NASA/JPL/University of Arizona
Multiple Levels of Gullies
This image shows groups of gullies at different elevations on the same crater wall. Although gullies are common in the mid-latitudes of Mars, they are rarely found to exist at such distinct elevations as visible here.
The mounds on the floor, one of which contains gullies, probably formed during a late stage of crater formation. Both levels of gullies appear to originate at layers. These layers might be ice-rich, or they might be capable of conducting water to the surface. The anaglyph image, providing a three-dimensional perspective, reveals the relative depth of the gullies in the crater walls and amount of alluvial material deposited at the bottom of the gullies.
The gullies visible here are good candidates for formation by subsurface water, as opposed to melting ice or snow originating on the surface. The rounded, theater-shaped alcove and tributary heads are typical of features formed by groundwater sapping on Earth. Surface runoff does not form this morphology.
This image contains possible evidence of subsurface piping, when soil pores connect to form a "pipe" that transports water. When piping occurs, water carries soil with it, leaving empty space beneath the surface. As this process continues, the overlying surface can no longer support itself, and it collapses to form a depression. Several depressions that could have formed this way are seen in this image. The depressions are also directly upslope of more developed alcoves. They also originate at upslope layers, and might be examples of developing alcoves.