Sand dunes and boulders on Mars. Sand dunes are shown covering the terrain, which has large boulders lying on flat areas between the dunes. The image was taken in Martian late winter in the Southern hemisphere and the dunes are getting enough sunlight to start defrosting their seasonal cover of carbon dioxide. Image obtained by the High Resolution Imaging Experiment (HiRISE) instrument on the Mars Reconnaissance Orbiter (MRO).