to calibrate the
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spectral ratio to TiO2 abundance. 7
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For each selected sample return site (Table 1), we derived a representative
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value of WAC photometrically normalized reflectance (nI/F ) at 321 and 415 nm
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respectively, through the following four steps explained hereafter: 1) collection
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of all the available WAC observations at each site and derivation of nI/F , 2)
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masking based on NAC albedo, 3) masking of sparse observation regions, and
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4) curve fitting to the nI/F histogram. Each step is described in detail below.
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2.1.1. WAC Pixel Data Preparation
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We used ∼62 months of WAC observations acquired from September 2009 to
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December 2014. The raw Digital Number (DN) of each pixel was converted to
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radiance factor (I/F ) (Hapke, 2012) through radiometric calibration (Mahanti
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et al., 2016), then photometrically normalized (emission angle = 0°, incidence
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angle = phase angle = 60°) with a Hapke function (Hapke, 2012) using spa-
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tially resolved Hapke parameter maps (Sato et al., 2014) to derive nI/F . The
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photometric angles were calculated relative to the local topography using a
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WAC-derived 100 m scale Digital Terrain Model (DTM) known as the GLD100
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(Scholten et al., 2012). The observations at incidence angle >75°, emission an-
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gle >30°, and phase angle >95° were excluded to minimize the uncertainties of
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photometric normalization. We normalized reflectance to 60° phase angle (0°
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emission angle, 60° incidence angle) instead of the traditional 30° because the
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lower value is not achieved in the latitudes over 30° under the WAC observation
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geometries (Sato et al., 2014). The nominal pixel scales (average within a frame
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for nadir observations) of the WAC from the 50 km altitude orbit of Lunar
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Reconnaissance Orbiter (LRO) (from September 2009 to December 2011) are
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402 m for the UV and 74.9 m for visible bands (Robinson et al., 2010). In the
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current elliptical orbit (after December 2011), the pixel scale ranges from 550
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to 1170 m/pixel in UV and from 107 to 228 m/pixel in visible bands within the
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latitudes of all the sample-return sites (9°S to 26°N). Only the pixels that were
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contained entirely within a 1000 by 1000 m box (hereafter called “site box”)
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centered at each sample-return location were collected from non map-projected
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WAC images for the 321 and 415 nm bands.
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2.1.2. NAC Albedo Masking Inside each site box, there are significant albedo variations due to local geo-
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logic and topographic features (e.g., fresh crater ejecta and slopes). From LROC
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NAC images (original pixel scale ∼50 cm, down-sampled to 2 m/pixel), we de-
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rived and applied an albedo mask to exclude WAC pixels that dominantly cover
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remarkably low or high albedo areas (Fig. 2). For the NAC images, we selected
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those that were acquired with relatively small incidence angles (from 7.3° to
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27.0°), and applied a photometric normalization using emission and incidence
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angles calculated relative to local topography derived from complementary NAC
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DTMs for each site (2 m/pixel, Henriksen et al., 2015). Although the NAC is a
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broadband imager (400 to 760 nm) it is useful as a generalized albedo map for
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comparison with the WAC observations. Within the NAC image cropped to the
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region of the site box, we masked (excluded) 10% in total of the area with high-
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and low-end reflectance relative to the reference value (Fig. 2). The reference
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value was obtained from the same NAC image, a median of a 10 by 10 pixel (20
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by 20 m) box centered at the exact location of each sample-return site. In order
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to avoid surfaces disturbed by human activities (e.g., rocket blast zone, tracks
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of astronauts and Lunar Roving Vehicles), at several sites, we relocated the 10
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by 10 pixel box to a nearby vicinity where there is a relatively uniform, undis-
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turbed area. The masked areas normally consist of multiple irregular patches
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whereas each WAC pixel is square. Thus, we count an overlapping fraction of
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each WAC pixel with the masked area, and if the fraction is more than 15% we
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excluded that WAC pixel.
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2.1.3. Sparse Observation Masking Owing to the four-times larger footprint of the 321 nm band relative to the
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415 nm band, and because we select only pixels entirely included in the site
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box, there is a zone along the margins of the site box where the number of
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observations is relatively sparse (Fig. 2a). This zone is much wider for the 321
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nm band (∼200 m, about half of the UV-band footprint size) than for the 415 nm
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band (∼50 m, half of visible-band footprint size). This offset, where there are 9
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numerous 415 nm observations but few 321 nm observations, results in spatially
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inconsistent weighting of the calculation of representative values inside each site
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box. To minimize such spatial weighting differences in the two bands, we reduced
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the number of 415 nm observations by applying an additional mask, which
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excludes the boundary regions where observations are sparse for the 321 nm
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band. We created a 321 nm observation accumulation map inside the site box
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and set the minimum threshold for a number of observations by the maximum
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number of observations divided by five, which was empirically determined to
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automatically mask the boundaries of the site box and the anomalous albedo
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areas. The areas that did not meet the threshold value in the accumulation map
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were masked. Examples of the distribution of 321 and 415 nm observations
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within a site box after application of the NAC albedo mask and the sparse
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observation mask are shown in Fig. 2a and 2b. On average, 111 and 3966 pixels
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were used for the 321 and 415 nm bands, respectively, at each sample site box.
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2.1.4. Curve Fitting to the Histogram
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A representative nI/F of each band at each site was derived from the modal
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value of the down-selected WAC observations. In order to avoid a singular-
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peaked mode of an irregularly shaped histogram, we fitted a curve to the his-
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togram by a least-square optimization, and the curve’s modal value was used as
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the representative nI/F . Since the histograms of nI/F for each site box did not
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always have a symmetric Gaussian shape, we employed a Skew Normal curve
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with an asymmetry adjustment for our fits (Fig. 2 bottom panels).
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Using the representative nI/F s of the two bands calculated in this final step,
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the 321/415 nm ratio was derived for each sample-return site. The uncertainty
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of each ratio value was estimated by 2-D standard deviation given as,
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σratio =
q 2 2 σ321/M + σM 415 321 /415
(1)
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where M321 and M415 are the representative nI/F of the 321 and 415 nm
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bands respectively, σ321/M415 is a standard deviation based on all the selected
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321 nm nI/F s divided by M415 , and σM321 /415 is a standard deviation based 10
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on M321 divided by the selected 415 nm nI/F s. We note that this uncertainty
270
includes the instrument calibration and photometric normalization uncertainties
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across the ∼62 months of repeat observations. The derived representative nI/F ,
the standard deviation of nI/F s of each band, the ratio, and the σratio for the
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selected sample-return sites are listed in Table 1.
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2.2. Correlation between WAC 321/415 and Soil TiO2
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For the lunar soil TiO2 values, we used the compositional data compiled and
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summarized by (Blewett et al., 1997) and (Jolliff, 1999). The selected WAC
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321/415 nm ratios and the lunar soil TiO2 show a positive linear correlation
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(Fig. 3). The least-squares linear fit has a slope = 0.010, y-intercept = 0.689,
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and R2 = 0.98.
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The samples from Luna 16 and Luna 24 are far off the Apollo sample trend
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line, thus these two samples were considered as outliers and excluded from the
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fitting. Previous works (Blewett et al., 1997; Lucey et al., 2000; Gillis et al.,
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2003) also reported these two samples as anomalies in their regression, which is
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consistent with our plots. The Luna 24 site is in close proximity to a fresh crater
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(Robinson et al., 2012). It is likely that Luna 24 samples are excavated materials
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from a buried unit, representing materials with a composition varying from the
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surface (Robinson et al., 2012; Denevi et al., 2014; Coman et al., 2016). The
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cause of Luna 16 sample discrepancy is not clear. The Luna 16 site is located
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at the northwestern edge of Langrenus crater ejecta (highland sourced deposits;
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see Fig. 14d). Mixtures of local mare materials with highland ejecta may have
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produced inhomogeneous soils, possibly resulting in the anomalous TiO2 content
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relative to the trend of Apollo samples.
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The spectra of Apollo soils revealed a clear positive correlation between
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the TiO2 content and the 321/415 nm ratio, except for low TiO2 soils (<∼2
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wt%) where pyroxene content begins to dominate the ratio, resulting in a poor
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correlation with the TiO2 content (Gillis-Davis et al., 2006; Coman et al., 2016).
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Therefore, we report only TiO2 values greater than 2 wt% using the WAC
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321/415 nm ratio, and consider lower values to be below the detection limit. 11
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In non-mare regions, the 321/415 nm ratio is likely controlled by various
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non-ilmenite sources, such as the shock degree (impact origin) of plagioclase
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and the presence of glass (Denevi et al., 2014) in addition to the other minerals
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(e.g., pyroxene). Therefore, our TiO2 map should be considered to be valid only
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for the mare regions.
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2.3. WAC TiO2 Map Overview
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From the correlation between the representative ratio values and the lunar
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soil TiO2 values, we converted the WAC 321/415 nm ratio map to a TiO2 (wt%)
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abundance map (Fig. 4). The ratio map was derived using the near-global WAC
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color mosaic (400 m/pixel) (Sato et al., 2014). This mosaic was photometrically
309
normalized (incidence = phase = 60° and emission = 0°) using emission and
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incidence angles computed at each pixel based on the GLD100 DTM (Scholten
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et al., 2012), resulting in reflectance on topographic slopes being more accurately
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normalized (e.g., Robinson and Jolliff, 2002).
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In our new TiO2 map (Fig. 4), the highest values are recognized in the
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northwestern portion of Mare Tranquillitatis (∼12.6 wt%), center of Oceanus
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Procellarum (∼11.6 wt%), and several Dark Mantle Deposits (DMD; Sinus Aes-
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tuum, Rima Bode, and Mare Vaporum; ∼12 wt%). The highest TiO2 values
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(18-20 wt%) were found at several small spots (∼10 by 10 pixels, ∼4 by 4 km
across) in Sinus Aestuum that likely correspond to freshly excavated DMDs as
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the ejecta of small craters (<1 km in diameter). However, the values associ-
320
ated with DMDs and other pyroclastic deposits are suspect since the increasing
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UV/visible ratio with increasing TiO2 content does not apply to glasses (Wells
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and Hapke, 1977; Wilcox et al., 2006).
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To focus on characterizing the global TiO2 distribution of the maria, we
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324
distinguished two groups (see Table 3 for a selected mare list and the group
325
affiliations). Group 1 comprises the 17 largest maria. Smaller mare deposits are
326
generally contaminated with highland ejecta due to the proximity of the high-
327
land/mare boundaries, and thus are excluded from Group 1. Group 2 comprises
328
the same maria as Group 1, but excludes Mare Frigoris, Mare Fecunditatis, and 12
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329
Mare Ingenii because these three maria have exceptionally large areas with TiO2
330
below the detection limit (<2 wt%). Comparing the two groups highlights the
331
areal fraction of these three very low-TiO2 maria out of the 17 maria. To define the mare areas we used mare boundaries as mapped from Clemen-
333
tine and LROC WAC data by Nelson et al. (2014). The Aristarchus plateau,
334
Montes Harbinger (east of the Aristarchus plateau), and red spots in the south-
335
ern portion of Oceanus Procellarum (included as mare areas in Nelson et al.,
336
2014) were classified as non-mare regions since these features are not associated
337
with mare basalts. Histograms of the global TiO2 estimates from the Group 1
338
and Group 2 maria are shown in Fig. 5. The Group 1 maria have an average
339
of 3.9 wt% TiO2 compared to 4.2 wt% for Group 2 mare regions (Fig. 5). The
340
areas with TiO2 below the detection limit (2 wt%) are 28% in Group 1 maria
341
(Fig. 5 cumulative plot) and 20% in Group 2 maria. Three geologic settings
342
are likely responsible for these low TiO2 areas: highland ejecta contamination,
343
swirls, and mare formed from low TiO2 basalts.
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Mare regions contaminated by highland impact ejecta exhibit lower TiO2
345
estimates not representative of the original underlying surface (Fig. 6, blue re-
346
gions). For example the rays of Aristarchus crater (40 km diameter) ejecta (Fig.
347
7) spread out underlying highland materials radially to the south and south-
348
west where the mare exhibit lower TiO2 values (mostly below 2 wt%) relative
349
to the adjacent mare surfaces (5-10 wt%) free from the ejecta. Lunar swirls
350
(e.g., Reiner Gamma and those in Mare Ingenii and Mare Marginis; Fig. 6,
351
green regions) have computed TiO2 values significantly lower than the detec-
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344
tion limit (negative values). Such extremely low values are due to the spectral
353
characteristics of immature swirl surfaces or the presence of glass, both of which
354
would decrease the 321/415 ratio irrespective of ilmenite content (Denevi et al.,
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355
2014, 2016). Several mare units exhibit low TiO2 abundance likely due to their
356
original composition (no clear sign of extensive ejecta contamination), such as
357
Mare Frigoris, the northern portion of Oceanus Procellarum, and the northeast-
358
ern portion of Mare Imbrium. Details of TiO2 distribution within each mare in
359
Group 1 are discussed in section 5.1. 13
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360
3. Comparison with Clementine TiO2 Maps We compared the new WAC TiO2 map (hereafter called WACTiO2 ) with
362
Clementine TiO2 abundance maps. Among various Clementine TiO2 maps de-
363
rived by their own approaches with different conversion parameters (Blewett
364
et al., 1997; Lucey et al., 1998; Jolliff, 1999; Lucey et al., 2000; Gillis et al.,
365
2003; Gillis-Davis et al., 2006), we compared the maps created by Lucey et al.
366
(2000) (hereafter called CLMTiO2 L; historically well known in the lunar science
367
community) and by Gillis et al. (2003) (hereafter called CLMTiO2 G; the latest
368
map with a revised algorithm). The CLMTiO2 L is derived by the following
369
algorithm,
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TiO2 = 3.708 × arctan
R415 R750
− 0.42
R750
!5.979
(2)
where R415 and R750 are the reflectance at 415 and 750 nm, respectively. The
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CLMTiO2 G is derived from a revised algorithm based on Lucey et al. (2000), R415 15.5 R750 −0.36 415 , if R415 ≤ 0.065 and R 0.14 + 0.24 × arctan R750 +0.012 R750 > 0.59 R415 9.8817 TiO2 = −0.36 1.7159 × arctan RR750+0.012 , otherwise 750 (3)
To minimize any geographic mismatch due to misregistration between the
371
WAC and original Clementine mosaics, we reduced the resolution of both TiO2
372
maps to 32 pixel/degree (947.6 m/pixel at the equator). We also excluded areas
373
with topographic slopes greater than 5° (computed from GLD100, Scholten
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370
et al., 2012) to minimize residual errors in photometric normalization on slopes
375
(McEwen, 1991) in the Clementine map products. For this comparison, in areas
376
where the WACTiO2 was below 2 wt% in the mare, the values were set to 1
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377
378
wt% with ±1 wt% uncertainty.
The CLMTiO2 L (Fig. 8) has higher TiO2 abundances relative to the WACTiO2 ,
379
and the offsets gradually increase with TiO2 values. On average for the entire
380
maria, the CLMTiO2 L is 0.8 wt% higher (Fig. 8a) relative to the WACTiO2 .
381
The difference map (Fig. 8b, CLMTiO2 L - WACTiO2 ; centered on the nearside 14
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maria) shows clear offsets in the high TiO2 abundance maria, such as Mare
383
Tranquillitatis (up to +6.9 wt%, +2.9 wt% in average), Mare Fecunditatis (up
384
to +4.6 wt%, +1.6 wt% in average), and Oceanus Procellarum (up to +4.8 wt%,
385
+1.1 wt% in average). On the other hand, the northern portions of Oceanus
386
Procellarum and Mare Imbrium have negative values (down to -2 wt%), mean-
387
ing that the CMLTiO2 L has slightly lower values relative to the WACTiO2 . All
388
of these areas with negative values are located in relatively high latitudes, which
389
may suggest a latitudinal trend possibly due to artifacts related to photometric
390
normalization or a latitude dependent maturity effect (Hemingway et al., 2015)
391
(see section 5.1.12). The other maria at high latitudes (e.g., Mare Australe,
392
Mare Frigoris, and Mare Humboldtianum) are small in size (and thus may be
393
contaminated with highland materials) or have low-TiO2 abundance (<2 wt%,
394
unable to compare with WACTiO2 ), both of which make it difficult to further
395
examine any possible latitudinal effect.
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The offset trends between WACTiO2 and CLMTiO2 G are clearly related
397
to the fact that CLMTiO2 G used two different equations to calculate TiO2
398
depending on the reflectance values (see equation 3). This results in some
399
values near or slightly above the 1:1 line with the WACTiO2 (those calculated
400
with the first version of equation 3), but a large portion that fall substantially
401
below the 1:1 line (TiO2 values calculated with the second version of equation
402
3). On average the difference of WACTiO2 - CLMTiO2 G is -0.3 wt% for the
403
entire maira.
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396
As seen in the difference maps of CLMTiO2 L - WACTiO2 (Fig. 8b) and
404
CLMTiO2 G - WACTiO2 (Fig. 9b), there are clear vertical stripes particularly
406
in high-TiO2 maria (e.g., Mare Tranquillitatis, Mare Fecunditatis, and Oceanus
407
Procellarum), which are due to artifacts in the Clementine global mosaic. The
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408
WAC near-global mosaic is derived from the median of 21 months of repeat ob-
409
servations, and the photometric normalization is optimized within each tile (1°
410
latitude by 1° longitude) (Sato et al., 2014), resulting in a significantly improved
411
accuracy relative to the Clementine mosaics. On the other hand, the WACTiO2
412
has a detection limit of 2 wt% TiO2 to avoid non-ilmenite source factors (see sec15
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tion 2.2). The algorithms of the Clementine-based TiO2 calculation are reported
414
to minimize the maturity effect (Lucey et al., 1998) , whereas significantly im-
415
mature materials may increase the uncertainties in WACTiO2 due to the nature
416
of the 321/415 ratio (Denevi et al., 2014; Coman et al., 2016). The WACTiO2
417
and CLMTiO2 L are derived from one set of parameters while the CLMTiO2 G
418
utilizes two parameter sets, resulting in the clear offsets of TiO2 values along
419
the condition-matching boundaries. In both the WAC and Clementine based
420
TiO2 maps, TiO2 abundances higher than the Apollo 17 sample-return sites
421
(>10 wt%) are derived simply by extrapolation with unknown uncertainties.
422
More soil samples particularly from the highest TiO2 maria (e.g., Mare Tran-
423
quillitatis, Oceanus Procellarum) in addition to medium TiO2 soils (4-6 wt%,
424
missing range in Fig. 3) are necessary for more accurate TiO2 estimates.
425
4. Comparison with Lunar Prospector TiO2 Map
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Next, we compared the WACTiO2 with the TiO2 abundance derived from
427
Lunar Prospector GRS observations (Elphic et al., 2002; Prettyman et al., 2006)
428
(hereafter called LPTiO2 ). The LPTiO2 was sampled at 2 degrees per pixel (61
429
km pixel scale at the equator) whereas the WACTiO2 is 400 m/pixel at the
430
equator. The area over which the GRS detector is sensitive is larger than the
431
mapped pixel scale, but the detector sensitivity gradually decreases with dis-
432
tance from the nadir point, which is difficult to directly compare with WACTiO2
433
data down-sampled by an average or median value of the footprint area. To ac-
434
curately simulate the spatial response function (footprint weighted by distance
435
from the nadir point) of the GRS (see Fig. 27 in Prettyman et al., 2006), we
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426
used a Gaussian weight function W given by (x/d)2 W (x) = a exp − 2c2
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436
437 438
(4)
where a = 0.48, d = 150, c = 0.6, and x is the distance from the nadir point in
kilometer. The weighted value Vw was then calculated by Pn Vi W (xi ) Vw = Pi=1 n i=1 W (xi ) 16
(5)
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where Vi and xi are the WACTiO2 value and the distance respectively for all the
440
map pixels (400 m/pixel) within 250 km radius circular area (in orthographic
441
projection) centered at each down-sampled pixel. As with the Clementine TiO2
442
vs. the WAC TiO2 comparison, we performed the comparison only for the
443
maria, and WACTiO2 values below 2 wt% in the mare were set to 1 wt% with
444
±1 wt% uncertainty.
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445
The LPTiO2 vs. WACTiO2 plot (Fig. 10a) shows significant scatter but
446
the points are clustered along a 1:1 line. The median of each 0.5 wt% bin lies
447
along the 1:1 line within ±0.7 wt% tolerance in the Group 1 maria (Table 3, see section 2.3 for the definition); the average of standard deviation for all bins
449
is 1.5 wt%; the R2 value of the 1:1 line is 0.52. The difference map (Fig. 10b,
450
LPTiO2 - WACTiO2 ) highlights that the LPTiO2 is lower (blue areas) in several
451
maria (e.g., Serenitatis, -0.5 wt%; Humorum, -0.5 wt%; Crisium, -0.5 wt% on
452
average) and is higher (red areas) in the eastern portion of Oceanus Procellarum
453
(+2.6 wt% on average), relative to the WACTiO2 . The eastern high LPTiO2
454
portion of Oceanus Procellarum corresponds to the region covered by the ejecta
455
and rays from Aristarchus and Kepler craters. The effective depth of the GRS
456
measurement is deeper (30 cm) (Lawrence et al., 2002) than the UV/visible
457
reflectance of the WAC (few millimeters) (Hapke, 2012). Thus, in the areas
458
extensively covered by a layer of ejecta thinner than the GRS sensible depth
459
(<30 cm, Lawrence et al., 2002), the TiO2 estimates by the two instruments
460
could result in an offset due to their different sampling depths.
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The correlation between the two TiO2 maps was further examined using
461
Mare Imbrium low- and high-TiO2 units that have compatible sizes with the
463
large pixel scale of the GRS (2 degrees per pixel). We defined seven units (Fig.
464
11a, H1-3 and L1-4) that outline the boundaries of TiO2 abundance units in
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465
the WACTiO2 . Generally, the units H1-3 indicate relatively high-TiO2 and
466
the units L1-4 are for low TiO2 . The WACTiO2 and LPTiO2 average values
467
(Table 4) for each unit fall near the 1:1 line (within ±0.8 wt% for average values
468
of each unit, R2 = 0.74). The units L2 and L4 fall on mare boundaries and
469
are thus likely contaminated with highland materials due to the GRS’s large 17
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footprint and the highland ejecta. The five major maria (Mare Tranquillitatis,
471
Oceanus Procellarum, Mare Imbrium, Mare Serenitatis, and Mare Crisium) are
472
also plotted in Fig. 11b. Each point corresponds to an average and standard
473
deviation of the entire area of each mare. All five maria are plotted within ±0.3
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470
474
wt%, showing a good agreement (R2 = 0.94 along the 1:1 line) between the
475
WACTiO2 and LPTiO2 .
