- Email: [email protected]
Lunar reconnaissance orbiter wide angle camera algorithm for TiO2 abundances on the lunar surface, including the highlands and low-ti maria
Accepted Manuscript
Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria Bruce Hapke , Hiroyuki Sato , Mark Robinson PII: DOI: Reference:
S0019-1035(18)30163-5 https://doi.org/10.1016/j.icarus.2018.10.001 YICAR 13045
To appear in:
Icarus
Received date: Revised date: Accepted date:
16 March 2018 28 September 2018 1 October 2018
Please cite this article as: Bruce Hapke , Hiroyuki Sato , Mark Robinson , Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria, Icarus (2018), doi: https://doi.org/10.1016/j.icarus.2018.10.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN US
CR IP T
Highlights A new algorithm for estimating titanium abundance on the lunar surface using lunar samples and Lunar Reconnaissance Orbiter lunar images is proposed.
1
ACCEPTED MANUSCRIPT
Title: Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria Authors: Bruce Hapke, Hiroyuki Sato, Mark Robinson
Declarations of interest: none.
Suggested authors: Paul Helfenstein, [email protected]
M
Jeffrey Johnson, [email protected]
AN US
Keywords: Moon, surface composition, spectrophotometry
CR IP T
Corresponding author: Bruce Hapke, Department of Geology and Environmental Science, University of Pittsburgh, 200 SRCC Bldg., 4107 O’Hara St., Pittsburgh, PA 15260. Email: [email protected] Phome: 1-412-715-1243
Paul Lucey, [email protected]
ED
Carle PIeters, [email protected]
AC
CE
PT
Yuriy Shkuratov, [email protected]
2
ACCEPTED MANUSCRIPT
Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria
Bruce Hapke1, Hiroyuki Sato2, Mark Robinson3 Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh. PA
CR IP T
1
15260. 2
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1
Yoshinodal, Chuo-Ku, Sagamihara, Kanagawa, 252-5210, JAPAN.
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287.
AN US
3
Abstract
M
A new algorithm is proposed for estimating TiO2 abundance on the moon using lunar reflectance values measured by the Wide Angle Camera on the Lunar Reconnaissance Orbiter
ED
spacecraft. The algorithm provides useful values for mature regoliths on the entire lunar surface including highlands and low titanium maria. However, it underestimates the abundances of
CE
and rays.
PT
immature regoliths, so that the algorithm returns a lower limit for such features as young craters
AC
1. Introduction
Several algorithms for estimating the abundance of TiO2 on the lunar surface based on
spectral reflectances measured by telescopes and spacecraft have been suggested (Charette et al, 1974; Pieters et al, 1978; Johnson et al, 1991; Lucy et al, 2000; Robinson et al, 2007; Sato et al, 2017). These previous efforts yield abundance estimates that are generally similar to each other
3
ACCEPTED MANUSCRIPT
and to Lunar Prospector values for maria. However, these algorithms are less satisfactory when applied to low titanium maria (< 2 wt%) and highlands, sometimes even giving negative values of TiO2. For example, Sato et al (2017) simply set the TiO2 abundance of any region with a derived value less than 2% equal to 1%.
CR IP T
Since highlands are the most common terrain on the Moon, a more accurate low titanium algorithm is highly desirable. This paper shows that such an algorithm is provided by a linear relationship between TiO2 abundance and r(566/689), defined as the ratio of I/F(566), the
reflectance of an area on the moon measured using the Lunar Reconnaissance Orbiter Camera
AN US
(LROC) Wide Angle Camera (WAC) band 4 filter (central wavelength = 566 nm, FWHM = 20 nm), to I/F(689), the reflectance measured through the band 7 filter (689 nm, 39 nm.). The proposed algorithm is combined with a revised Sato et al (2017) algorithm for high titanium
M
soils, which uses r(321/415), defined as the ratio of the reflectances in band 1 (321 nm, 32 nm) to band 3 (415 nm, 36 nm), to give a means of estimating theTiO2 abundance in mature regoliths
ED
over the entire Lunar surface. (Filter characteristics are from Robinson et al 2010.) The correlations between these spectral ratios and TiO2 abundances are empirical. It is
PT
the purpose of this paper to present the algorithm and explain qualitatively why it works. A
CE
paper in preparation will present and discuss TiO2 maps of the moon derived from the algorithm
AC
and compare them with results from other methods.
2. Spectral Systematics of the Lunar Regolith This section summarizes the various absorption bands present in materials of lunar
composition. The spectra in this section are shown in order to provide the reader with the
4
ACCEPTED MANUSCRIPT
necessary background to understand why the algorithm works and why two ratios are required. They were not utilized in the derivation of the algorithm. Figure 1 shows the reflectance spectra between 200 and 800 nm of pulverized ezaamples of the five
CR IP T
- - - - - --- -- - - - --- - -- - - - - - - - - - - - - - (Fig. 1 near here) - -- - - - - - - - -- - - - - - - - - - - - - most abundant materials found on the lunar surface: anorthite, augite, olivine, ilmenite and vacuum-melted glass (Wagner et al 1987: Wells and Hapke 1977). The samples in the figure are described in detail in the references. The anorthite is lunar; the augite, olivine and ilmenite are
AN US
terrestrial, chosen to be as free of Fe+3 as possible; the glass is artificial, made of pure oxides, melted in a vacuum, and of composition similar to Apollo 11 soil. Glass is important to this discussion, because it occurs as beads, irregular fragments, and in agglutinates, which make up
M
half of a mature regolith.
The absorption band assignments are summarized briefly here. They are discussed in
ED
more detail in the references. All these materials have a very strong exciton band (an electronhole pair in an insulator) below 200 nm, so that their reflectances are low in the UV. The augite,
PT
ilmenite, olivine and glass have a weak Fe+2 band (not shown) in the near infrared around 1000
CE
nm; in anorthite this band is at 1250 nm. There also are a number of Fe and Ti absorption bands in the near-UV and visible. The region around 700 nm lies between these bands and the 1000
AC
nm band, so that the translucent minerals and glasses are weakly absorbing there. Thus, their reflectances near 700 nm are high and decrease nonlinearly as the wavelength decreases to 200 nm.
Lunar regolith particles are covered with coatings containing nanometer-sized metallic Fe spheres created by the deposition of solar-wind-sputtered and meteorite-impact-generated vapor
5
ACCEPTED MANUSCRIPT
(Hapke 2001). These coatings are an additional cause of the decreasing reflectance of the regolith as the wavelength decreases over the whole range of the plot. The bands in the near-UV and visible are superimposed on this general spectral trend. If FeO and/or TiO2 are added to a translucent mineral or glass, its reflectance at 250 nm is
CR IP T
decreased by strong Fe+2-O-2 and Ti+4-O-2 charge-transfer bands. Although pure anorthite does not contain Fe, most lunar anorthites have some Fe and often Ti impurities. Augite containing FeO and TiO2 has the 250 nm charge-transfer bands and also Fe+2-Ti+4 charge-transfer bands at 340 and 470 nm. Olivine has the 250 nm Fe and Ti bands. Glass made from these materials has
AN US
all of these bands, but they are wider than the bands in the minerals because the bond lengths are variable in glass. Thus, adding TiO2 to the translucent regolith materials deepens these Ti bands but has less effect on the refletances at 700 nm, which lies between the bands and the 1000 nm
M
Fe_2 band. This causes the r(566/689) reflectance ratio of the soil to decrease with increasing TiO2.
