Vis reflectance

Icarus 296 (2017) 216–238

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Lunar mare TiO2 abundances estimated from UV/Vis reflectance Hiroyuki Sato a,∗, Mark S. Robinson a, Samuel J. Lawrence b, Brett W. Denevi c, Bruce Hapke d, Bradley L. Jolliff e, Harald Hiesinger f a

School of Earth and Space Exploration, Arizona State University, 1100 S. Cady Mall, INTDS A, Tempe, AZ 85287-3603, USA Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, USA c Johns Hopkins Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, 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 in St. Louis, 1 Brookings Drive, Saint Louis, MO 63130, USA f Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, Münster 48149, Germany b

a r t i c l e

i n f o

Article history: Received 11 October 2016 Revised 4 June 2017 Accepted 7 June 2017 Available online 8 June 2017 Keywords: Moon Volcanism Thermal histories Image processing Spectroscopy

a b s t r a c t The visible (40 0–70 0 nm) and near-infrared (70 0–280 0 nm) reflectance of the lunar regolith is dominantly controlled by variations in the abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g., ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced rims on soil grains, and Fe-metal). The same materials control reflectance into the near-UV (250–400 nm) with varying degrees of importance. A key difference is that while ilmenite is spectrally neutral in the visible to near-infrared, it exhibits a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC) filters were specifically designed to take advantage of this spectral feature to enable more accurate mapping of ilmenite within mare soils than previously possible. Using the reflectance measured at 321 and 415 nm during 62 months of repeated near-global WAC observations, first we found a linear correlation between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of each sample return site. We then used the coefficients from the linear regression and the near-global WAC multispectral mosaic to derive a new TiO2 map. The average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 values were found in Mare Tranquillitatis (∼12.6 wt%) and Oceanus Procellarum (∼11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the low-TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium) exhibit very low TiO2 values (<2 wt%). We find that the Clementine visible to near-infrared based TiO2 maps (Lucey et al., 20 0 0) have systematically higher values relative to the WAC estimates. The Lunar Prospector Gamma-Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map, although there are local offsets possibly due to the different depth sensitivities and large pixel scale of the GRS relative to the WAC. We find a wide variation of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga), whereas only medium to high TiO2 values (average = 6.8 wt%, minimum = 4.5 wt%) are found for younger mare units (<2.6 Ga). © 2017 Published by Elsevier Inc.

1. Introduction From lunar meteorites (Korotev et al., 2003) and Apollo and Luna returned samples (McKay et al., 1991), we have a preliminary understanding, albeit incomplete, of the major lunar rock types, particularly mare basalts. Since basalts form through partial melting of the lunar mantle, they provide our best view of the composition of the lunar interior; understanding the compositional range of mare basalts is, therefore, critical to understanding the geology ∗

Corresponding author. E-mail address: [email protected] (H. Sato).

http://dx.doi.org/10.1016/j.icarus.2017.06.013 0019-1035/© 2017 Published by Elsevier Inc.

and the interior of the Moon and thus the other terrestrial planets (Lunar Exploration Analysis Group, 2016). Apollo-era petrologic investigations of returned samples relied on titanium content as a primary tool for the first-order classification of mare basalt types, owing to the large range of titanium (0 to >10 wt% TiO2 ) among the basalt samples (Shearer et al., 2006). The Apollo mare samples have a bimodal distribution in the range of titanium concentrations, with relatively larger numbers of samples below 5 wt% TiO2 and higher than 9 wt% TiO2 (Papike et al., 1998; Taylor et al., 1991). Historically, lunar basalts were classified as either very low-Ti (VLT) basalts with <1 wt% TiO2 , low-Ti basalts with 1-5 wt%, and high-Ti basalts (9-14 wt%), with only a few sam-

H. Sato et al. / Icarus 296 (2017) 216–238

ples falling into the range of 5-9 wt% (Papike et al., 1976; Papike and Vaniman, 1978; Taylor et al., 1991; Papike et al., 1998); alternative simplified classification schemes also exist (Neal and Taylor, 1992). The recent Chang’e-3 mission measured basalts with 5.2 wt% TiO2 in northern Mare Imbrium (Ling et al., 2015). Earth-based telescopic observations combined with knowledge of sample chemistry were used to predict the TiO2 content of mare materials (Whitaker, 1972; McCord et al., 1972; Charette and McCord, 1974; Johnson et al., 1977; Pieters, 1978; Johnson et al., 1991). These early studies extended estimates of TiO2 abundance to unvisited mare surfaces across the nearside. Later, spacecraft multispectral observations were utilized to extend TiO2 abundance estimates to farside areas, such as Mariner 10 (Robinson et al., 1992), Galileo Solid State Imaging experiment (Lucey et al., 1998; Giguere et al., 20 0 0), and Clementine Ultraviolet/Visible (UVVIS) camera (Lucey et al., 1995; 1998; 20 0 0; Blewett et al., 1997; Jolliff, 1999; Giguere et al., 20 0 0; Gillis et al., 2003). Clementine UVVIS images (415-10 0 0 nm) provided the means for Lucey and coworkers to estimate FeO and TiO2 content of the surface at a pixel scale of 100 m (Lucey et al., 1995; 1998; 2000; Blewett et al., 1997). These products were the first global and uniformly processed maps that were broadly used to investigate the composition of the lunar crust. Giguere et al. (20 0 0) demonstrated that the TiO2 content in mare basalts has a continuous distribution ranging from 0-14 wt% with a mode near ∼2 wt%, not the bimodal distribution as suggested by the lunar samples. Recent spectral reflectance measurements from Chang’e-1 Interference Imaging Spectrometer (IIM) (Wu, 2012; Wu et al., 2012), and the SELENE Multiband Imager (Otake et al., 2012) increased the spatial resolution of TiO2 estimates up to 20 m/pixel. These series of works estimated TiO2 abundance from the visible to near-infrared spectral slopes acquired from Earth-based observations (e.g., 30 0–110 0 nm in Whitaker, 1972; 400–560 nm in McCord et al., 1972; Charette and McCord, 1974; Johnson et al., 1977; Pieters, 1978; Johnson et al., 1991), flyby observations (e.g., 480-580 nm by Mariner 10; 415-750 nm by Galileo), and orbital measurements (Clementine, Chang’e-1, and SELENE), based on the correlation with the TiO2 content of returned lunar samples. Since the sensitivity depth of reflectance spectra in these observations is the upper few millimeters (Hapke, 2012), these TiO2 estimates represent only the uppermost surface materials. The Lunar Prospector Gamma-Ray Spectrometer (GRS) and Neutron Spectrometer (NS) (Elphic et al., 20 0 0; 1998; 2002; Prettyman et al., 2006) provided an independent measure of titanium at coarser scales (data were binned at 2° and 5° per pixel). The NS/GRS senses into the regolith ∼30 cm (Lawrence et al., 2002), thus its TiO2 estimates are deeper relative to the spectral reflectance methods. In some areas, it is reported that the TiO2 estimates based on visible to near-infrared (400–750 nm) reflectance including Clementine UVVIS and Chang’e-1 IIM (Lucey et al., 20 0 0; Wu et al., 2012) are high relative to Lunar Prospector NS/GRS based TiO2 estimates (Elphic et al., 2002; Prettyman et al., 2006). The cause of this discrepancy is still not fully understood, but is in some cases may be related to the different sampling depths. Visible to near-infrared (40 0–280 0 nm) spectral reflectance of the lunar regolith is mainly controlled by variations in abundance of four components: plagioclase, iron-bearing silicate minerals, opaque minerals, and maturation products (e.g., agglutinate glass, Fe-metal, and amorphous radiation-damaged mineral grains) (Hapke et al., 1975; Pieters et al., 20 0 0; Hapke, 20 01). Additionally, it has long been known that the mare exhibit distinct color contrasts into the UV relative to visible wavelengths (Whitaker, 1972; McCord et al., 1972; Wells and Hapke, 1977; Wagner et al., 1987; Rava and Hapke, 1987; Robinson et al., 2007; Cloutis et al., 2008). While the spectral reflectance of most major lunar rock-forming minerals decreases toward shorter wavelengths (Fig. 1), the spec-

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Fig. 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. The reflectance of anorthosite is multiplied by 0.7 to fit on the plot.

trum of ilmenite is flat through the visible and increases with decreasing wavelength below ∼450 nm (Clark et al., 1993; Cloutis et al., 2008). A strong linear correlation was demonstrated between the visible-UV ratio (502/250 nm) of Apollo 17 sample stations as imaged by the Hubble Space Telescope and the TiO2 contents of the corresponding returned soil samples (Robinson et al., 2007). Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) near-global observations (± 70° latitude) provide the first opportunity to map all mare deposits in the near-UV to visible range (321 and 360 nm in UV bands; 415, 566, 604, 643, and 689 nm in visible bands) with uniform resolution (∼400 m in UV, ∼100 m in visible) (Robinson et al., 2010; Mahanti et al., 2016). An original science goal of the WAC was to investigate TiO2 abundance in the maria and the 321 nm and 415 nm bands were designed for this purpose (Robinson et al., 2010) based on promising results from Hubble Space Telescope observations of the Taurus Littrow valley (Apollo 17 landing site) (Robinson et al., 2007). Denevi et al. (2014) examined the WAC 321/415 ratio variations of the Moon in relation to the effects of space weathering, reporting that the fresh craters have low ratio values in the mare and in the moderate-iron highlands. However, they also found that the space weathering “saturates” in the UV relatively quickly (up to IS /FeO ∼ 40) compared to the visible and the near-infrared (Lucey et al., 20 0 0) (up to IS /FeO ∼ 60), suggesting that the UV is less dependent on the maturity variations. From the ratio of the 321 nm band to the 415 nm band, a new TiO2 abundance map is presented here and compared with two maps derived from Clementine UVVIS images (Lucey et al., 20 0 0; Gillis-Davis et al., 20 06) and global estimates derived from the Lunar Prospector GRS measurements (Elphic et al., 2002; Prettyman et al., 2006). We also discuss the distribution of ilmenite (the main carrier of TiO2 ) and its geologic implications within each mare. Finally, we discuss the variation of TiO2 abundance as a function of age to understand the evolution of lunar mare volcanism. 2. Derivation of WAC TiO2 map For the lunar mare, we assume that variations in the 321/415 nm ratio are dominantly controlled by ilmenite (and thus TiO2 ) abundance, based on laboratory and remote-sensing studies

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H. Sato et al. / Icarus 296 (2017) 216–238 Table 1 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 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. Sample Site

Soil TiO2 wt%

nI/F

St.Dev.

nI/F

St.Dev.