476
A pixel-by-pixel comparison of LPTiO2 and WACTiO2 indicates significant
477
scatter of up to ±4 wt% (Fig. 10), likely due to the large footprint and the
lower signal-to-noise ratio of GRS observations relative to the WAC observations
479
(high signal-to-noise ratio by repeated observations). The averaged TiO2 values
480
of each mare (and large mare units), however, showed good correlation between
481
the LPTiO2 and WACTiO2 (Fig. 11b), demonstrating the broadly consistent
482
estimates from the two instrument observations.
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To summarize the comparisons of TiO2 estimates based on three independent
484
datasets (the WAC, Clementine, and Lunar Prospector GRS), the CLMTiO2 L
485
have generally higher values than WACTiO2 , particularly in the high-TiO2
486
maria. The CLMTiO2 G has an average value close to the WACTiO2 (-0.5
487
wt% difference) and shows clear offsets along the boundary of the two calcula-
488
tion conditions. The LPTiO2 values are comparable with WACTiO2 in the five
489
major maria (R2 = 0.94) and in each unit of Mare Imbrium (R2 = 0.74). The
490
LPTiO2 is unaffected by maturity variations and is unique in that it is a mea-
491
sure of the integrated titanium abundance of the top ∼30 cm, minimizing effects
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483
from surface contamination by thin ejecta deposits. In terms of the observation
footprint (particularly compared to the GRS), signal-to-noise ratio, and the to-
494
pographic correction in the photometric normalization, the WACTiO2 has large
495
advantages.
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496
5. Discussion
497
Based on our WAC TiO2 abundance map, here we discuss the local TiO2
498
variations in each mare in the context of the surrounding geologic settings and 18
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the correlation between the TiO2 abundance and the surface age of the entire
500
maria.
501
5.1. TiO2 Variation within Major Maria
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We investigated the spatial variation of TiO2 in the WACTiO2 of 17 out of 23
503
maria defined in Andersson and Whitaker (1982). Each of the 17 maria (Group
504
1, Table 3) has at least one large mare unit that is not dominantly contaminated
505
by highland ejecta materials. For each mare, we derived the median, average,
506
standard deviation, and maximum values of TiO2 (Table 3), as well as the
507
histogram (Fig. 12) and the spatial distribution of TiO2 (Fig. 13-15) using the
508
whole mare area (defined by Nelson et al., 2014). We utilized absolute model
509
ages (derived from crater size frequency distributions; e.g., Morota et al., 2009;
510
Hiesinger et al., 2010) to discuss several flow units of the mare basalts outlined
511
by the WACTiO2 map. Although our flow units are not identical to the units
512
defined by the model age works due to the use of different basemaps (e.g., Galileo
513
and Clementine color ratio composite, Hiesinger et al., 2010), we used the ages
514
derived from the units geographically included or overlapping our flow units.
515
5.1.1. Oceanus Procellarum
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Oceanus Procellarum exhibits sharp TiO2 abundance boundaries likely due
517
to variations of the magma sources, and in some areas, mixing of ejecta from
518
Aristarchus and other craters (Fig. 13a). We note that TiO2 values near
519
Aristarchus crater (<190 km from the crater center) are low (<2 wt%) compared
520
to the surrounding mare surfaces (6-10 wt%) outside of the extent of ejecta rays.
521
Ejecta from Kepler crater (29 km in diameter) mixed with nearby mare (<120
522
km from the crater center) radially lowers the mare TiO2 abundance. Both of
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516
these Copernican craters (Wilhelms et al., 1987) excavated mare basalts along
524
with highland materials. There is a sharp boundary between high-TiO2 (>6
525
wt%) and medium-TiO2 (<6 wt%) units about 400 km west of the Aristarchus
526
plateau and it arcs about 1000 km from Seleucus crater to Rima Sharp (outlined
527
by black triangles). The high-TiO2 areas (6-12 wt%, southeastern side of this
AC 523
19
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boundary toward Aristarchus Plateau) have a younger model age (1.33–2.96 Ga)
529
relative to the northwestern mid-TiO2 areas (2-6 wt%, 3.40-3.47 Ga) (Hiesinger
530
et al., 2010). The far northern part of Oceanus Procellarum (denoted by "L")
531
shows low TiO2 abundances (<2 wt%), partially due to the ejecta of Pythagoras
532
crater (145 km in diameter, 250 km north from the northern edge of Procel-
533
larum). The Reiner Gamma albedo anomalies (swirls) show significantly lower
534
TiO2 values (although these values are not reliable due to the spectral charac-
535
teristics of immature swirl surfaces or the presence of glass, see section 2.3).
536
5.1.2. Mare Imbrium
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528
As highlighted in Fig. 11a, Mare Imbrium has large TiO2 units with rela-
538
tively sharp boundaries (Fig. 13b). The highest TiO2 (4-10 wt%) unit is found
539
in the center of the mare (H1, see Fig. 11a for the unit locations). The Chang’e-
540
3 spacecraft landed on and analyzed the regolith near the northeastern edge of
541
this unit (Neal et al., 2015; Ling et al., 2015). The measurements of the Ac-
542
tive Particle-induced X-ray Spectrometer (APXS) aboard Chang’e-3 reported
543
5 wt% TiO2 (sensitive to a depth of several microns; effective detection area
544
∼50 mm in diameter, Fu et al., 2014; Ling et al., 2015), close to the WACTiO2
M
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537
estimate of 6.6 wt% (at 400 m/pixel). The lowest TiO2 estimates (<2 wt%) are
546
found in the northeastern part of this mare (unit L1). The ejecta of Copernicus
547
crater extensively affects the southern third of Mare Imbrium, mostly unit H3.
548
The highland ejecta from Aristillus crater (55 km in diameter) located at the
549
eastern edge of this mare contaminates the eastern portion of unit L3 (Fig. 11),
550
decreasing the TiO2 value. The histogram of Imbrium TiO2 values (Fig. 12)
551
shows a semi-bimodal distribution that reflects the high (H1-3) and low (L1-4)
552
TiO2 units. Sharp low-high TiO2 boundaries (free from the highland ejecta con-
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545
tamination), particularly the H1-L1 boundary, outline flow units of mare basalt
554
possibly generated by multiple eruption events that each had distinct composi-
555
tions. The northern part of H1 unit is younger (2.96 Ga, Hiesinger et al., 2010;
556
I22 unit) than the western part of the L1 unit (sharing the boundary with H1;
557
3.01-3.52 Ga Hiesinger et al., 2010; I5, I11, I17, and I21 units), suggesting that
AC 553
20
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558
the TiO2 abundance was higher in the younger events.
559
5.1.3. Mare Cognitum Mare Cognitum can be divided into two spatially separated units (hereafter
561
called Cognitum East and Cognitum West). In Cognitum West (Fig. 13c),
562
the portion around the red spots (highland materials distinctly redder than
563
the surroundings, Whitaker, 1972; Bruno et al., 1991), Darney χ and Darney
564
τ , has the highest TiO2 abundance (6-8 wt%). The rays of Copernicus crater
565
(located at 450 km north of Cognitum) extensively affected the surface of the
566
northeastern portion of Cognitum West. The ejecta of craters Darney (15 km
567
in diameter, located outside of the mare near the southern edge) and Darney
568
C (13 km in diameter, near the southwestern edge of the mare) affected the
569
southern portion of Cognitum West. These ejecta and rays both contribute to
570
lowering the surface TiO2 content locally.
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560
In Cognitum East (Fig. 13d), a low TiO2 (<2 wt%) unit is dominated by
572
the red spot (Lassell Massif) and Lassell C crater (8.7 km in diameter). Ashley
573
et al. (2016) reported possible post-silicic volcanism at the Lassell Massif and
574
low concentrations of FeO and TiO2 (FeO < ∼2 wt%, TiO2 < ∼4 wt% from
M
571
their Fig. 8) on the Massif area based on Clementine data. It is still not clear if
576
such silicic volcanism is responsible for the low TiO2 unit (Fig. 13d) extending
577
about 50 km radius from the Lassell C crater. The TiO2 gradually increases
578
from both the western and eastern sides of this mare (2–4 wt%) without obvious
579
highland ejecta contamination toward the central high-TiO2 zone (up to 8 wt%)
580
meandering from the north to the south of Lassell Massif (indicated by dashed
581
line in Fig. 13d), suggesting that the varying TiO2 content reflects the changing
582
compositions of the mare basalt over time or within each eruption event.
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583
5.1.4. Mare Humorum
584
The northeastern portion of the largest mare unit in Mare Humorum has
585
the highest TiO2 abundance of all units in Humorum (8-10 wt%, see Fig. 13e).
586
In the western half and southeastern margin, TiO2 abundance ranges from 2-6
21
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wt% with no clear unit boundaries. The ejecta of small young craters (e.g.,
588
Gassendi J, Gassendi O, Gassendi Y, Gassendi L, Doppelmayer K, Puiseux
589
D; 5-8 km in diameter) show lower TiO2 content relative to the surrounding
590
surface, indicating that the uppermost layer (latest eruptions) contains more
591
TiO2 . The DMDs along the fractures at the southwestern margin (Hawke et al.,
592
2010) have moderate TiO2 abundances (2-6 wt%, suspicious value due to the
593
glass spectra, see section 2.3). The lowest TiO2 value (<2 wt%) along the
594
southeastern boundary is likely due to admixing of immature ejecta from Tycho
595
crater.
596
5.1.5. Mare Nubium
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587
Bullialdus crater (61 km in diameter) and its ejecta are located in the middle
598
of Mare Nubium and are masked in this analysis (Fig. 13f). A ray of Tycho
599
crater (350 km south of this mare) crosses the western portion in the SSE-
600
NNW direction, likely decreasing the derived TiO2 values (0-2 wt% TiO2 ). The
601
highest TiO2 values (6-10 wt%) are found in the northeastern portion (Fig.
602
13f, indicated by white arrow), in a ∼50 by 50 km spot surrounded by a low
M
597
TiO2 (<2 wt%) unit. The northwestern area of this mare has complicated
604
boundaries due to patchy kipukas of highland units and generally has medium
605
to low TiO2 values (2-6 wt%). The central spot, with a computed TiO2 value
606
substantially below the detection limit (Fig. 13f, indicated by black arrow), has
607
no topographic relief and no indication of contamination by ejecta, suggesting
608
very low TiO2 basalts with distinct spectral properties in this spot.
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609
5.1.6. Mare Serenitatis Most of Mare Serenitatis exhibits low to medium TiO2 abundance (2-6 wt%,
611
Fig. 14a). A ray crossing in the NNE-SSW direction, possibly from Tycho
612
crater (2000 km southwest of Serenitatis; Giamboni, 1959; Carr, 1966; Wilhelms
613
et al., 1987; Campbell et al., 1992), results in lower TiO2 values (<2 wt%) and
614
likely affected the areas surrounding the ray where TiO2 values are 2-4 wt%
615
by the mixing of original mare and ejecta materials. Other rays from Aristillus
AC
610
22
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crater (54 km in diameter, located to the west) and Eudoxus crater (70 km in
617
diameter, located to the north) affect the northern part of the mare, causing
618
TiO2 values lower than the nearby mare surface. The regional dark mantle
619
deposit (RDMD) (Weitz et al., 1998) found in an arc on the southeast margin
620
of the mare (indicated with a black arrow) has the highest TiO2 (>10 wt%);
621
however, this value is suspect due to the peculiar spectral response of pyroclastic
622
glasses (see section 2.3). Across the southern margin, including the boundary
623
with Mare Tranquillitatis, the basalts have the highest values (excluding suspect
624
values at RDMD) of TiO2 (6-10 wt%) . The sharp boundary (outlined by black
625
triangles in Fig. 14a) between these high TiO2 edges and the central mare unit
626
with medium TiO2 (4-6 wt%) is likely due to the compositional change of mare
627
basalt from eruption to eruption. The southern high TiO2 unit inside Serenitatis
628
and the northern high TiO2 unit inside Mare Tranquillitatis are probably the
629
same flow unit, which is slightly older (3.55 Ga (Hiesinger et al., 2010), unit
630
S11) relative to the adjacent medium TiO2 unit inside Serenitatis (∼3.28-3.44
631
Ga, unit S22 and S15). This is one of the sharp TiO2 offset boundaries showing
632
a direct contact of an older high-TiO2 unit and relatively young medium-TiO2
633
unit. See section 5.2 for a global analyses of the TiO2 variations over time.
634
5.1.7. Mare Crisium
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616
The eastern portion of Mare Crisium is high in TiO2 (6-8 wt%), with multiple
636
kipukas of highland material (Fig. 14b). The ejecta of an unnamed 2-km crater
637
has the highest local TiO2 value (∼8.5 wt%), indicating that the underlying layer
638
has higher TiO2 content relative to the uppermost surface. The ejecta of Picard
639
crater (22 km in diameter, located in southwestern Crisium) has higher TiO2
640
value (6-8 wt%) than the surrounding surface as well. The majority of Crisium
CE
PT
635
has intermediate TiO2 values (2-6 wt%). The ejecta of the highland crater
642
Proclus (27 km in diameter, 100 km west of Crisium) affects the northwestern
643
portion of Crisium, likely lowering TiO2 values (<2 wt%).
AC 641
23
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644
5.1.8. Mare Tranquillitatis The highest mare TiO2 abundances (>10 wt%) are in the northwestern
646
part of Mare Tranquillitatis (Fig. 14c), while in the southern half there are
647
patchy areas of lower TiO2 (4-8 wt%), likely due to both true compositional
648
variations of the mare and contamination by ejecta. The ejecta of Theophilus
649
crater (99 km in diameter, located about 100 km south of Mare Tranquillitatis)
650
overlies the southern portion of Tranquillitatis, likely decreasing the apparent
651
TiO2 abundance of the underlying mare. The northeastern part of the mare
652
has lower TiO2 values (<2 to 6 wt%). Römer crater (44 km in diameter) at
653
∼60 km north from the northern edge of Tranquillitatis also supplied ejecta to
AN US
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645
that region, decreasing the TiO2 as well (<2 wt%). There is no obvious sign of
655
overlying highland-sourced materials along the high-low TiO2 boundaries (8-10
656
wt% to 2-6 wt%) at the base of the horn-shaped area in the northeastern portion
657
(black arrows, Fig. 14c), likely indicating a compositional change in the mare
658
basalts with different flow units in a narrow time span (Hiesinger et al., 2000).
659
5.1.9. Mare Fecunditatis
M
654
Most of the southeastern part of Mare Fecunditatis (Fig. 14d) is extensively
661
covered by the ejecta from Langrenus (127 km in diameter) and Petavius B (33
662
km in diameter) craters, both located in the highlands close to the southeastern
663
margin of Mare Fecunditatis. The central part of this mare has patchy varia-
664
tions of TiO2 ranging from 2 to 8 wt%, thus likely resulting from mixtures of
665
ejecta from Langrenus and local compositional variations (4-8 wt%) of the mare
666
basalts. Both ejecta blankets resulted in the lowest TiO2 abundances (<2 wt%).
667
Ejecta of Taruntius crater (56 km in diameter) and Cameron crater (10 km in
668
diameter, located on the rim of Taruntius) also altered the northern surface of
AC
CE
PT
ED
660
669
this mare, resulting in low TiO2 (<2 to 4 wt%). The highest TiO2 values (8-10
670
wt%) are found at the northeastern edge of the mare.
24
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671
5.1.10. Mare Marginis The upper half of this mare is covered by swirls, resulting in unreliably low
673
(<2 wt%) computed TiO2 values (Fig. 14e), as described in section 2.3. Also,
674
the high reflectance ejecta materials from Goddard A crater (11 km in diameter)
675
contaminate the northeastern edge of the mare, likely lowering the TiO2 values.
676
The southern half of this mare is relatively unaffected by the swirls and ejecta
677
and has medium TiO2 values (2-6 wt%). The second large mare unit inside
678
Neper crater (144 km in diameter) also has an undisturbed surface, showing
679
low to medium TiO2 values (<2 to 4 wt%).
680
5.1.11. Mare Smythii
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672
Mare Smythii generally has intermediate TiO2 values (2-6 wt%) with little
682
spatial variation (Fig. 14f). Peek crater (12.5 km in diameter) located on the
683
northwestern side of this mare excavates the mare and probably the subsurface
684
highland materials, resulting in ejecta with lower (∼2 wt%) TiO2 values. The
685
ejecta of Hume Z crater (14.9 km in diameter) and an unnamed crater (3.5 km
686
in diameter) contaminate the southeastern edges of this mare, resulting in 2-4
687
wt% TiO2 . The central portion of Smythii probably shows the original TiO2
688
values (4-6 wt%).
689
5.1.12. Mare Frigoris
ED
M
681
Previous works have suggested that Mare Frigoris has unique compositional
691
characteristics of low TiO2 and low FeO values relative to other maria (Johnson
692
et al., 1977; Whitford-Stark, 1990; Lucey et al., 2000; Kramer et al., 2015). In
693
the WAC TiO2 , 93% of Mare Frigoris is lower than the detection limit (Fig.
694
15a). The western portion of this mare with about 4 wt% TiO2 may be an
CE
PT
690
extension of the unit that makes up the north end of Oceanus Procellarum, but
696
is separated by the ejecta of Harpalus crater (40 km in diameter). This mare is
697
a relatively narrow E-W strip, and thus the whole area is close to the highlands
698
relative to other maria, possibly resulting in more highland ejecta contamination
699
such as from the rays from Anaxagoras crater that cross much of Frigoris. Thus
AC 695
25
ACCEPTED MANUSCRIPT
the anomalously high albedo and lower TiO2 values may simply be a result of
701
highland contamination. Alternatively, Hemingway et al. (2015) suggest that
702
there is a latitudinal trend in maturity decreasing toward the pole, which is
703
responsible for the higher albedo of Mare Frigoris relative to the equatorial
704
mare with similar FeO and TiO2 content. The presence of this trend was also
705
confirmed using the active reflectance measurements by Lunar Orbiter Laser
706
Altimeter (LOLA) (Lemelin et al., 2016). This high latitude immaturity might
707
also be responsible for the low calculated TiO2 abundance in the majority of
708
Frigoris, similar to the immature swirls that result in anomalously low values of
709
TiO2 calculated from the WAC data.
710
5.1.13. Mare Tsiolkovskiy
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700
The southern and eastern portions of this mare have relatively high TiO2 (up
712
to 6 wt%, Fig. 15b). There is no clear indication of significant contamination
713
from craters outside of the mare. There are subtle TiO2 variations from 2 to 6
714
wt% without clear unit boundaries, possibly suggesting a gradual compositional
715
change as the mare basalts were emplaced.
716
5.1.14. Mare Moscoviense
ED
M
711
The majority of the eastern half of Mare Moscoviense (Fig. 15c) exhibits high
718
TiO2 values (6-10 wt%), whereas the western half has low to medium TiO2 (<2
719
to 4 wt%). The bimodal distribution of the Mare Moscoviense histogram (Fig.
720
12) is due to the eastern and western units having an obvious compositional
721
difference in their mare basalts. No clear age difference was found between the
722
eastern and western mare units (east: 2.6-3.6 Ga; west: 3.5 Ga, Morota et al.,
723
2009). The western edge of this mare is partially overlain by swirls, overprinting
724
low TiO2 (<2 wt%) on the western unit (originally 2 to 4 wt%). Rays of Steno
725
Q crater (33 km in diameter, 150 km to the east of Moscoviense) cross the
726
eastern high TiO2 unit, leaving multiple streaks that lower the surface TiO2
727
values about 2 to 4 wt% relative to the surroundings.
AC
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717
26
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728
5.1.15. Mare Ingenii Mare Ingenii embays the floors of Thomson crater (113 km in diameter),
730
Thomson M crater (114 km in diameter), and the central to the southern por-
731
tion of Ingenii basin (Fig. 15d). Most of the southern part of this mare and
732
roughly 1/3 of the mare in Thomson crater have very low computed TiO2 val-
733
ues, which correspond to the extensive system of swirls that overlies much of
734
the region (see section 2.3). The low-reflectance portions next to the winding
735
high-reflectance areas of the swirls have higher TiO2 values than the surround-
736
ing mare surface (about +2 wt% increase), which might be due to the relation
737
between maturity and the 321/415 ratio as suggested by Denevi et al. (2014)
738
and Coman et al. (2016). The southeastern edge of Thomson crater including
739
the portion extending out of the crater, which is relatively unaffected by swirls,
740
has 2-6 wt% of TiO2 .
741
5.1.16. Mare Orientale
AN US
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729
Mare Orientale consists of multiple small disconnected mare patches located
743
near the basin center and between the multi-ring structures (Fig. 15e). The
744
highest TiO2 values are 6-8 wt% at the southeastern portion of the largest
745
unit. The ejecta of Maunder crater (54 km in diameter, north side of the basin)
746
extensively overlays this unit, lowering the surface TiO2 values from <2 to 4
747
wt% for nearly all of the northern half of this unit. The southwestern portion
748
of this largest unit has low TiO2 values (<2 wt%), probably due to an actual
749
compositional variation in the basalts.
PT
ED
M
742
The second large mare unit, Lacus Veris, located along the inner ring, has
751
<2 to 6 wt% TiO2 . The ejecta of Maunder and other craters affect this unit,
752
decreasing the surface TiO2 values to <2 wt%. Other smaller units along the
AC
CE
750
753
northern portion of the inner ring are also covered by Maunder’s ejecta, and
754
show low TiO2 values (<2 wt%). The ejecta of an unnamed crater (∼2 km in
755
diameter, upper-left in Fig. 15e) exhibits larger TiO2 values (2-4 wt%) than the
756
surrounding mare surface (<2 wt%). This likely indicates that the uppermost
757
surface had been contaminated by highland ejecta first (from undefined multiple 27
ACCEPTED MANUSCRIPT
sources), then the more recent impact event of the unnamed crater exposed older
759
underlying mare basalts. The other units along the outer ring, including Lacus
760
Autumni, have TiO2 values up to 4 wt%.
761
5.2. Correlation between Age and TiO2 Abundance
CR IP T
758
The correlation between the age, locations, and the composition of mare
763
units has been discussed by several workers (e.g., Soderblom and Lebofsky,
764
1972; Head, 1976; Soderblom et al., 1977; Hiesinger et al., 2001; Kodama and
765
Yamaguchi, 2005; Morota et al., 2011). Early studies based on the Apollo and
766
Luna samples suggested that the low TiO2 basalts are generally younger than
767
high-TiO2 basalts (Taylor, 1982). Later interpretations of remote-sensing data
768
and lunar meteorites revealed the existence of relatively young, high-TiO2 and
769
old, low TiO2 basalts (e.g., Pieters et al., 1980; Cohen et al., 2000). In an exam-
770
ination using the global TiO2 map (CLMTiO2 L) and the crater size-frequency
771
distribution ages, no distinct correlation was observed between the deposit age
772
and the TiO2 values on a global scale (Hiesinger et al., 2001). However, several
773
regional studies suggest that the younger basalts tend to have relatively high-
774
TiO2 , and the low TiO2 basalts are relatively old (Imbrian Period, 3.2–3.8 Ga)
775
(Kodama and Yamaguchi, 2005; Morota et al., 2011).
