NASA/JPL/Malin Space Science Systems
Martian middle- and polar-latitude gullies are not the only places that 'alcoves' form by downslope erosion of debris. Even at equatorial latitudes, some craters exhibit these features. Alcoves at the heads of narrow, dry landslide scars are indicated in this Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image of a crater wall in Xanthe Terra. In both the middle/polar-latitude gully cases and in this example, alcoves form by undermining and collapse of material high on a relatively steep slope such as a crater wall. In this case, however, no fluid was involved, thus no gully or distinct apron formed. This crater wall is located near Shalbatana Vallis around 2.7°N, 43.1°W. The image is illuminated from the left; the 400 meter scale bar is about 437 yards long. For comparison, an example of martian gullies with alcoves, channels, and aprons can be seen in: Evidence for Recent Liquid Water on Mars: Basic Features of Martian Gullies, 22 June 2000.
Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, California. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, California and Denver, Colorado.