This image shows groups of gullies at different elevations on the same crater wall. Although gullies are common in the mid-latitudes of Mars, they are rarely found to exist at such distinct elevations as seen here.

The mounds on the floor, one of which contains gullies, probably formed during a late stage of crater formation. Both levels of gullies appear to originate at layers. These layers might be ice-rich, or they might be capable of conducting water to the surface.

The gullies visible here are good candidates for formation by subsurface water, as opposed to melting ice or snow originating on the surface. The rounded, theater-shaped alcove and tributary heads (see subimage, approximately 140 meters across; 550 x 1018; 1.6MB) are typical of features formed by groundwater sapping on Earth. Surface runoff does not form this morphology.

This image contains possible evidence of subsurface piping, when soil pores connect to form a "pipe" that transports water. When piping occurs, water carries soil with it, leaving empty space beneath the surface. As this process continues, the overlying surface can no longer support itself, and it collapses to form a depression. Several depressions that could have formed this way are seen in this image (see subimage). The depressions are also directly upslope of more developed alcoves. They also originate at upslope layers, and might be examples of developing alcoves.
Written by: Kelly Kolb

All of the images produced by HiRISE and accessible on this site are within the public domain: there are no restrictions on their usage by anyone in the public, including news or science organizations. We do ask for a credit line where possible: Image: NASA/JPL/University of Arizona

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.