Outflow channels are extremely long, wide swathes of scoured ground onMars.[1] They extend many hundreds of kilometers in length and are typically greater than one kilometer in width. They are thought to have been carved by huge outburst floods.
Crater counts indicate that most of the channels were cut since the earlyHesperian,[2] though the age of the features is variable between different regions of Mars. Some outflow channels in theAmazonis andElysium Planitiae regions have yielded ages of only tens of millions of years, extremely young by the standards of Martian topographic features.[3] The largest,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.
The outflow channels contrast with the Martian channel features known as "valley networks", which much more closely resemble the dendritic planform more typical of terrestrial riverdrainage basins.
Outflow channels tend to be named after the names for Mars in various ancient world languages, or more rarely for major terrestrial rivers.[1] The term outflow channels was introduced in planetology in 1975.[4]
On the basis of their geomorphology, locations and sources, the channels are today generally thought to have been carved byoutburst floods (huge, rare, episodic floods of liquidwater),[5][6] although some authors have made the case for formation by the action ofglaciers,[7]lava,[8] ordebris flows.[9][10] Calculations[11][12] 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 theChanneled Scablands in North America or those released during the re-flooding of the Mediterranean basin at the end of theMessinian Salinity Crisis).[5][13] 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 bymeteorite impact origneous activity.[6]
This is a partial list of named channel structures on Mars claimed as outflow channels in the literature, largely followingThe 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.
Chryse Planitia is a roughly circular volcanic plain east of theTharsis 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.
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:
Several channels flow either onto the plains ofAmazonis andElysium from thesouthern highlands, or originate at graben within the plains. This region contains some of the youngest channels.[3]
Several outflow channels rise in the region west of theElysium volcanic province and flow northwestward to theUtopia Planitia. As common in the Amazonis and Elysium Planitiae regions, these channels tend to originate in graben. Some of these channels may be influenced bylahars, as indicated by their surface textures and ridged, lobate deposits at their margins and termini.[14] The valleys of Hephaestus Fossae and Hebrus Valles are of extremely unusual form, and although sometimes claimed as outflow channels, are of enigmatic origin.[1]
Three valleys flow from east of its rim down onto the floor of theHellas basin.
It has been argued thatUzboi,Ladon, Margaritifer andAres Valles, although now separated by large craters, once comprised a single outflow channel flowing north intoChryse Planitia.[15] The source of this outflow has been suggested as overflow from theArgyre crater, formerly filled to the brim as alake 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 theSolar System. Under this suggestion, the extant form of the outflow channel Ares Vallis would thus be a remolding of a pre-existing structure.
The large troughs present in each pole,Chasma Boreale andChasma Australe, have both been argued to have been formed by meltwater release from beneath polar ice, as in a terrestrialjökulhlaup.[16] However, others have argued for aneolian origin, with them induced bykatabatic winds blowing down from the poles.[17]
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