ED
M
AN US
762
We re-examined the age vs. TiO2 abundance correlation of the mare basalts
777
at a global scale using the WACTiO2 map and the crater size frequency distribu-
778
tion model ages reported by various studies (Greeley et al., 1993; Hiesinger et al.,
779
2000, 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al.,
PT
776
2016). Instead of using the entire area of each mare unit defined by these works,
781
we used the subset areas (rectangular areas defined by Hiesinger et al., 2000,
782
2003, 2010) where the crater counts were performed (hereafter called “count
AC
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783
area”, see Fig. 16). Even a very thin dusting can change the optical character-
784
istics of the surface (thus WACTiO2 values) within each mare unit. Also in a
785
narrow sense, the model age can vary within a unit depending on the location of
786
count area (e.g., 1.0 to >2 Ga in P60 unit, Stadermann et al., 2015). Therefore,
787
comparing the age and the TiO2 values within the count areas minimizes possi28
ACCEPTED MANUSCRIPT
ble internal variations of age and TiO2 (due to actual compositional variations
789
and highland contamination) inside each unit. We note that in several mare
790
units (e.g., Mare Frigoris, Mare Fecunditatis, and Mare Humorum), we simply
791
used each unit area since no subset was identified as the count area in the model
792
age works. We also excluded several count areas (or the original unit areas) ob-
793
viously contaminated by highland ejecta, which do not represent the spectrum
794
of the original surface. Currently, a few major maria or mare units (e.g., Mare
795
Crisium, Mare Somniorum, and the southeastern unit of Mare Humorum) do
796
not have model ages, thus we excluded those maria. The TiO2 values below 2
797
wt% in WACTiO2 were set to 1 wt% before computing the median and standard
798
deviation for each count area. For the count areas that are dominantly below 2
799
wt% TiO2 (defined by the median = 1 wt%), ±1 wt% uncertainty was applied
AN US
800
CR IP T
788
instead of using the standard deviation.
801
The plot of age vs. TiO2 values (Fig. 17) using the count areas in all
802
maria exhibits a wide range in TiO2 (from <2 to 10 wt%) before ∼2.6 Ga,
and only medium- to high-TiO2 abundance (4-10 wt%) for ages <2.6 Ga. The
804
average before 2.6 Ga is 3.4 wt% TiO2 and after 2.6 Ga is 6.8 wt%. The
805
relatively high-TiO2 values for younger ages and low-TiO2 values with mostly
806
older ages are consistent with the trends observed by previous works (Pieters
807
et al., 1980; Kodama and Yamaguchi, 2005; Morota et al., 2011). In the plots for
808
the individual mare (Fig. 18), no clear trend was found except for several maria
809
(e.g., Oceanus Procellarum, Mare Humorum, and the southwestern nearside
810
maria). The dots clustered below 2 wt% (Fig. 17) dominantly consist of areas in
PT
ED
M
803
Mare Frigoris, Humboldtianum, and Australe Basin. Oceanus Procellarum has
812
a wider range of ages (from 3.63 to 1.21 Ga) relative to the other maria, and the
813
TiO2 values moderately increase with time (∼1 wt% per Gyr average). Although
AC
CE 811
814
the time range is limited, Mare Humorum and the southwestern nearside maria
815
also show increasing TiO2 trend over time from 3.8 to ∼3.0 Ga. After ∼2.8 Ga,
816
no count area has less than 4.5 wt% of TiO2 . The other maria have relatively
817
older ages ranging from 3.88 to ∼2.8 Ga, and some maria (e.g., Fecunditatis,
818
Serenitatis, and Tranquillitatis) have a wide range in TiO2 (from <2 to 10 wt%) 29
ACCEPTED MANUSCRIPT
while others (e.g., Vaporum, Marginis, and Nubium) have narrower TiO2 ranges.
820
These results suggest that old (>2.6 Ga) mare volcanic eruptions occurred in
821
various places with the widest variation in TiO2 contents, then younger (<2.6
822
Ga) eruptions mainly occurred in Oceanus Procellarum with relatively high-
823
TiO2 content.
824
5.3. Implications for mare volcanism
CR IP T
819
The nature of TiO2 enrichment in some mantle source regions is not well
826
understood. Presumably, at a late stage of magma ocean solidification, ilmenite
827
began to crystallize along with Fe-rich pyroxene, forming dense, gravitation-
828
ally unstable cumulates (Taylor and Jakes, 1974; Snyder et al., 1992; Hess and
829
Parmentier, 1995). Some portions of these cumulates were likely enriched in
830
incompatible elements, with relatively high concentrations of heat-producing
831
elements such as Th, U, and K (Haskin, 1998; Wieczorek and Phillips, 2000;
832
Korotev, 2000). Some of the high-TiO2 mare basalts are indeed enriched in
833
these elements, such as the high-K Apollo 11 basalts while other examples are
834
not (other high-TiO2 basalts from Apollo 11 and 17) (Taylor et al., 1991; Neal
835
and Taylor, 1992). Likewise, sampled KREEP basalts (enriched in potassium
836
(K), rare earth elements (REE), and phosphorous (P)) contain ∼2 wt% TiO2
ED
M
AN US
825
(e.g., Warren and Wasson, 1978; Salpas et al., 1987). Therefore it is possible
838
that Ti-rich cumulates and KREEP-rich residual melt were decoupled, probably
839
by gravitational separation, in some regions (e.g., Shearer et al., 2006). Alter-
840
natively, the basalts were enriched in incompatible elements by assimilation of
PT
837
KREEP at the base of the crust. Remote sensing by the Lunar Prospector
842
GRS shows Th enrichment over broad areas of the western Procellarum basalts
843
(Jolliff, 2004). We consider that this broadly enriched Th (3-5 ppm) over the
AC
CE 841
844
PKT area cannot be explained only by the direct excavation of sub-crustal
845
KREEP layer as inferred by the distributions of highest Th along the Imbrium-
846
circumferential highlands and the ejecta of several craters (e.g., Aristarchus,
847
Kepler, Lalande, and Aristillus). The mare basalt rock fragments with ilmenite
848
and high-Th content (6.9 ppm) from the Apollo 12 regolith samples also sug30
ACCEPTED MANUSCRIPT
gest possible Th enrichment in the mare basalts (Jolliff et al., 2005; Barra et al.,
850
2006; Korotev et al., 2011). Where the western Procellarum basalts dominate
851
the younger end of the age range for mare basalts, it is reasonable to infer, on
852
the basis of a correlation between TiO2 and incompatible trace elements (re-
853
flected by LP-GRS Th), a relationship between TiO2 and radiogenic-element
854
enrichment in their source regions.
CR IP T
849
In this case, we also infer that perhaps ilmenite cumulates and incompatible-
856
element-rich residual melt of the magma ocean were not decoupled as they
857
seem to have been in source regions of some of the older high-TiO2 basalts.
858
Having that enrichment in the source region might help to explain how basaltic
859
volcanism extended to ages in the range of 1-3 Ga on the Moon (see discussion
860
in Shearer et al., 2006, pp.467-468).
AN US
855
Understanding the accurate TiO2 distribution in mare basalts provides an
862
indication of TiO2 enrichment in underlying mantle source regions, and cor-
863
relations with Th enrichment in some areas suggests a possible petrogenetic
864
relationship, as inferred above for the western Procellarum basalts (e.g., Gillis
865
et al., 2002; Flor et al., 2002, 2003). Coupled with age data for various mare
866
units, we can gain an improved understanding of the thermal evolution of the
867
mantle. At about 2.6 Ga, the age-TiO2 relation (Fig. 17) suggests a transition
868
from a large range of TiO2 contents in widely distributed older maria to a more
869
restricted, medium to high TiO2 range for younger mare, mostly located in
870
Oceanus Procellarum (Fig. 17). What does this transition mean in the context
871
of mare volcanism?
PT
ED
M
861
The low TiO2 basalts (<5 wt%) represent the most abundant product of
873
partial melting of magma ocean cumulates (i.e., 70 vol% or more of the cumu-
874
late pile), whereas the medium- to high-TiO2 basalts (>5 wt%) are relatively
AC
CE 872
875
rare products from partial melting of late-stage cumulates (about 5-10 vol%
876
of the cumulate pile) based on the geochemical-thermal modeling (Taylor and
877
Jakes, 1974; Hess, 1991; Hess and Parmentier, 1995). Early mare eruptions (be-
878
fore the 2.6 Ga transition) were globally distributed, including several farside
879
examples (e.g., Mare Moscoviense, Mare Tsiolkovskiy, South Pole-Aitken). The 31
ACCEPTED MANUSCRIPT
large TiO2 variations in this period suggest multiple melt sources with a wide
881
range of TiO2 concentrations. Owing to sufficient heat in the early stage of
882
lunar thermal evolution (Wieczorek and Phillips, 2000), the melt source should
883
have existed at various depths including shallow regions. Indeed, petrology ex-
884
periments indicate that the Apollo 11 and 17 high-TiO2 basalts originated from
885
shallow mantle depths based on the equilibrium condition with a plausible cu-
886
mulate mineral assemblage using multiple saturation points (Longhi et al., 1974;
887
Walker et al., 1977; Grove and Krawczynski, 2009). The Th vs. age plot (Fig.
888
19, for all the count areas) exhibits larger variation in Th (0.1-11.3 ppm) in the
889
older basalts (>2.5 Ga), implying Th-independent melt production due to the
890
early hotter interior.
AN US
CR IP T
880
As the mantle cooled, partial melting zones were likely deeper and the flux
892
of basaltic melts to the surface should have diminished (Head and Wilson, 1992;
893
Hess and Parmentier, 2001; Shearer et al., 2006). The Ti-rich cumulates would
894
be enriched in the late-stage magma-ocean residual melt (urKREEP) (Warren
895
and Wasson, 1979), possibly deep seated as a result of mantle overturn (Hess and
896
Parmentier, 1995). In such cumulates, high Th (U and K as well) contents would
897
have prolonged the heat and the melt production (Wieczorek and Phillips, 2000),
898
possibly resulting in young high-TiO2 basalts (<2.6 Ga) in the Procellarum
899
KREEP Terrane (Jolliff et al., 2000). The Th contents in such young basalts
900
exhibit relatively high values (3.3-8.4 ppm) in the Th vs. age plot (Fig. 19).
PT
ED
M
891
901
We note here that our interpretation inferring the possible correlation be-
902
tween Th enrichment and titanium content of the relatively young basalts is based largely on the remote-sensing observations. Currently, there is no petro-
904
logical consensus supporting a direct connection of TiO2 and Th (or other ra-
905
dioactive elements) in the context of mare volcanism. The returned samples are
AC
CE 903
906
dominantly from the eastern older maria and not from the western younger high-
907
TiO2 maria, particularly Oceanus Procellarum. New samples from the western
908
young mare obtained by future missions will allow us to test our hypothesis and
909
to understand the mechanisms of long-lasting mare volcanism.
910
One of the long-standing questions is the discrepancy between TiO2 esti32
ACCEPTED MANUSCRIPT
mates based on the visible to NIR wavelength reflectance data (e.g., Clementine
912
UVVIS, Chang’e-1) and the GRS data (e.g., Elphic et al., 1998; Gillis et al.,
913
2003). In particular, the highest TiO2 values (up to 17 wt% in Clementine TiO2 ,
914
up to 11 wt% in LPTiO2 ) show a large difference. Our estimates using WAC
915
321/415 nm ratio are similar to the LPTiO2 values (see section 3 and 4), and
916
are also consistent with the range of TiO2 seen among Apollo and lunar mete-
917
orite basalt compositions (Lucey et al., 2006). Our high-end TiO2 estimates,
918
which match the GRS values and are significantly lower than the visible to NIR
919
estimates, represent >10 wt% at only 0.7% of the mare (Fig. 5, cumulative
920
histogram of Group 1 maria), indicating that very high-TiO2 basalt eruptions
921
were probably rare and limited in the history of lunar volcanism. This result
922
may add new constraints on the eruption mechanisms of the mare basalt.
923
6. Conclusion
AN US
CR IP T
911
We derived a new TiO2 abundance map of the Moon using the UV and
925
visible wavelengths of LROC WAC observations. We found a linear correla-
926
tion between the 321/415 nm band ratio of 62 months WAC observations and
927
the TiO2 content of 9 returned soil samples acquired in geologically homoge-
928
neous areas at the typical WAC pixel scale (400 m). Using this correlation
929
and a near-global WAC multispectral mosaic, we constructed a new WAC TiO2
930
abundance map. To avoid non-ilmenite source factors controlling the 321/415
931
ratio (e.g., the pyroxene content, shock degree of plagioclase, immaturity with
932
IS /FeO <∼40, and the presence of glass, Denevi et al., 2014; Gillis-Davis et al.,
933
2006; Coman et al., 2016), we set 2 wt% TiO2 as the detection limit. Since
934
the highlands have TiO2 abundances below this detection limit we excluded the
CE
PT
ED
M
924
highland terrain from our study.
AC 935
936
In the new WAC TiO2 map, the 17 major maria (Group 1) have an average
937
of 3.9 wt% TiO2 . The highest TiO2 values are observed in Mare Tranquilli-
938
tatis (∼12.6 wt%) and Oceanus Procellarum (∼11.6 wt%). Also, 30% of the
939
mare surfaces included in Group 1 maria are below the detection limit. Lunar 33
ACCEPTED MANUSCRIPT
940
swirls, low TiO2 basalts, and the regions contaminated by highland ejecta are
941
dominantly responsible for these low-TiO2 areas. A comparison of the WAC TiO2 map with the Clementine-based TiO2 map
943
(Lucey et al., 2000) revealed systematically higher TiO2 abundances in the
944
Clementine map, particularly in the high-TiO2 maria (e.g., up to +6.9 wt% in
945
Mare Tranquillitatis). Another Clementine-based TiO2 map created by Gillis
946
et al. (2003) has closer average TiO2 values to the WAC TiO2 map (-0.5 wt%
947
difference), albeit with a the clear TiO2 offsets along the condition-matching
948
boundaries used in their calculations. The Lunar Prospector GRS based TiO2
949
map (LPTiO2 ) is more consistent with WACTiO2 . In the comparisons for the
950
seven mare units with distinct TiO2 abundances within Mare Imbrium, and
951
for the five major maria (Mare Tranquillitatis, Oceanus Procellarum, Mare Im-
952
brium, Mare Serenitatis, and Mare Crisium), LPTiO2 and WACTiO2 showed
953
a good correlation that falls within ±0.8 wt% of a 1:1 line. The higher value
AN US
CR IP T
942
in the LPTiO2 relative to the WACTiO2 at the eastern portion of Oceanus
955
Procellarum could be due to the different depth sensitivities of the GRS (∼30
956
cm) and the WAC (few millimeters) in the areas of extensive ejecta deposits of
957
Aristarchus, Kepler, and Copernicus craters.
ED
M
954
The spatial variations of TiO2 abundance within each of the Group 1 maria
959
revealed significant contamination from highland ejecta, indicating that the orig-
960
inal mare basalts have potentially higher TiO2 content than the current upper-
961
most surface. Several maria (e.g., Mare Serenitatis, Mare Imbrium, and Oceanus
962
Procellarum) have distinctly high- and low-TiO2 units located next to each
PT
958
other, which also have clear age differences. For all of the mare units for which
964
model ages have been computed (using the count areas defined by Hiesinger
965
et al., 2000), a wide range of TiO2 values is found for relatively old (>2.6 Ga)
AC
CE 963
966
mare volcanism and only medium- to high-TiO2 values for the younger (<2.6
967
Ga) mare deposits, mostly in Oceanus Procellarum. This 2.6 Ga transition may
968
indicate a shift of lunar volcanism driven by the early hotter interior on a global
969
scale to one driven by local high-TiO2 cumulates enriched in radioactive element
970
heat sources. 34
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Our highest TiO2 estimates (excluding the DMDs) were found in the east-
972
ern portion of Mare Tranquillitatis. These values are higher than any returned
973
soils, meaning that they are the extrapolated estimates. Future lunar land-
974
ings or sample return missions on these highest TiO2 areas will increase the
975
accuracy of reflectance-based orbital TiO2 estimates by improving the correla-
976
tion fitting model (linear fit in this work). The small-sized or narrow-shaped
977
maria with low-TiO2 values (e.g., Mare Frigoris, Mare Moscoviense, and Mare
978
Humboldtianum) potentially have significant highland contamination, resulting
979
in reflectance-based estimates likely lower than the true mare values. These
980
maria are in key locations to understand farside and high latitude volcanism.
981
Higher resolution orbital images from future spacecraft missions, particularly
982
in the UV wavelength range (e.g., 250-400 nm, Cloutis et al., 2008), will allow
983
for the examination of small crater ejecta from the upper subsurface, which in
984
many cases could reveal the original surface TiO2 abundances before highland
985
contamination.
986
Acknowledgments
M
AN US
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971
This work was supported by the National Aeronautics and Space Administra-
988
tion Lunar Reconnaissance Orbiter Project. The United States Geological Sur-
989
vey Integrated Software for Imagers and Spectrometers (ISIS) (Anderson et al.,
990
2004) played a key role in processing the WAC observations. We acknowledge
991
Dr. Gillis-Davis for his help on analyzing the Clementine-based TiO2 maps.
992
We also acknowledge Oana Coman for her inputs that improved the quality of
993
this manuscript. We thank two anonymous reviewers who greatly improved our
994
manuscript.
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Figure 1: Spectra of typical lunar rock-forming minerals (from USGS spectral library, Clark et al., 1993). WAC’s seven wavelengths (321, 360, 415, 566, 604, and 689 nm) band center and FWHM of the WAC bandpasses are shown as vertical lines and gray bars, respectively.
Figure 2:
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The reflectance of anorthosite is multiplied by 0.7 to fit on the plot.
Example of WAC pixel selection using NAC reflectance mask in (a) 321 nm and
(b) 415 nm bands for the Apollo 11 landing site. Red box denotes the 1000 by 1000 meter
WAC sampling area (“site box”). Red and blue portions are the masked region. Colored dot and white cross indicate the center and the extent of each WAC pixel, and the dot color
corresponds to the photometrically normalized I/F (nI/F ). There are over 30 times more
observations at 415 nm than at 321 nm due to the higher spatial resolution in the visible bands.
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The histograms (blue line) of sampled WAC pixels in 321 and 415 nm bands are shown in the bottom panels. Red curve and dashed vertical line are the Skew Normal distribution curve fitting and its peak value location.
Figure 3:
WAC 321/415 nm nI/F ratio as a function of soil TiO2 content at the selected
sample return stations (see Table 1). Color and symbol indicate each sample return mission. The sample return site name is denoted at the top/bottom of each error bar, calculated by
Figure 4:
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Eq. 1. Solid and dashed line are the least-square fit and its 95% confidence band, respectively.
WAC TiO2 abundance map for lunar mare (latitude 70°S to 70°N, longitude
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180°W to 180°E in equirectangular map-projection). The highlands were masked using the mare/highland boundaries (Nelson et al., 2014). Color corresponds to 1 wt% bin of TiO2 values. All values less than ∼2 wt% (filled by dark blue) are below the detection limit of the
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reflectance variations related to ilmenite content.
Figure 5: Histograms of WAC TiO2 values for Group 1 (17 maria) and Group 2 (14 maria),
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listed in Table 3. Solid line is a cumulative histogram of Group 1 maria. Bin size is 0.1 wt%. Bold dashed line indicates the detection limit of TiO2 (<2 wt%). Narrow dashed line and
number indicate average value derived from the entire areas of maria in each group, with a simplification that the areas with <2 wt% TiO2 are uniformly 1 wt%. Dot with error bar
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indicates the fraction of TiO2 below 2 wt%, which dominantly corresponds to mare boundary pixels, highland ejecta (rays), swirls, and isolated highland massifs.
Figure 6: Distributions of swirls (green), dark mantle deposits (red), and the highland ejecta extensively contaminated the mare (blue) for latitude 70°S to 70°N, longitude 180°W to 180°E.
Gray zone corresponds to the highland terrains mapped by Nelson et al. (2014).
49
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Figure 7:
Southern portion of Aristarchus crater ejecta inside Oceanus Procellarum, in
WACTiO2 (400 m/pixel, upper panel) and in 643 nm band WAC global mosaic (100 m/pixel, lower panel). Images are centered at latitude 19.9°N, longitude 312.6°E, in equirectangular
Figure 8:
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map-projection. (a) Plot of CLMTiO2 L vs. WACTiO2 for the Group 1 maria (see Table 3) and
(b) difference map of CLMTiO2 L - WACTiO2 (40°S to 65°N, 90°W to 90°E). Errors increase below 2 wt% in the WACTiO2 (gray box in (a)). The difference map in (b) was derived by setting WACTiO2 values <2 wt% as 1 wt% before the subtraction. Black line in (b) highlights
boundaries of the Group 1 maria, and gray line for rest of the maria. The highland areas were masked.
(a) Plot of CLMTiO2 G vs. WACTiO2 for the Group 1 maria and (b) difference
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Figure 9:
map of CLMTiO2 G - WACTiO2 (40°S to 65°N, 90°W to 90°E). Errors increase below 2 wt% in the WACTiO2 (gray box in (a)). The difference map in (b) was derived by setting WACTiO2 values <2 wt% as 1 wt% before the subtraction. Black line in (b) highlights boundaries of the Group 1 maria, and gray line for rest of the maria. The highland areas were masked. Figure 10: (a) Plot of LPTiO2 vs. WACTiO2 for the Group 1 maria and (b) difference map
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of LPTiO2 - WACTiO2 in 2 pixels per degree (50°S to 70°N, 120°W to 120°E; same color stretch as Fig. 8 and 9). Dot with error bar in (a) correspond to a median and standard deviation (σ) at each bin (bin size: 0.5 wt%). Errors increase below 2 wt% in the WACTiO2
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(gray box in (a)). The difference map in (b) was derived by setting WACTiO2 values <2 wt% as 1 wt% before the subtraction. Black lines in (b) highlight boundaries of the Group 1 maria, and gray lines for rest of the maria. The highland areas were masked.
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Figure 11: (a) Mare Imbrium geologic units based on TiO2 abundance, and (b) TiO2 content from WACTiO2 and LPTiO2 for each Imbrium unit (listed in Table 4). The areas with WACTiO2 below the detection limit (2 wt%) were set to 1 wt%. The character and color of
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each unit in (a) correspond to the legend character and symbol color in (b). Black symbol with single character in (b) show the averaged TiO2 content in Mare Tranquillitatis (• T), Oceanus Procellarum ( P), Mare Imbrium (N I), Mare Serenitatis (H S), and Mare Crisium ( C).
