Martian landscapes - The Big Picture - Boston.com
A 4 km diameter feature near the edge of the south polar residual cap. The bright areas in this image are covered by carbon dioxide frost, and the “swiss cheese” terrain typical of the south polar residual cap covers much of the imaged area. The dark walls of the circular depression do not have as much frost on them, and are fractured in a polygonal pattern. Apparently the surface of the walls has been extensively modified by thermal expansion and contraction of water ice. It also appears that the “swiss cheese” terrain of the residual cap has buried the floor of the circular depression, as well as the terrain surrounding the feature, making it difficult to infer the origin of this depression.