Gullies on crater walls, Mars. Coloured satellite image of gullies on the walls of a Martian crater. 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 base of the crater wall contains light-coloured fan-shaped deposits. The colour difference may be due to variations in sand grain size or mineral content. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 18th July 2009. The width of this image is roughly two kilometres across.