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The shape of a crater can depend on factors including the angle of impact and the pre-impact slope and topography.

This image shows a roughly 3-kilometer impact crater, formed on the sloping walls of Tithonium Chasma, part of the large Valles Marineris canyon system. We can see that this crater is non-circular, measuring about 3 by 4 kilometers. The ejecta -- the debris that is generated and thrown out by an impact -- will typically distribute itself evenly around the outside of the crater rim where the pre-impact surface is flat and the angle of impact is not too low. However, due to the highly inclined nature of the surface here, the ejecta deposited preferentially downslope, forming a tongue-like deposit.

Additionally, when examining the area around the crater, we see smooth, dark-toned flow-like features superimposed on the ejecta blanket and flowing downslope. These are believed to be composed of impact -- or the mixture of rock and other material that melted upon impact -- and flowed down the slope before hardening. These potential impact melt deposits also occur as smooth ponds, which pooled on the surface of a portion of the ejecta that did not escape the crater interior.

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