24 November 2004
Ancient valleys that may once have been the conduits through which water flowed are common on the surface of Mars. They are also found--filled and buried--in the subsurface, preserved in the rock record. In addition, erosion may take what was once the floor of a valley and leave it as a high-standing, flat-topped ridge. This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a valley in eastern Arabia Terra that, in just one picture, exhibits both negative and positive relief forms. In negative relief, the valley is just that--a valley. In positive relief, instead of a valley, the former floor is now the top of a broad ridge. This MOC image is particularly instructive, because the transition from negative to positive (then back to negative and then again to positive) relief is captured in one small area. These landforms are located near 32.5°N, 314.1°W. The image covers an area about 3 km (1.9 mi) wide. Sunlight illuminates the scene from the lower left.