This image shows a part of the floor of a large, heavily modified crater in the Nilosyrtis Mensae region. The cliffs in the northern and southern parts of this image are walls of an irregular erosional pit and expose some of the infilling material. The crater itself was mostly filled in and then eroded.

An interesting aspect of the rocks at the edge of the pit is the widespread fracturing. The rocks of the upper layer are broken up along cracks called joints. On Earth, these commonly form when rocks are subjected to stresses, due to factors like cooling or the removal of weight due to erosion.

The upper layer forms the cliffs because it is relatively hard to erode, and protects the underlying material. This commonly happens when a lava flow covers weaker sedimentary rocks, but the joints at this site suggest that this resistant layer might be sedimentary. The rock appears to be breaking into broad, thin slabs; lava flows often fracture into columns or relatively uniform blocks as they cool.

For information about NASA and agency programs on the Web, visit: http://www.nasa.gov. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems is the prime contractor for the project and built the spacecraft. The HiRISE camera was built by Ball Aerospace and Technology Corporation and is operated by the University of Arizona. The image data were processed using the U.S. Geological Survey’s ISIS3 software.