A11 A12 A14 A15-S8 A15-09 A17-LM A17-LRV1 A17-LRV8 A17-LRV9 L16 L24

7.5 3.1 1.7 1.7 1.8 8.5 8.0 6.7 6.1 3.3 1.0

0.010 0.011 0.013 0.011 0.011 0.011 0.010 0.010 0.011 0.009 0.010

4.0E−4 3.7E−4 3.4E−4 3.0E−4 5.1E−4 3.9E−4 2.3E−4 3.0E−4 3.9E−4 3.8E−4 3.4E−4

0.0136 0.0153 0.0184 0.0158 0.0153 0.0140 0.0137 0.0137 0.0147 0.0119 0.0140

8.3E−4 8.2E−4 1.1E−3 8.7E−4 9.6E−4 1.6E−3 1.3E−3 1.1E−3 1.2E−3 7.1E−4 9.1E−4

321 nm

415 nm

(Charette and McCord, 1974; Robinson et al., 2007; Cloutis et al., 2008; Coman et al., 2016). In fact, the spectra of the returned regolith samples are known to show a clear correlation between the UV over visible wavelength ratio and the ilmenite-sourced TiO2 (Charette and McCord, 1974; Coman et al., 2016). First, we determined a correlation between TiO2 contents of returned sample soils and the WAC 321/415 ratios from appropriate sampling sites (Table 1). From the derived coefficients, we then constructed a new TiO2 abundance map from a near-global WAC multispectral mosaic. 2.1. WAC observations at sampling sites The areas from which returned samples were collected do not necessarily fully represent the surface observed from orbit, mostly due to gross differences in sampling size; remote sensing pixels measure areas hundreds to thousands of meters in size, whereas the returned samples are from areas typically much less than a square meter. Therefore, we only utilized samples acquired from sites deemed to be relatively geologically uniform (based on the albedo and morphological variations detected by the LROC Narrow Angle Camera (NAC) images with a pixel scale of ∼2 m/pixel) at the pixel scale of the WAC 321 nm 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 South Massif, Apollo 15-9A at the edge of Hadley Rille, and Apollo 17 Station 5 on Camelot crater ejecta, were all excluded (Table 2). Lunar soils with less than ∼2 wt% TiO2 do not show a strong correlation between their 321/415 nm ratio and TiO2 content (GillisDavis et al., 2006; Coman et al., 2016), therefore Apollo 16 and Luna 20 samples (highland materials with no or very low ilmenite content) were also excluded. In total, 11 sample sites were used to calibrate the spectral ratio to TiO2 abundance. For each selected sample return site (Table 1), we derived a representative value of WAC photometrically normalized reflectance (nI/F) at 321 and 415 nm, respectively, through the following four steps explained hereafter: 1) collection of all the available WAC observations at each site and derivation of nI/F, 2) masking based on NAC albedo, 3) masking of sparse observation regions, and 4) curve fitting to the nI/F histogram. Each step is described in detail below. 2.1.1. WAC pixel data preparation We used ∼62 months of WAC observations acquired from September 2009 to December 2014. The raw Digital Number (DN) of each pixel was converted to radiance factor (I/F) (Hapke, 2012) through radiometric calibration (Mahanti et al., 2016), then photometrically normalized (emission angle = 0° , incidence angle = phase angle = 60°) with Hapke photometric function (Hapke, 2012)

321/415

σ ratio

0.765 0.722 0.704 0.699 0.710 0.768 0.762 0.758 0.752 0.760 0.718

0.016 0.014 0.021 0.018 0.010 0.053 0.053 0.034 0.032 0.013 0.021

(Eq. 1)

using spatially resolved Hapke parameter maps (Sato et al., 2014) to derive nI/F. The photometric angles were calculated relative to the local topography using a WAC-derived 100 m scale Digital Terrain Model (DTM) known as the GLD100 (Scholten et al., 2012). The observations at incidence angle >75° , emission angle >30° , and phase angle >95° were excluded to minimize the uncertainties of photometric normalization. We normalized reflectance to 60° phase angle (0° emission angle, 60° incidence angle) instead of the traditional 30° because the lower value is not achieved in the latitudes over 30° under the WAC observation geometries (Sato et al., 2014). The nominal pixel scales (average within a frame for nadir observations) of the WAC from the 50 km altitude orbit of Lunar Reconnaissance Orbiter (LRO) (from September 2009 to December 2011) are 402 m for the UV and 74.9 m for visible bands (Robinson et al., 2010). In the current elliptical orbit (after December 2011), the pixel scale ranges from 550 to 1170 m in UV and from 107 to 228 m in visible bands within the latitudes of all the sample-return sites (9° S to 26° N). Only the pixels that were contained entirely within a 10 0 0 by 10 0 0 m box (hereafter called “site box”) centered at each sample-return location were collected from non map-projected WAC images for the 321 and 415 nm bands. 2.1.2. NAC albedo masking Inside each site box, there are significant albedo variations due to local geologic and topographic features (e.g., fresh crater ejecta and slopes). From LROC NAC images (original pixel scale ∼50 cm, down-sampled to 2 m/pixel), we derived and applied an albedo mask to exclude WAC pixels that dominantly cover remarkably low or high albedo areas (Fig. 2). For the NAC images, we selected those that were acquired with relatively small incidence angles (from 7.3° to 27.0°), and applied a photometric normalization using emission and incidence angles calculated relative to local topography derived from complementary NAC DTMs for each site (2 m/pixel, Henriksen et al., 2015). Although the NAC is a broadband imager (400 to 760 nm) it is useful as a generalized albedo map for comparison with the WAC observations. Within the NAC image cropped to the region of the site box, we masked (excluded) 10% in total of the area with high- and low-end reflectance relative to the reference value (Fig. 2). The reference value was obtained from the same NAC image, a median of a 10 by 10 pixel (20 by 20 m) box centered at the exact location of each sample-return site. In order to avoid surfaces disturbed by human activities (e.g., rocket blast zone, tracks of astronauts and Lunar Roving Vehicles), at several sites, we relocated the 10 by 10 pixel box to a nearby vicinity where there is a relatively uniform, undisturbed area. The masked areas normally consist of multiple irregular patches whereas each WAC pixel is square. Thus, we count an overlapping fraction of

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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 reliable representative value was not derived for several sites (e.g., slope sites with mixture of mare/highland materials). Excluded Sites

Soil TiO2 wt%

Site description

A15-S1 A15-S2 A15-S4 A15-S6 A15-S7 A15-9A A16 A17-S1 A17-S2 A17-S2A/LRV4 A17-S3 A17-S5 A17-S6 A17-S7 A17-S8 A17-S9 A17-LRV2 A17-LRV3 A17-LRV5 A17-LRV6 A17-LRV7 A17-LRV10 A17-LRV11 A17-LRV12 L20

1.6 1.3 1.2 1.5 1.1 2.0 – 9.6 1.5 1.3 1.8 9.9 3.4 3.9 4.4 6.4 4.4 5.5 2.6 2.6 6.8 3.7 4.5 10.0 0.5

Slope, gradual mixture of mare and highland materials (same as above) (same as above) (same as above) (same as above) Edge of Hadley rille Highland materials Rim of Steno-Apollo crater, excavated subsurface materials Slope bottom of South Massif On top of the Light Mantle deposit (same as above) Rim of Camelot crater, excavated subsurface materials Slope, gradual mixture of mare and highland materials (same as above) (same as above) Van Serg crater ejecta Light Mantle deposit Surrounded by Light Mantle deposit Light Mantle deposit (same as above) Surrounded by Light Mantle deposit Slope, gradual mixture of mare and highland materials Rim of unnamed ∼10 m diameter crater Rim of Sherlock crater, excavated subsurface materials Highland materials

each WAC pixel with the masked area, and if the fraction is more than 15% we excluded that WAC pixel. 2.1.3. Sparse observation masking Owing to the four-times larger footprint of the 321 nm band relative to the 415 nm band, and because we select only pixels entirely included in the site box, there is a zone along the margins of the site box where the number of observations is relatively sparse (Fig. 2a). This zone is much wider for the 321 nm band (∼200 m, about half of the UV-band footprint size) than for the 415 nm band (∼50 m, half of visible-band footprint size). This offset, where there are numerous 415 nm observations but few 321 nm observations, results in spatially inconsistent weighting of the calculation of representative values inside each site box. To minimize such spatial weighting differences in the two bands, we reduced the number of 415 nm observations by applying an additional mask, which excludes the boundary regions where observations are sparse for the 321 nm band. We created a 321 nm observation accumulation map inside the site box and set the minimum threshold for a number of observations by the maximum number of observations divided by five, which was empirically determined to automatically mask the boundaries of the site box and the anomalous albedo areas. The areas that did not meet the threshold value in the accumulation map were masked. Examples of the distribution of 321 and 415 nm observations within a site box after application of the NAC albedo mask and the sparse observation mask are shown in Fig. 2 upper panels. On average, 111 and 3966 pixels were used for the 321 and 415 nm bands, respectively, at each sample site box. 2.1.4. Curve fitting to the histogram A representative nI/F of each band at each site was derived from the modal value of the down-selected WAC observations. In order to avoid a singular-peaked mode of an irregularly shaped histogram, we fitted a curve to the histogram by a least-square optimization, and the curve’s modal value was used as the representative nI/F. Since the histograms of nI/F for each site box did not

always have a symmetric Gaussian shape, we employed a Skew Normal curve with an asymmetry adjustment for our fits (Fig. 2 bottom panels). Using the representative nI/Fs of the two bands calculated in this final step, the 321/415 nm ratio was derived for each samplereturn site. The uncertainty of each ratio value was estimated by 2-D standard deviation given as,

