Exposed rock layers on Mars. Coloured satellite image of layered rocks (top and lower left) near the Argyre impact basin on Mars. Sand covers the areas between the layered rocks. Martian rock layers may have been formed by a number of processes. For example, sedimentary layers can form when a region is covered by liquid water, or volcanic layers can form when volcanic activity is high. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 4th August 2009. The width of this image is roughly 1.2 kilometres across.