ED
By contrast, ilmenite has a large imaginary refractive index, so that it is opaque and its albedo is low over the whole wavelength range. Because it is opaque, all of the light it scatters
PT
comes from reflection from the grain surfaces, and absorption bands are manifested as maxima,
CE
rather than minima, as is the case with translucent materials (Hapke, 2012). The 200-800 nm spectrum of ilmenite is flat, except in the UV-blue (200-400 nm), where the reflectance increases
AC
as the wavelength decreases in the wings of its exciton, Fe-O and Ti-O bands. Thus, adding TiO2 in the form of ilmenite lowers the reflectance and decreases the slope of the regolith, causing the r(321/415) reflectance ratio to increase with increasing TiO2.
6
ACCEPTED MANUSCRIPT
In addition, adding ilmenite adds not only TiO2, but also an equal amount of FeO. As a soil matures space weathering converts some of this FeO into nanosized Fe, which further lowers the reflectances of all the translucent components,
CR IP T
Whether a reflectance ratio is increased or decreased by increasing the TiO2 depends critically on whether it is added to the translucent minerals and glass or as ilmenite.
3. The TiO2 Algorithm
AN US
Lunar regolith samples whose TiO2 abundances are known, along with WAC spectral measurements of the reflectances of the places on the moon from which the samples were acquired, were used to develop and calibrate the algorithm. The data are given in table 1.
M
- - - - - - - - - - - - - - - - - - - - - Table 1 near here - - - - - - - - - - - - - - - - - - - - - - - - - - - - - The reflectances are average radiance factors I/F (Hapke, 2012) of 1 km by 1 km areas on
ED
the moon corrected to a standard geometry of viewing angle = 0, incidence angle = 60o and phase angle = 60o. The procedures followed in obtaining the reflectances are described in Sato
PT
et al (2017). The standard deviations are the pixel-to-pixel variances within a sampling area.
CE
The TiO2 value of a given site is the average of the values of lunar samples from that site taken from the published literature (Merrill and Papike, 1978; Blewett et al, 1997; Joliff, 1999).
AC
Many sites and their samples were excluded for various reasons, for example, if the surface was non-uniform and highly variable over the measurement square. We tried to choose mature samples that were collected from geological units that were fairly uniform within the measurement square.
7
ACCEPTED MANUSCRIPT
Unfortunately, while chemical abundances of TiO2 in lunar samples are commonly listed in the literature, quantitative mineralogical descriptions, particularly of ilmenite content, are rare. (A notable exception is Taylor et all, 2001, 2010.) Hence, the following discussion of the reasons for the properties of the spectral ratios may be incomplete.
CR IP T
Figure 2 plots the reflectance in band 7, I/F(689), vs TiO2 content, and shows that there - - - - - - - - - - - - - - - - - - - - - - - - - - - -(Fig, 2 near here)- - - - - - - - - - - - - - - - - - - - - - - - - - -. are two distinct regimes in which the behavior of the reflectance with TiO2 differs markedly. Samples with lowTiO2 content have reflectances that are high and decrease rapidly with
AN US
increasing TiO2. The low-Ti regime consists of samples with TiO2 < 1% from the highland A16 and L20 sites, samples with 1% < TiO2 < 2% from the A15 and L24 low-Ti mare sites and material believed to be Mare Imbrium ejecta from the A14 site. Samples with high TiO2 content
M
have reflectanes that are low and flat, almost independent of TiO2. The high-Ti regime consists of samples with TiO2 > 3% from the A11, A12, A17 and L16 high-Ti mare sites. Apollo
ED
samples are denoted by A, Luna samples by L, and all abundances are weight percent. Between the two regimes is a gap that extends from 2% < TiO2 < 3%. Whether this gap is real or the
PT
result of insufficient sampling is unknown.
CE
The reflectances in the low-Ti regime are sensitive to both the translucent materials and the ilmenite. However, in the high-Ti regime the effects of adding ilmenite have saturated and
AC
the reflectance is almost constant and relatively insensitive to the translucent materials. Although not shown, plots of the reflectances at other wavelengths vs TiO2 are markedly
similar to figure 2. All show a low-Ti regime with a steep negative slope, a nearly flat high-Ti regime, and a 2 – 3% TiO2 gap. Straight lines fitted separately to the points in each regime, as
8
ACCEPTED MANUSCRIPT
illustrated in figure 2, intersect at or close to TiO2 = 2%. Therefore, we assume that the boundary between the regimes is at this abundance. Following a suggestion by Robinson et al (2007), Sato et al (2017) showed that there was a strong, positive, linear correlation between the TiO2 abundances and the LROC WAC ratios
CR IP T
r(321/415) of the lunar sites (figure 3a). This correlation occurs because the
- - - - - - - - - - - - - - - - - - - - - - - - - - - -(Fig. 3a and 3b near here)- - - - - - - - - - - - - - - - - - - - - reflectances of all the soil constituents are low in the UV-blue (200-450 nm), comparable with that of ilmenite, and he abundance of ilmenite is high in the high-Ti samples. Thus, this mineral
AN US
dominates the ratio, resulting in a strong positive correlation in the high-Ti regime (figure 3b). However, the low ilmenite abundance in the low-Ti samples allows the translucent materials to affect the ratios, causing the points to be scattered without a clear trend with TiO2.
M
In a search for an algorithm that might be useful in the low titanium regimes we investigated all 21 spectral ratios possible with the wavelengths of the 7 WAC filters. The most
ED
useful ratio was found to be r(566/689), which has a well-dispersed, strong, negative, linear correlation with TiO2 in the low-Ti regime (figures 4a and 4b). This correlation occurs
PT
- - - - - - - - - - - - - - - - - - - - - - - - - -(Fig. 4a and 4b near here)- - - - - - - - - - - - - - - - - - - - - - - -
CE
because the reflectances of the translucent minerals and glass are high in the green-red (500-700 nm), and the abundance of ilmenite is low in the low-Ti regime. Thus, the various Ti bands in
AC
the latter materials dominate the ratio, resulting in a strong negative correlation in the low-Ti regime (figure 4b). However, in the high-Ti regime the ilmenite abundance is sufficiently large that it competes spectrally with the translucent materials. This results in a positive correlation between r(566/689) and TiO2, but also a large scatter of points, making r(566/689) less satisfactory than r(321/415) in the high-Ti regime.
9
ACCEPTED MANUSCRIPT
Because there are two regimes with distinctly different spectral behavior, different algorithms are required in each regime. Thus, to make an algorithm for the entire lunar surface, lines were fitted separately to the data points shown in figure 4b for the low-Ti regime and to the data points shown in figure 3b for the high-Ti regime. While there are no points in our data set
CR IP T
within the 2 – 3% gap ,we allow for the possibility that there may be areas on the moon within the gap by assuming that they will be in the high-Ti regime, based on the arguments given above in connection with figure 2, and extrapolate the fitted line of figure 4b from 3% to 2% TiO2. (In Sato et al (2017) the four outlier points indicated by arrows in figure 3a were
AN US
suppressed when the line was fitted and brief justifications for their exclusion given. However, the two left points clearly belong in the low-Ti regime and should not be excluded. Also, we do not find the arguments against the two right points compelling and, thus, have included all four
M
points in our fits.)
The composite TiO2 algorithm for mature regoliths of the entire lunar surface is:
ED
TiO2 < 2%:
r(566/689) = 0.831 – 0.0241*TiO2(%),
(1a)
PT
which can be solved for TO2 abundance,
(1b)
CE
TiO2 = 34.48 - 41.49*r(566/689); TiO2 > 2%:
AC
r(321/415) = 0.718 + 0.00589*TiO2(%),
(2a)
or
TiO2 = 169.8*r(321/415) - 121.9.
