Gullies on Mars. Coloured satellite image of gullies on the wall of a crater in the South Aonia Terra region on Mars. The crater rim runs from lower left to middle top. The source of the gullies is not known. They may have been carved in the past by liquid water, or may be a result of recent landslides. The sand dune surface is rippled due to wind action. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 23rd July 2009. The width of this image is roughly 1.5 kilometres across.