Outflow channels

Outflow channels are extremely long, wide swathes of scoured ground on Mars, commonly containing the streamlined remnants of pre-existing topography and other linear erosive features indicating sculpting by fluids moving downslope. Channels extend many hundreds of kilometers in length and are typically greater than one kilometer in width; the largest valley (Kasei Vallis) is around 3,500 km (2,200 mi) long, greater than 400 km (250 mi) wide and exceeds 2.5 km (1.6 mi) in depth cut into the surrounding plains. These features tend to appear fully sized at fractures in the Martian surface, either from chaos terrains or from canyon systems or other tectonically controlled, deep graben, though there are exceptions. Besides their exceptional size, the channels are also characterized by low sinuosities and high width:depth ratios compared both to other Martian valley features and to terrestrial river channels. Crater counts indicate that most of the channels were cut since the early Hesperian, though the age of the features is variable between different regions of Mars. Some outflow channels in the Amazonis and Elysium Planitiae regions have yielded ages of only tens of million years, extremely young by the standards of Martian topographic features.