(2b)
The composite algorithm is shown in figure 5. To find the TiO2 value of a lunar area, - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -(Fig. 5 near here)- - - - - - - - - - - - - - - - - - - - - - - -
10
ACCEPTED MANUSCRIPT
equation (1) cannot be used by itself because r(566/689) is double-valued with respect to TiO2 (figure 4a). Equation (2) for r(321/415), which is single-valued (figure 3a), should be calculated first to find whether the resulting TiO2 is greater or less than 2%. If it is less than 2% then equation (1) for r(566/689) should be used.
CR IP T
The maturation process reddens a regolith. Thus, the r(566/689) ratio of an immature low Ti soil is larger than the ratio of a mature soil of the same composition, which causes the
algorithm to give TiO2 value that is too low. Similarly, the r(321.415) ratio of an immature highTi soil is smaller than the ratio of a mature soil, which causes the algorithm to give a low TiO2
AN US
value. Thus, for immature soils the TiO2 abundance estimated by the algorithm is a lower limit. In particular, immature, low-Ti craters or ejecta may cause a negative TiO2 abundance to be
M
returned by the algorithm.
4. Acknowledgements: We thank the anonymous reviewers for their thoughtful and constructive
ED
comments. This research is sponsored by the Lunar Reconnaissance Orbiter Program of the
CE
5. References
PT
National Aeronautics and Space Administration.
Blewett, D., P. Lucey, B. Hawke, B. Joliff (1997). Clementine images of the lunar sample-return
AC
stations. Refinement of FeO and TiO2 mapping techniques. J. Gephys. Res. Planets, 102, 16,321-16,325. Charette, M., T. Mc Cord (1974). Application of remote spectral reflectance measurements to lunar geology classification an determination of titanium content of lunar soils. J. Geophys. Res., 79, 1605-1613.
11
ACCEPTED MANUSCRIPT
Hapke, B. (2001), Space weathering from Mercury to the asteroid belt. J. Geophys. Res., 106, 39-73. Hapke, B. (2012). Theory of Reflectance and Emittance Spectroscopy, 2nd ed., Cambridge University Press, New York.
lunar mare Ti02 mapping, Geophys. Res. Lett., 18, 2153-2156.
CR IP T
Johnson, J. R., S. M. Larson, R. B., Singer (1991). A reevaluation of spectral ratio for
Joliff, B. (1999). Clementine UV/VIS multispectral data and the Apollo 17 landing site. What can we tell and how well? J. Geophys. Res. Planets, 104, 14,123-14,148.
AN US
Lucey, P.G., D. T. Blewett, B. L. Jolliff (2000). Lunar iron and titanium abundance algorithms based on final processing of Clementine ultraviolet-visible images. J. Geophys. Res. 105, 20297–20305. doi:10.1029/1999JE001117 .
M
Merrill, R. and J. Papike (1978). Mare Crisium: The view from Luna 24; Proceedings of the Conference, Houston, Tex., Dec. 1-3, 1977. Geochim. et Cosmochim. Acta, Suppl. 9.
ED
Pieters, C. (1978). New Basalt Types on the Front Side of the Moon: A Summary of Spectral Reflectance Data. Proc. Lunar Planet. Sci. Conf. 9th, pp 2825-2849.
PT
Robinson, M. S., B. W. Hapke, J. B. Garvin, D. Skillman, J. F. Bell III, M Ulmer, C. Pieters
CE
(2007). High resolution mapping of TiO2 abundances on the Moon using the Hubble Space Telescope. Geophys. Res. Lett. 34, 15–18, doi:10.1029/2007GL029754.
AC
Robinson, M.S., S.M. Brylow, M. Tschimmel, D. Humm, S.J. Lawrence, P.C. Thomas, B.W. Denevi, E. Bowman-Cisneros, J. Zerr, M.A. Ravine, M.A. Caplinger, F.T. Ghaemi · J.A. Schaffner · M.C. Malin · P. Mahanti · A. Bartels, J. Anderson, T.N. Tran, E.M. Eliason, A.S. McEwen, E. Turtle, B.L. Jolliff, H. Hiesinger (2010), Lunar Reconnaissance Orbiter Camera (LROC) Instrument Overview, Space Science Reviews, doi:10.1007/s11214-010-9634-2.
12
ACCEPTED MANUSCRIPT
Sato, H., M. Robinson, S. Lawrence, B. Denevi, B. Hapke, B Jolliff, H. Hiesinger (2017). Lunar mare TiO 2 abundances estimated from UV/Vis reflectance. Icarus 296, 216–238. Taylor, L., C. Pieters, L. Keller, R. Morris, S. McKay (2001). Lunar mare soils: space weathering and the major effects of surface-correlated nanophase Fe. J. Gophys. Res., 106
CR IP T
27,985-27,999.
Taylor, L., C.Pieters, A. Patchen, D. Taylor, R. Morris, L. Keller, D. McKay (2010).
Mineralogica and chemical characterization of lunar highland soils: insights into space
weathering of soil on airless bodies. J, Geophys. Res., 115, E02002. doi 10.1029/2009JE003427,
AN US
2010.
Wagner, J., B. Hapke, E. Wells (1987). Atlas of reflectance spectra of terrestrial, lunar, and meteoritic powders and frosts from 92 to 1800 nm. Icarus 69, 14–28.
M
Wells, E., B. Hapke (1977). Lunar soil: iron and titanium bands in the glass fraction. Science,
Figures Captions
ED
195, 977–979.
PT
Figure 1. Reflectance spectra of pulverized examples of the four most abundant minerals in the
CE
lunar regolith and vacuum-melted glass (Wagner et al, 1978; Wells and Hapke, 1977). The vertical lines are at LROC WAC band centers.
AC
Figure 2. TiO2 abundance of Apollo and Luna samples vs reflectance I/F at 689 nm of the area from which each sample was taken. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%. The straight lines are least-square fits to points in the two regimes. The letter next to each point is the site identifier in table 1.
13
ACCEPTED MANUSCRIPT
Figure 3a. TiO2 abundance of Apollo and Luna samples vs ratio r(321/415) = [I/F(321)]/[I/F(415)] of the area from which each sample was taken (from Sato et al, 2017, updated). The four outlier points (arrows) were suppressed when the line was fitted. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%.
CR IP T
Figure 3b. Portion of figure 3a with best-fit line and error bars for the high-Ti regime only. The error bars are the pixel variances within each measurement square. No data points were excluded from this fit. The letter next to each point is the site identifier in table 1. Figure 4a. TiO2 abundance of Apollo and Luna samples vs ratio r(566/689) =
AN US
[I/F(566)]/[I/F(689)] of the area from which each sample was taken. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%.
Figure 4b. Portion of figure 4a with best-fit line and error bars for the low-Ti regime only. The
M
error brs are the pixel variances within each measurement square. No data points were excluded from this fit. The letter next to each point is the site identifier in table 1.
ED
Figure 5. Composite algorithm for TiO2 abundances in mature lunar regoliths. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%. The two regimes are separated
PT
by the vertical line, with high-Ti points for r(321/415) on the right and low-Ti points for
AC
CE
r(566/689) on the left. The values on the left and right axes are identical.
14
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 1. Reflectance spectra of pulverized examples of the four most abundant minerals in the lunar regolith plus vacuum-melted silicate glass containing FeO and TiO2 (Data from Wagner et al, 1987 and Wells and Hapke, 1977). The vertical lines are at LROC WAC band centers.
15
PT
ED
M
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
Figure 2. TiO2 abundance of Apollo and Luna samples vs reflectance I/F at 689 nm of the area from which each sample was taken. Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%. The straight lines are least-square fits to points in the two regimes. The letter next to each point is the site identifier in table 1.