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Error bar indicates the standard deviation, except for unit L2 with ±1 wt% (uncertainty
below 2 wt% areas). Solid line is the 1:1 line.
Figure 12: Histograms of TiO2 abundance from the WACTiO2 for the Group 1 maria. Y-axis is a fraction normalized by the maximum count of each histogram. Color corresponds to <2 wt% (blue), 2-4 wt% (green), 4-6 wt% (yellow), 6-8 wt% (pink), 8-10 wt% (red), and >10 wt% (dark red).
50
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Figure 13: TiO2 variation inside each mare with the same color scheme of 2 wt% bin in TiO2 as Fig. 12. White solid lines outline the mare boundaries (see section 2.3). Background image is the WAC 643 nm monochrome mosaic (100 m/pixel). The scale bar in the lower left within
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each panel indicates 100 km. The direction towards the craters located out of image frame is
indicated by an arrow with the crater name. Images are centered at (a) Procellarum 304.5°E,
18.4°N; (b) Imbrium: 341.9°E, 32.8°N; (c) Cognitum West: 338.1°E, 10.6°S; (d) Cognitum East: 349.9°E, 12.4°S; (e) Humorum: 323.9°E, 27.1°S; (f) Nubium: 341.4°E, 23.0°S. Black triangles and symbol "L" in (a) indicate the boundary of sharp TiO2 offset and a low-TiO2
area respectively. Dashed line in (d) traces central high-TiO2 zone. Black and white bold
Figure 14:
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arrows in (f) point out the low-TiO2 and highest TiO2 spots, respectively.
TiO2 variation inside each mare (continued from Fig. 13). See Fig. 13 caption
for symbol definitions. Images are centered at (a) Serenitatis: 19.3°E, 26.9°N; (b) Crisium: 60.5°E, 18.3°N; (c) Tranquillitatis: 30.7°E, 9.0°N; (d) Fecunditatis: 51.9°E, 6.9°S; (e) Marginis: 85.2°E, 13.6°N; (f) Smythii: 87.8°E, 1.6°S. Black bold arrows in (a) and (c) point out the RDMD (section 5.1.6) and the base of the horn-shaped area (section 5.1.8) respectively. Black
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triangles in (a) outline the high- and low-TiO2 boundaries. Polygons hatched and outlined by narrow black lines in (e) indicate the distribution of the swirls mapped by Denevi et al.
TiO2 variation inside each mare (continued from Fig. 14). See Fig. 13 caption
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Figure 15:
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(2016).
for symbol definitions. Imaged are centered at (a) Frigoris: 5.4°E, 56.2°N; (b) Tsiolkovskiy: 128.9°E, 20.5°S; (c) Moscoviense: 148.4°E, 27.7°N; (e) Ingenii: 164.8°E, 32.9°S; (f) Orientale: 270.3°E, 16.6°S. Polygons hatched and outlined by narrow black lines in (c) and (d) indicate
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the distribution of the swirls mapped by Denevi et al. (2016).
Figure 16:
Distribution of the count areas (areas where crater counting was performed).
Blue count areas were included in the plots in Fig. 17-19, and red count areas were excluded. Square shaped count area corresponds to a representative subset of each mare unit defined by Hiesinger et al. (2000, 2003). Background image is the WAC 643 nm monochrome mosaic. The highlands (gray zone) were masked using mare/highland boundaries (Nelson et al., 2014).
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Figure 17:
TiO2 abundance as a function of crater size-frequency distribution model ages
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(Greeley et al., 1993; Hiesinger et al., 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016). Dot and bar indicate the median and standard deviation of WACTiO2 values within each count area (sub-divided mare unit or the square-shaped sub-
set area (Hiesinger et al., 2000, 2003) where the crater counts were actually performed). Age unit is gigayear ago (Ga, 109 years). Dashed line indicates the WACTiO2 detection limit (2 wt%). The count areas with WACTiO2 less than 2 wt% in median were set to 1 wt% with
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±1 wt% uncertainty.
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Figure 18: TiO2 abundance vs. crater size-frequency distribution model ages (Greeley et al., 1993; Hiesinger et al., 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016) inside each mare (see Fig. 17 for symbol definitions). The “Southwestern Nearside”
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corresponds to several mare patches inside individual craters and smooth plains to the south
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of Oceanus Procellarum.
Figure 19: Thorium weight fraction as a function of crater size-frequency distribution model
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age (Greeley et al., 1993; Hiesinger et al., 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016). Dot and bar indicate the median and standard deviation of the Th weight fraction derived by Lunar Prospector GRS (Prettyman et al., 2006) within each count area. Color indicates the median of WACTiO2 within each count area.
52
Table 1:
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Representative WAC nI/F and standard deviation at 321 and 415 nm as well
as 321/451 nm ratio over each Apollo ("Axx") and Luna (“Lxx”) sampling site. The TiO2
content of each soil compiled by Blewett et al. (1997) and Jolliff (1999) is also given. The
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representative nI/F value corresponds to the modal value of multiple WAC observations (on average 111 at 321 nm and 3966 at 415 nm), photometrically normalized to 0° emission and 60° incidence angles.
Soil TiO2
321 nm
wt%
nI/F
A11
7.5
0.010
A12
3.1
0.011
A14
1.7
0.013
A15-S8
1.7
A15-09
415 nm
321/415
σratio
nI/F
St.Dev.
4.0E-4
0.0136
8.3E-4
0.765
0.016
3.7E-4
0.0153
8.2E-4
0.722
0.014
3.4E-4
0.0184
1.1E-3
0.704
0.021
0.011
3.0E-4
0.0158
8.7E-4
0.699
0.018
1.8
0.011
5.1E-4
0.0153
9.6E-4
0.710
0.010
8.5
0.011
3.9E-4
0.0140
1.6E-3
0.768
0.053
8.0
0.010
2.3E-4
0.0137
1.3E-3
0.762
0.053
6.7
0.010
3.0E-4
0.0137
1.1E-3
0.758
0.034
A17-LRV9
6.1
0.011
3.9E-4
0.0147
1.2E-3
0.752
0.032
L16
3.3
0.009
3.8E-4
0.0119
7.1E-4
0.760
0.013
L24
1.0
0.010
3.4E-4
0.0140
9.1E-4
0.718
0.021
A17-LRV1
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CE
A17-LRV8
ED
A17-LM
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St.Dev.
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Sample Site
53
(Eq.1)
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Table 2: List of excluded sample sites with each TiO2 contents (Blewett et al., 1997; Jolliff, 1999) and a brief site description. “Axx” and “Lxx” denote Apollo and Luna missions, respectively. Representative WAC nI/F and standard deviation at 321 nm, 415 nm, and 321/451 nm ratio for these excluded sites are included in supplemental materials. We note that a
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reliable representative value was not derived for several sites (e.g., slope sites with mixture of mare/highland materials).
Excluded Sites
Soil TiO2
Site description
wt% 1.6
Slope, gradual mixture of mare and highland materials
A15-S2
1.3
(same as above)
A15-S4
1.2
(same as above)
A15-S6
1.5
A15-S7
1.1
A15-9A
2.0
A16
-
A17-S1
9.6
Rim of Steno-Apollo crater, excavated subsurface materials
A17-S2
1.5
Slope bottom of South Massif
A17-S2A/LRV4
1.3
A17-S3
1.8
(same as above) (same as above)
Edge of Hadley rille
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Highland materials
On top of the Light Mantle deposit (same as above)
9.9
Rim of Camelot crater, excavated subsurface materials
3.4
Slope, gradual mixture of mare and highland materials
3.9
(same as above)
A17-S8
4.4
(same as above)
A17-S9
6.4
Van Serg crater ejecta
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A17-S5
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A15-S1
A17-LRV2
4.4
Light Mantle deposit
A17-LRV3
5.5
Surrounded by Light Mantle deposit
A17-LRV5
2.6
Light Mantle deposit
A17-LRV6
2.6
(same as above)
A17-LRV7
6.8
Surrounded by Light Mantle deposit
A17-LRV10
3.7
Slope, gradual mixture of mare and highland materials
A17-LRV11
4.5
A17-LRV12
10.0
Rim of unnamed ∼10 m diameter crater
L20
0.5
A17-S6
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A17-S7
Rim of Sherlock crater, excavated subsurface materials 54
Highland materials
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Table 3: TiO2 abundance statistics for the 17 selected maria (see corresponding histograms
in Fig. 12 and map views in Fig. 13-15). Latitude and longitude indicate the center of each mare. Affiliation to Group 1 (17 maria) and Group 2 (14 maria) is listed. The areas with WACTiO2 below the detection limit (2 wt%) were set to 1 wt%.
Group 1,2
Mare Cognitum (east)
1,2
Mare Crisium
1,2
Mare Fecunditatis
1
Mare Frigoris
1
Mare Humorum
1,2
Mare Imbrium
Median
Average
St.Dev.
Max
wt%
wt%
wt%
wt%
3.7
3.5
1.6
8.0
338.17°E
12.39°S
349.87°E
3.3
3.4
1.8
8.3
18.40°N
60.78°E
2.7
2.6
1.4
8.1
6.90°S
51.95°E
1.0
2.4
1.7
9.2
56.21°N
5.36°E
1.0
1.1
0.6
8.0
27.14°S
323.91°E
3.8
3.9
2.1
10.3
1,2
32.79°N
341.89°E
3.4
3.7
2.3
10.2
1
32.91°S
164.79°E
1.0
1.6
1.0
7.2
Mare Marginis
1,2
13.61°N
85.23°E
1.0
1.8
1.1
6.1
Mare Moscoviense
1,2
27.72°N
148.45°E
3.1
3.6
2.2
9.4
Mare Nubium
1,2
23.04°S
341.43°E
3.2
3.2
1.7
8.5
Mare Orientale
1,2
16.47°S
270.53°E
2.4
2.5
1.4
7.5
Oceanus Procellarum
1,2
18.40°N
304.54°E
4.7
4.7
2.4
11.7
Mare Serenitatis
1,2
26.94°N
19.28°E
3.5
3.6
1.8
11.4
Mare Smythii
1,2
1.63°S
87.77°E
2.8
2.7
1.2
6.3
Mare Tranqillitatis
1,2
9.04°N
30.66°E
6.8
6.4
2.5
12.6
Mare Tsiolkovskiy
1,2
20.49°S
128.90°E
2.2
2.2
1.2
6.7
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Mare Ingenii
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Longitude
10.59°S
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Mare Cognitum (west)
Latitude
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Major Maria
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Table 4:
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TiO2 abundance of the seven mare units inside Mare Imbrium (Fig. 11) and five
major maria from the WACTiO2 (down-sampled to the LPTiO2 resolution: 2 degrees per were set to 1 wt%.
Unit Name
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pixel) and the LPTiO2 values. The areas with WACTiO2 below the detection limit (2 wt%)
WACTiO2 [wt%] Average
LPTiO2 [wt%]
St.Dev.
Average
St.Dev.
1.0
5.2
1.8
0.6
4.8
1.5
0.8
3.5
1.8
1.0
2.8
1.1
5.1
H2
4.2
H3
3.5
L1
1.9
L2
1.0
1.0
1.9
0.8
L3
2.1
0.7
2.8
1.7
2.5
0.8
2.7
1.4
Mare Tranqillitatis
5.2
2.0
5.4
2.6
Oceanus Procellarum
4.3
1.7
4.4
2.0
Mare Imbrium
3.5
1.6
3.8
1.9
Mare Serenitatis
3.6
1.0
3.2
1.6
Mare Crisium
1.9
0.8
1.4
0.9
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AC
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L4
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H1
56
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Figure 1
1344
0.5
Anorthosite (x0.7)
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Reflectance
0.4
Olivine
0.3
Augite
0.2
Pigeonite
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0.1
Ilmenite
0.0
300
400
1345
Figure 2
1346
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(a)
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500 Wavelength [nm]
1348
1349
Figure 3
57
(b)
600
700
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Apollo 11 Apollo 12 Apollo 14 Apollo 15
0.82
0.78
A17-LRV8 A17-LRV9 A11
(L16)
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WAC ratio (321/415 nm)
0.80
A17-LM A17-LRV1
Apollo 17 Luna 16 Luna 24
0.76 (L24)
0.74
A12 A15-09
0.72 0.70
0
2
4 6 Lunar Soil TiO2 [wt%]
1350
Figure 4
1352
30
PT
120
90
60
30
0 30 Longitude
Figure 5
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1354
150
58
TiO2 [wt%]
0
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Latitude
30
1353
10 9 8 7 6 5 4 3 2
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60
60 180
RMS = 3.80e-03 95% Confidence Band 8 10
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A14 A15-S8
0.68
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y = 0.010x + 0.689 R2 = 0.980
60
90
120
150
180
4.2 3.9
2
4
1355
6 TiO2 [wt%]
Figure 6
1358
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Figure 7
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1357
1359
8
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100 90 Group 1 (17 maria) 80 Group 2 (14 maria) 70 Group 1 Cumulative 60 50 40 30 20 10
Cumulative Fraction [%]
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
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Fraction [%]
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59
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1360
[wt%]
8.0
AC
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2.0
50 km
1361
1362
Figure 8
60
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(a)
1363
Figure 9
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1364
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(b)
61
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(a)
1365
Figure 10
AC
CE
1366
PT
ED
M
(b)
62
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(a)
1367
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Figure 11
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(b)
63
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(a)
L2
L3
H1 H2 H3
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(b)
T
P
I
M
S
PT
Figure 12
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1370
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C
1369
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L1
L4
64
Mare Tranqillitatis
Mare Cognitum (east)
Mare Tsiolkovskiy
Oceanus Procellarum
Mare Nubium
Mare Fecunditatis
Mare Humorum
Mare Moscoviense
Mare Cognitum (west)
Mare Smythii
Mare Serenitatis
Mare Crisium
Mare Imbrium 6 4 2 0 2 4 6 8 10
Mare Ingenii
6 4 2 0 2 4 6 8 10 WACTiO2 [wt%]
AC
CE
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Figure 13
Mare Marginis
Mare Frigoris
Mare Orientale
1371
1372
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0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0
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Normalized Fraction
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65
6 4 2 0 2 4 6 8 10
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M
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L
Figure 14
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1374
TiO2 [wt%]
PT
1373
66
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M
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Figure 15
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1376
TiO2 [wt%]
PT
1375
67
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Figure 16
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CE
1378
TiO2 [wt%]
PT
1377
68
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1379
Figure 17
CE
1382
PT
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1381
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1380
Figure 18
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1383
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Figure 19
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1384
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Lunar Mare TiO2 Abundances Estimated from UV/Vis Reflectance Hiroyuki Sato, Mark S. Robinson, Samuel J. Lawrence, Brett W. Denevi, Bruce Hapke, Bradley L. Jolliff, Harald Hiesinger PII: DOI: Reference:
S0019-1035(16)30659-5 10.1016/j.icarus.2017.06.013 YICAR 12497
To appear in:
Icarus
Received date: Revised date: Accepted date:
11 October 2016 4 June 2017 7 June 2017
Please cite this article as: Hiroyuki Sato, Mark S. Robinson, Samuel J. Lawrence, Brett W. Denevi, Bruce Hapke, Bradley L. Jolliff, Harald Hiesinger, Lunar Mare TiO2 Abundances Estimated from UV/Vis Reflectance, Icarus (2017), doi: 10.1016/j.icarus.2017.06.013
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Highlights • We constructed a new TiO2 map from LROC WAC near-global mosaic.
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• TiO2 contents of lunar soil samples and 321/415 nm ratio are linearly correlated.
• Clementine-based TiO2 estimates are higher relative to WAC TiO2.
• Lunar Prospector GRS-based TiO2 values are comparable with WAC TiO2 .
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• Trend of mare volcanism may have shifted at 2.6 Ga.
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3 4 5 6 7 8 9 10 11 12 13 14 15
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Hiroyuki Satoa,∗, Mark S. Robinsona , Samuel J. Lawrenceb , Brett W. Denevic , Bruce Hapked , Bradley L. Jolliffe , Harald Hiesingerf
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Lunar Mare TiO2 Abundances Estimated from UV/Vis Reflectance
a School
of Earth and Space Exploration, Arizona State University, 1100 S. Cady Mall, INTDS A, Tempe, AZ 85287-3603, USA. b Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, USA. c Applied Physics Laboratory, Johns Hopkins University, 11100 John Hopkins Rd, Laurel, MD 20723-6005, USA. d Department of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara Street, Pittsburgh, PA 15260, USA. e Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University, One Brookings Drive, St Louis, Missouri 63130, USA. f Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany.
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Abstract
The visible (VIS; 400-700 nm) and near-infrared (NIR; 700-2800 nm) re-
18
flectance of the lunar regolith is dominantly controlled by variations in the
19
abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g.,
20
ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced
21
rims on soil grains, and Fe-metal). The same materials control reflectance into
22
the near-UV (250-400 nm) with varying degrees of importance. A key differ-
23
ence is that while ilmenite is spectrally neutral in the VIS and NIR, it exhibits
24
a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than
25
about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC)
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filters were specifically designed to take advantage of this spectral feature to
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enable more accurate mapping of ilmenite within mare soils than previously
28
possible. Using the reflectance measured at 321 and 415 nm during 62 months
29
of repeated near-global WAC observations, first we found a linear correlation
30
between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of
31
each sample return site. We then used the coefficients from the linear regression
32
and the near-global WAC multispectral mosaic to derive a new TiO2 map. The
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∗ Corresponding author at: Arizona State University, 1100 S. Cady Mall, INTDS A, Tempe, AZ 85287-3603, USA. Email address: [email protected] (Hiroyuki Sato). Phone Preprint submitted to Icarus June number: 8, 2017 480-727-0099
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average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 val-
34
ues were found in Mare Tranquillitatis (∼12.6 wt%) and Oceanus Procellarum
35
(∼11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the
36
low TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium)
37
exhibit very low TiO2 values (<2 wt%). We find that the Clementine visi-
38
ble to near-infrared based TiO2 maps (Lucey et al., 2000) have systematically
39
higher values relative to the WAC estimates. The Lunar Prospector Gamma-
40
Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map,
41
although there are local offsets possibly due to the different depth sensitivities
42
and large pixel scale of the GRS relative to the WAC. We find a wide variation
43
of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga),
44
whereas only medium- to high-TiO2 values (average = 6.8 wt%, minimum =
45
4.5 wt%) are found for younger mare units (<2.6 Ga).
46
Keywords: Moon, Volcanism, Thermal histories, Image processing,
47
Spectroscopy
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1. Introduction
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From lunar meteorites (Korotev et al., 2003) and Apollo and Luna returned
50
samples (McKay et al., 1991), we have a preliminary understanding, albeit in-
51
complete, of the major lunar rock types, particularly mare basalts. Since basalts
52
form through partial melting of the lunar mantle, they provide our best view of
53
the composition of the lunar interior; understanding the compositional range of
54
mare basalts is, therefore, critical to understanding the geology and the interior
55
of the Moon and thus the other terrestrial planets (Lunar Exploration Analysis
56
Group, 2016).
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Apollo-era petrologic investigations of returned samples relied on titanium
58
content as a primary tool for the first-order classification of mare basalt types,
59
owing to the large range of titanium (0 to >10 wt% TiO2 ) among the basalt
60
samples (Shearer et al., 2006). The Apollo mare samples have a bimodal distri-
61
bution in the range of titanium concentrations, with relatively larger numbers
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3
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of samples below 5 wt% TiO2 and higher than 9 wt% TiO2 (Papike et al., 1998;
63
Taylor et al., 1991). Historically, lunar basalts were classified as either very
64
low-Ti (VLT) basalts with <1 wt% TiO2 , low-Ti basalts with 1-5 wt%, and
65
high-Ti basalts (9-14 wt%), with only a few samples falling into the range of 5-9
66
wt% (Papike et al., 1976; Papike and Vaniman, 1978; Taylor et al., 1991; Papike
67
et al., 1998); alternative simplified classification schemes also exist (Neal and
68
Taylor, 1992). The recent Chang’e-3 mission measured basalts with 5.2 wt%
69
TiO2 in northern Mare Imbrium (Ling et al., 2015).
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Earth-based telescopic observations combined with knowledge of sample
71
chemistry were used to predict the TiO2 content of mare materials (Whitaker,
72
1972; McCord et al., 1972; Charette and McCord, 1974; Johnson et al., 1977;
73
Pieters, 1978; Johnson et al., 1991). These early studies extended estimates of
74
TiO2 abundance to unvisited mare surfaces across the nearside. Later, space-
75
craft multispectral observations were utilized to extend TiO2 abundance esti-
76
mates to farside areas, such as Mariner 10 (Robinson et al., 1992), Galileo Solid
77
State Imaging experiment (Lucey et al., 1998; Giguere et al., 2000), and Clemen-
78
tine Ultraviolet/Visible (UVVIS) camera (Lucey et al., 1995, 1998; Blewett
79
et al., 1997; Jolliff, 1999; Lucey et al., 2000; Giguere et al., 2000; Gillis et al.,
80
2003). Clementine UVVIS images (415-1000 nm) provided the means for Lucey
81
and coworkers to estimate FeO and TiO2 content of the surface at a pixel scale
82
of 100 m (Lucey et al., 1995; Blewett et al., 1997; Lucey et al., 1998, 2000).
83
These products were the first global and uniformly processed maps that were
84
broadly used to investigate the composition of the lunar crust. Giguere et al.
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(2000) demonstrated that the TiO2 content in mare basalts has a continuous
86
distribution ranging from 0-14 wt% with a mode near ∼2 wt%, not the bimodal
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distribution as suggested by the lunar samples. Recent spectral reflectance mea-
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surements from Chang’e-1 Interference Imaging Spectrometer (IIM) (Wu, 2012;
89
Wu et al., 2012), and the SELENE Multiband Imager (Otake et al., 2012) in-
90
creased the spatial resolution of TiO2 estimates up to 20 m/pixel. These series
91
of works estimated TiO2 abundance from the visible to NIR spectral slopes
92
acquired from Earth-based observations (e.g., 300-1100 nm in Whitaker, 1972; 4
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400-560 nm in McCord et al., 1972; Charette and McCord, 1974; Johnson et al.,
94
1977; Pieters, 1978; Johnson et al., 1991), flyby observations (e.g., 480-580 nm
95
by Mariner 10; 415-750 nm by Galileo), and orbital measurements (Clementine,
96
Chang’e-1, and SELENE), based on the correlation with the TiO2 content of re-
97
turned lunar samples. Since the sensitivity depth of reflectance spectra in these
98
observations is the upper few millimeters (Hapke, 2012), these TiO2 estimates
99
represent only the uppermost surface materials.
