Mars' polar cap. Satellite image of the South Polar residual cap on the surface of Mars. The residual cap is the part of the ice cap that lasts through the summer. On Mars it is composed of carbon dioxide ice. Although the cap survives each warm summer season, it is constantly changing its shape due to sublimation of carbon dioxide from steep slopes and deposition onto flat areas. Imaged by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) satellite, on 11th August 2009.