Rabe Crater, Noachis
This Viking 3-D view of Mars features
Rabe, a 110-kilometer-wide impact crater in the cratered highlands
west of the Hellas impact basin. Most of the craters in this
appear to be very degraded. Raised rims, terraces, central peaks,
and other morphologic features and structures that characterize
relatively fresh craters are virtually absent or highly muted.
Only one crater, at the bottom of the scene,
looks fresh. The process of degrading the topography of features
on Mars is termed "terrain softening."
Terrain softening may be due to the slow but steady downslope
movement of soil, a process called creep. This process may indicate
the presence of ground ice on Mars
because ice in the soil would enhance creep. In this and other
areas of the highlands, dark spots are found in the floors of
craters. At higher resolution, these spots are
resolved into fields of dunes. The dunes may be the remnants
of crater-filling deposits or accumulations of windblown sand
that formed in the topographic trap at the
bottom of the crater.
Imaging from the Viking 1 Orbiter and Lunar and Planetary Institute