The mapping of ilmenite on the surface of the moon is a necessary precursor to the investigation of prospective lunar base sites. Telescopic observations of the moon using a variety of narrow bandpass optical interference filters are being performed as a preliminary means of achieving this goal. Specifically, ratios of images obtained using filters centered at 0.40 and 0.56 microns provide quantitative estimates of TiO2 abundances. Analysis of preliminary distribution maps of TiO2 concentrations allows identification of specific high-Ti areas. Investigations of these areas using slit spectra in the range 0.03 to 0.85 microns are underway to search for discrete spectral signatures attributable to ilmenite

Johnson, J. R.

Larson, S. M.

Singer, Robert B.

English

NASA Technical Reports Server

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Determination of Lunar Ilmenite Abundances
From Remotely Sensed Data p ,/_
J. R. Johnson, S. M. Larson, and R. B. Singer
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Department of Geosciences/Lunar and Planetary Laboratory 75
The University of Arizona #_ _',_.,,._7'
Abstract
The mapping of ilmenite on the surface of the Moon is a necessary precursor to
the investigation of prospective lunar base sites. Telescopic observations of the Moon
using a variety of narrow bandpass optical interference filters are being performed as a
preliminary means of achieving this goal. Specifically, ratios of images obtained using
filters centered at 0.40 #m and 0.56 #m provide quantitative estimates of TiO 2
abundances. Analysis of preliminary distribution maps of TiO 2 concentrations allows
identification of specific high-Ti areas. Investigations of these areas using slit spectra
in the range 0.30-0.85 #m are underway to search for discrete spectral signatures
attributable to ilmenite.
Introduction and Background
An important criterion during selection of lunar base sites will be the availability of
ilmenite (FeTiO3) for use as a local resource. The distribution of ilmenite across the
surface of the Moon must therefore be well known. Earth-based telescopic remote
sensing of the lunar surface in combination with laboratory spectral reflectance
measurements of returned lunar samples has proven to be a valuable tool, both in
discerning relative differences between lunar material and in establishing more
quantitative estimates of surface compositions (e.g., Head et al. 1978).
Lunar ilmenites most often contain greater tha,z 50 wt% titanium dioxide (TiO2),
making ilmenite the dominant titanium-bearing mineral On the surface of the Moon.
Surface distribution maps of TiO 2 abundances are consequently valuable in determining
regions of probable high-ilmenite concentrations.
Charette et al. (1974) measured TiO 2 contents of sampled bulk lunar soils and
compared their spectral reflectivities in the laboratory. An empirical relation between
weight percent TiO 2 and the reflectance slope between 0.40 /zm and 0.56 #m was
established (Figure 3.1). The slope of the lunar reflectance spectrum in this region is
primarily affected by the absorptions due to Fe and Ti in the lunar soil, i.e., the
agglutinates and glasses. The relation is therefore more accurate for mature soils
(those that have been extensively reworked by micrometeorite impacts), particularly in
the lunar maria. Charette et al. also found the relation to hold for telescopic spectra of
lunar landing sites (Figure 3.2).
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Figure 3.1 Plot of TiO 2 percentage of the bulk lunar soils as a function of 0.40 #m to
0.56 #m slope. (From Charette et al. 1974.)
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Figure 3.2 Plot of TiO 2 percentage versus 0.40 #m to 0.56 #m slope for telescopic
spectra. The TiO 2 contents of telescopic areas are averages of sampled
soils used in Figure 3.1. (From Charette et al. 1974.)
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Johnson etal. (1977) used a silicon vidicon imaging system to obtain multispectral
images of the Moon through narrow bandpass interference filters centered at 0.38 #m
and 0.56 #m. An abundance map of TiO 2 for the northern maria was produced by
converting the 0.38/0.56 #m ratio (normalized to a standard area in Mare Serenitatis
named MS-2) to weight percent TiO 2 (Figure 3.3).
Figure 3.4 shows the most recent version of the relation between the 0.40/0.56 #m
ratio and weight percent TiO 2 (Pieters 1978). At values of TiO 2 less than about 4 wt%,
the absorbing effects of the Ti-Fe opaque phases can be obscured by the effects of
other contaminants such as Fe-metal, plagioclase, and non-opaque (homogeneous) glass
compositions in the soil. Thus, a ratio value of 1.00 may correspond to a range of
0-2.5 wt% TiO 2.
Multispectral Image Acquisition and Results
Previous efforts to obtain multispectral images of the Moon used television-type
silicon vidicon tubes (e.g., McCord etal. 1976, 1979). These images possessed spatial
resolutions of about 2 km per picture element (pixel) but covered relatively small areas
on the lunar surface. Mosaicking of photographs taken of the individual vidicon images
was necessary to achieve greater areal perspective. Contrast differences between the