16
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 3a. TiO2 abundance of Apollo and Luna samples vs ratio r(321/415) = [I/F(321)]/[I/F(415)] of the area from which each sample was taken (from Sato et al, 2018, updated). The four outlier points (arrows) were suppressed when the line was fitted: Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%.
Figure 3b. Portion of figure 3a with a line fitted to the points in the high-Ti regime only. The error brs are the pixel variances within each measurement square. No points were excluded from this fit. The letter next to each point is the site identifier in table 1. 17
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 4a. TiO2 abundance of Apollo and Luna samples vs ratio r(566/689) = [I/F(566)]/[I/F(689)] of the area from which each sample was taken. Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%.
Figure 4b. Portion of figure 4a with line fitted to points in the low-Ti regime only. The error brs are the pixel variances within each measurement square. No points were excluded from this fit. The letter next to each point is the site identifier in table 1.
18
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 5. Composite algorithm for TiO2 abundances in mature lunar regoliths. Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%.. The vertical line separates the low-Ti and high-Ti regimes, with high-Ti points for r(321/415) on the right and low-Ti points for r(566/689) on the left. The values on the left and rightt axes are identical.
19
ACCEPTED MANUSCRIPT
Table 1. Data TiO2( wt% )
I/F( 321)
std( 321)
I/F( 415)
std( 415)
I/F( 566 )
std( 566)
I/F( 689 )
std( 689 )
A1 6S4
644 21, 645 01 633 41, 635 01
0.5
0.02 139
0.00 097
0.03 061
0.00 238
0.0 418 2
0.00 289
0.0 514 3
0.00 258
0.69 0.06 879 2899
0.81 0.06 314 9438
0.5
0.02 881
0.00 365
0.04 122
0.00 685
0.0 577
0.00 726
0.0 693 8
0.00 909
0.69 0.14 893 605
0.83 0.15 165 107
7.5
0.5
0.02 514 6 0.03 257 1
0.00 214 4 0.00 200 4
0.0 455
0.00 274
0.70 0.07 019 5155
0.80 0.06 901 9105
0.6
0.00 114 8 0.00 104 5
0.00 223
605 01, 606 01
0.01 760 7 0.02 388 3
0.00 162
0.0 544 3
0.00 191
0.73 0.05 326 536
0.82 0.04 381 1491
665 01 657 01 659 01 19. 6
0.7
154 71
E
L2 4
G
A1 5S2 A1 5S1
AC
F
H
I
A1 5S8
J
A1 4
150 71, 150 81 150 13, 150 20 140 03,
AN US
A1 6S1 0 & L M A1 6S5
std(32 1/415 )
0.0 368 1 0.0 448 4
r(566 /689 )
std(56 6/689 )
M
D
1
0.02 032 1
ED
C
A1 6S1 3 L2 0
0.00 089 2
PT
B
r(321 /415 )
CR IP T
Sa mp le ID
0.02 917 8
0.00 177 9
0.0 423
0.00 241
0.0 515
0.00 428
0.69 0.05 645 2323
0.82 0.08 136 2761
0.0 198 2 0.0 289 7 0.0 247 9
0.00 065
0.0 249 1 0.0 364
0.00 089
0.71 857
0.79 566
0.00 402
0.70 0.07 092 9087
0.79 0.12 588 393
0.00 82
0.0 311 3
0.00 164
0.70 0.07 526 378
0.79 0.04 634 95
0.01 006
0.00 034
0.01 4
0.00 091
1.2
0.01 289
0.00 09
0.01 839
0.00 163
1.6
0.01 201 7
0.00 069
0.01 703 9
0.00 149
1.7
0.01 105
0.00 03
0.01 581
0.00 087
0.0 230 7
0.00 057
0.0 292 2
0.00 083
0.69 0.04 892 2887
0.78 0.02 953 9724
1.7
0.01 299
0.00 034
0.01 844
0.00 106
0.0 270
0.00 088
0.0 336
0.00 101
0.70 0.04
0.80 0.03
CE
Lo wTi A
Sit e
20
0.00 318
0.052 64
0.038 6
ACCEPTED MANUSCRIPT
N
A1 7LR V1 1
782 21, 782 31, 784
0.00 096
3.1
0.01 101
0.00 037
0.01 526
0.00 082
3.3
0.00 903
0.00 038
0.01 189
0.00 071
4.5
0.01 182 7
0.00 045 7
0.01 596 5
0.00 143 6
0.0 236 5
0.00 451
0.0 216 8
321
5526
CR IP T
0.01 528
0.0 303 3
0.00 434
0.00 102 2
0.0 269 2
0.00 133
0.72 0.04 149 5727
0.80 0.05 535 4995
0.00 057
0.0 203 4 0.0 225 5
0.00 06
0.75 0.05 946 5481
0.80 0.03 875 6803
0.00 055
0.74 0.07 081 2522
0.81 0.03 33 1784
0.70 0.05 942 5683
AN US
L1 6
0.00 051
4494
0.77 0.18 976 59
M
M
120 01, 120 23, 120 30, 120 32, 120 33, 120 37, 120 41, 120 42, 120 44, 106 0, 120 70 16. 7
0.01 084
445
AC
CE
A1 2
1.8
9
ED
Hi gh -Ti L
A1 5S9
6
PT
K
141 41, 141 48, 142 20, 142 30, 142 40, 142 59, 142 60, 142 63 155 01
0.0 164 5 0.0 183 4
21
0.00 056
ACCEPTED MANUSCRIPT
R
S
T
U
A1 7S5
0.00 119
0.0 204 6
0.00 09
0.0 249 7
0.00 102
6.1
0.01 107
0.00 039
0.01 472
0.00 119
0.0 191 3
0.00 061
0.0 236 2
0.00 068
0.75 0.06 204 6319
0.80 0.03 991 4794
751 11, 751 21 100 02, 100 10, 100 84 721 31
6.7
0.01 042
0.00 03
0.01 375
0.00 105
0.0 183 7
0.00 059
0.0 226 6
0.00 064
0.75 0.06 782 1846
0.81 0.03 068 4672
7.5
0.01 037
0.00 04
0.01 356
0.00 083
8
0.01 043
0.00 042 3
0.01 368
0.00 134
700 11, 701 81 711 31, 715 01 750 61, 750 81
8.5
0.01 076
9.6
0.01 153 6
9.9
0.01 059 4
0.81 0.04 938 9188
CR IP T
0.74 0.06 821 7945
0.0 178 7
0.00 061
0.0 217 2
0.00 082
0.76 0.05 475 5329
0.82 0.04 274 1875
0.0 215 7
0.00 09
0.0 264 7
0.00 103
0.76 0.07 243 6551
0.81 0.04 488 6492
0.00 039
0.01 4
0.00 157
0.0 195 3
0.00 079
0.0 238 1
0.00 091
0.76 0.09 857 058
0.82 0.04 024 5647
0.00 038
0.01 459 6
0.00 136
0.0 191 1
0.00 227
0.0 233 2
0.00 301
0.79 0.07 035 8109
0.81 0.14 947 375
0.00 054
0.01 384 8
0.00 209
0.0 186 5
0.00 263
0.0 229 1
0.00 33
0.76 0.12 502 187
0.81 0.16 406 41
AC
V
A1 7LR V1 1 A1 7L M A1 7S1
0.01 466 3
AN US
Q
0.00 044 7
M
A1 7LR V9 A1 7LR V8 A1 1
0.01 097 1
ED
P
5.5
PT
A1 7LR V3
CE
O
41, 784 61, 781 21 721 50, 721 51, 721 61 761 21
22
Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria Bruce Hapke , Hiroyuki Sato , Mark Robinson PII: DOI: Reference:
S0019-1035(18)30163-5 https://doi.org/10.1016/j.icarus.2018.10.001 YICAR 13045
To appear in:
Icarus
Received date: Revised date: Accepted date:
16 March 2018 28 September 2018 1 October 2018
Please cite this article as: Bruce Hapke , Hiroyuki Sato , Mark Robinson , Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria, Icarus (2018), doi: https://doi.org/10.1016/j.icarus.2018.10.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
AN US
CR IP T
Highlights A new algorithm for estimating titanium abundance on the lunar surface using lunar samples and Lunar Reconnaissance Orbiter lunar images is proposed.