σratio =



2 σ321 + σM2 321 /415 /M415

(1)

where M321 and M415 are the representative nI/F of the 321 and 415 nm bands respectively, σ321/M415 is a standard deviation based on all the selected 321 nm nI/Fs divided by M415 , and σM321 /415 is a standard deviation based on M321 divided by the selected 415 nm nI/Fs. We note that this uncertainty includes the instrument calibration and photometric normalization uncertainties across the ∼62 months of repeat observations. The derived representative nI/F, the standard deviation of nI/Fs of each band, the ratio, and the σ ratio for the selected sample-return sites are listed in Table 1. 2.2. Correlation between WAC 321/415 and soil TiO2 For the lunar soil TiO2 values, we used the compositional data compiled and summarized by Blewett et al. (1997) and Jolliff (1999). The selected WAC 321/415 nm ratios and the lunar soil TiO2 show a positive linear correlation (Fig. 3). The least-squares linear fit has a slope = 0.010, y-intercept = 0.689, and R2 = 0.98. The samples from Luna 16 and Luna 24 are far off the Apollo sample trend line, thus these two samples were considered as outliers and excluded from the fitting. Previous works (Blewett et al., 1997; Lucey et al., 20 0 0; Gillis et al., 20 03) also reported these two samples as anomalies in their regression, which is consistent with our plots. The Luna 24 site is in close proximity to a fresh crater (Robinson et al., 2012). It is likely that Luna 24 samples are excavated materials from a buried unit, representing materials with a composition varying from the surface (Robinson et al., 2012; Denevi et al., 2014; Coman et al., 2016). The cause of Luna 16

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H. Sato et al. / Icarus 296 (2017) 216–238

Fig. 2. 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 10 0 0 by 10 0 0 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. 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 position.

sample discrepancy is not clear. The Luna 16 site is located at the northwestern edge of Langrenus crater ejecta (highland sourced deposits; see Fig. 14d). Mixtures of local mare materials with highland ejecta may have produced inhomogeneous soils, possibly resulting in the anomalous TiO2 content relative to the trend of Apollo samples. The spectra of Apollo soils revealed a clear positive correlation between the TiO2 content and the 321/415 nm ratio, except for low-TiO2 soils (<∼2 wt%) where pyroxene content begins to dominate the ratio, resulting in a poor correlation with the TiO2 content (Gillis-Davis et al., 2006; Coman et al., 2016). Therefore, we report only TiO2 values greater than 2 wt% using the WAC 321/415 nm ratio, and consider lower values to be below the detection limit. In non-mare regions, the 321/415 nm ratio is likely controlled by various non-ilmenite sources, such as the shock degree (impact origin) of plagioclase and the presence of glass (Denevi et al., 2014) in addition to the other minerals (e.g., pyroxene). Therefore, our TiO2 map should be considered to be valid only for the mare regions. Fig. 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 Eq. (1). Solid and dashed line are the least-squares fit and its 95% confidence band, respectively.

2.3. WAC TiO2 map overview From the correlation between the representative ratio values and the lunar soil TiO2 values, we converted the WAC 321/415 nm ratio map to a TiO2 (wt%) abundance map (Fig. 4). The ra-

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Fig. 4. WAC TiO2 abundance map for lunar mare (latitude 70° S to 70° N, longitude 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 reflectance variations related to ilmenite content.

Table 3 TiO2 abundance statistics for the 17 selected maria (see corresponding histograms in Fig. 12 and map views in Figs. 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 TiO2 below the detection limit (2 wt%) were set to 1 wt%. Major Maria

Group

Latitude

Longitude

Median wt%

Average wt%

St.Dev. wt%

Max wt%

Mare Cognitum (west) Mare Cognitum (east) Mare Crisium Mare Fecunditatis Mare Frigoris Mare Humorum Mare Imbrium Mare Ingenii Mare Marginis Mare Moscoviense Mare Nubium Mare Orientale Oceanus Procellarum Mare Serenitatis Mare Smythii Mare Tranqillitatis Mare Tsiolkovskiy

1,2 1,2 1,2 1 1 1,2 1,2 1 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2 1,2

10.59° S 12.39° S 18.40° N 6.90° S 56.21° N 27.14° S 32.79° N 32.91° S 13.61° N 27.72° N 23.04° S 16.47° S 18.40° N 26.94° N 1.63° S 9.04° N 20.49° S

338.17° E 349.87° E 60.78° E 51.95° E 5.36° E 323.91° E 341.89° E 164.79° E 85.23° E 148.45° E 341.43° E 270.53° E 304.54° E 19.28° E 87.77° E 30.66° E 128.90° E

3.7 3.3 2.7 1.0 1.0 3.8 3.4 1.0 1.0 3.1 3.2 2.4 4.7 3.5 2.8 6.8 2.2

3.5 3.4 2.6 2.4 1.1 3.9 3.7 1.6 1.8 3.6 3.2 2.5 4.7 3.6 2.7 6.4 2.2

1.6 1.8 1.4 1.7 0.6 2.1 2.3 1.0 1.1 2.2 1.7 1.4 2.4 1.8 1.2 2.5 1.2

8.0 8.3 8.1 9.2 8.0 10.3 10.2 7.2 6.1 9.4 8.5 7.5 11.7 11.4 6.3 12.6 6.7

tio map was derived using the near-global WAC color mosaic (400 m/pixel) (Sato et al., 2014). This mosaic was photometrically normalized (incidence = phase = 60° and emission = 0°) using emission and incidence angles computed at each pixel based on the GLD100 DTM (Scholten et al., 2012), resulting in reflectance on topographic slopes being more accurately normalized (e.g., Robinson and Jolliff, 2002). In our new TiO2 map (Fig. 4), the highest values are recognized in the northwestern portion of Mare Tranquillitatis (∼12.6 wt%), center of Oceanus Procellarum (∼11.6 wt%), and several Dark Mantle Deposits (DMD; Sinus Aestuum, Rima Bode, and Mare Vaporum; ∼12 wt%). The highest TiO2 values (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 the ejecta of small craters (<1 km in diameter). However, the values associated with DMDs and other pyroclastic deposits are suspect since the increasing UV/visible ratio with increasing TiO2 content does not apply to glasses (Wells and Hapke, 1977; Wilcox et al., 2006). To focus on characterizing the global TiO2 distribution of the maria, we distinguished two groups (see Table 3 for a selected mare list and the group affiliations). Group 1 comprises the 17 largest maria. Smaller mare deposits are generally contaminated

with highland ejecta due to the proximity of the highland/mare boundaries, and thus are excluded from Group 1. Group 2 comprises the same maria as Group 1, but excludes Mare Frigoris, Mare Fecunditatis, and Mare Ingenii because these three maria have exceptionally large areas with TiO2 below the detection limit (<2 wt%). Comparing the two groups highlights the 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 Clementine and LROC WAC data by Nelson et al. (2014). The Aristarchus plateau, Montes Harbinger (east of the Aristarchus plateau), and red spots in the southern portion of Oceanus Procellarum (included as mare areas in Nelson et al., 2014) were classified as non-mare regions since these features are not associated with mare basalts. Histograms of the global TiO2 estimates from the Group 1 and Group 2 maria are shown in Fig. 5. The Group 1 maria have an average of 3.9 wt% TiO2 compared to 4.2 wt% for Group 2 mare regions. The areas with TiO2 below the detection limit (2 wt%) are 28% in Group 1 maria (Fig. 5 cumulative plot) and 20% in Group 2 maria. Three geologic settings are likely responsible for these low-TiO2 areas: highland ejecta contamination, swirls, and mare formed from low-TiO2 basalts. Mare regions contaminated by highland impact ejecta exhibit lower TiO2 estimates not representative of the original underlying

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Fig. 5. Histograms of WAC TiO2 values for Group 1 (17 maria) and Group 2 (14 maria), 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 indicates the fraction of TiO2 below 2 wt%.

surface (Fig. 6, blue regions). For example the rays of Aristarchus crater (40 km diameter) ejecta (Fig. 7) spread out underlying highland materials radially to the south and southwest where the mare exhibit lower TiO2 values (mostly below 2 wt%) relative to the adjacent mare surfaces (5-10 wt%) free from the ejecta. Lunar swirls (e.g., Reiner Gamma and those in Mare Ingenii and Mare Marginis; Fig. 6, green regions) have computed TiO2 values significantly lower than the detection limit (negative values). Such extremely low values are due to the spectral characteristics of immature swirl surfaces or the presence of glass, both of which would decrease the 321/415 ratio irrespective of ilmenite content (Denevi et al., 2014; 2016). Several mare units exhibit low TiO2 abundance likely due to their original composition (no clear sign of extensive ejecta contamination), such as Mare Frigoris, the northern portion of Oceanus Procellarum, and the northeastern portion of Mare Imbrium. Details of TiO2 distribution within each mare in Group 1 are discussed in Section 5.1.

