LUNAR IMPACT BASINS - NEW DATA FOR THE WESTERN LIMB AND FAR SIDE (ORIENTALE AND SOUTH-POLE AITKEN-BASINS) FROM THE 1ST GALILEO FLYBY

Compositional aspects of impact basin materials can be analyzed using multispectral image data acquired by the Galileo solid state imaging ( SSI) experiment during the December 1990 lunar encounter. These data p rovide important information on the spectral properties of the western lunar limb and parts of the far side. The SSI images cover the wavele ngth range 0.4-1.0 mum, allowing measurement of spectral slope and est imation of the strength of the 1 mum absorption due to iron in the maf ic minerals olivine and pyroxene. Among deposits of the 930-km-diamete r Orientale basin, exterior ejecta comprising the Hevelius Formation i s relatively homogeneous and spectrally similar to mature Apollo 16 so ils, suggesting an upper crustal source. The centrally located Maunder Formation is distinct from the younger mare basalts but comparable to the Hevelius Formation in its spectral reflectance properties, suppor ting an interpretation as basin impact melt. The Montes Rook Formation , located in an annulus between the Maunder and the Hevelius, shows a slightly stronger mafic absorption and may be the deepest crustal mate rial excavated. The distal Orientale deposits show local mafic enhance ments (in the Schiller-Schickard and Mendel-Rydberg regions) interpret ed to represent pre-Orientale mare deposits, or cryptomaria, intermixe d with overlying basin ejecta. In this case, maria of sizes comparable to those presently observed were widespread in this region before the Orientale impact. Mixing-model analyses are consistent with the balli stic erosion and sedimentation model for ejecta emplacement in the dis tal regions beyond the continuous ejecta deposit. On the southern luna r farside, a huge area with an enhanced mafic absorption corresponds t o the interior and rim of the pre-Nectarian South Pole-Aitken impact b asin, 2000-2500 km in diameter. The anomaly is interpreted to be due t o several factors, including excavation into the more mafic lower crus t, and the presence of extensive early volcanic fill (cryptomare), sim ilar to that seen in ancient basins such as Smythii and Australe. Thes e results show that although basin-forming events are an important fac tor in producing lateral heterogeneities in crustal composition, and i n modifying preexisting deposits (such as cryptomaria), the majority o f material in even the largest basins was excavated from crustal level s. Our results suggest a gradational vertical crustal stratigraphy con sisting of an uppermost mixed crustal layer of anorthosite, basin ejec ta, and cryptomaria deposits (generally corresponding to the megaregol ith), an upper crustal layer of anorthosite, and a lower more noritic layer. Many of the basic questions remaining from this study could be addressed by global high-resolution geochemical and mineralogical data obtained by polar orbiting spacecraft.