This is an oblique image of the lunar farside. It shows the impact basin
that holds Mare Moscoviense. Like Mare Marginis, this mare appears to be
fairly thin. However, it is clearly centered within a large impact basin.
It is also much lower than either the outer basin floor or the farside
highlands. The great depth of this mare beneath the nearby highlands
probably explains why mare units are so rare on the lunar farside. Very
few basins on the farside were deep enough to allow mare volcanism. Such a
contrast in mare and highland elevations also exists on the nearside.
Still, it is much smaller than that found on the farside. This may be
because the Moon's crust is much thinner on the nearside. Thus, while
large impact basins are found on both the nearside and farside, large maria
are mostly found on the nearside. Mare lavas apparently could reach the
surface more often and more easily there. (Lunar Orbiter image IV-103-M)