Fig. 7. Southern portion of Aristarchus crater ejecta inside Oceanus Procellarum, in the WAC TiO2 map (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 map-projection.

3. Comparison with Clementine TiO2 maps We compared the new WAC TiO2 map (hereafter called WACTiO2 ) with Clementine TiO2 abundance maps. Among various Clementine TiO2 maps derived by their own approaches with different conversion parameters (Blewett et al., 1997; Lucey et al.,

Fig. 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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1998; Jolliff, 1999; Lucey et al., 20 0 0; Gillis et al., 2003; GillisDavis et al., 2006), we compared the maps created by Lucey et al. (20 0 0) (hereafter called CLMTiO2 L; historically well known in the lunar science community) and by Gillis et al. (2003) (hereafter called CLMTiO2 G; the latest map with a revised algorithm). The CLMTiO2 L is derived by the following algorithm,



TiO2 = 3.708 × arctan

R415 R750

− 0.42

5.979

(2)

R750

where R415 and R750 are the reflectance tively. The CLMTiO2 G is derived from a Lucey et al. (20 0 0), ⎧ R 15.5 415 −0.36 ⎪ ⎪ R750 ⎪ ⎪ 0 . 14 + 0 . 24 ×arctan , ⎪ R750 +0.012 ⎨ TiO2 = R 9.8817 ⎪ 415 −0.36 ⎪ ⎪ R750 ⎪ , ⎪1.7159 × arctan R +0.012 ⎩ 750

at 415 and 750 nm, respecrevised algorithm based on

if R415 ≤ 0.065 and

R415 R750

> 0.59

otherwise

(3) To minimize any geographic mismatch due to misregistration between the WAC and original Clementine mosaics, we reduced the resolution of both TiO2 maps to 32 pixel/degree (947.6 m/pixel at the equator). We also excluded areas with topographic slopes greater than 5° (computed from GLD100, Scholten et al., 2012) to minimize residual errors in photometric normalization on slopes (McEwen, 1991) in the Clementine map products. For this comparison, in areas where the WACTiO2 was below 2 wt% in the mare, the values were set to 1 wt% with ± 1 wt% uncertainty. The CLMTiO2 L (Fig. 8) has higher TiO2 abundances relative to the WACTiO2 , and the offsets gradually increase with TiO2 values. On average for the entire maria, the CLMTiO2 L is 0.8 wt% higher (Fig. 8a) relative to the WACTiO2 . The difference map (Fig. 8b, CLMTiO2 L - WACTiO2 ; centered on the nearside maria) shows clear offsets in the high-TiO2 maria, such as Mare Tranquillitatis (up to +6.9 wt%, +2.9 wt% in average), Mare Fecunditatis (up to +4.6 wt%, +1.6 wt% in average), and Oceanus Procellarum (up to +4.8 wt%, +1.1 wt% in average). On the other hand, the northern portions of Oceanus Procellarum and Mare Imbrium have negative values (down to -2 wt%), meaning that the CMLTiO2 L has slightly lower values relative to the WACTiO2 . All of these areas with negative values are located in relatively high latitudes, which may suggest a latitudinal trend possibly due to artifacts related to photometric normalization or a latitude dependent maturity effect (Hemingway et al., 2015) (see Section 5.1.12). The other maria at high latitudes (e.g., Mare Australe, Mare Frigoris, and Mare Humboldtianum) are small in size (and thus may be contaminated with highland materials) or have low TiO2 abundance (<2 wt%, unable to compare with WACTiO2 ), both of which make it difficult to further examine any possible latitudinal effect. The offset trends between WACTiO2 and CLMTiO2 G are clearly related to the fact that CLMTiO2 G used two different equations to calculate TiO2 depending on the reflectance values (see Eq. 3). This results in some values near or slightly above the 1:1 line with the WACTiO2 (those calculated with the second version of Eq. 3), but a large portion that fall substantially below the 1:1 line (TiO2 values calculated with the first version of Eq. 3). On average the difference of CLMTiO2 G - WACTiO2 is -0.3 wt% for the entire maira. As seen in the difference maps of CLMTiO2 L - WACTiO2 (Fig. 8b) and CLMTiO2 G - WACTiO2 (Fig. 9b), there are clear vertical stripes particularly in high-TiO2 maria (e.g., Mare Tranquillitatis, Mare Fecunditatis, and Oceanus Procellarum), which are due to artifacts in the Clementine global mosaic. The WAC near-global mosaic is derived from the median of 21 months of repeat observations, and the photometric normalization is optimized within each tile

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(1° latitude by 1° longitude) (Sato et al., 2014), resulting in a significantly improved accuracy relative to the Clementine mosaics. On the other hand, the WACTiO2 has a detection limit of 2 wt% TiO2 to avoid non-ilmenite source factors (see Section 2.2). The algorithms of the Clementine-based TiO2 calculation are reported to minimize the maturity effect (Lucey et al., 1998) , whereas significantly immature materials may increase the uncertainties in WACTiO2 due to the nature of the 321/415 ratio (Denevi et al., 2014; Coman et al., 2016). The WACTiO2 and CLMTiO2 L are derived from one set of parameters while the CLMTiO2 G utilizes two parameter sets, resulting in the clear offsets of TiO2 values along the conditionmatching boundaries. In both the WAC and Clementine based TiO2 maps, TiO2 abundances higher than the Apollo 17 sample-return sites (>10 wt%) are derived simply by extrapolation with unknown uncertainties. More soil samples particularly from the highest TiO2 maria (e.g., Mare Tranquillitatis, Oceanus Procellarum) in addition to medium-TiO2 soils (4-6 wt%, missing range in Fig. 3) are necessary for more accurate TiO2 estimates. 4. Comparison with Lunar Prospector TiO2 map Next, we compared the WACTiO2 with the TiO2 abundance derived from Lunar Prospector GRS observations (Elphic et al., 2002; Prettyman et al., 2006) (hereafter called LPTiO2 ). The LPTiO2 was sampled at 2° per pixel (61 km pixel scale at the equator) whereas the WACTiO2 is 400 m/pixel at the equator. The area over which the GRS detector is sensitive (footprint) is larger than the mapped pixel scale, but the detector sensitivity gradually decreases with distance from the nadir point, which is difficult to directly compare with WACTiO2 data down-sampled by an average or median value of the footprint area. To accurately simulate the spatial response function (footprint weighted by distance from the nadir point) of the GRS (see Fig. 27 in Prettyman et al., 2006), we used a Gaussian weight function W given by



W (x ) = a exp −

(x/d )2 2c 2



(4)

where a = 0.48, d = 150, c = 0.6, and x is the distance from the nadir point in kilometers. The weighted value Vw was then calculated by

n ViW (xi ) Vw = i=1 n i=1 W (xi )

(5)

where Vi and xi are the WACTiO2 value and the distance, respectively, for all the map pixels (400 m/pixel) within 250 km radius circular area (in orthographic projection) centered at each downsampled pixel. As with the Clementine TiO2 vs. the WAC TiO2 comparison, we performed the comparison only for the maria, and WACTiO2 values below 2 wt% in the mare were set to 1 wt% with ± 1 wt% uncertainty. The LPTiO2 vs. WACTiO2 plot (Fig. 10a) shows significant scatter but the points are clustered along a 1:1 line. The median of each 0.5 wt% bin lies 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 is 1.5 wt%; the R2 value of the 1:1 line is 0.52. The difference map (Fig. 10b, LPTiO2 - WACTiO2 ) highlights that the LPTiO2 is lower (blue areas) in several maria (e.g., Serenitatis, −0.5 wt%; Humorum, −0.5 wt%; Crisium, −0.5 wt% on average) and is higher (red areas) in the eastern portion of Oceanus Procellarum (+2.6 wt% on average), relative to the WACTiO2 . The eastern high LPTiO2 portion of Oceanus Procellarum corresponds to the region covered by the ejecta and rays from Aristarchus and Kepler craters. The effective depth of the GRS measurement is deeper (30 cm, Lawrence et al., 2002) than the UV/visible reflectance of the WAC (few millimeters, Hapke, 2012).

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Fig. 8. (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.

Thus, in the areas extensively covered by a layer of ejecta thinner than the GRS sensible depth (<30 cm), the TiO2 estimates by the two instruments could result in an offset due to their different sampling depths. The correlation between the two TiO2 maps was further examined using Mare Imbrium low- and high-TiO2 units that have compatible sizes with the large pixel scale of the GRS (2° per pixel). We defined seven units (Fig. 11a, H1-3 and L1-4) that outline the boundaries of TiO2 abundance units in the WACTiO2 . Generally, the units H1-3 indicate relatively high TiO2 and the units L1-4 are for low TiO2 . The WACTiO2 and LPTiO2 average values (Table 4) for each unit fall near the 1:1 line (within ±0.8 wt% for average values of each unit, R2 = 0.74). The units L2 and L4 fall on mare boundaries and are thus likely contaminated with highland materials due to the GRS’s large footprint and the highland ejecta. The five major maria (Mare Tranquillitatis, Oceanus Procellarum, Mare Imbrium, Mare Serenitatis, and Mare Crisium) are also plotted in Fig. 11b. Each point corresponds to an average and standard deviation of the entire area of each mare. All five maria are plotted within