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The Lunar Prospector Gamma-Ray Spectrometer (GRS) and Neutron Spec-
101
trometer (NS) (Elphic et al., 2000, 1998, 2002; Prettyman et al., 2006) provided
102
an independent measure of titanium at coarser scales (data were binned at 2 and
103
5 degrees per pixel). The NS/GRS senses into the regolith ∼30 cm (Lawrence
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et al., 2002), thus its TiO2 estimates are deeper relative to the spectral re-
105
flectance methods. In some areas, it is reported that the TiO2 estimates based
106
on visible (400-750 nm) reflectance including Clementine UVVIS and Chang’e-1
107
IIM (Lucey et al., 2000; Wu et al., 2012) are high relative to Lunar Prospector
108
NS/GRS based TiO2 estimates (Elphic et al., 2002; Prettyman et al., 2006).
109
The cause of this discrepancy is still not fully understood, but is in some cases
110
may be related to the different sampling depths.
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Visible to near-infrared (400-2800 nm) spectral reflectance of the lunar re-
112
golith is mainly controlled by variations in abundance of four components: pla-
113
gioclase, iron-bearing silicate minerals, opaque minerals, and maturation prod-
114
ucts (e.g., agglutinate glass, Fe metal, and amorphous radiation-damaged min-
115
eral grains) (Hapke et al., 1975; Pieters et al., 2000; Hapke, 2001). Additionally,
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it has long been known that the mare exhibit distinct color contrasts into the
117
UV relative to visible wavelengths (Whitaker, 1972; McCord et al., 1972; Wells
118
and Hapke, 1977; Wagner et al., 1987; Rava and Hapke, 1987; Robinson et al.,
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119
2007; Cloutis et al., 2008). While the spectral reflectance of most major lu-
120
nar rock-forming minerals decreases toward shorter wavelengths (Fig. 1), the
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spectrum of ilmenite is flat through the visible and increases with decreasing
122
wavelength below ∼450 nm (Clark et al., 1993; Cloutis et al., 2008). A strong
123
linear correlation was demonstrated between the visible-UV ratio (502/250 nm) 5
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of Apollo 17 sample stations as imaged by the Hubble Space Telescope and
125
the TiO2 contents of the corresponding returned soil samples (Robinson et al.,
126
2007). Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC)
128
near-global observations (±70° latitude) provide the first opportunity to map
129
all mare deposits in the near-ultraviolet to visible range (321 and 360 nm in
130
UV bands; 415, 566, 604, 643, and 689 nm in visible bands) with uniform
131
resolution (∼400 m in UV, ∼100 m in visible) (Robinson et al., 2010; Mahanti
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et al., 2016). An original science goal of the WAC was to investigate TiO2
133
abundance in the maria and the 321 nm and 415 nm bands were designed for this
134
purpose (Robinson et al., 2010) based on promising results from Hubble Space
135
Telescope observations of the Taurus Littrow valley (Apollo 17 landing site)
136
(Robinson et al., 2007). Denevi et al. (2014) examined the WAC 321/415 ratio
137
variations of the Moon in relation to the effects of space weathering, reporting
138
that the fresh craters have low ratio values in the mare and in the moderate-iron
139
highlands. However, they also found that the space weathering “saturates” in
140
the UV relatively quickly (up to IS /FeO ∼40) compared to the visible and the
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near-infrared (Lucey et al., 2000) (up to IS /FeO ∼60), suggesting that the UV
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is less dependent on the maturity variations. From the ratio of the 321 nm
143
band to the 415 nm band, a new TiO2 abundance map is presented here and
144
compared with two maps derived from Clementine near-infrared spectral images
145
(Lucey et al., 2000; Gillis-Davis et al., 2006) and global estimates derived from
146
the Lunar Prospector GRS measurements (Elphic et al., 2002; Prettyman et al.,
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2006). We also discuss the distribution of ilmenite (the main carrier of TiO2 )
148
and its geologic implications within each mare. Finally, we discuss the variation
149
of TiO2 abundance as a function of age to understand the evolution of lunar
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150
mare volcanism.
6
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151
2. Derivation of WAC TiO2 Map For the lunar mare, we assume that variations in the 321/415 nm ratio
153
are dominantly controlled by ilmenite (and thus TiO2 ) abundance, based on
154
laboratory and remote-sensing studies (Charette and McCord, 1974; Robinson
155
et al., 2007; Cloutis et al., 2008; Coman et al., 2016). In fact, the spectra of
156
the returned regolith samples are known to show a clear correlation between
157
the UV over visible wavelength ratio and the ilmenite-sourced TiO2 (Charette
158
and McCord, 1974; Coman et al., 2016). First, we determined a correlation
159
between TiO2 contents of returned sample soils and the WAC 321/415 ratios
160
from appropriate sampling sites (Table 1). From the derived coefficients, we then
161
constructed a new TiO2 abundance map from a near-global WAC multispectral
162
mosaic.
163
2.1. WAC Observations at Sampling Sites
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The areas from which returned samples were collected do not necessarily
165
fully represent the surface observed from orbit, mostly due to gross differences
166
in sampling size; remote sensing pixels measure areas hundreds to thousands
167
of meters in size, whereas the returned samples are from areas typically much
168
less than a square meter. Therefore, we only utilized samples acquired from
169
sites deemed to be relatively geologically uniform (based on the albedo and
170
morphological variations detected by the LROC Narrow Angle Camera (NAC)
171
images with a pixel scale of ∼2 m/pixel) at the pixel scale of the WAC 321 nm
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band (typically 400 m) (Table 1). Sites with complicated geology at the scale of WAC pixels, such as Apollo 17-LRV2 on a narrow lobe of the landslide from
174
South Massif, Apollo 15-9A at the edge of Hadley Rille, and Apollo 17 Station
175
5 on Camelot crater ejecta, were all excluded (Table 2). Lunar soils with less
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176
than ∼2 wt% TiO2 do not show a strong correlation between their 321/415 nm
177
ratio and TiO2 content (Gillis-Davis et al., 2006; Coman et al., 2016), therefore
178
Apollo 16 and Luna 20 samples (highland materials with no or very low ilmenite
179
content) were also excluded. In total, 11 sample sites were used to calibrate the
180
spectral ratio to TiO2 abundance. 7
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For each selected sample return site (Table 1), we derived a representative
182
value of WAC photometrically normalized reflectance (nI/F ) at 321 and 415 nm
183
respectively, through the following four steps explained hereafter: 1) collection
184
of all the available WAC observations at each site and derivation of nI/F , 2)
185
masking based on NAC albedo, 3) masking of sparse observation regions, and
186
4) curve fitting to the nI/F histogram. Each step is described in detail below.
187
2.1.1. WAC Pixel Data Preparation
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We used ∼62 months of WAC observations acquired from September 2009 to
188
December 2014. The raw Digital Number (DN) of each pixel was converted to
190
radiance factor (I/F ) (Hapke, 2012) through radiometric calibration (Mahanti
191
et al., 2016), then photometrically normalized (emission angle = 0°, incidence
192
angle = phase angle = 60°) with a Hapke function (Hapke, 2012) using spa-
193
tially resolved Hapke parameter maps (Sato et al., 2014) to derive nI/F . The
194
photometric angles were calculated relative to the local topography using a
195
WAC-derived 100 m scale Digital Terrain Model (DTM) known as the GLD100
196
(Scholten et al., 2012). The observations at incidence angle >75°, emission an-
197
gle >30°, and phase angle >95° were excluded to minimize the uncertainties of
198
photometric normalization. We normalized reflectance to 60° phase angle (0°
199
emission angle, 60° incidence angle) instead of the traditional 30° because the
200
lower value is not achieved in the latitudes over 30° under the WAC observation
201
geometries (Sato et al., 2014). The nominal pixel scales (average within a frame
202
for nadir observations) of the WAC from the 50 km altitude orbit of Lunar
203
Reconnaissance Orbiter (LRO) (from September 2009 to December 2011) are
204
402 m for the UV and 74.9 m for visible bands (Robinson et al., 2010). In the
205
current elliptical orbit (after December 2011), the pixel scale ranges from 550
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to 1170 m/pixel in UV and from 107 to 228 m/pixel in visible bands within the
207
latitudes of all the sample-return sites (9°S to 26°N). Only the pixels that were
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contained entirely within a 1000 by 1000 m box (hereafter called “site box”)
209
centered at each sample-return location were collected from non map-projected
210
WAC images for the 321 and 415 nm bands.
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2.1.2. NAC Albedo Masking Inside each site box, there are significant albedo variations due to local geo-
213
logic and topographic features (e.g., fresh crater ejecta and slopes). From LROC
214
NAC images (original pixel scale ∼50 cm, down-sampled to 2 m/pixel), we de-
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rived and applied an albedo mask to exclude WAC pixels that dominantly cover
216
remarkably low or high albedo areas (Fig. 2). For the NAC images, we selected
217
those that were acquired with relatively small incidence angles (from 7.3° to
218
27.0°), and applied a photometric normalization using emission and incidence
219
angles calculated relative to local topography derived from complementary NAC
220
DTMs for each site (2 m/pixel, Henriksen et al., 2015). Although the NAC is a
221
broadband imager (400 to 760 nm) it is useful as a generalized albedo map for
222
comparison with the WAC observations. Within the NAC image cropped to the
223
region of the site box, we masked (excluded) 10% in total of the area with high-
224
and low-end reflectance relative to the reference value (Fig. 2). The reference
225
value was obtained from the same NAC image, a median of a 10 by 10 pixel (20
226
by 20 m) box centered at the exact location of each sample-return site. In order
227
to avoid surfaces disturbed by human activities (e.g., rocket blast zone, tracks
228
of astronauts and Lunar Roving Vehicles), at several sites, we relocated the 10
229
by 10 pixel box to a nearby vicinity where there is a relatively uniform, undis-
230
turbed area. The masked areas normally consist of multiple irregular patches
231
whereas each WAC pixel is square. Thus, we count an overlapping fraction of
232
each WAC pixel with the masked area, and if the fraction is more than 15% we
233
excluded that WAC pixel.
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2.1.3. Sparse Observation Masking Owing to the four-times larger footprint of the 321 nm band relative to the
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236
415 nm band, and because we select only pixels entirely included in the site
237
box, there is a zone along the margins of the site box where the number of
238
observations is relatively sparse (Fig. 2a). This zone is much wider for the 321
239
nm band (∼200 m, about half of the UV-band footprint size) than for the 415 nm
240
band (∼50 m, half of visible-band footprint size). This offset, where there are 9
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numerous 415 nm observations but few 321 nm observations, results in spatially
242
inconsistent weighting of the calculation of representative values inside each site
243
box. To minimize such spatial weighting differences in the two bands, we reduced
244
the number of 415 nm observations by applying an additional mask, which
245
excludes the boundary regions where observations are sparse for the 321 nm
246
band. We created a 321 nm observation accumulation map inside the site box
247
and set the minimum threshold for a number of observations by the maximum
248
number of observations divided by five, which was empirically determined to
249
automatically mask the boundaries of the site box and the anomalous albedo
250
areas. The areas that did not meet the threshold value in the accumulation map
251
were masked. Examples of the distribution of 321 and 415 nm observations
252
within a site box after application of the NAC albedo mask and the sparse
253
observation mask are shown in Fig. 2a and 2b. On average, 111 and 3966 pixels
254
were used for the 321 and 415 nm bands, respectively, at each sample site box.
255
2.1.4. Curve Fitting to the Histogram
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A representative nI/F of each band at each site was derived from the modal
257
value of the down-selected WAC observations. In order to avoid a singular-
258
peaked mode of an irregularly shaped histogram, we fitted a curve to the his-
259
togram by a least-square optimization, and the curve’s modal value was used as
260
the representative nI/F . Since the histograms of nI/F for each site box did not
261
always have a symmetric Gaussian shape, we employed a Skew Normal curve
262
with an asymmetry adjustment for our fits (Fig. 2 bottom panels).
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Using the representative nI/F s of the two bands calculated in this final step,
264
the 321/415 nm ratio was derived for each sample-return site. The uncertainty
265
of each ratio value was estimated by 2-D standard deviation given as,
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σratio =
q 2 2 σ321/M + σM 415 321 /415
(1)
266
where M321 and M415 are the representative nI/F of the 321 and 415 nm
267
bands respectively, σ321/M415 is a standard deviation based on all the selected
268
321 nm nI/F s divided by M415 , and σM321 /415 is a standard deviation based 10
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269
on M321 divided by the selected 415 nm nI/F s. We note that this uncertainty
270
includes the instrument calibration and photometric normalization uncertainties
271
across the ∼62 months of repeat observations. The derived representative nI/F ,
the standard deviation of nI/F s of each band, the ratio, and the σratio for the
273
selected sample-return sites are listed in Table 1.
274
2.2. Correlation between WAC 321/415 and Soil TiO2
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For the lunar soil TiO2 values, we used the compositional data compiled and
276
summarized by (Blewett et al., 1997) and (Jolliff, 1999). The selected WAC
277
321/415 nm ratios and the lunar soil TiO2 show a positive linear correlation
278
(Fig. 3). The least-squares linear fit has a slope = 0.010, y-intercept = 0.689,
279
and R2 = 0.98.
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The samples from Luna 16 and Luna 24 are far off the Apollo sample trend
281
line, thus these two samples were considered as outliers and excluded from the
282
fitting. Previous works (Blewett et al., 1997; Lucey et al., 2000; Gillis et al.,
283
2003) also reported these two samples as anomalies in their regression, which is
284
consistent with our plots. The Luna 24 site is in close proximity to a fresh crater
285
(Robinson et al., 2012). It is likely that Luna 24 samples are excavated materials
286
from a buried unit, representing materials with a composition varying from the
287
surface (Robinson et al., 2012; Denevi et al., 2014; Coman et al., 2016). The
288
cause of Luna 16 sample discrepancy is not clear. The Luna 16 site is located
289
at the northwestern edge of Langrenus crater ejecta (highland sourced deposits;
290
see Fig. 14d). Mixtures of local mare materials with highland ejecta may have
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produced inhomogeneous soils, possibly resulting in the anomalous TiO2 content
292
relative to the trend of Apollo samples.
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The spectra of Apollo soils revealed a clear positive correlation between
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294
the TiO2 content and the 321/415 nm ratio, except for low TiO2 soils (<∼2
295
wt%) where pyroxene content begins to dominate the ratio, resulting in a poor
296
correlation with the TiO2 content (Gillis-Davis et al., 2006; Coman et al., 2016).
297
Therefore, we report only TiO2 values greater than 2 wt% using the WAC
298
321/415 nm ratio, and consider lower values to be below the detection limit. 11
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In non-mare regions, the 321/415 nm ratio is likely controlled by various
300
non-ilmenite sources, such as the shock degree (impact origin) of plagioclase
301
and the presence of glass (Denevi et al., 2014) in addition to the other minerals
302
(e.g., pyroxene). Therefore, our TiO2 map should be considered to be valid only
303
for the mare regions.
304
2.3. WAC TiO2 Map Overview
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From the correlation between the representative ratio values and the lunar
306
soil TiO2 values, we converted the WAC 321/415 nm ratio map to a TiO2 (wt%)
307
abundance map (Fig. 4). The ratio map was derived using the near-global WAC
308
color mosaic (400 m/pixel) (Sato et al., 2014). This mosaic was photometrically
309
normalized (incidence = phase = 60° and emission = 0°) using emission and
310
incidence angles computed at each pixel based on the GLD100 DTM (Scholten
311
et al., 2012), resulting in reflectance on topographic slopes being more accurately
312
normalized (e.g., Robinson and Jolliff, 2002).
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In our new TiO2 map (Fig. 4), the highest values are recognized in the
314
northwestern portion of Mare Tranquillitatis (∼12.6 wt%), center of Oceanus
315
Procellarum (∼11.6 wt%), and several Dark Mantle Deposits (DMD; Sinus Aes-
316
tuum, Rima Bode, and Mare Vaporum; ∼12 wt%). The highest TiO2 values
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(18-20 wt%) were found at several small spots (∼10 by 10 pixels, ∼4 by 4 km
across) in Sinus Aestuum that likely correspond to freshly excavated DMDs as
319
the ejecta of small craters (<1 km in diameter). However, the values associ-
320
ated with DMDs and other pyroclastic deposits are suspect since the increasing
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UV/visible ratio with increasing TiO2 content does not apply to glasses (Wells
322
and Hapke, 1977; Wilcox et al., 2006).
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To focus on characterizing the global TiO2 distribution of the maria, we
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distinguished two groups (see Table 3 for a selected mare list and the group
325
affiliations). Group 1 comprises the 17 largest maria. Smaller mare deposits are
326
generally contaminated with highland ejecta due to the proximity of the high-
327
land/mare boundaries, and thus are excluded from Group 1. Group 2 comprises
328
the same maria as Group 1, but excludes Mare Frigoris, Mare Fecunditatis, and 12
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329
Mare Ingenii because these three maria have exceptionally large areas with TiO2
330
below the detection limit (<2 wt%). Comparing the two groups highlights the
331
areal fraction of these three very low-TiO2 maria out of the 17 maria. To define the mare areas we used mare boundaries as mapped from Clemen-
333
tine and LROC WAC data by Nelson et al. (2014). The Aristarchus plateau,
334
Montes Harbinger (east of the Aristarchus plateau), and red spots in the south-
335
ern portion of Oceanus Procellarum (included as mare areas in Nelson et al.,
336
2014) were classified as non-mare regions since these features are not associated
337
with mare basalts. Histograms of the global TiO2 estimates from the Group 1
338
and Group 2 maria are shown in Fig. 5. The Group 1 maria have an average
339
of 3.9 wt% TiO2 compared to 4.2 wt% for Group 2 mare regions (Fig. 5). The
340
areas with TiO2 below the detection limit (2 wt%) are 28% in Group 1 maria
341
(Fig. 5 cumulative plot) and 20% in Group 2 maria. Three geologic settings
342
are likely responsible for these low TiO2 areas: highland ejecta contamination,
343
swirls, and mare formed from low TiO2 basalts.
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Mare regions contaminated by highland impact ejecta exhibit lower TiO2
345
estimates not representative of the original underlying surface (Fig. 6, blue re-
346
gions). For example the rays of Aristarchus crater (40 km diameter) ejecta (Fig.
347
7) spread out underlying highland materials radially to the south and south-
348
west where the mare exhibit lower TiO2 values (mostly below 2 wt%) relative
349
to the adjacent mare surfaces (5-10 wt%) free from the ejecta. Lunar swirls
350
(e.g., Reiner Gamma and those in Mare Ingenii and Mare Marginis; Fig. 6,
351
green regions) have computed TiO2 values significantly lower than the detec-
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tion limit (negative values). Such extremely low values are due to the spectral
353
characteristics of immature swirl surfaces or the presence of glass, both of which
354
would decrease the 321/415 ratio irrespective of ilmenite content (Denevi et al.,
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355
2014, 2016). Several mare units exhibit low TiO2 abundance likely due to their
356
original composition (no clear sign of extensive ejecta contamination), such as
357
Mare Frigoris, the northern portion of Oceanus Procellarum, and the northeast-
358
ern portion of Mare Imbrium. Details of TiO2 distribution within each mare in
359
Group 1 are discussed in section 5.1. 13
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360
3. Comparison with Clementine TiO2 Maps We compared the new WAC TiO2 map (hereafter called WACTiO2 ) with
362
Clementine TiO2 abundance maps. Among various Clementine TiO2 maps de-
363
rived by their own approaches with different conversion parameters (Blewett
364
et al., 1997; Lucey et al., 1998; Jolliff, 1999; Lucey et al., 2000; Gillis et al.,
365
2003; Gillis-Davis et al., 2006), we compared the maps created by Lucey et al.
366
(2000) (hereafter called CLMTiO2 L; historically well known in the lunar science
367
community) and by Gillis et al. (2003) (hereafter called CLMTiO2 G; the latest
368
map with a revised algorithm). The CLMTiO2 L is derived by the following
369
algorithm,
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TiO2 = 3.708 × arctan
R415 R750
− 0.42
R750
!5.979
(2)
where R415 and R750 are the reflectance at 415 and 750 nm, respectively. The
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CLMTiO2 G is derived from a revised algorithm based on Lucey et al. (2000), R415 15.5 R750 −0.36 415 , if R415 ≤ 0.065 and R 0.14 + 0.24 × arctan R750 +0.012 R750 > 0.59 R415 9.8817 TiO2 = −0.36 1.7159 × arctan RR750+0.012 , otherwise 750 (3)
To minimize any geographic mismatch due to misregistration between the
371
WAC and original Clementine mosaics, we reduced the resolution of both TiO2
372
maps to 32 pixel/degree (947.6 m/pixel at the equator). We also excluded areas
373
with topographic slopes greater than 5° (computed from GLD100, Scholten
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370
et al., 2012) to minimize residual errors in photometric normalization on slopes
375
(McEwen, 1991) in the Clementine map products. For this comparison, in areas
376
where the WACTiO2 was below 2 wt% in the mare, the values were set to 1
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377
378
wt% with ±1 wt% uncertainty.
The CLMTiO2 L (Fig. 8) has higher TiO2 abundances relative to the WACTiO2 ,
379
and the offsets gradually increase with TiO2 values. On average for the entire
380
maria, the CLMTiO2 L is 0.8 wt% higher (Fig. 8a) relative to the WACTiO2 .
381
The difference map (Fig. 8b, CLMTiO2 L - WACTiO2 ; centered on the nearside 14
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maria) shows clear offsets in the high TiO2 abundance maria, such as Mare
383
Tranquillitatis (up to +6.9 wt%, +2.9 wt% in average), Mare Fecunditatis (up
384
to +4.6 wt%, +1.6 wt% in average), and Oceanus Procellarum (up to +4.8 wt%,
385
+1.1 wt% in average). On the other hand, the northern portions of Oceanus
386
Procellarum and Mare Imbrium have negative values (down to -2 wt%), mean-
387
ing that the CMLTiO2 L has slightly lower values relative to the WACTiO2 . All
388
of these areas with negative values are located in relatively high latitudes, which
389
may suggest a latitudinal trend possibly due to artifacts related to photometric
390
normalization or a latitude dependent maturity effect (Hemingway et al., 2015)
391
(see section 5.1.12). The other maria at high latitudes (e.g., Mare Australe,
392
Mare Frigoris, and Mare Humboldtianum) are small in size (and thus may be
393
contaminated with highland materials) or have low-TiO2 abundance (<2 wt%,
394
unable to compare with WACTiO2 ), both of which make it difficult to further
395
examine any possible latitudinal effect.