1
ACCEPTED MANUSCRIPT
Title: Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria Authors: Bruce Hapke, Hiroyuki Sato, Mark Robinson
Declarations of interest: none.
Suggested authors: Paul Helfenstein, [email protected]
M
Jeffrey Johnson, [email protected]
AN US
Keywords: Moon, surface composition, spectrophotometry
CR IP T
Corresponding author: Bruce Hapke, Department of Geology and Environmental Science, University of Pittsburgh, 200 SRCC Bldg., 4107 O’Hara St., Pittsburgh, PA 15260. Email: [email protected] Phome: 1-412-715-1243
Paul Lucey, [email protected]
ED
Carle PIeters, [email protected]
AC
CE
PT
Yuriy Shkuratov, [email protected]
2
ACCEPTED MANUSCRIPT
Lunar Reconnaissance Orbiter Wide Angle Camera Algorithm for TiO2 Abundances on the Lunar Surface, including the Highlands and Low-Ti Maria
Bruce Hapke1, Hiroyuki Sato2, Mark Robinson3 Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh. PA
CR IP T
1
15260. 2
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1
Yoshinodal, Chuo-Ku, Sagamihara, Kanagawa, 252-5210, JAPAN.
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287.
AN US
3
Abstract
M
A new algorithm is proposed for estimating TiO2 abundance on the moon using lunar reflectance values measured by the Wide Angle Camera on the Lunar Reconnaissance Orbiter
ED
spacecraft. The algorithm provides useful values for mature regoliths on the entire lunar surface including highlands and low titanium maria. However, it underestimates the abundances of
CE
and rays.
PT
immature regoliths, so that the algorithm returns a lower limit for such features as young craters
AC
1. Introduction
Several algorithms for estimating the abundance of TiO2 on the lunar surface based on
spectral reflectances measured by telescopes and spacecraft have been suggested (Charette et al, 1974; Pieters et al, 1978; Johnson et al, 1991; Lucy et al, 2000; Robinson et al, 2007; Sato et al, 2017). These previous efforts yield abundance estimates that are generally similar to each other
3
ACCEPTED MANUSCRIPT
and to Lunar Prospector values for maria. However, these algorithms are less satisfactory when applied to low titanium maria (< 2 wt%) and highlands, sometimes even giving negative values of TiO2. For example, Sato et al (2017) simply set the TiO2 abundance of any region with a derived value less than 2% equal to 1%.
CR IP T
Since highlands are the most common terrain on the Moon, a more accurate low titanium algorithm is highly desirable. This paper shows that such an algorithm is provided by a linear relationship between TiO2 abundance and r(566/689), defined as the ratio of I/F(566), the
reflectance of an area on the moon measured using the Lunar Reconnaissance Orbiter Camera
AN US
(LROC) Wide Angle Camera (WAC) band 4 filter (central wavelength = 566 nm, FWHM = 20 nm), to I/F(689), the reflectance measured through the band 7 filter (689 nm, 39 nm.). The proposed algorithm is combined with a revised Sato et al (2017) algorithm for high titanium
M
soils, which uses r(321/415), defined as the ratio of the reflectances in band 1 (321 nm, 32 nm) to band 3 (415 nm, 36 nm), to give a means of estimating theTiO2 abundance in mature regoliths
ED
over the entire Lunar surface. (Filter characteristics are from Robinson et al 2010.) The correlations between these spectral ratios and TiO2 abundances are empirical. It is
PT
the purpose of this paper to present the algorithm and explain qualitatively why it works. A
CE
paper in preparation will present and discuss TiO2 maps of the moon derived from the algorithm
AC
and compare them with results from other methods.
2. Spectral Systematics of the Lunar Regolith This section summarizes the various absorption bands present in materials of lunar
composition. The spectra in this section are shown in order to provide the reader with the
4
ACCEPTED MANUSCRIPT
necessary background to understand why the algorithm works and why two ratios are required. They were not utilized in the derivation of the algorithm. Figure 1 shows the reflectance spectra between 200 and 800 nm of pulverized ezaamples of the five
CR IP T
- - - - - --- -- - - - --- - -- - - - - - - - - - - - - - (Fig. 1 near here) - -- - - - - - - - -- - - - - - - - - - - - - most abundant materials found on the lunar surface: anorthite, augite, olivine, ilmenite and vacuum-melted glass (Wagner et al 1987: Wells and Hapke 1977). The samples in the figure are described in detail in the references. The anorthite is lunar; the augite, olivine and ilmenite are
AN US
terrestrial, chosen to be as free of Fe+3 as possible; the glass is artificial, made of pure oxides, melted in a vacuum, and of composition similar to Apollo 11 soil. Glass is important to this discussion, because it occurs as beads, irregular fragments, and in agglutinates, which make up
M
half of a mature regolith.
The absorption band assignments are summarized briefly here. They are discussed in
ED
more detail in the references. All these materials have a very strong exciton band (an electronhole pair in an insulator) below 200 nm, so that their reflectances are low in the UV. The augite,
PT
ilmenite, olivine and glass have a weak Fe+2 band (not shown) in the near infrared around 1000
CE
nm; in anorthite this band is at 1250 nm. There also are a number of Fe and Ti absorption bands in the near-UV and visible. The region around 700 nm lies between these bands and the 1000
AC
nm band, so that the translucent minerals and glasses are weakly absorbing there. Thus, their reflectances near 700 nm are high and decrease nonlinearly as the wavelength decreases to 200 nm.
Lunar regolith particles are covered with coatings containing nanometer-sized metallic Fe spheres created by the deposition of solar-wind-sputtered and meteorite-impact-generated vapor
5
ACCEPTED MANUSCRIPT
(Hapke 2001). These coatings are an additional cause of the decreasing reflectance of the regolith as the wavelength decreases over the whole range of the plot. The bands in the near-UV and visible are superimposed on this general spectral trend. If FeO and/or TiO2 are added to a translucent mineral or glass, its reflectance at 250 nm is
CR IP T
decreased by strong Fe+2-O-2 and Ti+4-O-2 charge-transfer bands. Although pure anorthite does not contain Fe, most lunar anorthites have some Fe and often Ti impurities. Augite containing FeO and TiO2 has the 250 nm charge-transfer bands and also Fe+2-Ti+4 charge-transfer bands at 340 and 470 nm. Olivine has the 250 nm Fe and Ti bands. Glass made from these materials has
AN US
all of these bands, but they are wider than the bands in the minerals because the bond lengths are variable in glass. Thus, adding TiO2 to the translucent regolith materials deepens these Ti bands but has less effect on the refletances at 700 nm, which lies between the bands and the 1000 nm
M
Fe_2 band. This causes the r(566/689) reflectance ratio of the soil to decrease with increasing TiO2.