±0.3 wt%, showing a good agreement (R2 = 0.94 along the 1:1 line) between the WACTiO2 and LPTiO2 . A pixel-by-pixel comparison of LPTiO2 and WACTiO2 indicates significant 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 (high signal-to-noise ratio by repeated observations). The averaged TiO2 values of each mare (and large mare units), however, showed good correlation between the LPTiO2 and WACTiO2 (Fig. 11b), demonstrating the broadly consistent estimates from the two instrument observations. To summarize the comparisons of TiO2 estimates based on three independent datasets (the WAC, Clementine, and Lunar Prospector GRS), the CLMTiO2 L have generally higher values than WACTiO2 , particularly in the high-TiO2 maria. The CLMTiO2 G has an average value close to the WACTiO2 (-0.3 wt% difference) and shows clear offsets along the boundary of the two calculation conditions. The LPTiO2 values are comparable with WACTiO2 in the five major maria (R2 = 0.94) and in each unit of Mare Imbrium (R2 = 0.74). The LPTiO2 is unaffected by maturity variations and is unique in that it is a measure of the integrated titanium abundance

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Fig. 9. (a) Plot of CLMTiO2 G vs. WACTiO2 for the Group 1 maria and (b) difference 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.

of the top ∼30 cm, minimizing effects from surface contamination by thin ejecta deposits. In terms of the observation footprint (particularly compared to the GRS), signal-to-noise ratio, and the topographic correction in the photometric normalization, the WACTiO2 has large advantages. 5. Discussion Based on our WAC TiO2 abundance map, here we discuss the local TiO2 variations in each mare in the context of the surrounding geologic settings and the correlation between the TiO2 abundance and the surface age of the entire maria. 5.1. TiO2 variation within major maria We investigated the spatial variation of TiO2 of 17 out of 23 maria defined in Andersson and Whitaker (1982). Each of the 17 maria (Group 1, Table 3) has at least one large mare unit that is not dominantly contaminated by highland ejecta materials. For each mare, we present the median, average, standard deviation,

and maximum values of TiO2 (Table 3), as well as the histogram (Fig. 12) and the spatial distribution of TiO2 (Figs. 13–15) using the whole mare area (defined by Nelson et al., 2014). We utilized absolute model ages (derived from crater size frequency distributions; e.g., Morota et al., 2009; Hiesinger et al., 2010) to discuss several flow units of the mare basalts outlined by the WACTiO2 map. Although our flow units are not identical to the units defined by the model age works due to the use of different basemaps (e.g., Galileo and Clementine color ratio composite, Hiesinger et al., 2010), we used the ages derived from the units geographically included or overlapping our flow units. 5.1.1. Oceanus Procellarum Oceanus Procellarum exhibits sharp TiO2 abundance boundaries likely due to variations of the magma sources, and in some areas, mixing of ejecta from Aristarchus and other craters (Fig. 13a). We note that TiO2 values near Aristarchus crater (<190 km from the crater center) are low (<2 wt%) compared to the surrounding mare surfaces (6-10 wt%) outside of the extent of ejecta rays. Ejecta from Kepler crater (29 km in diameter) mixed with nearby

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Fig. 10. (a) Plot of LPTiO2 vs. WACTiO2 for the Group 1 maria and (b) difference map of LPTiO2 - WACTiO2 in 2 pixels per degree (50° S to 70° N, 120° W to 120° E; same color stretch as Figs. 8b and 9b). 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 (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.

mare (<120 km from the crater center) radially lowers the mare TiO2 abundance. Both of these Copernican craters (Wilhelms et al., 1987) excavated mare basalts along with highland materials. There is a sharp boundary between high-TiO2 (>6 wt%) and mediumTiO2 (<6 wt%) units about 400 km west of the Aristarchus plateau and it arcs about 10 0 0 km from Seleucus crater to Rima Sharp (outlined by black triangles). The high-TiO2 areas (6-12 wt%, southeastern side of this boundary toward Aristarchus Plateau) have a younger model age (1.33-2.96 Ga) relative to the northwestern mid-TiO2 areas (2-6 wt%, 3.40-3.47 Ga) (Hiesinger et al., 2010). The far northern part of Oceanus Procellarum (denoted by “L”) shows low TiO2 abundances (<2 wt%), partially due to the ejecta of Pythagoras crater (145 km in diameter, 250 km north from the northern edge of Procellarum). The Reiner Gamma albedo anomalies (swirls) show significantly lower TiO2 values (although these values are not reliable due to the spectral characteristics of immature swirl surfaces or the presence of glass, see Section 2.3). 5.1.2. Mare Imbrium As highlighted in Fig. 11a, Mare Imbrium has large TiO2 units with relatively sharp boundaries (Fig. 13b). The highest TiO2

(4-10 wt%) unit is found in the center of the mare (H1, see Fig. 11a for the unit locations). The Chang’e-3 spacecraft landed on and analyzed the regolith near the northeastern edge of this unit (Neal et al., 2015; Ling et al., 2015). The measurements of the Active Particle-induced X-ray Spectrometer (APXS) aboard Chang’e-3 reported 5 wt% TiO2 (sensitive to a depth of several microns; effective detection area ∼50 mm in diameter, Fu et al., 2014; Ling et al., 2015), close to the WACTiO2 estimate of 6.6 wt% (at 400 m/pixel). The lowest TiO2 estimates (<2 wt%) are found in the northeastern part of this mare (unit L1). The ejecta of Copernicus crater extensively affects the southern third of Mare Imbrium, mostly unit H3. The highland ejecta from Aristillus crater (54 km in diameter) located at the eastern edge of this mare contaminates the eastern portion of unit L3 (Fig. 11), decreasing the TiO2 value. The histogram of Imbrium TiO2 values (Fig. 12) shows a semi-bimodal distribution that reflects the high (H1-3) and low (L1-4) TiO2 units. Sharp low-high TiO2 boundaries (free from the highland ejecta contamination), particularly the H1-L1 boundary, outline flow units of mare basalt possibly generated by multiple eruption events that each had distinct compositions. The northern part of H1 unit is

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younger (2.96 Ga, Hiesinger et al., 2010; I22 unit) than the western part of the L1 unit (sharing the boundary with H1; 3.01–3.52 Ga Hiesinger et al., 2010; I5, I11, I17, and I21 units), suggesting that the TiO2 abundance was higher in the younger events. 5.1.3. Mare Cognitum Mare Cognitum can be divided into two spatially separated units (hereafter called Cognitum East and Cognitum West). In Cognitum West (Fig. 13c), the portion around the red spots (highland materials distinctly redder than the surroundings, Whitaker, 1972; Bruno et al., 1991), Darney χ and Darney τ , has the highest TiO2 abundance (6-8 wt%). The rays of Copernicus crater (located at 450 km north of Cognitum) extensively affected the surface of the northeastern portion of Cognitum West. The ejecta of craters Darney (15 km in diameter, located outside of the mare near the southern edge) and Darney C (13 km in diameter, near the southwestern edge of the mare) affected the southern portion of Cognitum West. These ejecta and rays both contribute to lowering the surface TiO2 content locally. In Cognitum East (Fig. 13d), a low-TiO2 (<2 wt%) unit is dominated by the red spot (Lassell Massif) and Lassell C crater (8.7 km in diameter). Ashley et al. (2016) reported possible post-silicic volcanism at the Lassell Massif and low concentrations of FeO and TiO2 (FeO < ∼2 wt%, TiO2 < ∼4 wt% from their Fig. 8) on the Massif area based on Clementine data. It is still not clear if such silicic volcanism is responsible for the low-TiO2 unit (Fig. 13d) extending about 50 km radius from the Lassell C crater. The TiO2 gradually increases from both the western and eastern sides of this mare (2–4 wt%) without obvious highland ejecta contamination toward the central high-TiO2 zone (up to 8 wt%) meandering from the north to the south of Lassell Massif (indicated by dashed line in Fig. 13d), suggesting that the varying TiO2 content reflects the changing compositions of the mare basalt over time or within each eruption event.

Fig. 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 each unit in (a) correspond to the legend character and symbol color in (b). Black symbols with single character in (b) show the averaged TiO2 content in Mare Tranquillitatis (• T), Oceanus Procellarum (✦ P), Mare Imbrium ( I), Mare Serenitatis ( S), and Mare Crisium ( C). 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.

Table 4 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° per pixel) and the LPTiO2 values. The areas with WACTiO2 below the detection limit (2 wt%) were set to 1 wt%. Unit Name

H1 H2 H3 L1 L2 L3 L4 Mare Tranqillitatis Oceanus Procellarum Mare Imbrium Mare Serenitatis Mare Crisium

WACTiO2 [wt%]

LPTiO2 [wt%]

Average

St.Dev.

Average

St.Dev.

5.1 4.2 3.5 1.9 1.0 2.1 2.5 5.2 4.3 3.5 3.6 1.9

1.0 0.6 0.8 1.0 1.0 0.7 0.8 2.0 1.7 1.6 1.0 0.8

5.2 4.8 3.5 2.8 1.9 2.8 2.7 5.4 4.4 3.8 3.2 1.4

1.8 1.5 1.8 1.1 0.8 1.7 1.4 2.6 2.0 1.9 1.6 0.9

5.1.4. Mare Humorum The northeastern portion of the largest mare unit in Mare Humorum has the highest TiO2 abundance of all units in Humorum (8–10 wt%, see Fig. 13e). In the western half and southeastern margin, TiO2 abundance ranges from 2–6 wt% with no clear unit boundaries. The ejecta of small young craters (e.g., Gassendi J, Gassendi O, Gassendi Y, Gassendi L, Doppelmayer K, Puiseux D; 5–8 km in diameter) show lower TiO2 content relative to the surrounding surface, indicating that the uppermost layer (latest eruptions) contains more TiO2 . The DMDs along the fractures at the southwestern margin (Hawke et al., 2010) have moderate TiO2 abundances (2–6 wt%, suspicious value due to the glass spectra, see Section 2.3). The lowest TiO2 value (<2 wt%) along the southeastern boundary is likely due to admixing of immature ejecta from Tycho crater. 5.1.5. Mare Nubium Bullialdus crater (61 km in diameter) and its ejecta are located in the middle of Mare Nubium and are masked in this analysis (Fig. 13f). A ray of Tycho crater (350 km south of this mare) crosses the western portion in the SSE-NNW direction, likely decreasing the derived TiO2 values (0–2 wt% TiO2 ). The highest TiO2 values (6–10 wt%) are found in the northeastern portion (Fig. 13f, indicated by white arrow), in a ∼50 by 50 km spot surrounded by a low-TiO2 (<2 wt%) unit. The northwestern area of this mare has complicated boundaries due to patchy kipukas of highland units and generally has medium to low TiO2 values (2–6 wt%). The central spot, with a computed TiO2 value substantially below the detection limit (Fig. 13f, indicated by black arrow), has no topographic relief and no indication of contamination by ejecta,

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Fig. 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).

suggesting very low-TiO2 basalts with distinct spectral properties in this spot.

