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Gullies are common on steep slopes of many impact craters on Mars. When gullies were first observed, there was a lot of excitement surrounding them because similar features form on Earth through the action of liquid water.

However, liquid water is currently unstable on the surface of Mars. Long-term observations have prompted many scientists to question a liquid water origin for the gullies, and alternative ideas have been suggested. These include flows of salt-rich (briny) water, as the salt would allow water to be liquid under lower temperatures than those for pure water. Also "dry" processes, which do not require the action of liquid water at all.

Monitoring of gullies by HiRISE could help scientists better understand the conditions where the gullies are active, and in doing so, help understand how they form.

The map is projected here at a scale of 50 centimeters (19.7 inches) per pixel. (The original image scale is 50.2 centimeters [19.8 inches] per pixel [with 2 x 2 binning]; objects on the order of 151 centimeters [59.4 inches] across are resolved.) North is up.

The University of Arizona, in Tucson, operates HiRISE, which was built by Ball Aerospace & Technologies Corp., in Boulder, Colorado. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Mars Reconnaissance Orbiter Project for NASA's Science Mission Directorate, Washington.