CRUSTAL DIVERSITY OF THE MOON - COMPOSITIONAL ANALYSES OF GALILEO SOLID-STATE IMAGING DATA

The multispectral images of the lunar limb and farside obtained by the solid state imaging (SSI) system on board the Galileo spacecraft provide the first new pulse of compositional data of the Moon by a spacecraft in well over a decade. The wavelength range covered by SSI filters (0.4-1.0 mum) is particularly sensitive to the composition of mare basalts, the abundance of mafic (ferrous) minerals, and the maturity of the regolith. To a first order, the limb and farside material is consistent with previous characterization of nearside lunar spectral types for mare and highland soils and craters. Most basalts are of an intermediate TiO2 composition and most of the highland crust is feldspathic with local variations in mafic content identified principally at impact craters. Dark mantling material on the farside can be interpreted in terms of known properties of lunar pyroclastic glass. Regions of cryptomare are shown to have spectral properties intermediate between those of highland and mare soils, as would be expected from a mixture of the two. There are several important exceptions and surprises, however. Unlike the basalt types identified on the nearside, limb and farside basalts exhibit an exceptionally weak 1 mum ferrous absorption band. This may indicate a compositionally distinct lunar basalt group that, for example, is more Mg-rich than most basalts of the nearside. Some of the most notable compositional anomalies are associated with South Pole-Aitken Basin. This large region has a much lower albedo than surrounding highlands. The inner, darkest, portion of the basin exhibits optical properties indistinguishable from low-Ti basalts. Deposits to the south exhibit unique properties with a strong and broad ferrous 1 mum absorption, most consistent with abundant olivine. The unusual compositions associated with South Pole-Aitken and their spatial extent suggests the impact creating this huge lunar basin excavated mafic-rich lower crust or perhaps mantle material.