5.1.6. Mare Serenitatis Most of Mare Serenitatis exhibits low to medium TiO2 abundance (2–6 wt%, Fig. 14a). A ray crossing in the NNE-SSW direction, possibly from Tycho crater (20 0 0 km southwest of Serenitatis; Giamboni, 1959; Carr, 1966; Wilhelms et al., 1987; Campbell et al., 1992), results in lower TiO2 values (<2 wt%) and likely affected the areas surrounding the ray where TiO2 values are 2– 4 wt% by the mixing of original mare and ejecta materials. Other rays from Aristillus crater (54 km in diameter, located to the west) and Eudoxus crater (70 km in diameter, located to the north) affect the northern part of the mare, causing TiO2 values lower than the nearby mare surface. The regional dark mantle deposit (RDMD) (Weitz et al., 1998) found in an arc on the southeast margin of the mare (indicated with a black arrow) has the highest TiO2 (>10 wt%); however, this value is suspect due to the peculiar spectral response of pyroclastic glasses (see Section 2.3). Across the southern margin, including the boundary with Mare Tranquillitatis, the basalts have the highest values (excluding suspect values at RDMD) of TiO2 (6–10 wt%) . The sharp boundary (outlined by black triangles in Fig. 14a) between these high-TiO2 edges and the central mare unit with medium TiO2 (4–6 wt%) is likely due to the compositional change of mare basalt from eruption to eruption. The southern high-TiO2 unit inside Serenitatis and the northern highTiO2 unit inside Mare Tranquillitatis are probably the same flow unit, which is slightly older (3.55 Ga, Hiesinger et al., 2010; unit S11) relative to the adjacent medium-TiO2 unit inside Serenitatis (∼3.28–3.44 Ga, unit S22 and S15). This is one of the sharp TiO2 offset boundaries showing a direct contact of an older high-TiO2 unit and relatively young medium-TiO2 unit. See section 5.2 for a global analyses of the TiO2 variations over time.

5.1.7. Mare Crisium The eastern portion of Mare Crisium is high in TiO2 (6–8 wt%), with multiple kipukas of highland material (Fig. 14b). The ejecta of an unnamed 2-km crater has the highest local TiO2 value (∼8.5 wt%), indicating that the underlying layer has higher TiO2 content relative to the uppermost surface. The ejecta of Picard crater (22 km in diameter, located in southwestern Crisium) has higher TiO2 value (6–8 wt%) than the surrounding surface as well. The majority of Crisium has intermediate TiO2 values (2–6 wt%). The ejecta of the highland crater Proclus (27 km in diameter, 100 km west of Crisium) affects the northwestern portion of Crisium, likely lowering TiO2 values (<2 wt%).

5.1.8. Mare Tranquillitatis The highest mare TiO2 abundances (>10 wt%) are in the northwestern part of Mare Tranquillitatis (Fig. 14c), while in the southern half there are patchy areas of lower TiO2 (4-8 wt%), likely due to both true compositional variations of the mare and contamination by ejecta. The ejecta of Theophilus crater (99 km in diameter, located about 100 km south of Mare Tranquillitatis) overlies the southern portion of Tranquillitatis, likely decreasing the apparent TiO2 abundance of the underlying mare. The northeastern part of the mare has lower TiO2 values (<2 to 6 wt%). Römer crater (44 km in diameter) at ∼60 km north from the northern edge of Tranquillitatis also supplied ejecta to that region, decreasing the TiO2 as well (<2 wt%). There is no obvious sign of overlying highland-sourced materials along the high-low TiO2 boundaries (8–10 wt% to 2–6 wt%) at the base of the hornshaped area in the northeastern portion (black arrows, Fig. 14c), likely indicating a compositional change in the mare basalts with different flow units in a narrow time span (Hiesinger et al., 20 0 0).

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Fig. 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 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 arrows in (f) point out the low-TiO2 and highest TiO2 spots, respectively.

5.1.9. Mare Fecunditatis Most of the southeastern part of Mare Fecunditatis (Fig. 14d) is extensively covered by the ejecta from Langrenus (132 km in diameter) and Petavius B (32 km in diameter) craters, both located in the highlands close to the southeastern margin of Mare

Fecunditatis. The central part of this mare has patchy variations of TiO2 ranging from 2 to 8 wt%, thus likely resulting from mixtures of ejecta from Langrenus and local compositional variations (4–8 wt%) of the mare basalts. Both ejecta blankets resulted in the lowest TiO2 abundances (<2 wt%). Ejecta of Taruntius crater

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Fig. 14. 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 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. (2016).

(57 km in diameter) and Cameron crater (11 km in diameter, located on the rim of Taruntius) also altered the northern surface of this mare, resulting in low TiO2 (<2 to 4 wt%). The highest TiO2 values (8–10 wt%) are found at the northeastern edge of the mare.

5.1.10. Mare Marginis The upper half of this mare is covered by swirls, resulting in unreliably low (<2 wt%) computed TiO2 values (Fig. 14e), as described in Section 2.3. Also, the high reflectance ejecta materials from Goddard A crater (11 km in diameter) contaminate the northeastern edge of the mare, likely lowering the TiO2 values. The

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Fig. 15. TiO2 variation inside each mare (continued from Fig. 14). See Fig. 13 caption for symbol definitions. Images 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 the distribution of the swirls mapped by Denevi et al. (2016).

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southern half of this mare is relatively unaffected by the swirls and ejecta and has medium TiO2 values (2–6 wt%). The second large mare unit inside Neper crater (144 km in diameter) also has an undisturbed surface, showing low to medium TiO2 values (<2 to 4 wt%). 5.1.11. Mare Smythii Mare Smythii generally has intermediate TiO2 values (2-6 wt%) with little spatial variation (Fig. 14f). Peek crater (13 km in diameter) located on the northwestern side of this mare excavates the mare and probably the subsurface highland materials, resulting in ejecta with lower (∼2 wt%) TiO2 values. The ejecta of Hume Z crater (14 km in diameter) and an unnamed crater (3.5 km in diameter) contaminate the southeastern edges of this mare, resulting in 2-4 wt% TiO2 . The central portion of Smythii probably shows the original TiO2 values (4-6 wt%). 5.1.12. Mare Frigoris Previous works have suggested that Mare Frigoris has unique compositional characteristics of low TiO2 and low FeO values relative to other maria (Johnson et al., 1977; Whitford-Stark, 1990; Lucey et al., 20 0 0; Kramer et al., 2015). In the WAC TiO2 , 93% of Mare Frigoris is lower than the detection limit (Fig. 15a). The western portion of this mare with about 4 wt% TiO2 may be an extension of the unit that makes up the north end of Oceanus Procellarum, but is separated by the ejecta of Harpalus crater (40 km in diameter). This mare is a relatively narrow E-W strip, and thus the whole area is close to the highlands relative to other maria, possibly resulting in more highland ejecta contamination such as from the rays from Anaxagoras crater that cross much of Frigoris. Thus the anomalously high albedo and lower TiO2 values may simply be a result of highland contamination. Alternatively, Hemingway et al. (2015) suggest that there is a latitudinal trend in maturity decreasing toward the pole, which is responsible for the higher albedo of Mare Frigoris relative to the equatorial mare with similar FeO and TiO2 content. The presence of this trend was also confirmed using the active reflectance measurements by Lunar Orbiter Laser Altimeter (LOLA) (Lemelin et al., 2016). This high latitude immaturity might also be responsible for the low calculated TiO2 abundance in the majority of Frigoris, similar to the immature swirls that result in anomalously low values of TiO2 calculated from the WAC data. 5.1.13. Mare Tsiolkovskiy The southern and eastern portions of this mare have relatively high TiO2 (up to 6 wt%, Fig. 15b). There is no clear indication of significant contamination from craters outside of the mare. There are subtle TiO2 variations from 2 to 6 wt% without clear unit boundaries, possibly suggesting a gradual compositional change as the mare basalts were emplaced. 5.1.14. Mare Moscoviense The majority of the eastern half of Mare Moscoviense (Fig. 15c) exhibits high TiO2 values (6–10 wt%), whereas the western half has low to medium TiO2 (<2 to 4 wt%). The bimodal distribution of the Mare Moscoviense histogram (Fig. 12) is due to the eastern and western units having an obvious compositional difference in their mare basalts. No clear age difference was found between the eastern and western mare units (east: 2.6–3.6 Ga; west: 3.5 Ga, Morota et al., 2009). The western edge of this mare is partially overlain by swirls, overprinting low TiO2 (<2 wt%) on the western unit (originally 2 to 4 wt%). Rays of Steno Q crater (33 km in diameter, 150 km to the east of Moscoviense) cross the eastern high-TiO2 unit, leaving multiple streaks that lower the surface TiO2 values about 2 to 4 wt% relative to the surroundings.

