The Galileo spacecraft imaged parts of the western limb and far side o
f the Moon in December 1990. Ratios of 0.41/0.56 mum filter images fro
m the Solid State Imaging (SSI) experiment provided information on the
titanium content of mare deposits; ratios of the 0.76/0.99 mum images
indicated 1 mum absorptions associated with Fe2+ in mafic minerals. M
are ages were derived from crater statistics obtained from Lunar Orbit
er images. Results on mare compositions in western Oceanus Procellarum
and the Humorum basin are consistent with previous Earth-based observ
ations, thus providing confidence in the use of Galileo data to extrac
t compositional information. Mare units in the Grimaldi and Riccioli b
asins range in age from 3.25 to 3.48 Ga and consist of medium- to medi
um-high titanium (<4 to 7% TiO2) content lavas. The Schiller-Zucchius
basin shows a higher 0.76/0.99 mum ratio than the surrounding highland
s, indicating a potentially higher mafic mineral content consistent wi
th previous interpretations that the area includes mare deposits blank
eted by highland ejecta and light plains materials. The oldest mare ma
terials in the Orientale basin occur in south-central Mare Orientale a
nd are 3.7 Ga old; youngest mare materials are in Lacus Autumni and ar
e 2.85 Ga old; these units are medium- to medium-high titanium (<4 to
7% TiO2) basalts. Thus, volcanism was active in Orientale for 0.85 Ga,
but lavas were relatively constant in composition. Galileo data sugge
st that Mendel-Rydberg mare is similar to Mare Orientale; cryptomare a
re present as well. Thus, the mare lavas on the western limb and far s
ide (to 178-degrees-E) are remarkably uniform in composition, being ge
nerally of medium- to medium-high titanium content and having relative
ly low 0.76/0.99 mum ratios. This region of the Moon is between two po
stulated large impact structures, the Procellarum and the South Pole-A
itken basins, and may have a relatively thick crust. In areas underlai
n by an inferred thinner crust, i.e., zones within large basins (as at
Apollo), titanium content is often higher. However, no mare deposits
with titanium abundances approaching those of the high-titanium (9 to
14% TiO2) Apollo 11 and 17 basalts nor of the high-titanium regions of
central Oceanus Procellarum are seen on the western limb or eastern f
ar side. Light plains deposits are generally indistinct from the surro
unding highlands in the SSI data and are inferred to be derived primar
ily from the same material that forms the highlands. Some of the light
plains are too young to be related to basin-forming impacts, suggesti
ng possible volcanic origin. Dark mantle deposit compositions derived
from SSI data are consistent with Earth-based observations of similar
near-side deposits and are interpreted to be pyroclastic materials. Ho
wever, the moderate albedo and 1 mum absorption of the dark mantle dep
osit on the southwest margin of the Orientale basin suggest it is a lo
cal pyroclastic deposit contaminated with underlying highland material
s from the Orientale impact.