Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p51b0493r&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P51B-0493
Physics
5410 Composition (1060, 3672), 5455 Origin And Evolution, 5480 Volcanism (6063, 8148, 8450), 6094 Instruments And Techniques, 6250 Moon (1221)
Scientific paper
Fine-grained particles (≤60μm) dominate the particle size distributions of many planetary surfaces. Systematic studies of particle size effects within the fine fraction have typically concentrated on transparent silicate minerals [1,2]. We have acquired a synthetic powdered ilmenite sample from Sigma-Alrich and verified the composition via X-ray diffraction and Mössbauer spectroscopy. The sample was separated into six size fractions between 5 and 63μm using ultrasonic sieving techniques (Gilson Company). Reflectance spectra, taken at RELAB, between 0.3 and 2.5μm show systematic changes in spectral contrast with particle size.
A long-standing goal of the lunar science community has been the remote mapping of TiO2 abundance on the lunar surface to enable detailed studies of lunar crustal composition, surface volcanism and temporal and spatial variations in mantle composition. Returned lunar regolith samples have large variations in TiO2 abundance (0-10wt%), dominantly in the opaque oxide mineral ilmenite (FeTiO3). There is a correlation between TiO2 abundance and the ratio of ultraviolet (UV) to visible (VIS) reflectance of lunar soils but the accuracy and universal applicability of the correlation persists. Gillis-Davis and coworkers [3], demonstrated the effect of ilmenite grain size on the TiO2-UVVIS color correlation of modeled mixtures of ilmenite with low-Ti lunar mare soil for variations in ilmenite grain size from the rest of the lunar soil. They found that reducing the modeled grain size of ilmenite increased its cross sectional area and thus its spectral effectiveness. Utilizing the new ilmenite size separates we present the single scattering albedo as a function of particle size (5-63μm) in the wavelength range 0.3-2.5μm. These data can be used to improve modeled mixtures of ilmenite as a function of grain size. We show that spectral variations in ilmenite with grain size introduces scatter into the TiO2-UVVIS color correlation even if the grain size of the ilmenite is the same as the lunar soil in which it is contained. We also show that the spectral changes in ilmenite due to grain sizes will temper the magnitude of the grain size effects demonstrated by Gillis-Davis et al. [3] and makes it less likely that the UVVIS-TiO2 correlation can be explained by ilmenite alone. Grain size effects cannot explain the full range of TiO2 variability but do introduce significant scatter in UVVIS - TiO2 correlation. These findings emphasize the need to understand secondary controls on the UVVIS color of lunar soils in order to yield accurate TiO2 estimates. [1] Mustard and Hayes, Icarus, 1997; [2] Adams and Felice, JGR, 1967; [3] Gillis-Davis et al., GCA, 2006.
Gillis-Davis Jeffery J.
Lucey Paul G.
Riner Miriam A.
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