5.1.15. Mare Ingenii Mare Ingenii embays the floors of Thomson crater (117 km in diameter), Thomson M crater (114 km in diameter), and the central to the southern portion of Ingenii basin (Fig. 15d). Most of the southern part of this mare and roughly 1/3 of the mare in Thomson crater have very low computed TiO2 values, which correspond to the extensive system of swirls that overlies much of the region (see Section 2.3). The low-reflectance portions next to the winding high-reflectance areas of the swirls have higher TiO2 values than the surrounding mare surface (about +2 wt% increase), which might be due to the relation between maturity and the 321/415 ratio as suggested by Denevi et al. (2014) and Coman et al. (2016). The southeastern edge of Thomson crater including the portion extending out of the crater, which is relatively unaffected by swirls, has 2–6 wt% of TiO2 . 5.1.16. Mare Orientale Mare Orientale consists of multiple small disconnected mare patches located near the basin center and between the multiring structures (Fig. 15e). The highest TiO2 values are 6–8 wt% at the southeastern portion of the largest unit. The ejecta of Maunder crater (54 km in diameter, north side of the basin) extensively overlays this unit, lowering the surface TiO2 values from <2 to 4 wt% for nearly all of the northern half of this unit. The southwestern portion of this largest unit has low TiO2 values (<2 wt%), probably due to an actual compositional variation in the basalts. The second large mare unit, Lacus Veris, located along the inner ring, has <2 to 6 wt% TiO2 . The ejecta of Maunder and other craters affect this unit, decreasing the surface TiO2 values to <2 wt%. Other smaller units along the northern portion of the inner ring are also covered by Maunder’s ejecta, and show low TiO2 values (<2 wt%). The ejecta of an unnamed crater (∼2 km in diameter, upper-left in Fig. 15e) exhibits larger TiO2 values (2-4 wt%) than the surrounding mare surface (<2 wt%). This likely indicates that the uppermost surface had been contaminated by highland ejecta first (from undefined multiple sources), then the more recent impact event of the unnamed crater exposed older underlying mare basalts. The other units along the outer ring, including Lacus Autumni, have TiO2 values up to 4 wt%. 5.2. Correlation between age and TiO2 abundance The correlation between the age, locations, and the composition of mare units has been discussed by several workers (e.g., Soderblom and Lebofsky, 1972; Head, 1976; Soderblom et al., 1977; Hiesinger et al., 2001; Kodama and Yamaguchi, 2005; Morota et al., 2011). Early studies based on the Apollo and Luna samples suggested that the low-TiO2 basalts are generally younger than highTiO2 basalts (Taylor, 1982). Later interpretations of remote-sensing data and lunar meteorites revealed the existence of relatively young, high-TiO2 and old, low-TiO2 basalts (e.g., Pieters et al., 1980; Cohen et al., 20 0 0). In an examination using the global TiO2 map (CLMTiO2 L) and the crater size-frequency distribution ages, no distinct correlation was observed between the deposit age and the TiO2 values on a global scale (Hiesinger et al., 2001). However, several regional studies suggest that the younger basalts tend to have relatively high-TiO2 , and the low-TiO2 basalts are relatively old (Imbrian Period, 3.2–3.8 Ga) (Kodama and Yamaguchi, 2005; Morota et al., 2011). We re-examined the age vs. TiO2 abundance correlation of the mare basalts at a global scale using the WAC TiO2 map and the crater size frequency distribution model ages reported by various studies (Greeley et al., 1993; Hiesinger et al., 20 0 0; 20 03; 20 06; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016). Instead of using the entire area of each mare unit defined by these

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Fig. 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. (20 0 0; 20 03). Background image is the WAC 643 nm monochrome mosaic. The highlands (gray zone) were masked using mare/highland boundaries (Nelson et al., 2014).

works, we used the subset areas (rectangular areas defined by Hiesinger et al., 20 0 0; 20 03; 2010) where the crater counts were performed (hereafter called “count area”, see Fig. 16). Even a very thin dusting can change the optical characteristics of the surface (thus WACTiO2 values) within each mare unit. Also in a narrow sense, the model age can vary within a unit depending on the location of count area (e.g., 1.0 to >2 Ga in P60 unit, Stadermann et al., 2015). Therefore, comparing the age and the TiO2 values within the count areas minimizes possible internal variations of age and TiO2 (due to actual compositional variations and highland contamination) inside each unit. We note that in several mare units (e.g., Mare Frigoris, Mare Fecunditatis, and Mare Humorum), we simply used each unit area since no subset was identified as the count area in the model age works. We also excluded several count areas (or the original unit areas) obviously contaminated by highland ejecta, which do not represent the spectrum of the original surface. Currently, a few major maria or mare units (e.g., Mare Crisium, Mare Somniorum, and the southeastern unit of Mare Humorum) do not have model ages, thus we excluded those maria. The TiO2 values below 2 wt% in WACTiO2 were set to 1 wt% before computing the median and standard deviation for each count area. For the count areas that are dominantly below 2 wt% TiO2 (defined by the median = 1 wt%), ± 1 wt% uncertainty was applied instead of using the standard deviation. The plot of age vs. TiO2 values (Fig. 17) using the count areas in all 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 average before 2.6 Ga is 3.4 wt% TiO2 and after 2.6 Ga is 6.8 wt%. The relatively high TiO2 values for younger ages and low TiO2 values with mostly older ages are consistent with the trends observed by previous works (Pieters et al., 1980; Kodama and Yamaguchi, 2005; Morota et al., 2011). In the plots for the individual mare (Fig. 18), no clear trend was found except for several maria (e.g., Oceanus Procellarum, Mare Humorum, and the southwestern nearside maria). The dots clustered below 2 wt% (Fig. 17) dominantly consist of areas in Mare Frigoris, Humboldtianum, and Australe Basin. Oceanus Procellarum has a wider range of ages (from 3.63 to 1.21 Ga) relative to the other maria, and the TiO2 values moderately increase with time (∼1 wt% per Gyr average). Although the time range is limited, Mare Humorum and the southwestern nearside maria also show increasing TiO2 trend over time from 3.8 to ∼3.0 Ga. After ∼2.8 Ga, no count area has less than 4.5 wt% of TiO2 . The other maria have relatively older ages ranging from 3.88 to ∼2.8 Ga, and some maria (e.g., Fecunditatis,

Fig. 17. TiO2 abundance as a function of crater size-frequency distribution model ages (Greeley et al., 1993; Hiesinger et al., 20 03; 20 06; 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 subset area (Hiesinger et al., 20 0 0; 20 03) 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 ± 1 wt% uncertainty.

Serenitatis, and Tranquillitatis) have a wide range in TiO2 (from <2 to 10 wt%) while others (e.g., Vaporum, Marginis, and Nubium) have narrower TiO2 ranges. These results suggest that old (>2.6 Ga) mare volcanic eruptions occurred in various places with the widest variation in TiO2 contents, then younger (<2.6 Ga) eruptions mainly occurred in Oceanus Procellarum with relatively high TiO2 content. 5.3. Implications for mare volcanism The nature of TiO2 enrichment in some mantle source regions is not well understood. Presumably, at a late stage of magma ocean solidification, ilmenite began to crystallize along with Ferich pyroxene, forming dense, gravitationally unstable cumulates (Taylor and Jakes, 1974; Snyder et al., 1992; Hess and Parmentier, 1995). Some portions of these cumulates were likely enriched in

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Fig. 18. TiO2 abundance vs. crater size-frequency distribution model ages (Greeley et al., 1993; Hiesinger et al., 20 03; 20 06; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016) inside each mare (see Fig. 17 for symbol definitions). The “Southwestern Nearside” corresponds to several mare patches inside individual craters and smooth plains to the south of Oceanus Procellarum.

incompatible elements, with relatively high concentrations of heatproducing elements such as Th, U, and K (Haskin, 1998; Wieczorek and Phillips, 20 0 0; Korotev, 20 0 0). Some of the high-TiO2 mare basalts are indeed enriched in these elements, such as the highK Apollo 11 basalts, while other examples are not (other high-TiO2 basalts from Apollo 11 and 17) (Taylor et al., 1991; Neal and Taylor, 1992). Likewise, sampled KREEP basalts (enriched in potassium (K), rare earth elements (REE), and phosphorous (P)) contain ∼2 wt% TiO2 (e.g., Warren and Wasson, 1978; Salpas et al., 1987). Therefore it is possible that Ti-rich cumulates and KREEP-rich residual melt were decoupled, probably by gravitational separation, in some regions (e.g., Shearer et al., 2006). Alternatively, the basalts were enriched in incompatible elements by assimilation of KREEP at the base of the crust. Remote sensing by the Lunar Prospector GRS shows Th enrichment over broad areas of the western Procellarum basalts

(Jolliff, 2004). We consider that this broadly enriched Th (3–5 ppm) over the PKT area cannot be explained only by the direct excavation of a sub-crustal KREEP layer as inferred by the distributions of highest Th along the Imbrium-circumferential highlands and the ejecta of several craters (e.g., Aristarchus, Kepler, Lalande, and Aristillus). A mare basalt rock fragment with ilmenite and a high Th content (6.9 ppm) from the Apollo 12 regolith samples also suggest possible Th enrichment in the mare basalts (Jolliff et al., 20 05; Barra et al., 20 06; Korotev et al., 2011). Where the western Procellarum basalts dominate the younger end of the age range for mare basalts, it is reasonable to infer, on the basis of a correlation between TiO2 and incompatible trace elements (reflected by GRS Th), a relationship between TiO2 and radiogenic-element enrichment in their source regions. In this case, we also infer that perhaps ilmenite cumulates and incompatible-element-rich residual melt of the magma ocean were not decoupled as they seem to

