This image taken by the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter satellite shows the wall of a crater in the Northern Lowlands that has several gullies incising it.

How gullies are formed on Mars is unclear. Mars is globally covered with permafrost, where soil temperatures are permanently below the freezing point of water. Therefore, any water present in the Martian soil or atop the soil surface must be in the form of ice.