Martian sand dunes. Coloured satellite image of north polar sand dunes blown into crescent shapes by the wind. The dunes are covered by carbon dioxide frost except dark-coloured patches where the carbon dioxide has sublimated (turned from solid directly to gas) and exposed the basaltic sand beneath. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 27th January 2008. The width of this image is roughly 1.3 kilometres across.