South polar ice on Mars. Coloured satellite image of frozen carbon dioxide on the south polar ice cap of Mars. During spring and summer, carbon dioxide sublimates (transforming from solid direct to gas) to form spots and polygonal cracks. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 27th March 2009. The width of this image is roughly one kilometre across.