Martian sand dunes. Martian sand dunes. Coloured satellite image of north polar sand dunes blown into crescent shapes by the wind. The surface is covered by carbon dioxide frost (light brown) except the darker areas 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 24th August 2009. The width of this image is roughly one kilometre across.