ED
By contrast, ilmenite has a large imaginary refractive index, so that it is opaque and its albedo is low over the whole wavelength range. Because it is opaque, all of the light it scatters
PT
comes from reflection from the grain surfaces, and absorption bands are manifested as maxima,
CE
rather than minima, as is the case with translucent materials (Hapke, 2012). The 200-800 nm spectrum of ilmenite is flat, except in the UV-blue (200-400 nm), where the reflectance increases
AC
as the wavelength decreases in the wings of its exciton, Fe-O and Ti-O bands. Thus, adding TiO2 in the form of ilmenite lowers the reflectance and decreases the slope of the regolith, causing the r(321/415) reflectance ratio to increase with increasing TiO2.
6
ACCEPTED MANUSCRIPT
In addition, adding ilmenite adds not only TiO2, but also an equal amount of FeO. As a soil matures space weathering converts some of this FeO into nanosized Fe, which further lowers the reflectances of all the translucent components,
CR IP T
Whether a reflectance ratio is increased or decreased by increasing the TiO2 depends critically on whether it is added to the translucent minerals and glass or as ilmenite.
3. The TiO2 Algorithm
AN US
Lunar regolith samples whose TiO2 abundances are known, along with WAC spectral measurements of the reflectances of the places on the moon from which the samples were acquired, were used to develop and calibrate the algorithm. The data are given in table 1.
M
- - - - - - - - - - - - - - - - - - - - - Table 1 near here - - - - - - - - - - - - - - - - - - - - - - - - - - - - - The reflectances are average radiance factors I/F (Hapke, 2012) of 1 km by 1 km areas on
ED
the moon corrected to a standard geometry of viewing angle = 0, incidence angle = 60o and phase angle = 60o. The procedures followed in obtaining the reflectances are described in Sato
PT
et al (2017). The standard deviations are the pixel-to-pixel variances within a sampling area.
CE
The TiO2 value of a given site is the average of the values of lunar samples from that site taken from the published literature (Merrill and Papike, 1978; Blewett et al, 1997; Joliff, 1999).
AC
Many sites and their samples were excluded for various reasons, for example, if the surface was non-uniform and highly variable over the measurement square. We tried to choose mature samples that were collected from geological units that were fairly uniform within the measurement square.
7
ACCEPTED MANUSCRIPT
Unfortunately, while chemical abundances of TiO2 in lunar samples are commonly listed in the literature, quantitative mineralogical descriptions, particularly of ilmenite content, are rare. (A notable exception is Taylor et all, 2001, 2010.) Hence, the following discussion of the reasons for the properties of the spectral ratios may be incomplete.
CR IP T
Figure 2 plots the reflectance in band 7, I/F(689), vs TiO2 content, and shows that there - - - - - - - - - - - - - - - - - - - - - - - - - - - -(Fig, 2 near here)- - - - - - - - - - - - - - - - - - - - - - - - - - -. are two distinct regimes in which the behavior of the reflectance with TiO2 differs markedly. Samples with lowTiO2 content have reflectances that are high and decrease rapidly with
AN US
increasing TiO2. The low-Ti regime consists of samples with TiO2 < 1% from the highland A16 and L20 sites, samples with 1% < TiO2 < 2% from the A15 and L24 low-Ti mare sites and material believed to be Mare Imbrium ejecta from the A14 site. Samples with high TiO2 content
M
have reflectanes that are low and flat, almost independent of TiO2. The high-Ti regime consists of samples with TiO2 > 3% from the A11, A12, A17 and L16 high-Ti mare sites. Apollo
ED
samples are denoted by A, Luna samples by L, and all abundances are weight percent. Between the two regimes is a gap that extends from 2% < TiO2 < 3%. Whether this gap is real or the
PT
result of insufficient sampling is unknown.
CE
The reflectances in the low-Ti regime are sensitive to both the translucent materials and the ilmenite. However, in the high-Ti regime the effects of adding ilmenite have saturated and
AC
the reflectance is almost constant and relatively insensitive to the translucent materials. Although not shown, plots of the reflectances at other wavelengths vs TiO2 are markedly
similar to figure 2. All show a low-Ti regime with a steep negative slope, a nearly flat high-Ti regime, and a 2 – 3% TiO2 gap. Straight lines fitted separately to the points in each regime, as
8
ACCEPTED MANUSCRIPT
illustrated in figure 2, intersect at or close to TiO2 = 2%. Therefore, we assume that the boundary between the regimes is at this abundance. Following a suggestion by Robinson et al (2007), Sato et al (2017) showed that there was a strong, positive, linear correlation between the TiO2 abundances and the LROC WAC ratios
CR IP T
r(321/415) of the lunar sites (figure 3a). This correlation occurs because the
- - - - - - - - - - - - - - - - - - - - - - - - - - - -(Fig. 3a and 3b near here)- - - - - - - - - - - - - - - - - - - - - reflectances of all the soil constituents are low in the UV-blue (200-450 nm), comparable with that of ilmenite, and he abundance of ilmenite is high in the high-Ti samples. Thus, this mineral
AN US
dominates the ratio, resulting in a strong positive correlation in the high-Ti regime (figure 3b). However, the low ilmenite abundance in the low-Ti samples allows the translucent materials to affect the ratios, causing the points to be scattered without a clear trend with TiO2.
M
In a search for an algorithm that might be useful in the low titanium regimes we investigated all 21 spectral ratios possible with the wavelengths of the 7 WAC filters. The most
ED
useful ratio was found to be r(566/689), which has a well-dispersed, strong, negative, linear correlation with TiO2 in the low-Ti regime (figures 4a and 4b). This correlation occurs
PT
- - - - - - - - - - - - - - - - - - - - - - - - - -(Fig. 4a and 4b near here)- - - - - - - - - - - - - - - - - - - - - - - -
CE
because the reflectances of the translucent minerals and glass are high in the green-red (500-700 nm), and the abundance of ilmenite is low in the low-Ti regime. Thus, the various Ti bands in
AC
the latter materials dominate the ratio, resulting in a strong negative correlation in the low-Ti regime (figure 4b). However, in the high-Ti regime the ilmenite abundance is sufficiently large that it competes spectrally with the translucent materials. This results in a positive correlation between r(566/689) and TiO2, but also a large scatter of points, making r(566/689) less satisfactory than r(321/415) in the high-Ti regime.
9
ACCEPTED MANUSCRIPT
Because there are two regimes with distinctly different spectral behavior, different algorithms are required in each regime. Thus, to make an algorithm for the entire lunar surface, lines were fitted separately to the data points shown in figure 4b for the low-Ti regime and to the data points shown in figure 3b for the high-Ti regime. While there are no points in our data set
CR IP T
within the 2 – 3% gap ,we allow for the possibility that there may be areas on the moon within the gap by assuming that they will be in the high-Ti regime, based on the arguments given above in connection with figure 2, and extrapolate the fitted line of figure 4b from 3% to 2% TiO2. (In Sato et al (2017) the four outlier points indicated by arrows in figure 3a were
AN US
suppressed when the line was fitted and brief justifications for their exclusion given. However, the two left points clearly belong in the low-Ti regime and should not be excluded. Also, we do not find the arguments against the two right points compelling and, thus, have included all four
M
points in our fits.)
The composite TiO2 algorithm for mature regoliths of the entire lunar surface is:
ED
TiO2 < 2%:
r(566/689) = 0.831 – 0.0241*TiO2(%),
(1a)
PT
which can be solved for TO2 abundance,
(1b)
CE
TiO2 = 34.48 - 41.49*r(566/689); TiO2 > 2%:
AC
r(321/415) = 0.718 + 0.00589*TiO2(%),
(2a)
or
TiO2 = 169.8*r(321/415) - 121.9.
