This observation shows northern hemisphere gullies on a layered crater wall.

Many channels are visible emanating from beneath layers suggesting that the layers are permeable and carried water to the slope face via the subsurface. It is also possible that the source of water came from the surface. The gullies that do not originate at a layer likely did at one time and have subsequently experienced headward erosion, eroding the layers upslope of their original location.

A mantled unit (smooth terrain) is visible the sources of and within many of the gullies in this image. The mantled unit has been proposed to be remnant snowpack that melts at its bottom to carve gullies. The mantled unit is less abundant in locations where the gullies are most deeply incised, which supports the melting snowpack theory.

Deeper incision typically involves more water and/or more flow events. If the mantled unit is the source of the liquid for the gullies, then it is expected that locations with evidence of larger or more frequent flows would be associated with regions of less mantled unit. It is unknown whether the mantled unit can insulate the surface sufficiently to allow temperatures and pressures appropriate for liquid water formation. An answer to this awaits future modeling of snowpack under Martian conditions.

Within the subimage, channels can clearly be seen to originate at a variety of layers. Also noticeable is the smooth, mantled material located between layers above these gullies.
Written by: Kelly Kolb   (10 March 2010)

This is a stereo pair with PSP_002214_2210.