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The offset trends between WACTiO2 and CLMTiO2 G are clearly related
397
to the fact that CLMTiO2 G used two different equations to calculate TiO2
398
depending on the reflectance values (see equation 3). This results in some
399
values near or slightly above the 1:1 line with the WACTiO2 (those calculated
400
with the first version of equation 3), but a large portion that fall substantially
401
below the 1:1 line (TiO2 values calculated with the second version of equation
402
3). On average the difference of WACTiO2 - CLMTiO2 G is -0.3 wt% for the
403
entire maira.
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As seen in the difference maps of CLMTiO2 L - WACTiO2 (Fig. 8b) and
404
CLMTiO2 G - WACTiO2 (Fig. 9b), there are clear vertical stripes particularly
406
in high-TiO2 maria (e.g., Mare Tranquillitatis, Mare Fecunditatis, and Oceanus
407
Procellarum), which are due to artifacts in the Clementine global mosaic. The
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408
WAC near-global mosaic is derived from the median of 21 months of repeat ob-
409
servations, and the photometric normalization is optimized within each tile (1°
410
latitude by 1° longitude) (Sato et al., 2014), resulting in a significantly improved
411
accuracy relative to the Clementine mosaics. On the other hand, the WACTiO2
412
has a detection limit of 2 wt% TiO2 to avoid non-ilmenite source factors (see sec15
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tion 2.2). The algorithms of the Clementine-based TiO2 calculation are reported
414
to minimize the maturity effect (Lucey et al., 1998) , whereas significantly im-
415
mature materials may increase the uncertainties in WACTiO2 due to the nature
416
of the 321/415 ratio (Denevi et al., 2014; Coman et al., 2016). The WACTiO2
417
and CLMTiO2 L are derived from one set of parameters while the CLMTiO2 G
418
utilizes two parameter sets, resulting in the clear offsets of TiO2 values along
419
the condition-matching boundaries. In both the WAC and Clementine based
420
TiO2 maps, TiO2 abundances higher than the Apollo 17 sample-return sites
421
(>10 wt%) are derived simply by extrapolation with unknown uncertainties.
422
More soil samples particularly from the highest TiO2 maria (e.g., Mare Tran-
423
quillitatis, Oceanus Procellarum) in addition to medium TiO2 soils (4-6 wt%,
424
missing range in Fig. 3) are necessary for more accurate TiO2 estimates.
425
4. Comparison with Lunar Prospector TiO2 Map
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Next, we compared the WACTiO2 with the TiO2 abundance derived from
427
Lunar Prospector GRS observations (Elphic et al., 2002; Prettyman et al., 2006)
428
(hereafter called LPTiO2 ). The LPTiO2 was sampled at 2 degrees per pixel (61
429
km pixel scale at the equator) whereas the WACTiO2 is 400 m/pixel at the
430
equator. The area over which the GRS detector is sensitive is larger than the
431
mapped pixel scale, but the detector sensitivity gradually decreases with dis-
432
tance from the nadir point, which is difficult to directly compare with WACTiO2
433
data down-sampled by an average or median value of the footprint area. To ac-
434
curately simulate the spatial response function (footprint weighted by distance
435
from the nadir point) of the GRS (see Fig. 27 in Prettyman et al., 2006), we
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426
used a Gaussian weight function W given by (x/d)2 W (x) = a exp − 2c2
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436
437 438
(4)
where a = 0.48, d = 150, c = 0.6, and x is the distance from the nadir point in
kilometer. The weighted value Vw was then calculated by Pn Vi W (xi ) Vw = Pi=1 n i=1 W (xi ) 16
(5)
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where Vi and xi are the WACTiO2 value and the distance respectively for all the
440
map pixels (400 m/pixel) within 250 km radius circular area (in orthographic
441
projection) centered at each down-sampled pixel. As with the Clementine TiO2
442
vs. the WAC TiO2 comparison, we performed the comparison only for the
443
maria, and WACTiO2 values below 2 wt% in the mare were set to 1 wt% with
444
±1 wt% uncertainty.
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445
The LPTiO2 vs. WACTiO2 plot (Fig. 10a) shows significant scatter but
446
the points are clustered along a 1:1 line. The median of each 0.5 wt% bin lies
447
along the 1:1 line within ±0.7 wt% tolerance in the Group 1 maria (Table 3, see section 2.3 for the definition); the average of standard deviation for all bins
449
is 1.5 wt%; the R2 value of the 1:1 line is 0.52. The difference map (Fig. 10b,
450
LPTiO2 - WACTiO2 ) highlights that the LPTiO2 is lower (blue areas) in several
451
maria (e.g., Serenitatis, -0.5 wt%; Humorum, -0.5 wt%; Crisium, -0.5 wt% on
452
average) and is higher (red areas) in the eastern portion of Oceanus Procellarum
453
(+2.6 wt% on average), relative to the WACTiO2 . The eastern high LPTiO2
454
portion of Oceanus Procellarum corresponds to the region covered by the ejecta
455
and rays from Aristarchus and Kepler craters. The effective depth of the GRS
456
measurement is deeper (30 cm) (Lawrence et al., 2002) than the UV/visible
457
reflectance of the WAC (few millimeters) (Hapke, 2012). Thus, in the areas
458
extensively covered by a layer of ejecta thinner than the GRS sensible depth
459
(<30 cm, Lawrence et al., 2002), the TiO2 estimates by the two instruments
460
could result in an offset due to their different sampling depths.
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The correlation between the two TiO2 maps was further examined using
461
Mare Imbrium low- and high-TiO2 units that have compatible sizes with the
463
large pixel scale of the GRS (2 degrees per pixel). We defined seven units (Fig.
464
11a, H1-3 and L1-4) that outline the boundaries of TiO2 abundance units in
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465
the WACTiO2 . Generally, the units H1-3 indicate relatively high-TiO2 and
466
the units L1-4 are for low TiO2 . The WACTiO2 and LPTiO2 average values
467
(Table 4) for each unit fall near the 1:1 line (within ±0.8 wt% for average values
468
of each unit, R2 = 0.74). The units L2 and L4 fall on mare boundaries and
469
are thus likely contaminated with highland materials due to the GRS’s large 17
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footprint and the highland ejecta. The five major maria (Mare Tranquillitatis,
471
Oceanus Procellarum, Mare Imbrium, Mare Serenitatis, and Mare Crisium) are
472
also plotted in Fig. 11b. Each point corresponds to an average and standard
473
deviation of the entire area of each mare. All five maria are plotted within ±0.3
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474
wt%, showing a good agreement (R2 = 0.94 along the 1:1 line) between the
475
WACTiO2 and LPTiO2 .
476
A pixel-by-pixel comparison of LPTiO2 and WACTiO2 indicates significant
477
scatter of up to ±4 wt% (Fig. 10), likely due to the large footprint and the
lower signal-to-noise ratio of GRS observations relative to the WAC observations
479
(high signal-to-noise ratio by repeated observations). The averaged TiO2 values
480
of each mare (and large mare units), however, showed good correlation between
481
the LPTiO2 and WACTiO2 (Fig. 11b), demonstrating the broadly consistent
482
estimates from the two instrument observations.
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To summarize the comparisons of TiO2 estimates based on three independent
484
datasets (the WAC, Clementine, and Lunar Prospector GRS), the CLMTiO2 L
485
have generally higher values than WACTiO2 , particularly in the high-TiO2
486
maria. The CLMTiO2 G has an average value close to the WACTiO2 (-0.5
487
wt% difference) and shows clear offsets along the boundary of the two calcula-
488
tion conditions. The LPTiO2 values are comparable with WACTiO2 in the five
489
major maria (R2 = 0.94) and in each unit of Mare Imbrium (R2 = 0.74). The
490
LPTiO2 is unaffected by maturity variations and is unique in that it is a mea-
491
sure of the integrated titanium abundance of the top ∼30 cm, minimizing effects
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492
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483
from surface contamination by thin ejecta deposits. In terms of the observation
footprint (particularly compared to the GRS), signal-to-noise ratio, and the to-
494
pographic correction in the photometric normalization, the WACTiO2 has large
495
advantages.
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496
5. Discussion
497
Based on our WAC TiO2 abundance map, here we discuss the local TiO2
498
variations in each mare in the context of the surrounding geologic settings and 18
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the correlation between the TiO2 abundance and the surface age of the entire
500
maria.
501
5.1. TiO2 Variation within Major Maria
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We investigated the spatial variation of TiO2 in the WACTiO2 of 17 out of 23
503
maria defined in Andersson and Whitaker (1982). Each of the 17 maria (Group
504
1, Table 3) has at least one large mare unit that is not dominantly contaminated
505
by highland ejecta materials. For each mare, we derived the median, average,
506
standard deviation, and maximum values of TiO2 (Table 3), as well as the
507
histogram (Fig. 12) and the spatial distribution of TiO2 (Fig. 13-15) using the
508
whole mare area (defined by Nelson et al., 2014). We utilized absolute model
509
ages (derived from crater size frequency distributions; e.g., Morota et al., 2009;
510
Hiesinger et al., 2010) to discuss several flow units of the mare basalts outlined
511
by the WACTiO2 map. Although our flow units are not identical to the units
512
defined by the model age works due to the use of different basemaps (e.g., Galileo
513
and Clementine color ratio composite, Hiesinger et al., 2010), we used the ages
514
derived from the units geographically included or overlapping our flow units.
515
5.1.1. Oceanus Procellarum
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Oceanus Procellarum exhibits sharp TiO2 abundance boundaries likely due
517
to variations of the magma sources, and in some areas, mixing of ejecta from
518
Aristarchus and other craters (Fig. 13a). We note that TiO2 values near
519
Aristarchus crater (<190 km from the crater center) are low (<2 wt%) compared
520
to the surrounding mare surfaces (6-10 wt%) outside of the extent of ejecta rays.
521
Ejecta from Kepler crater (29 km in diameter) mixed with nearby mare (<120
522
km from the crater center) radially lowers the mare TiO2 abundance. Both of
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these Copernican craters (Wilhelms et al., 1987) excavated mare basalts along
524
with highland materials. There is a sharp boundary between high-TiO2 (>6
525
wt%) and medium-TiO2 (<6 wt%) units about 400 km west of the Aristarchus
526
plateau and it arcs about 1000 km from Seleucus crater to Rima Sharp (outlined
527
by black triangles). The high-TiO2 areas (6-12 wt%, southeastern side of this
AC 523
19
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boundary toward Aristarchus Plateau) have a younger model age (1.33–2.96 Ga)
529
relative to the northwestern mid-TiO2 areas (2-6 wt%, 3.40-3.47 Ga) (Hiesinger
530
et al., 2010). The far northern part of Oceanus Procellarum (denoted by "L")
531
shows low TiO2 abundances (<2 wt%), partially due to the ejecta of Pythagoras
532
crater (145 km in diameter, 250 km north from the northern edge of Procel-
533
larum). The Reiner Gamma albedo anomalies (swirls) show significantly lower
534
TiO2 values (although these values are not reliable due to the spectral charac-
535
teristics of immature swirl surfaces or the presence of glass, see section 2.3).
536
5.1.2. Mare Imbrium
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As highlighted in Fig. 11a, Mare Imbrium has large TiO2 units with rela-
538
tively sharp boundaries (Fig. 13b). The highest TiO2 (4-10 wt%) unit is found
539
in the center of the mare (H1, see Fig. 11a for the unit locations). The Chang’e-
540
3 spacecraft landed on and analyzed the regolith near the northeastern edge of
541
this unit (Neal et al., 2015; Ling et al., 2015). The measurements of the Ac-
542
tive Particle-induced X-ray Spectrometer (APXS) aboard Chang’e-3 reported
543
5 wt% TiO2 (sensitive to a depth of several microns; effective detection area
544
∼50 mm in diameter, Fu et al., 2014; Ling et al., 2015), close to the WACTiO2
M
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537
estimate of 6.6 wt% (at 400 m/pixel). The lowest TiO2 estimates (<2 wt%) are
546
found in the northeastern part of this mare (unit L1). The ejecta of Copernicus
547
crater extensively affects the southern third of Mare Imbrium, mostly unit H3.
548
The highland ejecta from Aristillus crater (55 km in diameter) located at the
549
eastern edge of this mare contaminates the eastern portion of unit L3 (Fig. 11),
550
decreasing the TiO2 value. The histogram of Imbrium TiO2 values (Fig. 12)
551
shows a semi-bimodal distribution that reflects the high (H1-3) and low (L1-4)
552
TiO2 units. Sharp low-high TiO2 boundaries (free from the highland ejecta con-
CE
PT
ED
545
tamination), particularly the H1-L1 boundary, outline flow units of mare basalt
554
possibly generated by multiple eruption events that each had distinct composi-
555
tions. The northern part of H1 unit is younger (2.96 Ga, Hiesinger et al., 2010;
556
I22 unit) than the western part of the L1 unit (sharing the boundary with H1;
557
3.01-3.52 Ga Hiesinger et al., 2010; I5, I11, I17, and I21 units), suggesting that
AC 553
20
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558
the TiO2 abundance was higher in the younger events.
559
5.1.3. Mare Cognitum Mare Cognitum can be divided into two spatially separated units (hereafter
561
called Cognitum East and Cognitum West). In Cognitum West (Fig. 13c),
562
the portion around the red spots (highland materials distinctly redder than
563
the surroundings, Whitaker, 1972; Bruno et al., 1991), Darney χ and Darney
564
τ , has the highest TiO2 abundance (6-8 wt%). The rays of Copernicus crater
565
(located at 450 km north of Cognitum) extensively affected the surface of the
566
northeastern portion of Cognitum West. The ejecta of craters Darney (15 km
567
in diameter, located outside of the mare near the southern edge) and Darney
568
C (13 km in diameter, near the southwestern edge of the mare) affected the
569
southern portion of Cognitum West. These ejecta and rays both contribute to
570
lowering the surface TiO2 content locally.
AN US
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560
In Cognitum East (Fig. 13d), a low TiO2 (<2 wt%) unit is dominated by
572
the red spot (Lassell Massif) and Lassell C crater (8.7 km in diameter). Ashley
573
et al. (2016) reported possible post-silicic volcanism at the Lassell Massif and
574
low concentrations of FeO and TiO2 (FeO < ∼2 wt%, TiO2 < ∼4 wt% from
M
571
their Fig. 8) on the Massif area based on Clementine data. It is still not clear if
576
such silicic volcanism is responsible for the low TiO2 unit (Fig. 13d) extending
577
about 50 km radius from the Lassell C crater. The TiO2 gradually increases
578
from both the western and eastern sides of this mare (2–4 wt%) without obvious
579
highland ejecta contamination toward the central high-TiO2 zone (up to 8 wt%)
580
meandering from the north to the south of Lassell Massif (indicated by dashed
581
line in Fig. 13d), suggesting that the varying TiO2 content reflects the changing
582
compositions of the mare basalt over time or within each eruption event.
AC
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575
583
5.1.4. Mare Humorum
584
The northeastern portion of the largest mare unit in Mare Humorum has
585
the highest TiO2 abundance of all units in Humorum (8-10 wt%, see Fig. 13e).
586
In the western half and southeastern margin, TiO2 abundance ranges from 2-6
21
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wt% with no clear unit boundaries. The ejecta of small young craters (e.g.,
588
Gassendi J, Gassendi O, Gassendi Y, Gassendi L, Doppelmayer K, Puiseux
589
D; 5-8 km in diameter) show lower TiO2 content relative to the surrounding
590
surface, indicating that the uppermost layer (latest eruptions) contains more
591
TiO2 . The DMDs along the fractures at the southwestern margin (Hawke et al.,
592
2010) have moderate TiO2 abundances (2-6 wt%, suspicious value due to the
593
glass spectra, see section 2.3). The lowest TiO2 value (<2 wt%) along the
594
southeastern boundary is likely due to admixing of immature ejecta from Tycho
595
crater.
596
5.1.5. Mare Nubium
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587
Bullialdus crater (61 km in diameter) and its ejecta are located in the middle
598
of Mare Nubium and are masked in this analysis (Fig. 13f). A ray of Tycho
599
crater (350 km south of this mare) crosses the western portion in the SSE-
600
NNW direction, likely decreasing the derived TiO2 values (0-2 wt% TiO2 ). The
601
highest TiO2 values (6-10 wt%) are found in the northeastern portion (Fig.
602
13f, indicated by white arrow), in a ∼50 by 50 km spot surrounded by a low
M
597
TiO2 (<2 wt%) unit. The northwestern area of this mare has complicated
604
boundaries due to patchy kipukas of highland units and generally has medium
605
to low TiO2 values (2-6 wt%). The central spot, with a computed TiO2 value
606
substantially below the detection limit (Fig. 13f, indicated by black arrow), has
607
no topographic relief and no indication of contamination by ejecta, suggesting
608
very low TiO2 basalts with distinct spectral properties in this spot.
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603
609
5.1.6. Mare Serenitatis Most of Mare Serenitatis exhibits low to medium TiO2 abundance (2-6 wt%,
611
Fig. 14a). A ray crossing in the NNE-SSW direction, possibly from Tycho
612
crater (2000 km southwest of Serenitatis; Giamboni, 1959; Carr, 1966; Wilhelms
613
et al., 1987; Campbell et al., 1992), results in lower TiO2 values (<2 wt%) and
614
likely affected the areas surrounding the ray where TiO2 values are 2-4 wt%
615
by the mixing of original mare and ejecta materials. Other rays from Aristillus
AC
610
22
ACCEPTED MANUSCRIPT
crater (54 km in diameter, located to the west) and Eudoxus crater (70 km in
617
diameter, located to the north) affect the northern part of the mare, causing
618
TiO2 values lower than the nearby mare surface. The regional dark mantle
619
deposit (RDMD) (Weitz et al., 1998) found in an arc on the southeast margin
620
of the mare (indicated with a black arrow) has the highest TiO2 (>10 wt%);
621
however, this value is suspect due to the peculiar spectral response of pyroclastic
622
glasses (see section 2.3). Across the southern margin, including the boundary
623
with Mare Tranquillitatis, the basalts have the highest values (excluding suspect
624
values at RDMD) of TiO2 (6-10 wt%) . The sharp boundary (outlined by black
625
triangles in Fig. 14a) between these high TiO2 edges and the central mare unit
626
with medium TiO2 (4-6 wt%) is likely due to the compositional change of mare
627
basalt from eruption to eruption. The southern high TiO2 unit inside Serenitatis
628
and the northern high TiO2 unit inside Mare Tranquillitatis are probably the
629
same flow unit, which is slightly older (3.55 Ga (Hiesinger et al., 2010), unit
630
S11) relative to the adjacent medium TiO2 unit inside Serenitatis (∼3.28-3.44
631
Ga, unit S22 and S15). This is one of the sharp TiO2 offset boundaries showing
632
a direct contact of an older high-TiO2 unit and relatively young medium-TiO2
633
unit. See section 5.2 for a global analyses of the TiO2 variations over time.
634
5.1.7. Mare Crisium
ED
M
AN US
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616
The eastern portion of Mare Crisium is high in TiO2 (6-8 wt%), with multiple
636
kipukas of highland material (Fig. 14b). The ejecta of an unnamed 2-km crater
637
has the highest local TiO2 value (∼8.5 wt%), indicating that the underlying layer
638
has higher TiO2 content relative to the uppermost surface. The ejecta of Picard
639
crater (22 km in diameter, located in southwestern Crisium) has higher TiO2
640
value (6-8 wt%) than the surrounding surface as well. The majority of Crisium
CE
PT
635
has intermediate TiO2 values (2-6 wt%). The ejecta of the highland crater
642
Proclus (27 km in diameter, 100 km west of Crisium) affects the northwestern
643
portion of Crisium, likely lowering TiO2 values (<2 wt%).
AC 641
23
ACCEPTED MANUSCRIPT
644
5.1.8. Mare Tranquillitatis The highest mare TiO2 abundances (>10 wt%) are in the northwestern
646
part of Mare Tranquillitatis (Fig. 14c), while in the southern half there are
647
patchy areas of lower TiO2 (4-8 wt%), likely due to both true compositional
648
variations of the mare and contamination by ejecta. The ejecta of Theophilus
649
crater (99 km in diameter, located about 100 km south of Mare Tranquillitatis)
650
overlies the southern portion of Tranquillitatis, likely decreasing the apparent
651
TiO2 abundance of the underlying mare. The northeastern part of the mare
652
has lower TiO2 values (<2 to 6 wt%). Römer crater (44 km in diameter) at
653
∼60 km north from the northern edge of Tranquillitatis also supplied ejecta to
AN US
CR IP T
645
that region, decreasing the TiO2 as well (<2 wt%). There is no obvious sign of
655
overlying highland-sourced materials along the high-low TiO2 boundaries (8-10
656
wt% to 2-6 wt%) at the base of the horn-shaped area in the northeastern portion
657
(black arrows, Fig. 14c), likely indicating a compositional change in the mare
658
basalts with different flow units in a narrow time span (Hiesinger et al., 2000).
659
5.1.9. Mare Fecunditatis
M
654
Most of the southeastern part of Mare Fecunditatis (Fig. 14d) is extensively
661
covered by the ejecta from Langrenus (127 km in diameter) and Petavius B (33
662
km in diameter) craters, both located in the highlands close to the southeastern
663
margin of Mare Fecunditatis. The central part of this mare has patchy varia-
664
tions of TiO2 ranging from 2 to 8 wt%, thus likely resulting from mixtures of
665
ejecta from Langrenus and local compositional variations (4-8 wt%) of the mare
666
basalts. Both ejecta blankets resulted in the lowest TiO2 abundances (<2 wt%).
667
Ejecta of Taruntius crater (56 km in diameter) and Cameron crater (10 km in
668
diameter, located on the rim of Taruntius) also altered the northern surface of
AC
CE
PT
ED
660
669
this mare, resulting in low TiO2 (<2 to 4 wt%). The highest TiO2 values (8-10
670
wt%) are found at the northeastern edge of the mare.
24
ACCEPTED MANUSCRIPT
671
5.1.10. Mare Marginis The upper half of this mare is covered by swirls, resulting in unreliably low
673
(<2 wt%) computed TiO2 values (Fig. 14e), as described in section 2.3. Also,
674
the high reflectance ejecta materials from Goddard A crater (11 km in diameter)
675
contaminate the northeastern edge of the mare, likely lowering the TiO2 values.
676
The southern half of this mare is relatively unaffected by the swirls and ejecta
677
and has medium TiO2 values (2-6 wt%). The second large mare unit inside
678
Neper crater (144 km in diameter) also has an undisturbed surface, showing
679
low to medium TiO2 values (<2 to 4 wt%).
