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

The lunar soil characterization consortium, a group of lunar-sample and rem ote-sensing scientists, has undertaken the extensive task of characterizati on of the finest fractions of lunar soils, with respect to their mineralogi cal and chemical makeup. These compositional data form the basis for integr ation and modeling with the reflectance spectra of these same soil fraction s. 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 un derstanding of the complexities involved in the formation of lunar soil. Several significant findings have been documented in the study of the <45 < mu>m size fractions of selected Apollo 17 mare soils. As grain size decreas es, the abundance of agglutinitic glass increases, as does the plagioclase, whereas the other minerals decrease. The composition of the agglutinitic g lass 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 compo sition of each size fraction continuously changes, becoming more Al-rich an d Fe-poor, and approaches the composition of the agglutinitic glasses. Betw een the smallest grain sizes ( 10-20 and <10 <mu>m), the I-S/FeO values (am ount of total iron present as nanophase Fe-0) 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 nanoph ase Fe-0 to the I-S/FeO values, particularly in the <10 <mu>m size fraction . The surface nanophase Fe-0 is present largely as vapor-deposited patinas on the surfaces of almost every particle of the mature soils, and to a less er degree for the immature soils (Keller ef nl., 1999a). It is reasoned tha t the vapor-deposited patinas may have far greater effects upon reflectance spectra of mare soils than the agglutinitic Fe-0.