Lunar surface aluminum and iron concentration from Galileo solid state imaging data, and the mixing of mare and highland materials

Abstract

Apollo X ray spectrometer data provide chemical information for 9% of the lunar surface. Galileo solid state imaging system (SSI) multispectral data for the Moon are employed to reexamine the long‐accepted positive correlation between lunar surface reflectance (or albedo) and aluminum concentration, derived from Apollo X ray spectrometer data. The overall goal of the analysis is to quantify the relationship between reflectance and aluminum, and to take advantage of the extensive spatial coverage of the SSI data (∼75% of the lunar surface) to calculate aluminum concentration from SSI reflectance for the majority of the lunar surface. After removing nonmature highlands from the analysis, it is found that the relationship between lunar surface reflectance and X ray spectrometer‐derived aluminum concentration is described by two diffuse endmembers, representing highland and mare materials, and an apparent mixing line suggestive of mixtures of mare and highland materials. Regression analysis is utilized to show that whereas the correlation between reflectance and aluminum for the entire lunar soil system is fairly good, the correlations for mare soils alone and for highland soils alone are extremely low. The low precision of the X‐ray data may at least in part be responsible for the observed poor correlations. Although the low correlation for the individual soil types precludes the precise calculation of aluminum concentration from reflectance, approximate aluminum contents can be determined. The excellent inverse correlation between aluminum and iron concentration for returned lunar soils allows an estimation of iron content to be made as well. An extensive zone of mixtures of mare and highland soils exists in the vicinity of mare‐filled impact basins, and around smaller craters. This zone occurs in morphologically defined mare and highland units. Physical mixing of more mafic and less mafic material due to vertical and lateral transport by impact and downslope movement can account for the widespread mixing zones.