Outflow channels

On the basis of their geomorphology, locations and sources, the channels are today generally thought to have been carved by outburst floods (huge, rare, episodic floods of liquid water), although some authors have made the case for formation by the action of glaciers, lava, or debris flows. Calculations indicate that the volumes of water required to cut such channels at least equal and most likely exceed by several orders of magnitude the present discharges of the largest terrestrial rivers, and are probably comparable to the largest floods known to have ever occurred on Earth (e.g., those that cut the Channeled Scablands in North America or those released during the re-flooding of the Mediterranean basin at the end of the Messinian Salinity Crisis). Such exceptional flow rates and the implied associated volumes of water released could not be sourced by precipitation but rather demand the release of water from some long-term store, probably a subsurface aquifer sealed by ice and subsequently breached by meteorite impact or igneous activity. ==List of outflow channels by region==

This is a partial list of named channel structures on Mars claimed as outflow channels in the literature, largely following The Surface of Mars by Carr. The channels tend to cluster in certain regions on the Martian surface, often associated with volcanic provinces, and the list reflects this. Originating structures at the head of the channels, if clear and named, are noted in parentheses and in italics after each entry. Circum-Chryse region Chryse Planitia is a roughly circular volcanic plain east of the Tharsis bulge and its associated volcanic systems. This region contains the most prominent and numerous outflow channels on Mars. The channels flow east or north into the plain. • Ares Vallis (Aram Chaos; Iani Chaos) • Kasei Vallis (Echus Chasma) • Maja Valles (Juventae Chasma) • Mawrth Vallis (no obvious source) • Ravi Vallis (Aromatum Chaos) • Shalbatana Vallis (chaos in Orson Welles crater; Ganges Chasma?) • Simud Valles (Hydraotes Chaos; Aureum Chaos?; Arsinoes Chaos?) • Tiu Valles (Aram Chaos?; Aureum Chaos?) Tharsis region In this region it is particularly difficult to distinguish outflow channels from lava channels but the following features have been suggested as at least overprinted by outflow channel floods: • Parts of the Olympica Fossae • Valleys adjacent to the southeast margin of Olympus Mons (nameless graben) Amazonis and Elysium Planitiae Several channels flow either onto the plains of Amazonis and Elysium from the southern highlands, or originate at graben within the plains. This region contains some of the youngest channels. The valleys of Hephaestus Fossae and Hebrus Valles are of extremely unusual form, and although sometimes claimed as outflow channels, are of enigmatic origin. The source of this outflow has been suggested as overflow from the Argyre crater, formerly filled to the brim as a lake by channels (Surius, Dzigai, and Palacopus Valles) draining down from the south pole. If real, the full length of this drainage system would be over 8000 km, the longest known drainage path in the Solar System. Under this suggestion, the extant form of the outflow channel Ares Vallis would thus be a remolding of a pre-existing structure. Polar regions The large troughs present in each pole, Chasma Boreale and Chasma Australe, have both been argued to have been formed by meltwater release from beneath polar ice, as in a terrestrial jökulhlaup. However, others have argued for an eolian origin, with them induced by katabatic winds blowing down from the poles. ==See also==