(2b)
The composite algorithm is shown in figure 5. To find the TiO2 value of a lunar area, - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -(Fig. 5 near here)- - - - - - - - - - - - - - - - - - - - - - - -
10
ACCEPTED MANUSCRIPT
equation (1) cannot be used by itself because r(566/689) is double-valued with respect to TiO2 (figure 4a). Equation (2) for r(321/415), which is single-valued (figure 3a), should be calculated first to find whether the resulting TiO2 is greater or less than 2%. If it is less than 2% then equation (1) for r(566/689) should be used.
CR IP T
The maturation process reddens a regolith. Thus, the r(566/689) ratio of an immature low Ti soil is larger than the ratio of a mature soil of the same composition, which causes the
algorithm to give TiO2 value that is too low. Similarly, the r(321.415) ratio of an immature highTi soil is smaller than the ratio of a mature soil, which causes the algorithm to give a low TiO2
AN US
value. Thus, for immature soils the TiO2 abundance estimated by the algorithm is a lower limit. In particular, immature, low-Ti craters or ejecta may cause a negative TiO2 abundance to be
M
returned by the algorithm.
4. Acknowledgements: We thank the anonymous reviewers for their thoughtful and constructive
ED
comments. This research is sponsored by the Lunar Reconnaissance Orbiter Program of the
CE
5. References
PT
National Aeronautics and Space Administration.
Blewett, D., P. Lucey, B. Hawke, B. Joliff (1997). Clementine images of the lunar sample-return
AC
stations. Refinement of FeO and TiO2 mapping techniques. J. Gephys. Res. Planets, 102, 16,321-16,325. Charette, M., T. Mc Cord (1974). Application of remote spectral reflectance measurements to lunar geology classification an determination of titanium content of lunar soils. J. Geophys. Res., 79, 1605-1613.
11
ACCEPTED MANUSCRIPT
Hapke, B. (2001), Space weathering from Mercury to the asteroid belt. J. Geophys. Res., 106, 39-73. Hapke, B. (2012). Theory of Reflectance and Emittance Spectroscopy, 2nd ed., Cambridge University Press, New York.
lunar mare Ti02 mapping, Geophys. Res. Lett., 18, 2153-2156.
CR IP T
Johnson, J. R., S. M. Larson, R. B., Singer (1991). A reevaluation of spectral ratio for
Joliff, B. (1999). Clementine UV/VIS multispectral data and the Apollo 17 landing site. What can we tell and how well? J. Geophys. Res. Planets, 104, 14,123-14,148.
AN US
Lucey, P.G., D. T. Blewett, B. L. Jolliff (2000). Lunar iron and titanium abundance algorithms based on final processing of Clementine ultraviolet-visible images. J. Geophys. Res. 105, 20297–20305. doi:10.1029/1999JE001117 .
M
Merrill, R. and J. Papike (1978). Mare Crisium: The view from Luna 24; Proceedings of the Conference, Houston, Tex., Dec. 1-3, 1977. Geochim. et Cosmochim. Acta, Suppl. 9.
ED
Pieters, C. (1978). New Basalt Types on the Front Side of the Moon: A Summary of Spectral Reflectance Data. Proc. Lunar Planet. Sci. Conf. 9th, pp 2825-2849.
PT
Robinson, M. S., B. W. Hapke, J. B. Garvin, D. Skillman, J. F. Bell III, M Ulmer, C. Pieters
CE
(2007). High resolution mapping of TiO2 abundances on the Moon using the Hubble Space Telescope. Geophys. Res. Lett. 34, 15–18, doi:10.1029/2007GL029754.
AC
Robinson, M.S., S.M. Brylow, M. Tschimmel, D. Humm, S.J. Lawrence, P.C. Thomas, B.W. Denevi, E. Bowman-Cisneros, J. Zerr, M.A. Ravine, M.A. Caplinger, F.T. Ghaemi · J.A. Schaffner · M.C. Malin · P. Mahanti · A. Bartels, J. Anderson, T.N. Tran, E.M. Eliason, A.S. McEwen, E. Turtle, B.L. Jolliff, H. Hiesinger (2010), Lunar Reconnaissance Orbiter Camera (LROC) Instrument Overview, Space Science Reviews, doi:10.1007/s11214-010-9634-2.
12
ACCEPTED MANUSCRIPT
Sato, H., M. Robinson, S. Lawrence, B. Denevi, B. Hapke, B Jolliff, H. Hiesinger (2017). Lunar mare TiO 2 abundances estimated from UV/Vis reflectance. Icarus 296, 216–238. Taylor, L., C. Pieters, L. Keller, R. Morris, S. McKay (2001). Lunar mare soils: space weathering and the major effects of surface-correlated nanophase Fe. J. Gophys. Res., 106
CR IP T
27,985-27,999.
Taylor, L., C.Pieters, A. Patchen, D. Taylor, R. Morris, L. Keller, D. McKay (2010).
Mineralogica and chemical characterization of lunar highland soils: insights into space
weathering of soil on airless bodies. J, Geophys. Res., 115, E02002. doi 10.1029/2009JE003427,
AN US
2010.
Wagner, J., B. Hapke, E. Wells (1987). Atlas of reflectance spectra of terrestrial, lunar, and meteoritic powders and frosts from 92 to 1800 nm. Icarus 69, 14–28.
M
Wells, E., B. Hapke (1977). Lunar soil: iron and titanium bands in the glass fraction. Science,
Figures Captions
ED
195, 977–979.
PT
Figure 1. Reflectance spectra of pulverized examples of the four most abundant minerals in the
CE
lunar regolith and vacuum-melted glass (Wagner et al, 1978; Wells and Hapke, 1977). The vertical lines are at LROC WAC band centers.
AC
Figure 2. TiO2 abundance of Apollo and Luna samples vs reflectance I/F at 689 nm of the area from which each sample was taken. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%. The straight lines are least-square fits to points in the two regimes. The letter next to each point is the site identifier in table 1.
13
ACCEPTED MANUSCRIPT
Figure 3a. TiO2 abundance of Apollo and Luna samples vs ratio r(321/415) = [I/F(321)]/[I/F(415)] of the area from which each sample was taken (from Sato et al, 2017, updated). The four outlier points (arrows) were suppressed when the line was fitted. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%.
CR IP T
Figure 3b. Portion of figure 3a with best-fit line and error bars for the high-Ti regime only. The error bars are the pixel variances within each measurement square. No data points were excluded from this fit. The letter next to each point is the site identifier in table 1. Figure 4a. TiO2 abundance of Apollo and Luna samples vs ratio r(566/689) =
AN US
[I/F(566)]/[I/F(689)] of the area from which each sample was taken. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%.
Figure 4b. Portion of figure 4a with best-fit line and error bars for the low-Ti regime only. The
M
error brs are the pixel variances within each measurement square. No data points were excluded from this fit. The letter next to each point is the site identifier in table 1.
ED
Figure 5. Composite algorithm for TiO2 abundances in mature lunar regoliths. Open circles: samples with TiO2 < 2%; filled circles: samples with TiO2 > 2%. The two regimes are separated
PT
by the vertical line, with high-Ti points for r(321/415) on the right and low-Ti points for
AC
CE
r(566/689) on the left. The values on the left and right axes are identical.
14
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 1. Reflectance spectra of pulverized examples of the four most abundant minerals in the lunar regolith plus vacuum-melted silicate glass containing FeO and TiO2 (Data from Wagner et al, 1987 and Wells and Hapke, 1977). The vertical lines are at LROC WAC band centers.
15
PT
ED
M
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
Figure 2. TiO2 abundance of Apollo and Luna samples vs reflectance I/F at 689 nm of the area from which each sample was taken. Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%. The straight lines are least-square fits to points in the two regimes. The letter next to each point is the site identifier in table 1.