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Fig. 19. Thorium weight fraction as a function of crater size-frequency distribution model age (Greeley et al., 1993; Hiesinger et al., 20 03; 20 06; Morota et al., 2009; Hiesinger et al., 2010; Greenhagen et al., 2016). Dots and bars 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.

have been in source regions of some of the older high-TiO2 basalts. Having that enrichment in the source region might help to explain how basaltic volcanism extended to ages in the range of 1-3 Ga on the Moon (see discussion in Shearer et al., 2006, pp.467–468). Understanding the accurate TiO2 distribution in mare basalts provides an indication of TiO2 enrichment in underlying mantle source regions, and correlations with Th enrichment in some areas suggests a possible petrogenetic relationship, as inferred above for the western Procellarum basalts (e.g., Gillis et al., 2002; Flor et al., 20 02; 20 03). Coupled with age data for various mare units, we can gain an improved understanding of the thermal evolution of the mantle. At about 2.6 Ga, the age-TiO2 relation (Fig. 17) suggests a transition from a large range of TiO2 contents in widely distributed older maria to a more restricted, medium to high TiO2 range for younger mare, mostly located in Oceanus Procellarum (Fig. 17). What does this transition mean in the context of mare volcanism? The low-TiO2 basalts (<5 wt%) represent the most abundant product of partial melting of magma ocean cumulates (i.e., 70 vol% or more of the cumulate pile), whereas the medium- to highTiO2 basalts (>5 wt%) are relatively rare products from partial melting of late-stage cumulates (about 5-10 vol% of the cumulate pile) based on the geochemical-thermal modeling (Taylor and Jakes, 1974; Hess, 1991; Hess and Parmentier, 1995). Early mare eruptions (before the 2.6 Ga transition) were globally distributed, including several farside examples (e.g., Mare Moscoviense, Mare Tsiolkovskiy, South Pole-Aitken). The large TiO2 variations in this period suggest multiple melt sources with a wide range of TiO2 concentrations. Owing to sufficient heat in the early stage of lunar thermal evolution (Wieczorek and Phillips, 20 0 0), the melt source should have existed at various depths including shallow regions. Indeed, petrology experiments indicate that the Apollo 11 and 17 high-TiO2 basalts originated from shallow mantle depths based on equilibrium conditions, with a plausible cumulate mineral assemblage and using multiple saturation (Longhi et al., 1974; Walker et al., 1977; Grove and Krawczynski, 2009). The Th vs. age plot (Fig. 19, for all the count areas) exhibits larger variation in Th (0.111.3 ppm) in the older basalts (>2.5 Ga), implying Th-independent melt production due to the early hotter interior. As the mantle cooled, partial melting zones were likely deeper and the flux of basaltic melts to the surface should have dimin-

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ished (Head and Wilson, 1992; Hess and Parmentier, 2001; Shearer et al., 2006). The Ti-rich cumulates would be enriched in the latestage magma-ocean residual melt (urKREEP) (Warren and Wasson, 1979), possibly deep seated as a result of mantle overturn (Hess and Parmentier, 1995). In such cumulates, high Th (U and K as well) contents would have prolonged the heat and the melt production (Wieczorek and Phillips, 20 0 0), possibly resulting in young high-TiO2 basalts (<2.6 Ga) in the Procellarum KREEP Terrane (Jolliff et al., 20 0 0). The Th contents in such young basalts exhibit relatively high values (3.3–8.4 ppm) in the Th vs. age plot (Fig. 19). We note here that our interpretation inferring the possible correlation between Th enrichment and titanium content of the relatively young basalts is based largely on the remote-sensing observations. Currently, there is no petrological consensus supporting a direct connection of titanium and Th (or other radioactive elements) in the context of mare volcanism. The returned samples are dominantly from the eastern older maria and not from the western younger high-TiO2 maria, particularly Oceanus Procellarum. New samples from the western young mare obtained by future missions will allow us to test our hypothesis and to understand the mechanisms of long-lasting mare volcanism. One of the long-standing questions is the discrepancy between TiO2 estimates based on the visible to near-infrared wavelength reflectance data (e.g., Clementine UVVIS, Chang’e-1) and the GRS data (e.g., Elphic et al., 1998; Gillis et al., 2003). In particular, the highest TiO2 values (up to 17 wt% in Clementine TiO2 , up to 11 wt% in LPTiO2 ) show a large difference. Our estimates using WAC 321/415 nm ratio are similar to the LPTiO2 values (see Sections 3 and 4), and are also consistent with the range of TiO2 seen among Apollo and lunar meteorite basalt compositions (Lucey et al., 2006). Our high-end TiO2 estimates, which match the GRS values and are significantly lower than the visible to near-infrared estimates, represent >10 wt% at only 0.7% of the mare (Fig. 5, cumulative histogram of Group 1 maria), indicating that very highTiO2 basalt eruptions were probably rare and limited in the history of lunar volcanism. This result may add new constraints on the eruption mechanisms of the mare basalts. 6. Conclusions We derived a new TiO2 abundance map of the Moon from the UV and visible reflectance acquired by LROC WAC. We found a linear correlation between the 321/415 nm band ratio derived from 62 months of WAC observations and the TiO2 content of 9 returned soil samples from geologically homogeneous areas at the typical WAC pixel scale (400 m). Using this correlation and a nearglobal WAC multispectral mosaic, we constructed a new WAC TiO2 abundance map. To avoid non-ilmenite source factors controlling the 321/415 ratio (e.g., the pyroxene content, shock degree of plagioclase, immaturity with IS /FeO <∼40, and the presence of glass, Denevi et al., 2014; Gillis-Davis et al., 2006; Coman et al., 2016), we set 2 wt% TiO2 as the detection limit. Since the highlands have TiO2 abundances below this detection limit we excluded the highland terrain from our study. In the new WAC TiO2 map, the 17 major maria (Group 1) have an average of 3.9 wt% TiO2 . The highest TiO2 values are observed in Mare Tranquillitatis (∼12.6 wt%) and Oceanus Procellarum (∼11.6 wt%). Also, 28% of the mare surfaces included in Group 1 maria exhibit TiO2 abundances below the detection limit; low-TiO2 basalts, swirls, and the regions contaminated by highland ejecta compose these low-TiO2 areas. A comparison of the WAC TiO2 map with the Clementinebased TiO2 map (Lucey et al., 20 0 0) revealed systematically higher TiO2 abundances in the Clementine map, particularly in the highTiO2 maria (e.g., up to +6.9 wt% in Mare Tranquillitatis). Another

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Clementine-based TiO2 map created by Gillis et al. (2003) has closer average TiO2 values to the WAC TiO2 map (-0.3 wt% difference), albeit with a the clear TiO2 offsets along the conditionmatching boundaries used in their calculations. The Lunar Prospector GRS based TiO2 map (LPTiO2 ) is more consistent with WACTiO2 . In the comparisons for the seven mare units with distinct TiO2 abundances within Mare Imbrium, and for the five major maria (Mare Tranquillitatis, Oceanus Procellarum, Mare Imbrium, Mare Serenitatis, and Mare Crisium), LPTiO2 and WACTiO2 showed a good correlation that falls within ±0.8 wt% of a 1:1 line. The higher value in the LPTiO2 relative to the WACTiO2 at the eastern portion of Oceanus Procellarum could be due to the different depth sensitivities of the GRS (∼30 cm) and the WAC (few millimeters) in the areas of extensive ejecta deposits of Aristarchus, Kepler, and Copernicus craters. The spatial variations of TiO2 abundance within each of the Group 1 maria revealed significant contamination from highland ejecta, indicating that the original mare basalts have potentially higher TiO2 content than the current uppermost surface. Several maria (e.g., Mare Serenitatis, Mare Imbrium, and Oceanus Procellarum) have distinctly high- and low-TiO2 units located next to each other, which also have clear age differences. For all of the mare units for which model ages have been computed (using the count areas defined by Hiesinger et al., 20 0 0), a wide range of TiO2 values is found for relatively old (>2.6 Ga) mare volcanism and only medium to high TiO2 values for the younger (<2.6 Ga) mare deposits, mostly in Oceanus Procellarum. This 2.6 Ga transition may indicate a shift of lunar volcanism driven by the early hotter interior on a global scale to one driven by local high-TiO2 cumulates enriched in radioactive element heat sources. Our highest TiO2 estimates (excluding the DMDs) occur in the eastern portion of Mare Tranquillitatis. These values are higher than any returned soils, meaning that they are the extrapolated estimates. Future lunar landings or sample return missions on these highest TiO2 areas will increase the accuracy of reflectance-based orbital TiO2 estimates by improving the correlation fitting model (linear fit in this work). The small-sized or narrow-shaped maria with low TiO2 values (e.g., Mare Frigoris, Mare Moscoviense, and Mare Humboldtianum) potentially have significant highland contamination, resulting in reflectance-based estimates likely lower than the true mare values. These maria are in key locations to understand farside and high latitude volcanism. Higher resolution orbital images from future spacecraft missions, particularly in the UV wavelength range (e.g., 250-400 nm, Cloutis et al., 2008), will allow for the examination of small crater ejecta from the upper subsurface, which in many cases could reveal the original surface TiO2 abundances before highland contamination. Acknowledgments This work was supported by the National Aeronautics and Space Administration Lunar Reconnaissance Orbiter Project. The United States Geological Survey Integrated Software for Imagers and Spectrometers (ISIS) (Anderson et al., 2004) played a key role in processing the WAC observations. We acknowledge Dr. Gillis-Davis for his help on analyzing the Clementine-based TiO2 maps. We also acknowledge Oana Coman for her inputs that improved the quality of this manuscript. We thank two anonymous reviewers who greatly improved our manuscript. Supplementary Materials Supplementary material associated with this article can be found, in the online version, at dx.doi.org/10.1016/j.icarus.2017.06. 013.

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