Lunar Mare Soils: Space weathering and the major effects of surface‐correlated nanophase Fe

Abstract

Lunar soils form the “ground truth” for calibration and modeling of reflectance spectra for quantitative remote sensing. The Lunar Soil Characterization Consortium, a group of lunar sample and remote sensing scientists, has undertaken the extensive task of characterization of lunar soils, with respect to their mineralogical and chemical makeup. This endeavor is aimed at deciphering the effects of space weathering of soils from the Moon, and these results should apply to other airless bodies. Modal abundances and chemistries of minerals and glasses in the <45 μm size fractions of nine selected mare soils have been determined, along with the bulk chemistry of each size fraction, and their I S/FeO values. These data can be addressed at http:/web.utk.edu/∼pgi/data.html. As grain size decreases, the bulk composition of each size fraction continuously changes and approaches the composition of the agglutinitic glasses. Past dogma had it that the majority of the nanophase Fe 0 resides in the agglutinitic glasses. However, as grain size of a soil decreases, the percentage of the total iron present as nanophase‐sized Fe 0 increases dramatically, while the agglutinitic glass content rises only slightly. This is evidence for a large contribution to the I S/FeO values from surface‐correlated nanophase Fe 0, particularly in the <10 μm size fraction. This surficial nanophase Fe 0 is present largely as vapor‐deposited patinas on the surfaces of almost every particle of the mature soils. It is proposed that these vapor‐deposited, nanophase Fe 0‐bearing patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe 0.