MGS MOC Release No. MOC2-554, 24 November 2003

Middle- and polar-latitude gullies are common on crater and trough walls in the martian southern hemisphere. Some also occur in the north. One of the controversies surrounding gullies is whether they involved fluid flow (such as liquid water) or were formed by dry landsliding processes. The occurrence of banked and leveed channels in many gullies argues that they formed by fluid flow. Another question centers on whether gullies are "one-shot deals" or involved more than one episode of fluid flow. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image, acquired in November 2003, shows a gully with an apron, near the center of the frame, that formed by multiple fluid flows. The apron of debris at the base of the gully near the center of the picture is not just one apron, it is three. Three separate aprons formed at three different times. First, the larger, left-most apron formed. Later, another event caused fluid to cut through that apron and create a new one (the middle of the three). Later, a third event cut both aprons and formed a small, third one. This image shows the wall of an impact crater located near 51.3°S, 326.6°W. The image covers an area 3 km (1.9 mi) wide, and is illuminated by sunlight from the upper left.