3 May 2004
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; see PIA01031.