This image shows the degraded rim of a large impact crater which is partly filled with material that is fractured in several places, especially near the crater rim.
Pits of various sizes have formed along most of the fractures, and some of the pits have merged into elongated depressions. The association of the pits with the fractures suggests that the pits were formed by removal of material deep within the fractures. A plausible reason for the formation of these features is that the crater floor material includes substantial ground ice. When the fractures open, perhaps due to glacial-like flow of the ice-rich floor material toward the center of the crater, water ice is exposed to the Martian atmosphere.
The ice then evaporates into the dry atmosphere, enlarging the fracture at depth. As the icy “cave” grows, ice-free material near the surface collapses into the cave, forming the pits we see here.
Written by: Ken Herkenhoff (5 May 2010)
This is a stereo pair with ESP_018092_2255 .
More info and image formats at http://hirise.lpl.arizona.edu/ESP_017103_2255
Image: NASA/JPL/University of Arizona