16
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 3a. TiO2 abundance of Apollo and Luna samples vs ratio r(321/415) = [I/F(321)]/[I/F(415)] of the area from which each sample was taken (from Sato et al, 2018, updated). The four outlier points (arrows) were suppressed when the line was fitted: Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%.
Figure 3b. Portion of figure 3a with a line fitted to the points in the high-Ti regime only. The error brs are the pixel variances within each measurement square. No points were excluded from this fit. The letter next to each point is the site identifier in table 1. 17
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 4a. TiO2 abundance of Apollo and Luna samples vs ratio r(566/689) = [I/F(566)]/[I/F(689)] of the area from which each sample was taken. Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%.
Figure 4b. Portion of figure 4a with line fitted to points in the low-Ti regime only. The error brs are the pixel variances within each measurement square. No points were excluded from this fit. The letter next to each point is the site identifier in table 1.
18
AN US
CR IP T
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
M
Figure 5. Composite algorithm for TiO2 abundances in mature lunar regoliths. Open circles: sites with TiO2 < 2%; filled circles: sites with TiO2 > 2%.. The vertical line separates the low-Ti and high-Ti regimes, with high-Ti points for r(321/415) on the right and low-Ti points for r(566/689) on the left. The values on the left and rightt axes are identical.
19
ACCEPTED MANUSCRIPT
Table 1. Data TiO2( wt% )
I/F( 321)
std( 321)
I/F( 415)
std( 415)
I/F( 566 )
std( 566)
I/F( 689 )
std( 689 )
A1 6S4
644 21, 645 01 633 41, 635 01
0.5
0.02 139
0.00 097
0.03 061
0.00 238
0.0 418 2
0.00 289
0.0 514 3
0.00 258
0.69 0.06 879 2899
0.81 0.06 314 9438
0.5
0.02 881
0.00 365
0.04 122
0.00 685
0.0 577
0.00 726
0.0 693 8
0.00 909
0.69 0.14 893 605
0.83 0.15 165 107
7.5
0.5
0.02 514 6 0.03 257 1
0.00 214 4 0.00 200 4
0.0 455
0.00 274
0.70 0.07 019 5155
0.80 0.06 901 9105
0.6
0.00 114 8 0.00 104 5
0.00 223
605 01, 606 01
0.01 760 7 0.02 388 3
0.00 162
0.0 544 3
0.00 191
0.73 0.05 326 536
0.82 0.04 381 1491
665 01 657 01 659 01 19. 6
0.7
154 71
E
L2 4
G
A1 5S2 A1 5S1
AC
F
H
I
A1 5S8
J
A1 4
150 71, 150 81 150 13, 150 20 140 03,
AN US
A1 6S1 0 & L M A1 6S5
std(32 1/415 )
0.0 368 1 0.0 448 4
r(566 /689 )
std(56 6/689 )
M
D
1
0.02 032 1
ED
C
A1 6S1 3 L2 0
0.00 089 2
PT
B
r(321 /415 )
CR IP T
Sa mp le ID
0.02 917 8
0.00 177 9
0.0 423
0.00 241
0.0 515
0.00 428
0.69 0.05 645 2323
0.82 0.08 136 2761
0.0 198 2 0.0 289 7 0.0 247 9
0.00 065
0.0 249 1 0.0 364
0.00 089
0.71 857
0.79 566
0.00 402
0.70 0.07 092 9087
0.79 0.12 588 393
0.00 82
0.0 311 3
0.00 164
0.70 0.07 526 378
0.79 0.04 634 95
0.01 006
0.00 034
0.01 4
0.00 091
1.2
0.01 289
0.00 09
0.01 839
0.00 163
1.6
0.01 201 7
0.00 069
0.01 703 9
0.00 149
1.7
0.01 105
0.00 03
0.01 581
0.00 087
0.0 230 7
0.00 057
0.0 292 2
0.00 083
0.69 0.04 892 2887
0.78 0.02 953 9724
1.7
0.01 299
0.00 034
0.01 844
0.00 106
0.0 270
0.00 088
0.0 336
0.00 101
0.70 0.04
0.80 0.03
CE
Lo wTi A
Sit e
20
0.00 318
0.052 64
0.038 6
ACCEPTED MANUSCRIPT
N
A1 7LR V1 1
782 21, 782 31, 784
0.00 096
3.1
0.01 101
0.00 037
0.01 526
0.00 082
3.3
0.00 903
0.00 038
0.01 189
0.00 071
4.5
0.01 182 7
0.00 045 7
0.01 596 5
0.00 143 6
0.0 236 5
0.00 451
0.0 216 8
321
5526
CR IP T
0.01 528
0.0 303 3
0.00 434
0.00 102 2
0.0 269 2
0.00 133
0.72 0.04 149 5727
0.80 0.05 535 4995
0.00 057
0.0 203 4 0.0 225 5
0.00 06
0.75 0.05 946 5481
0.80 0.03 875 6803
0.00 055
0.74 0.07 081 2522
0.81 0.03 33 1784
0.70 0.05 942 5683
AN US
L1 6
0.00 051
4494
0.77 0.18 976 59
M
M
120 01, 120 23, 120 30, 120 32, 120 33, 120 37, 120 41, 120 42, 120 44, 106 0, 120 70 16. 7
0.01 084
445
AC
CE
A1 2
1.8
9
ED
Hi gh -Ti L
A1 5S9
6
PT
K
141 41, 141 48, 142 20, 142 30, 142 40, 142 59, 142 60, 142 63 155 01
0.0 164 5 0.0 183 4
21
0.00 056
ACCEPTED MANUSCRIPT
R
S
T
U
A1 7S5
0.00 119
0.0 204 6
0.00 09
0.0 249 7
0.00 102
6.1
0.01 107
0.00 039
0.01 472
0.00 119
0.0 191 3
0.00 061
0.0 236 2
0.00 068
0.75 0.06 204 6319
0.80 0.03 991 4794
751 11, 751 21 100 02, 100 10, 100 84 721 31
6.7
0.01 042
0.00 03
0.01 375
0.00 105
0.0 183 7
0.00 059
0.0 226 6
0.00 064
0.75 0.06 782 1846
0.81 0.03 068 4672
7.5
0.01 037
0.00 04
0.01 356
0.00 083
8
0.01 043
0.00 042 3
0.01 368
0.00 134
700 11, 701 81 711 31, 715 01 750 61, 750 81
8.5
0.01 076
9.6
0.01 153 6
9.9
0.01 059 4
0.81 0.04 938 9188
CR IP T
0.74 0.06 821 7945
0.0 178 7
0.00 061
0.0 217 2
0.00 082
0.76 0.05 475 5329
0.82 0.04 274 1875
0.0 215 7
0.00 09
0.0 264 7
0.00 103
0.76 0.07 243 6551
0.81 0.04 488 6492
0.00 039
0.01 4
0.00 157
0.0 195 3
0.00 079
0.0 238 1
0.00 091
0.76 0.09 857 058
0.82 0.04 024 5647
0.00 038
0.01 459 6
0.00 136
0.0 191 1
0.00 227
0.0 233 2
0.00 301
0.79 0.07 035 8109
0.81 0.14 947 375
0.00 054
0.01 384 8
0.00 209
0.0 186 5
0.00 263
0.0 229 1
0.00 33
0.76 0.12 502 187
0.81 0.16 406 41
AC
V
A1 7LR V1 1 A1 7L M A1 7S1
0.01 466 3
AN US
Q
0.00 044 7
M
A1 7LR V9 A1 7LR V8 A1 1
0.01 097 1
ED
P
5.5
PT
A1 7LR V3
CE
O
41, 784 61, 781 21 721 50, 721 51, 721 61 761 21
22