South polar ice on Mars. Coloured satellite image of frozen carbon dioxide on Ultimi Scopuli in the south polar ice cap of Mars. During spring and summer, carbon dioxide sublimates (transforming from solid direct to gas) to form these radially patterned troughs, known as 'spiders'. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 2nd July 2009. The width of this image is roughly 1.2 kilometres across.