Ice cap erosion on Mars. Coloured satellite image of pits on the south polar ice cap. The ice cap is made from frozen carbon dioxide. Spring and summer heat causes the carbon dioxide to sublime (transform from solid to gas) and form large oval-shaped pits. The pit walls are stratified which reveals variations in global Martian climate over the past few million years. Image obtained by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) on 18th August 2009. The width of this image is roughly one kilometre across.