File:TerraMeridiani ValleyNetwork.gif

English: A portion of a typical valley network system in central Terra Meridiani near the martian Equator and Prime Meridian. The surface into which the valley has formed has a low albedo (that is, it is dark) and exhibits many circular impact craters and pits. The valley appears brighter than its surroundings, owing to sand and dust deposits; bright sand can also be seen on the floors of craters. The outline of the valley wall shows attributes--for example, various protrusions and alcoves--that suggest the margin of a lava flow (which would be consistent with the dark surface). Crater-like rings within the valley adjacent to the wall are the resistant portions of impact craters that survived the retreat of the wall as the valley widened. This is evidence that processes of valley widening were relatively gentle, probably related to groundwater seeping from beneath a resistant cap of volcanic rock. Image was taken by Mars Orbiter Camera (MOC) on April 14, 1998. The subframe shown here covers an area 11.5 km by 27.4 km (7.1 miles by 17.0 miles) in size. The image as presented here has a resolution of about 22.4 meters (74 feet) per pixel. The subframe is centered at 6.2°S latitude and 357.4°W longitude. North is approximately up, illumination is from the lower right.