680
5.1.11. Mare Smythii
AN US
CR IP T
672
Mare Smythii generally has intermediate TiO2 values (2-6 wt%) with little
682
spatial variation (Fig. 14f). Peek crater (12.5 km in diameter) located on the
683
northwestern side of this mare excavates the mare and probably the subsurface
684
highland materials, resulting in ejecta with lower (∼2 wt%) TiO2 values. The
685
ejecta of Hume Z crater (14.9 km in diameter) and an unnamed crater (3.5 km
686
in diameter) contaminate the southeastern edges of this mare, resulting in 2-4
687
wt% TiO2 . The central portion of Smythii probably shows the original TiO2
688
values (4-6 wt%).
689
5.1.12. Mare Frigoris
ED
M
681
Previous works have suggested that Mare Frigoris has unique compositional
691
characteristics of low TiO2 and low FeO values relative to other maria (Johnson
692
et al., 1977; Whitford-Stark, 1990; Lucey et al., 2000; Kramer et al., 2015). In
693
the WAC TiO2 , 93% of Mare Frigoris is lower than the detection limit (Fig.
694
15a). The western portion of this mare with about 4 wt% TiO2 may be an
CE
PT
690
extension of the unit that makes up the north end of Oceanus Procellarum, but
696
is separated by the ejecta of Harpalus crater (40 km in diameter). This mare is
697
a relatively narrow E-W strip, and thus the whole area is close to the highlands
698
relative to other maria, possibly resulting in more highland ejecta contamination
699
such as from the rays from Anaxagoras crater that cross much of Frigoris. Thus
AC 695
25
ACCEPTED MANUSCRIPT
the anomalously high albedo and lower TiO2 values may simply be a result of
701
highland contamination. Alternatively, Hemingway et al. (2015) suggest that
702
there is a latitudinal trend in maturity decreasing toward the pole, which is
703
responsible for the higher albedo of Mare Frigoris relative to the equatorial
704
mare with similar FeO and TiO2 content. The presence of this trend was also
705
confirmed using the active reflectance measurements by Lunar Orbiter Laser
706
Altimeter (LOLA) (Lemelin et al., 2016). This high latitude immaturity might
707
also be responsible for the low calculated TiO2 abundance in the majority of
708
Frigoris, similar to the immature swirls that result in anomalously low values of
709
TiO2 calculated from the WAC data.
710
5.1.13. Mare Tsiolkovskiy
AN US
CR IP T
700
The southern and eastern portions of this mare have relatively high TiO2 (up
712
to 6 wt%, Fig. 15b). There is no clear indication of significant contamination
713
from craters outside of the mare. There are subtle TiO2 variations from 2 to 6
714
wt% without clear unit boundaries, possibly suggesting a gradual compositional
715
change as the mare basalts were emplaced.
716
5.1.14. Mare Moscoviense
ED
M
711
The majority of the eastern half of Mare Moscoviense (Fig. 15c) exhibits high
718
TiO2 values (6-10 wt%), whereas the western half has low to medium TiO2 (<2
719
to 4 wt%). The bimodal distribution of the Mare Moscoviense histogram (Fig.
720
12) is due to the eastern and western units having an obvious compositional
721
difference in their mare basalts. No clear age difference was found between the
722
eastern and western mare units (east: 2.6-3.6 Ga; west: 3.5 Ga, Morota et al.,
723
2009). The western edge of this mare is partially overlain by swirls, overprinting
724
low TiO2 (<2 wt%) on the western unit (originally 2 to 4 wt%). Rays of Steno
725
Q crater (33 km in diameter, 150 km to the east of Moscoviense) cross the
726
eastern high TiO2 unit, leaving multiple streaks that lower the surface TiO2
727
values about 2 to 4 wt% relative to the surroundings.
AC
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717
26
ACCEPTED MANUSCRIPT
728
5.1.15. Mare Ingenii Mare Ingenii embays the floors of Thomson crater (113 km in diameter),
730
Thomson M crater (114 km in diameter), and the central to the southern por-
731
tion of Ingenii basin (Fig. 15d). Most of the southern part of this mare and
732
roughly 1/3 of the mare in Thomson crater have very low computed TiO2 val-
733
ues, which correspond to the extensive system of swirls that overlies much of
734
the region (see section 2.3). The low-reflectance portions next to the winding
735
high-reflectance areas of the swirls have higher TiO2 values than the surround-
736
ing mare surface (about +2 wt% increase), which might be due to the relation
737
between maturity and the 321/415 ratio as suggested by Denevi et al. (2014)
738
and Coman et al. (2016). The southeastern edge of Thomson crater including
739
the portion extending out of the crater, which is relatively unaffected by swirls,
740
has 2-6 wt% of TiO2 .
741
5.1.16. Mare Orientale
AN US
CR IP T
729
Mare Orientale consists of multiple small disconnected mare patches located
743
near the basin center and between the multi-ring structures (Fig. 15e). The
744
highest TiO2 values are 6-8 wt% at the southeastern portion of the largest
745
unit. The ejecta of Maunder crater (54 km in diameter, north side of the basin)
746
extensively overlays this unit, lowering the surface TiO2 values from <2 to 4
747
wt% for nearly all of the northern half of this unit. The southwestern portion
748
of this largest unit has low TiO2 values (<2 wt%), probably due to an actual
749
compositional variation in the basalts.
PT
ED
M
742
The second large mare unit, Lacus Veris, located along the inner ring, has
751
<2 to 6 wt% TiO2 . The ejecta of Maunder and other craters affect this unit,
752
decreasing the surface TiO2 values to <2 wt%. Other smaller units along the
AC
CE
750
753
northern portion of the inner ring are also covered by Maunder’s ejecta, and
754
show low TiO2 values (<2 wt%). The ejecta of an unnamed crater (∼2 km in
755
diameter, upper-left in Fig. 15e) exhibits larger TiO2 values (2-4 wt%) than the
756
surrounding mare surface (<2 wt%). This likely indicates that the uppermost
757
surface had been contaminated by highland ejecta first (from undefined multiple 27
ACCEPTED MANUSCRIPT
sources), then the more recent impact event of the unnamed crater exposed older
759
underlying mare basalts. The other units along the outer ring, including Lacus
760
Autumni, have TiO2 values up to 4 wt%.
761
5.2. Correlation between Age and TiO2 Abundance
CR IP T
758
The correlation between the age, locations, and the composition of mare
763
units has been discussed by several workers (e.g., Soderblom and Lebofsky,
764
1972; Head, 1976; Soderblom et al., 1977; Hiesinger et al., 2001; Kodama and
765
Yamaguchi, 2005; Morota et al., 2011). Early studies based on the Apollo and
766
Luna samples suggested that the low TiO2 basalts are generally younger than
767
high-TiO2 basalts (Taylor, 1982). Later interpretations of remote-sensing data
768
and lunar meteorites revealed the existence of relatively young, high-TiO2 and
769
old, low TiO2 basalts (e.g., Pieters et al., 1980; Cohen et al., 2000). In an exam-
770
ination using the global TiO2 map (CLMTiO2 L) and the crater size-frequency
771
distribution ages, no distinct correlation was observed between the deposit age
772
and the TiO2 values on a global scale (Hiesinger et al., 2001). However, several
773
regional studies suggest that the younger basalts tend to have relatively high-
774
TiO2 , and the low TiO2 basalts are relatively old (Imbrian Period, 3.2–3.8 Ga)
775
(Kodama and Yamaguchi, 2005; Morota et al., 2011).
ED
M
AN US
762
We re-examined the age vs. TiO2 abundance correlation of the mare basalts
777
at a global scale using the WACTiO2 map and the crater size frequency distribu-
778
tion model ages reported by various studies (Greeley et al., 1993; Hiesinger et al.,
779
2000, 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al.,
PT
776
2016). Instead of using the entire area of each mare unit defined by these works,
781
we used the subset areas (rectangular areas defined by Hiesinger et al., 2000,
782
2003, 2010) where the crater counts were performed (hereafter called “count
AC
CE 780
783
area”, see Fig. 16). Even a very thin dusting can change the optical character-
784
istics of the surface (thus WACTiO2 values) within each mare unit. Also in a
785
narrow sense, the model age can vary within a unit depending on the location of
786
count area (e.g., 1.0 to >2 Ga in P60 unit, Stadermann et al., 2015). Therefore,
787
comparing the age and the TiO2 values within the count areas minimizes possi28
ACCEPTED MANUSCRIPT
ble internal variations of age and TiO2 (due to actual compositional variations
789
and highland contamination) inside each unit. We note that in several mare
790
units (e.g., Mare Frigoris, Mare Fecunditatis, and Mare Humorum), we simply
791
used each unit area since no subset was identified as the count area in the model
792
age works. We also excluded several count areas (or the original unit areas) ob-
793
viously contaminated by highland ejecta, which do not represent the spectrum
794
of the original surface. Currently, a few major maria or mare units (e.g., Mare
795
Crisium, Mare Somniorum, and the southeastern unit of Mare Humorum) do
796
not have model ages, thus we excluded those maria. The TiO2 values below 2
797
wt% in WACTiO2 were set to 1 wt% before computing the median and standard
798
deviation for each count area. For the count areas that are dominantly below 2
799
wt% TiO2 (defined by the median = 1 wt%), ±1 wt% uncertainty was applied
AN US
800
CR IP T
788
instead of using the standard deviation.
801
The plot of age vs. TiO2 values (Fig. 17) using the count areas in all
802
maria exhibits a wide range in TiO2 (from <2 to 10 wt%) before ∼2.6 Ga,
and only medium- to high-TiO2 abundance (4-10 wt%) for ages <2.6 Ga. The
804
average before 2.6 Ga is 3.4 wt% TiO2 and after 2.6 Ga is 6.8 wt%. The
805
relatively high-TiO2 values for younger ages and low-TiO2 values with mostly
806
older ages are consistent with the trends observed by previous works (Pieters
807
et al., 1980; Kodama and Yamaguchi, 2005; Morota et al., 2011). In the plots for
808
the individual mare (Fig. 18), no clear trend was found except for several maria
809
(e.g., Oceanus Procellarum, Mare Humorum, and the southwestern nearside
810
maria). The dots clustered below 2 wt% (Fig. 17) dominantly consist of areas in
PT
ED
M
803
Mare Frigoris, Humboldtianum, and Australe Basin. Oceanus Procellarum has
812
a wider range of ages (from 3.63 to 1.21 Ga) relative to the other maria, and the
813
TiO2 values moderately increase with time (∼1 wt% per Gyr average). Although
AC
CE 811
814
the time range is limited, Mare Humorum and the southwestern nearside maria
815
also show increasing TiO2 trend over time from 3.8 to ∼3.0 Ga. After ∼2.8 Ga,
816
no count area has less than 4.5 wt% of TiO2 . The other maria have relatively
817
older ages ranging from 3.88 to ∼2.8 Ga, and some maria (e.g., Fecunditatis,
818
Serenitatis, and Tranquillitatis) have a wide range in TiO2 (from <2 to 10 wt%) 29
ACCEPTED MANUSCRIPT
while others (e.g., Vaporum, Marginis, and Nubium) have narrower TiO2 ranges.
820
These results suggest that old (>2.6 Ga) mare volcanic eruptions occurred in
821
various places with the widest variation in TiO2 contents, then younger (<2.6
822
Ga) eruptions mainly occurred in Oceanus Procellarum with relatively high-
823
TiO2 content.
824
5.3. Implications for mare volcanism
CR IP T
819
The nature of TiO2 enrichment in some mantle source regions is not well
826
understood. Presumably, at a late stage of magma ocean solidification, ilmenite
827
began to crystallize along with Fe-rich pyroxene, forming dense, gravitation-
828
ally unstable cumulates (Taylor and Jakes, 1974; Snyder et al., 1992; Hess and
829
Parmentier, 1995). Some portions of these cumulates were likely enriched in
830
incompatible elements, with relatively high concentrations of heat-producing
831
elements such as Th, U, and K (Haskin, 1998; Wieczorek and Phillips, 2000;
832
Korotev, 2000). Some of the high-TiO2 mare basalts are indeed enriched in
833
these elements, such as the high-K Apollo 11 basalts while other examples are
834
not (other high-TiO2 basalts from Apollo 11 and 17) (Taylor et al., 1991; Neal
835
and Taylor, 1992). Likewise, sampled KREEP basalts (enriched in potassium
836
(K), rare earth elements (REE), and phosphorous (P)) contain ∼2 wt% TiO2
ED
M
AN US
825
(e.g., Warren and Wasson, 1978; Salpas et al., 1987). Therefore it is possible
838
that Ti-rich cumulates and KREEP-rich residual melt were decoupled, probably
839
by gravitational separation, in some regions (e.g., Shearer et al., 2006). Alter-
840
natively, the basalts were enriched in incompatible elements by assimilation of
PT
837
KREEP at the base of the crust. Remote sensing by the Lunar Prospector
842
GRS shows Th enrichment over broad areas of the western Procellarum basalts
843
(Jolliff, 2004). We consider that this broadly enriched Th (3-5 ppm) over the
AC
CE 841
844
PKT area cannot be explained only by the direct excavation of sub-crustal
845
KREEP layer as inferred by the distributions of highest Th along the Imbrium-
846
circumferential highlands and the ejecta of several craters (e.g., Aristarchus,
847
Kepler, Lalande, and Aristillus). The mare basalt rock fragments with ilmenite
848
and high-Th content (6.9 ppm) from the Apollo 12 regolith samples also sug30
ACCEPTED MANUSCRIPT
gest possible Th enrichment in the mare basalts (Jolliff et al., 2005; Barra et al.,
850
2006; Korotev et al., 2011). Where the western Procellarum basalts dominate
851
the younger end of the age range for mare basalts, it is reasonable to infer, on
852
the basis of a correlation between TiO2 and incompatible trace elements (re-
853
flected by LP-GRS Th), a relationship between TiO2 and radiogenic-element
854
enrichment in their source regions.
CR IP T
849
In this case, we also infer that perhaps ilmenite cumulates and incompatible-
856
element-rich residual melt of the magma ocean were not decoupled as they
857
seem to have been in source regions of some of the older high-TiO2 basalts.
858
Having that enrichment in the source region might help to explain how basaltic
859
volcanism extended to ages in the range of 1-3 Ga on the Moon (see discussion
860
in Shearer et al., 2006, pp.467-468).
AN US
855
Understanding the accurate TiO2 distribution in mare basalts provides an
862
indication of TiO2 enrichment in underlying mantle source regions, and cor-
863
relations with Th enrichment in some areas suggests a possible petrogenetic
864
relationship, as inferred above for the western Procellarum basalts (e.g., Gillis
865
et al., 2002; Flor et al., 2002, 2003). Coupled with age data for various mare
866
units, we can gain an improved understanding of the thermal evolution of the
867
mantle. At about 2.6 Ga, the age-TiO2 relation (Fig. 17) suggests a transition
868
from a large range of TiO2 contents in widely distributed older maria to a more
869
restricted, medium to high TiO2 range for younger mare, mostly located in
870
Oceanus Procellarum (Fig. 17). What does this transition mean in the context
871
of mare volcanism?
PT
ED
M
861
The low TiO2 basalts (<5 wt%) represent the most abundant product of
873
partial melting of magma ocean cumulates (i.e., 70 vol% or more of the cumu-
874
late pile), whereas the medium- to high-TiO2 basalts (>5 wt%) are relatively
AC
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875
rare products from partial melting of late-stage cumulates (about 5-10 vol%
876
of the cumulate pile) based on the geochemical-thermal modeling (Taylor and
877
Jakes, 1974; Hess, 1991; Hess and Parmentier, 1995). Early mare eruptions (be-
878
fore the 2.6 Ga transition) were globally distributed, including several farside
879
examples (e.g., Mare Moscoviense, Mare Tsiolkovskiy, South Pole-Aitken). The 31
ACCEPTED MANUSCRIPT
large TiO2 variations in this period suggest multiple melt sources with a wide
881
range of TiO2 concentrations. Owing to sufficient heat in the early stage of
882
lunar thermal evolution (Wieczorek and Phillips, 2000), the melt source should
883
have existed at various depths including shallow regions. Indeed, petrology ex-
884
periments indicate that the Apollo 11 and 17 high-TiO2 basalts originated from
885
shallow mantle depths based on the equilibrium condition with a plausible cu-
886
mulate mineral assemblage using multiple saturation points (Longhi et al., 1974;
887
Walker et al., 1977; Grove and Krawczynski, 2009). The Th vs. age plot (Fig.
888
19, for all the count areas) exhibits larger variation in Th (0.1-11.3 ppm) in the
889
older basalts (>2.5 Ga), implying Th-independent melt production due to the
890
early hotter interior.
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880
As the mantle cooled, partial melting zones were likely deeper and the flux
892
of basaltic melts to the surface should have diminished (Head and Wilson, 1992;
893
Hess and Parmentier, 2001; Shearer et al., 2006). The Ti-rich cumulates would
894
be enriched in the late-stage magma-ocean residual melt (urKREEP) (Warren
895
and Wasson, 1979), possibly deep seated as a result of mantle overturn (Hess and
896
Parmentier, 1995). In such cumulates, high Th (U and K as well) contents would
897
have prolonged the heat and the melt production (Wieczorek and Phillips, 2000),
898
possibly resulting in young high-TiO2 basalts (<2.6 Ga) in the Procellarum
899
KREEP Terrane (Jolliff et al., 2000). The Th contents in such young basalts
900
exhibit relatively high values (3.3-8.4 ppm) in the Th vs. age plot (Fig. 19).
PT
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891
901
We note here that our interpretation inferring the possible correlation be-
902
tween Th enrichment and titanium content of the relatively young basalts is based largely on the remote-sensing observations. Currently, there is no petro-
904
logical consensus supporting a direct connection of TiO2 and Th (or other ra-
905
dioactive elements) in the context of mare volcanism. The returned samples are
AC
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906
dominantly from the eastern older maria and not from the western younger high-
907
TiO2 maria, particularly Oceanus Procellarum. New samples from the western
908
young mare obtained by future missions will allow us to test our hypothesis and
909
to understand the mechanisms of long-lasting mare volcanism.
910
One of the long-standing questions is the discrepancy between TiO2 esti32
ACCEPTED MANUSCRIPT
mates based on the visible to NIR wavelength reflectance data (e.g., Clementine
912
UVVIS, Chang’e-1) and the GRS data (e.g., Elphic et al., 1998; Gillis et al.,
913
2003). In particular, the highest TiO2 values (up to 17 wt% in Clementine TiO2 ,
914
up to 11 wt% in LPTiO2 ) show a large difference. Our estimates using WAC
915
321/415 nm ratio are similar to the LPTiO2 values (see section 3 and 4), and
916
are also consistent with the range of TiO2 seen among Apollo and lunar mete-
917
orite basalt compositions (Lucey et al., 2006). Our high-end TiO2 estimates,
918
which match the GRS values and are significantly lower than the visible to NIR
919
estimates, represent >10 wt% at only 0.7% of the mare (Fig. 5, cumulative
920
histogram of Group 1 maria), indicating that very high-TiO2 basalt eruptions
921
were probably rare and limited in the history of lunar volcanism. This result
922
may add new constraints on the eruption mechanisms of the mare basalt.
923
6. Conclusion
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911
We derived a new TiO2 abundance map of the Moon using the UV and
925
visible wavelengths of LROC WAC observations. We found a linear correla-
926
tion between the 321/415 nm band ratio of 62 months WAC observations and
927
the TiO2 content of 9 returned soil samples acquired in geologically homoge-
928
neous areas at the typical WAC pixel scale (400 m). Using this correlation
929
and a near-global WAC multispectral mosaic, we constructed a new WAC TiO2
930
abundance map. To avoid non-ilmenite source factors controlling the 321/415
931
ratio (e.g., the pyroxene content, shock degree of plagioclase, immaturity with
932
IS /FeO <∼40, and the presence of glass, Denevi et al., 2014; Gillis-Davis et al.,
933
2006; Coman et al., 2016), we set 2 wt% TiO2 as the detection limit. Since
934
the highlands have TiO2 abundances below this detection limit we excluded the
CE
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924
highland terrain from our study.
AC 935
936
In the new WAC TiO2 map, the 17 major maria (Group 1) have an average
937
of 3.9 wt% TiO2 . The highest TiO2 values are observed in Mare Tranquilli-
938
tatis (∼12.6 wt%) and Oceanus Procellarum (∼11.6 wt%). Also, 30% of the
939
mare surfaces included in Group 1 maria are below the detection limit. Lunar 33
ACCEPTED MANUSCRIPT
940
swirls, low TiO2 basalts, and the regions contaminated by highland ejecta are
941
dominantly responsible for these low-TiO2 areas. A comparison of the WAC TiO2 map with the Clementine-based TiO2 map
943
(Lucey et al., 2000) revealed systematically higher TiO2 abundances in the
944
Clementine map, particularly in the high-TiO2 maria (e.g., up to +6.9 wt% in
945
Mare Tranquillitatis). Another Clementine-based TiO2 map created by Gillis
946
et al. (2003) has closer average TiO2 values to the WAC TiO2 map (-0.5 wt%
947
difference), albeit with a the clear TiO2 offsets along the condition-matching
948
boundaries used in their calculations. The Lunar Prospector GRS based TiO2
949
map (LPTiO2 ) is more consistent with WACTiO2 . In the comparisons for the
950
seven mare units with distinct TiO2 abundances within Mare Imbrium, and
951
for the five major maria (Mare Tranquillitatis, Oceanus Procellarum, Mare Im-
952
brium, Mare Serenitatis, and Mare Crisium), LPTiO2 and WACTiO2 showed
953
a good correlation that falls within ±0.8 wt% of a 1:1 line. The higher value
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942
in the LPTiO2 relative to the WACTiO2 at the eastern portion of Oceanus
955
Procellarum could be due to the different depth sensitivities of the GRS (∼30
956
cm) and the WAC (few millimeters) in the areas of extensive ejecta deposits of
957
Aristarchus, Kepler, and Copernicus craters.
ED
M
954
The spatial variations of TiO2 abundance within each of the Group 1 maria
959
revealed significant contamination from highland ejecta, indicating that the orig-
960
inal mare basalts have potentially higher TiO2 content than the current upper-
961
most surface. Several maria (e.g., Mare Serenitatis, Mare Imbrium, and Oceanus
962
Procellarum) have distinctly high- and low-TiO2 units located next to each
PT
958
other, which also have clear age differences. For all of the mare units for which
964
model ages have been computed (using the count areas defined by Hiesinger
965
et al., 2000), a wide range of TiO2 values is found for relatively old (>2.6 Ga)
AC
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966
mare volcanism and only medium- to high-TiO2 values for the younger (<2.6
967
Ga) mare deposits, mostly in Oceanus Procellarum. This 2.6 Ga transition may
968
indicate a shift of lunar volcanism driven by the early hotter interior on a global
969
scale to one driven by local high-TiO2 cumulates enriched in radioactive element
970
heat sources. 34
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Our highest TiO2 estimates (excluding the DMDs) were found in the east-
972
ern portion of Mare Tranquillitatis. These values are higher than any returned
973
soils, meaning that they are the extrapolated estimates. Future lunar land-
974
ings or sample return missions on these highest TiO2 areas will increase the
975
accuracy of reflectance-based orbital TiO2 estimates by improving the correla-
976
tion fitting model (linear fit in this work). The small-sized or narrow-shaped
977
maria with low-TiO2 values (e.g., Mare Frigoris, Mare Moscoviense, and Mare
978
Humboldtianum) potentially have significant highland contamination, resulting
979
in reflectance-based estimates likely lower than the true mare values. These
980
maria are in key locations to understand farside and high latitude volcanism.
