Jw. Head et al., LUNAR IMPACT BASINS - NEW DATA FOR THE WESTERN LIMB AND FAR SIDE (ORIENTALE AND SOUTH-POLE AITKEN-BASINS) FROM THE 1ST GALILEO FLYBY, J GEO R-PLA, 98(E9), 1993, pp. 17149-17181
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.