This observation shows gullies in a semi-circular trough in Noachis Terra. The gullies are observed to face all directions.
It is interesting to note that the gully morphology seen here depends on the orientation of the gullies. The morphology differences are most pronounced on the sunlit slope, with the gullies facing south (down) being more deeply incised than those facing the west. It is unknown what caused the different gully morphologies, but there are several possibilities.
Gullies are proposed to form at locations determined by the availability of a forming liquid (thought to be water) and/or the amount of insolation the slope receives, among other factors. It is possible that the deeper gullies experienced more erosional events or that their erosional events were more effective for undetermined reasons. It is also possible that the gullies formed at different times such that they did not have the same amount of water either for an individual flow or totalavailable to them. Also, the underlying topography could make the gullies appear relatively more incised without this actually being the case.
The majority of the gullies on both sides of the trough appear to originate at a boulder-rich layer visible in the subimage . The layer appears dark on the sunlit slope because the boulders sticking out from the slopes cast shadows. If these gullies formed by water from the subsurface, then it is possible that this layer is a permeable layer that conducted water to the surface. The layer is deteriorating and traveling down slope in the form of boulders. These boulders can clearly be seen in the alcoves of the gullies on both sides of the trough.
Note that the alternating stripes on the left side of the image are an artifact from camera noise. They are not real features.
Written by: Kelly Kolb (28 April 2010)
This is a stereo pair with PSP_001823_1320 .
More info and image formats at http://hirise.lpl.arizona.edu/PSP_001691_1320
Image: NASA/JPL/University of Arizona