981
Higher resolution orbital images from future spacecraft missions, particularly
982
in the UV wavelength range (e.g., 250-400 nm, Cloutis et al., 2008), will allow
983
for the examination of small crater ejecta from the upper subsurface, which in
984
many cases could reveal the original surface TiO2 abundances before highland
985
contamination.
986
Acknowledgments
M
AN US
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971
This work was supported by the National Aeronautics and Space Administra-
988
tion Lunar Reconnaissance Orbiter Project. The United States Geological Sur-
989
vey Integrated Software for Imagers and Spectrometers (ISIS) (Anderson et al.,
990
2004) played a key role in processing the WAC observations. We acknowledge
991
Dr. Gillis-Davis for his help on analyzing the Clementine-based TiO2 maps.
992
We also acknowledge Oana Coman for her inputs that improved the quality of
993
this manuscript. We thank two anonymous reviewers who greatly improved our
994
manuscript.
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Figure 1: Spectra of typical lunar rock-forming minerals (from USGS spectral library, Clark et al., 1993). WAC’s seven wavelengths (321, 360, 415, 566, 604, and 689 nm) band center and FWHM of the WAC bandpasses are shown as vertical lines and gray bars, respectively.
Figure 2:
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The reflectance of anorthosite is multiplied by 0.7 to fit on the plot.
Example of WAC pixel selection using NAC reflectance mask in (a) 321 nm and
(b) 415 nm bands for the Apollo 11 landing site. Red box denotes the 1000 by 1000 meter
WAC sampling area (“site box”). Red and blue portions are the masked region. Colored dot and white cross indicate the center and the extent of each WAC pixel, and the dot color
corresponds to the photometrically normalized I/F (nI/F ). There are over 30 times more
observations at 415 nm than at 321 nm due to the higher spatial resolution in the visible bands.
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The histograms (blue line) of sampled WAC pixels in 321 and 415 nm bands are shown in the bottom panels. Red curve and dashed vertical line are the Skew Normal distribution curve fitting and its peak value location.
Figure 3:
WAC 321/415 nm nI/F ratio as a function of soil TiO2 content at the selected
sample return stations (see Table 1). Color and symbol indicate each sample return mission. The sample return site name is denoted at the top/bottom of each error bar, calculated by
Figure 4:
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Eq. 1. Solid and dashed line are the least-square fit and its 95% confidence band, respectively.
WAC TiO2 abundance map for lunar mare (latitude 70°S to 70°N, longitude
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180°W to 180°E in equirectangular map-projection). The highlands were masked using the mare/highland boundaries (Nelson et al., 2014). Color corresponds to 1 wt% bin of TiO2 values. All values less than ∼2 wt% (filled by dark blue) are below the detection limit of the
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reflectance variations related to ilmenite content.
Figure 5: Histograms of WAC TiO2 values for Group 1 (17 maria) and Group 2 (14 maria),
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listed in Table 3. Solid line is a cumulative histogram of Group 1 maria. Bin size is 0.1 wt%. Bold dashed line indicates the detection limit of TiO2 (<2 wt%). Narrow dashed line and
number indicate average value derived from the entire areas of maria in each group, with a simplification that the areas with <2 wt% TiO2 are uniformly 1 wt%. Dot with error bar
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indicates the fraction of TiO2 below 2 wt%, which dominantly corresponds to mare boundary pixels, highland ejecta (rays), swirls, and isolated highland massifs.
Figure 6: Distributions of swirls (green), dark mantle deposits (red), and the highland ejecta extensively contaminated the mare (blue) for latitude 70°S to 70°N, longitude 180°W to 180°E.
Gray zone corresponds to the highland terrains mapped by Nelson et al. (2014).
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Figure 7:
Southern portion of Aristarchus crater ejecta inside Oceanus Procellarum, in
WACTiO2 (400 m/pixel, upper panel) and in 643 nm band WAC global mosaic (100 m/pixel, lower panel). Images are centered at latitude 19.9°N, longitude 312.6°E, in equirectangular
Figure 8:
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map-projection. (a) Plot of CLMTiO2 L vs. WACTiO2 for the Group 1 maria (see Table 3) and
(b) difference map of CLMTiO2 L - WACTiO2 (40°S to 65°N, 90°W to 90°E). Errors increase below 2 wt% in the WACTiO2 (gray box in (a)). The difference map in (b) was derived by setting WACTiO2 values <2 wt% as 1 wt% before the subtraction. Black line in (b) highlights
boundaries of the Group 1 maria, and gray line for rest of the maria. The highland areas were masked.
(a) Plot of CLMTiO2 G vs. WACTiO2 for the Group 1 maria and (b) difference
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Figure 9:
map of CLMTiO2 G - WACTiO2 (40°S to 65°N, 90°W to 90°E). Errors increase below 2 wt% in the WACTiO2 (gray box in (a)). The difference map in (b) was derived by setting WACTiO2 values <2 wt% as 1 wt% before the subtraction. Black line in (b) highlights boundaries of the Group 1 maria, and gray line for rest of the maria. The highland areas were masked. Figure 10: (a) Plot of LPTiO2 vs. WACTiO2 for the Group 1 maria and (b) difference map
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of LPTiO2 - WACTiO2 in 2 pixels per degree (50°S to 70°N, 120°W to 120°E; same color stretch as Fig. 8 and 9). Dot with error bar in (a) correspond to a median and standard deviation (σ) at each bin (bin size: 0.5 wt%). Errors increase below 2 wt% in the WACTiO2
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(gray box in (a)). The difference map in (b) was derived by setting WACTiO2 values <2 wt% as 1 wt% before the subtraction. Black lines in (b) highlight boundaries of the Group 1 maria, and gray lines for rest of the maria. The highland areas were masked.
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Figure 11: (a) Mare Imbrium geologic units based on TiO2 abundance, and (b) TiO2 content from WACTiO2 and LPTiO2 for each Imbrium unit (listed in Table 4). The areas with WACTiO2 below the detection limit (2 wt%) were set to 1 wt%. The character and color of
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each unit in (a) correspond to the legend character and symbol color in (b). Black symbol with single character in (b) show the averaged TiO2 content in Mare Tranquillitatis (• T), Oceanus Procellarum ( P), Mare Imbrium (N I), Mare Serenitatis (H S), and Mare Crisium ( C).
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Error bar indicates the standard deviation, except for unit L2 with ±1 wt% (uncertainty
below 2 wt% areas). Solid line is the 1:1 line.
Figure 12: Histograms of TiO2 abundance from the WACTiO2 for the Group 1 maria. Y-axis is a fraction normalized by the maximum count of each histogram. Color corresponds to <2 wt% (blue), 2-4 wt% (green), 4-6 wt% (yellow), 6-8 wt% (pink), 8-10 wt% (red), and >10 wt% (dark red).
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Figure 13: TiO2 variation inside each mare with the same color scheme of 2 wt% bin in TiO2 as Fig. 12. White solid lines outline the mare boundaries (see section 2.3). Background image is the WAC 643 nm monochrome mosaic (100 m/pixel). The scale bar in the lower left within
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each panel indicates 100 km. The direction towards the craters located out of image frame is
indicated by an arrow with the crater name. Images are centered at (a) Procellarum 304.5°E,
18.4°N; (b) Imbrium: 341.9°E, 32.8°N; (c) Cognitum West: 338.1°E, 10.6°S; (d) Cognitum East: 349.9°E, 12.4°S; (e) Humorum: 323.9°E, 27.1°S; (f) Nubium: 341.4°E, 23.0°S. Black triangles and symbol "L" in (a) indicate the boundary of sharp TiO2 offset and a low-TiO2
area respectively. Dashed line in (d) traces central high-TiO2 zone. Black and white bold
Figure 14:
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arrows in (f) point out the low-TiO2 and highest TiO2 spots, respectively.
TiO2 variation inside each mare (continued from Fig. 13). See Fig. 13 caption
for symbol definitions. Images are centered at (a) Serenitatis: 19.3°E, 26.9°N; (b) Crisium: 60.5°E, 18.3°N; (c) Tranquillitatis: 30.7°E, 9.0°N; (d) Fecunditatis: 51.9°E, 6.9°S; (e) Marginis: 85.2°E, 13.6°N; (f) Smythii: 87.8°E, 1.6°S. Black bold arrows in (a) and (c) point out the RDMD (section 5.1.6) and the base of the horn-shaped area (section 5.1.8) respectively. Black
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triangles in (a) outline the high- and low-TiO2 boundaries. Polygons hatched and outlined by narrow black lines in (e) indicate the distribution of the swirls mapped by Denevi et al.
TiO2 variation inside each mare (continued from Fig. 14). See Fig. 13 caption
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Figure 15:
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(2016).
for symbol definitions. Imaged are centered at (a) Frigoris: 5.4°E, 56.2°N; (b) Tsiolkovskiy: 128.9°E, 20.5°S; (c) Moscoviense: 148.4°E, 27.7°N; (e) Ingenii: 164.8°E, 32.9°S; (f) Orientale: 270.3°E, 16.6°S. Polygons hatched and outlined by narrow black lines in (c) and (d) indicate
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the distribution of the swirls mapped by Denevi et al. (2016).
Figure 16:
Distribution of the count areas (areas where crater counting was performed).
Blue count areas were included in the plots in Fig. 17-19, and red count areas were excluded. Square shaped count area corresponds to a representative subset of each mare unit defined by Hiesinger et al. (2000, 2003). Background image is the WAC 643 nm monochrome mosaic. The highlands (gray zone) were masked using mare/highland boundaries (Nelson et al., 2014).
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Figure 17:
TiO2 abundance as a function of crater size-frequency distribution model ages
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(Greeley et al., 1993; Hiesinger et al., 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016). Dot and bar indicate the median and standard deviation of WACTiO2 values within each count area (sub-divided mare unit or the square-shaped sub-
set area (Hiesinger et al., 2000, 2003) where the crater counts were actually performed). Age unit is gigayear ago (Ga, 109 years). Dashed line indicates the WACTiO2 detection limit (2 wt%). The count areas with WACTiO2 less than 2 wt% in median were set to 1 wt% with
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±1 wt% uncertainty.
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Figure 18: TiO2 abundance vs. crater size-frequency distribution model ages (Greeley et al., 1993; Hiesinger et al., 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016) inside each mare (see Fig. 17 for symbol definitions). The “Southwestern Nearside”
ED
corresponds to several mare patches inside individual craters and smooth plains to the south
CE
PT
of Oceanus Procellarum.
Figure 19: Thorium weight fraction as a function of crater size-frequency distribution model
AC
age (Greeley et al., 1993; Hiesinger et al., 2003, 2006; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016). Dot and bar indicate the median and standard deviation of the Th weight fraction derived by Lunar Prospector GRS (Prettyman et al., 2006) within each count area. Color indicates the median of WACTiO2 within each count area.
52
Table 1:
CR IP T
ACCEPTED MANUSCRIPT
Representative WAC nI/F and standard deviation at 321 and 415 nm as well
as 321/451 nm ratio over each Apollo ("Axx") and Luna (“Lxx”) sampling site. The TiO2
content of each soil compiled by Blewett et al. (1997) and Jolliff (1999) is also given. The
AN US
representative nI/F value corresponds to the modal value of multiple WAC observations (on average 111 at 321 nm and 3966 at 415 nm), photometrically normalized to 0° emission and 60° incidence angles.
Soil TiO2
321 nm
wt%
nI/F
A11
7.5
0.010
A12
3.1
0.011
A14
1.7
0.013
A15-S8
1.7
A15-09
415 nm
321/415
σratio
nI/F
St.Dev.
4.0E-4
0.0136
8.3E-4
0.765
0.016
3.7E-4
0.0153
8.2E-4
0.722
0.014
3.4E-4
0.0184
1.1E-3
0.704
0.021
0.011
3.0E-4
0.0158
8.7E-4
0.699
0.018
1.8
0.011
5.1E-4
0.0153
9.6E-4
0.710
0.010
8.5
0.011
3.9E-4
0.0140
1.6E-3
0.768
0.053
8.0
0.010
2.3E-4
0.0137
1.3E-3
0.762
0.053
6.7
0.010
3.0E-4
0.0137
1.1E-3
0.758
0.034
A17-LRV9
6.1
0.011
3.9E-4
0.0147
1.2E-3
0.752
0.032
L16
3.3
0.009
3.8E-4
0.0119
7.1E-4
0.760
0.013
L24
1.0
0.010
3.4E-4
0.0140
9.1E-4
0.718
0.021
A17-LRV1
AC
CE
A17-LRV8
ED
A17-LM
M
St.Dev.
PT
Sample Site
53
(Eq.1)
ACCEPTED MANUSCRIPT
Table 2: List of excluded sample sites with each TiO2 contents (Blewett et al., 1997; Jolliff, 1999) and a brief site description. “Axx” and “Lxx” denote Apollo and Luna missions, respectively. Representative WAC nI/F and standard deviation at 321 nm, 415 nm, and 321/451 nm ratio for these excluded sites are included in supplemental materials. We note that a
CR IP T
reliable representative value was not derived for several sites (e.g., slope sites with mixture of mare/highland materials).
Excluded Sites
Soil TiO2
Site description
wt% 1.6
Slope, gradual mixture of mare and highland materials
A15-S2
1.3
(same as above)
A15-S4
1.2
(same as above)
A15-S6
1.5
A15-S7
1.1
A15-9A
2.0
A16
-
A17-S1
9.6
Rim of Steno-Apollo crater, excavated subsurface materials
A17-S2
1.5
Slope bottom of South Massif
A17-S2A/LRV4
1.3
A17-S3
1.8
(same as above) (same as above)
Edge of Hadley rille
ED
M
Highland materials
On top of the Light Mantle deposit (same as above)
9.9
Rim of Camelot crater, excavated subsurface materials
3.4
Slope, gradual mixture of mare and highland materials
3.9
(same as above)
A17-S8
4.4
(same as above)
A17-S9
6.4
Van Serg crater ejecta
CE
A17-S5
AN US
A15-S1
A17-LRV2
4.4
Light Mantle deposit
A17-LRV3
5.5
Surrounded by Light Mantle deposit
A17-LRV5
2.6
Light Mantle deposit
A17-LRV6
2.6
(same as above)
A17-LRV7
6.8
Surrounded by Light Mantle deposit
A17-LRV10
3.7
Slope, gradual mixture of mare and highland materials
A17-LRV11
4.5
A17-LRV12
10.0
Rim of unnamed ∼10 m diameter crater
L20
0.5
A17-S6
AC
PT
A17-S7
Rim of Sherlock crater, excavated subsurface materials 54
Highland materials
CR IP T
ACCEPTED MANUSCRIPT
Table 3: TiO2 abundance statistics for the 17 selected maria (see corresponding histograms
in Fig. 12 and map views in Fig. 13-15). Latitude and longitude indicate the center of each mare. Affiliation to Group 1 (17 maria) and Group 2 (14 maria) is listed. The areas with WACTiO2 below the detection limit (2 wt%) were set to 1 wt%.
Group 1,2
Mare Cognitum (east)
1,2
Mare Crisium
1,2
Mare Fecunditatis
1
Mare Frigoris
1
Mare Humorum
1,2
Mare Imbrium
Median
Average
St.Dev.
Max
wt%
wt%
wt%
wt%
3.7
3.5
1.6
8.0
338.17°E
12.39°S
349.87°E
3.3
3.4
1.8
8.3
18.40°N
60.78°E
2.7
2.6
1.4
8.1
6.90°S
51.95°E
1.0
2.4
1.7
9.2
56.21°N
5.36°E
1.0
1.1
0.6
8.0
27.14°S
323.91°E
3.8
3.9
2.1
10.3
1,2
32.79°N
341.89°E
3.4
3.7
2.3
10.2
1
32.91°S
164.79°E
1.0
1.6
1.0
7.2
Mare Marginis
1,2
13.61°N
85.23°E
1.0
1.8
1.1
6.1
Mare Moscoviense
1,2
27.72°N
148.45°E
3.1
3.6
2.2
9.4
Mare Nubium
1,2
23.04°S
341.43°E
3.2
3.2
1.7
8.5
Mare Orientale
1,2
16.47°S
270.53°E
2.4
2.5
1.4
7.5
Oceanus Procellarum
1,2
18.40°N
304.54°E
4.7
4.7
2.4
11.7
Mare Serenitatis
1,2
26.94°N
19.28°E
3.5
3.6
1.8
11.4
Mare Smythii
1,2
1.63°S
87.77°E
2.8
2.7
1.2
6.3
Mare Tranqillitatis
1,2
9.04°N
30.66°E
6.8
6.4
2.5
12.6
Mare Tsiolkovskiy
1,2
20.49°S
128.90°E
2.2
2.2
1.2
6.7
CE
PT
ED
Mare Ingenii
AC
Longitude
10.59°S
M
Mare Cognitum (west)
Latitude
AN US
Major Maria
55
Table 4:
CR IP T
ACCEPTED MANUSCRIPT
TiO2 abundance of the seven mare units inside Mare Imbrium (Fig. 11) and five
major maria from the WACTiO2 (down-sampled to the LPTiO2 resolution: 2 degrees per were set to 1 wt%.
Unit Name
AN US
pixel) and the LPTiO2 values. The areas with WACTiO2 below the detection limit (2 wt%)
WACTiO2 [wt%] Average
LPTiO2 [wt%]
St.Dev.
Average
St.Dev.
1.0
5.2
1.8
0.6
4.8
1.5
0.8
3.5
1.8
1.0
2.8
1.1
5.1
H2
4.2
H3
3.5
L1
1.9
L2
1.0
1.0
1.9
0.8
L3
2.1
0.7
2.8
1.7
2.5
0.8
2.7
1.4
Mare Tranqillitatis
5.2
2.0
5.4
2.6
Oceanus Procellarum
4.3
1.7
4.4
2.0
Mare Imbrium
3.5
1.6
3.8
1.9
Mare Serenitatis
3.6
1.0
3.2
1.6
Mare Crisium
1.9
0.8
1.4
0.9
ED
AC
CE
PT
L4
M
H1
56
ACCEPTED MANUSCRIPT
Figure 1
1344
0.5
Anorthosite (x0.7)
CR IP T
Reflectance
0.4
Olivine
0.3
Augite
0.2
Pigeonite
AN US
0.1
Ilmenite
0.0
300
400
1345
Figure 2
1346
M
(a)
AC
CE
PT
ED
1347
500 Wavelength [nm]
1348
1349
Figure 3
57
(b)
600
700
ACCEPTED MANUSCRIPT
Apollo 11 Apollo 12 Apollo 14 Apollo 15
0.82
0.78
A17-LRV8 A17-LRV9 A11
(L16)
CR IP T
WAC ratio (321/415 nm)
0.80
A17-LM A17-LRV1
Apollo 17 Luna 16 Luna 24
0.76 (L24)
0.74
A12 A15-09
0.72 0.70
0
2
4 6 Lunar Soil TiO2 [wt%]
1350
Figure 4
1352
30
PT
120
90
60
30
0 30 Longitude
Figure 5
AC
CE
1354
150
58
TiO2 [wt%]
0
ED
Latitude
30
1353
10 9 8 7 6 5 4 3 2
M
60
60 180
RMS = 3.80e-03 95% Confidence Band 8 10
AN US
A14 A15-S8
0.68
1351
y = 0.010x + 0.689 R2 = 0.980
60
90
120
150
180
4.2 3.9
2
4
1355
6 TiO2 [wt%]
Figure 6
1358
AC
CE
PT
Figure 7
ED
M
1357
1359
8
AN US
1356
100 90 Group 1 (17 maria) 80 Group 2 (14 maria) 70 Group 1 Cumulative 60 50 40 30 20 10
Cumulative Fraction [%]
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
CR IP T
Fraction [%]
ACCEPTED MANUSCRIPT
59
ACCEPTED MANUSCRIPT
AN US
CR IP T
1360
[wt%]
8.0
AC
CE
PT
ED
M
2.0
50 km
1361
1362
Figure 8
60
ACCEPTED MANUSCRIPT
AN US
CR IP T
(a)
1363
Figure 9
AC
CE
1364
PT
ED
M
(b)
61
ACCEPTED MANUSCRIPT
AN US
CR IP T
(a)
1365
Figure 10
AC
CE
1366
PT
ED
M
(b)
62
ACCEPTED MANUSCRIPT
AN US
CR IP T
(a)
1367
PT
Figure 11
AC
CE
1368
ED
M
(b)
63
ACCEPTED MANUSCRIPT
(a)
L2
L3
H1 H2 H3
AN US
(b)
T
P
I
M
S
PT
Figure 12
AC
CE
1370
ED
C
1369
CR IP T
L1
L4
64
Mare Tranqillitatis
Mare Cognitum (east)
Mare Tsiolkovskiy
Oceanus Procellarum
Mare Nubium
Mare Fecunditatis
Mare Humorum
Mare Moscoviense
Mare Cognitum (west)
Mare Smythii
Mare Serenitatis
Mare Crisium
Mare Imbrium 6 4 2 0 2 4 6 8 10
Mare Ingenii
6 4 2 0 2 4 6 8 10 WACTiO2 [wt%]
AC
CE
PT
ED
M
Figure 13
Mare Marginis
Mare Frigoris
Mare Orientale
1371
1372
CR IP T
0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0
AN US
Normalized Fraction
ACCEPTED MANUSCRIPT
65
6 4 2 0 2 4 6 8 10
ACCEPTED MANUSCRIPT
ED
M
AN US
CR IP T
L
Figure 14
AC
CE
1374
TiO2 [wt%]
PT
1373
66
ED
M
AN US
CR IP T
ACCEPTED MANUSCRIPT
Figure 15
AC
CE
1376
TiO2 [wt%]
PT
1375
67
ED
M
AN US
CR IP T
ACCEPTED MANUSCRIPT
Figure 16
AC
CE
1378
TiO2 [wt%]
PT
1377
68
ACCEPTED MANUSCRIPT
CR IP T
1379
Figure 17
CE
1382
PT
ED
M
1381
AN US
1380
Figure 18
AC
1383
69
Figure 19
AC
CE
1385
PT
1384
ED
M
AN US
CR IP T
ACCEPTED MANUSCRIPT
70
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN US
1386
71