mosaic frames suffer from some uncertainties, which discourage comparison of
apparently similar gray-level intensities between frames not contiguous.
Newer imaging technologies use charge-coupled device (CCD) systems that
provide better stability and higher photometric precision. For our project, an RCA 320
x 512-pixel CCD chip has been used at The University of Arizona Tumamoc Hill Station
O.5-m telescope. In October 1989, multispectral images of the Moon were obtained at
the f/4 Newtonian focus using a variety of narrowband interference filters (Table 3.1).
In addition to the 0.40 p.m and 0.56 #m filters, ultraviolet filters were used to discern
reflectance differences in that interesting but seldom-imaged portion of the lunar
spectrum. The near-infrared filters can be used to investigate additional surface
properties; for example, the 0.95/0.56 #m ratio can be used as a relative indicator of
surface maturity (McCord et al. 1976).
Five images are necessary to cover the full Moon (Figure 3.5). The spatial
resolution of the images is about 5.3 km per pixel on the Moon. Figure 3.6 shows four
of the image ratios obtained by dividing the 0.40 #m images by the 0.56 #m images in
which the brighter areas correspond to higher ratio values. Full-size versions of the
0.40 #m image and 0.40/0.56 #m ratio for the eastern section of the Moon are shown in
Figures 3.7 and 3.8. Using an average of the plot in Figure 3.4 (dotted line) to calibrate
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Figure 3.3
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0.40/0.56um i_,XTlO qRELATIVE TO MS-21
Weight percent TiO 2 versus 0.40/0.56 #m ratio (relative to MS-2).
Pieters and McCord 1976.)
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0.92 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10
TELESCOPE 0.40/0.56 _,m (RELATIVE TO MS-2)
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Figure 3.4 Relationship between weight percent TiO 2 in lunar mare soils and the the
0.40/0.56 p,m ratio for telescopic spectra relative to MS-2. The stippled
area is the estimated range of TiO 2 that can be derived from a 0.40/0.56
p,m ratio measurement of mature mare regions. Dotted curve approximates
the curve used for calibration of ratio values to wt% TiO 2 in present study.
(After Pieters, 1978.)
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Figure 3.5 images of the Moon obtained with bandpass interference filter centered at
0.40 p.m. (Reduced-size postscript laser printer copies.)
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Figure 3•7 Image of eastern section of Moon taken through 0.40 #m filter• (Full-sized
postscript laser printer copy).
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Figure 3.8
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Preliminary 0.40/0.56 #m ratio image of the eastern section of the moon.
(Full-sized postscript laser printer copy.)
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Table 3.1 Bandpass interference filters.
Center Wavelength (nm) ............... Bar_dwidt5 [FWHM] (A)
309 75
319 102
338 106
365 80
389 48
400 95
427 80
560 95
729 128
902 289
948 184
images, a TiO 2 wt% abundance map can be constructed (Figure 3.9). The value for
MS-2 was taken from the eastern raticed image section in Figure 3.8.
Variability in observed TiO 2 concentrations in some overlapping areas between
adjacent image ratio sections rarely exceeds 2 wt% but is nevertheless problematic.
Investigations are underway to calibrate the MS-2 standard to areas common to the
overlapping portions of individual images in order to provide a standard ("tied" to MS-
2) for those images in which MS-2 is not in view.
Spectroscopic Studies
Those areas highest in TiO 2 content (as determined from analysis of the
preliminary maps of TiO 2 concentration) are being studied spectroscopically as well.
Initial attempts were made in November 1989 (again at Tumamoc Hill) to obtain slit
spectra for several regions of interest using the CCD spectrograph/camera designed by
one of us (S.M.L.). It includes two blue'blazed gratingsl one giving a spectral range
from 0.30 #m to about 0.88 #m w!th 1! A per Pixel and=_the other_ providing spectra
from 0.30 p.m to about 0.56 #m at 5 A per pixel. The 6 × 0.2 mm slit used
corresponded to 130.0 x 4.3 km on the Moon. In December 1989, further spectra were
obtained using a 20 x 0.25 mm slit with the spectrograph, corresponding to 442.3 x 5.5
km on the Moon. For the December run, a new reflecting slit allowed the image of the
spectrographic slit on the Moon to be simultaneously videotaped for all lunar spectra
so that the precise region being analyzed could be recorded accurately.
For both the November and December observations, each spectral observation was
followed immediately by acquisition of spectra for the MS-2 area. In this way, relative
reflectance spectra (ratioed to MS-2) free from instrumental and atmospheric effects
were collected.
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Figure 3.9 TiO 2 abundance map of the eastern section of the Moon based on
0.40/0.56 #m ratio image. Dark gray areas correspond to <5 wt %. Light
gray areas correspond to 5-8 wt%. Brightest areas correspond to >8 wt%.
