This observation shows an area in the northern mid-latitudes of Mars marked by large depressions in the mantle.

The large depressions have scalloped edges and appear to have formed from multiple, smaller scalloped depressions that have coalesced together. The color in this image (see subimage) clearly reveals several layers present within the depressions.

Scalloped pits such as these are typical features of the mid-latitude mantle and are most commonly found at approximately 55 degrees north and south latitude. The presence of scalloped pits has led to hypotheses of the removal of subsurface material, possibly interstitial ice, by sublimation (the process of a material going directly from a solid state to a gaseous state).

Typically, scalloped pits have a steep pole-facing scarp consisting of small pits and fractures and a gentler, smoother, equator-facing slope. This is most likely due to differences in solar heating.

A polygonal pattern of fractures (see subimage), commonly associated with “scalloped terrain,” can be found on the surrounding surface and within the depressions. The fractures indicate that the surface has undergone stress that may have been caused by subsidence, desiccation, or thermal contraction. These polygonal fractures are similar to permafrost (permanently frozen) polygons that form on Earth and are indicative of the presence of ground ice.

Scallop formation may be an ongoing process at the present time.

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.