Icy Impact (CTX Frame - Natural Colors; credits: Lunexit)

A thick (approx. 3 Km or a little less than 2 miles) sequence of ice and dust, stacked like a layer cake, covers the South Pole of Mars. Impact craters that form here experience slightly different processes of modification and degradation than those that form in rocky areas.

One example of such a process is sublimation. Most of the material that makes up an icy crater is volatile, meaning it can melt or sublimate (change directly from a solid to a gas) if heated. Sublimation is more common on Mars because of its thin and dry Atmosphere.
Crater walls facing the Sun receive more direct light than their surroundings, and are therefore more easily warmed. If the ice in the walls sublimates, the rim structure of the crater becomes degraded.