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Figure 3.10 shows relative reflectance plots obtained in December with the lower
resolution grating for four lunar areas. The spectra have been normalized to unity at
5600 A. The Tranquillitatis and Iridium mare regions show a characteristic upturn in
slope toward the near-UV and a noticeable upturn in the near-IR (Iongward of about
7800 A). From preliminary maps, Tranquillitatis was shown to have larger TiO 2
"concentrations than Iridium. The relative reflectance spectra are consistent with this
comparison, as the slope of the spectrum for Tranquillitatis is steeper than that for
Iridium, i.e., Tranquillitatis has a greater 0.40/0.56 #m ratio and a corresponding higher
TiO 2 value (Figure 3.4). For comparison with mature mare areas, the craters Tycho
(located in the southern highlands) and Aristarchus (located in northern Oceanus
Procellarum) show a more constant relative reflectance in the near-UV and a steady
decrease toward the near-IR. These two spectra also exhibit an abrupt downturn in
relative reflectance at about 4800 A
The relative reflectance spectra can be more readily compared to laboratory
spectral reflectance measurements of lunar soils by calibrating them to absolute
reflectance. In order to accomplish this, further spectra were obtained at The
University of Arizona Catalina Observatory t.54-m telescope in January 1990. Spectra
were recorded for a solar analog star (i.e., one with spectral characteristics
indistinguishable from the Sun) and then ratioed to spectra of the MS-2 region for the
same air mass in order to provide an absolute spectrum for MS-2. This can be applied
to the December relative reflectance ratios in order to provide absolute reflectance
spectra for specific lunar areas. Reduction of the spectra to this form is presently
underway.
Conclusions
The multispectral image data set taken in October 1989 is of sufficient quality to
be used for analysis of both TiO 2 variations in the lunar maria and relative maturity
differences of the lunar surface. Initial results from the October images show most of
western Mare Tranquillitatis to be a region of high TiO 2 concentrations (>8 wt%) (Figure
3.10); this region is being investigated more fully with high-resolution spectra. Other
smaller areas that show high concentrations, such as the "dark spots" east of the crater
Copernicus and in the region of Sinus Aestuum (south of Mare Imbrium), are also under
spectral scrutiny.
The 0.95/0.56 /_m maturity index may be of interest to other SERC investigators
when used in conjunction with the TiO 2 abundance map. M. Hutson, J. Ruiz, and J.
Lewis conclude that high-Ti soils with low agglutinate content (immature soils) are best
for processing and separation of ilmenite grains. T. Swindle and C. Glass note that the
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highest Ti soils are apparently the best sites for 3He implantation. Since 3He will be
more abundant in soils that have been long exposed to the solar wind, they prefer
high-Ti soils with abundant agglutinates (mature soils).
Final products of this project will be TiO 2 abundance maps of the entire lunar
near-side maria. Other ratio images [including the relative maturity index (0.95/0.56
#m)] may be of enough interest in comparison with each other to warrant additional
analysis. Qualitative analysis of relative and/or spectral reflectance measurements of
several areas will provide further details regarding the mineralogical and chemical
makeup of high-Ti regions. The Apollo 15 and 16 orbital gamma-ray data set for
titanium is also being used as an additional resource. Comparisons of the two
methods' estimates of Ti concentrations will give further insight to the precision of the
spectral studies.
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References
Charette, M. P., McCord, T. B., Pieters, C., and Adams, J. B., 1974, "Application of
Remote Spectral Reflectance Measurements to Lunar Geology Classification and
Determination of Titanium Content of Lunar Soils," J. Geophys Res., Vol. 79, p.
1605.
Head, J. W., Pieters, C., McCord, T., Adams, J., and Zisk, S., 1978, "Definition and
Detailed Characterization of Lunar Surface Units Using Remote Observations,"
Icarus, Vol. 33, pp. 145-172.
Johnson, T. V., Saunders, R. S., Matson, D. L., and Mosher, J. A., 1977, "A TiO 2
Abundance Map for the Northern Maria," Proc. Lunar Sci. Conf. 8th, pp.
1029-1036.
McCord, T. B., Pieters, C., and Feierberg, M. A., 1976, "Multispectral Mapping of the
Lunar Surface Using Groundbased Telescopes," icarus, VoI. 29, pp. 1-34.
McCord, T. B., Grabow, M., Feierberg, M., Maclaskey, O, and Pieters, C., 1979, "Lunar
Multispectral Maps: Part [I of the Lunar Nearside," icarus, Vol. 37, pp. 1-28.
Pieters, C. and McCord, T. B., 1976, "Characterization of Lunar Mare Basalt-Types: Luna
24 Landing Areas as Derived From Remote Observations," Geophys. Res. Lett.,
Vol. 3, pp. 697-700.
Pieters, C., 1978, "Mare Basalt Types on the Front Side of the Moon: A Summary of
Spectral Reflectance Data," Proc. Lunar Planet. Sci. Con/. £th, pp. 2825-2849.


Determination of lunar ilmentite abundances from remotely sensed data

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