The effects of space weathering on Apollo 17 mare soils: Petrographie and chemical characterization



Abstract— The lunar soil characterization consortium, a group of lunar-sample and remote-sensing scientists, has undertaken the extensive task of characterization of the finest fractions of lunar soils, with respect to their mineralogical and chemical makeup. These compositional data form the basis for integration and modeling with the reflectance spectra of these same soil fractions. This endeavor is aimed at deciphering the effects of space weathering of soils on airless bodies with quantification of the links between remotely sensed reflectance spectra and composition. A beneficial byproduct is an understanding of the complexities involved in the formation of lunar soil.

Several significant findings have been documented in the study of the <45 μm size fractions of selected Apollo 17 mare soils. As grain size decreases, the abundance of agglutinitic glass increases, as does the plagioclase, whereas the other minerals decrease. The composition of the agglutinitic glass is relatively constant for all size fractions, being more feldspathic than any of the bulk compositions; notably, TiO2 is substantially depleted in the agglutinitic glass. However, as grain size decreases, the bulk composition of each size fraction continuously changes, becoming more Al-rich and Fe-poor, and approaches the composition of the agglutinitic glasses. Between the smallest grain sizes (10–20 and < 10 μm), the IS/FeO values (amount of total iron present as nanophase Fe0) increase by greater than 100% (>2x), whereas the abundance of agglutinitic glass increases by only 10–15%. This is evidence for a large contribution from surface-correlated nanophase Fe0 to the IS/FeO values, particularly in the <10 μm size fraction. The surface nanophase Fe0 is present largely as vapor-deposited patinas on the surfaces of almost every particle of the mature soils, and to a lesser degree for the immature soils (Keller et al., 1999a). It is reasoned that the vapor-deposited patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe0.