Mars Reconnaissance Orbiter (MRO) image of circular sand dunes on Mars. Sand dunes are common on Mars, though the relatively symmetrical shape of these dunes is unusual. The south sides of the dunes are steeper, suggesting that wind in this area predominantly blows south. In early Martian winter, this area is typically covered in frost, and images such as these are helping scientists to monitor the way frost cover changes over the Martian year. Photographed by the Mars Reconnaissance Orbiters High Resolution Imaging Science Experiment (HiRISE).