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This HiRISE image (PSP_001691_1320) 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 total -- available 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 seen in this 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.
Acquisition date: 12 December 2006
Local Mars time: 3:39 PM
Degrees latitude (centered): :-47.5°
Degrees longitude (East): 4.4°
Range to target site: 258.5 km (161.6 miles)
Original image scale range: 51.7 cm/pixel (with 2 x 2 binning) so objects ~155 cm across are resolved
Map-projected scale: 50 cm/pixel and north is up
Emission angle: 14.0°
Phase angle: 89.2°
Solar incidence angle: 78°, with the Sun about 12° above the horizon
Solar longitude: 146.1°, Northern Summer
NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft. The High Resolution Imaging Science Experiment is operated by the University of Arizona, Tucson, and the instrument was built by Ball Aerospace and Technology Corp., Boulder, Colo.