This Mars Global Surveyor Mars Orbiter Camera image shows a gullied crater wall in the Terra Sirenum region of Mars. The fluids that formed the gully channels and deposited debris in aprons at the base of the crater wall may have percolated through layers in the layered bedrock exposed in the crater wall. As fluid seeped out of the ground, it undermined overlying layers of rock, and caused the formation of several compex alcoves, higher in the crater wall. The formation of alcoves requires the undermining and collapse of layered material at a point where the channel begins; this is a key observation supporting the hypothesis that martian gullies require groundwater, not snowmelt or other exotic processes or fluids, to form.

NASA / JPL / MSSS