HR: 0800h
AN: P51B-0493    [Abstracts]
TI: Grain Size Effects (5-63μm) of the UV-VIS-NIR Spectra of Ilmenite-Bearing Assemblages: Implications for Mapping TiO₂ in the Lunar Maria
AU: * Riner, M A
EM: riner@asu.edu
AF: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, United States
AU: Lucey, P G
EM: lucey@higp.hawaii.edu
AF: Hawaii Institute of Geophysics and Planetology, University of Hawai'i, Manoa, Honolulu, HI 96822, United States
AU: Gillis-Davis, J J
EM: gillis@higp.hawaii.edu
AF: Hawaii Institute of Geophysics and Planetology, University of Hawai'i, Manoa, Honolulu, HI 96822, United States
AB: 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.
DE: 5410 Composition (1060, 3672)
DE: 5455 Origin and evolution
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 6094 Instruments and techniques
DE: 6250 Moon (1221)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting