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[31] CHANNELS are among the more puzzling and intriguing features of the Martian surface. The most controversial aspect of the channels is whether they were formed by running water. Present climatic conditions on Mars prevent the existence of liquid water at the surface, so a water-worn origin implies that very different climatic conditions prevailed in the past. A denser atmosphere and higher temperature are both required. Because of the difficulty in explaining how climatic conditions could have changed so drastically, alternative methods of erosion, such as by wind and lava, have been suggested.


Three main types of charmers have been recognized: (1) runoff channels appear as dendritic networks, or arrays of relatively small channels or valleys located mainly in the old, densely cratered terrain; (2) outflow channels appear as large scale tributaries; and (3) fretted channels appear as long, relatively wide, fiat-floored valleys that possess tributaries and increase in size downstream.


Much of the old, cratered terrain, particularly in the equatorial regions, is dissected by channels of some type, the most common of which is the simple gully, typically a few tens of kilometers long. The gullies generally have few tributaries which, if present, have small junction angles. The numerous well-integrated tributary networks provide the strongest evidence for water erosion at some period in Martian history, because they are unlikely to form by wind action or lava erosion. The runoff channels are largely restricted to the oldest terrain and are themselves commonly degraded. Most of these channels therefore appear to have formed early in the planet's history.


Most outflow channels occur around the Chryse basin. They commonly emerge full size from chaotic terrain that has seemingly collapsed to form areas of jostled blocks as many as 3 km below the surroundings. The channels extend from the chaotic terrain downslope several hundred kilometers into the plain of Chryse Planitia. They may be tens of kilometers wide and more than a kilometer deep, a size indicating erosion on an enormous scale.


Within the channels are many features, such as teardrop-shaped islands, longitudinal grooves, terraced margins, and inner channel cataracts, that are also found in regions on Earth affected by large floods. The dimensions of the Martian channels suggest peak flood discharges of 107 to 109 m3/sec. By comparison, the average discharge of the Amazon is 105 m3/sec, and the largest known terrestrial flood, the Lake Missoula flood that occurred in eastern Washington in the late Pleistocene, had a peak discharge of [32] 107 m3/sec. Thus the Chryse outflow channels, and similar ones elsewhere, provide evidence of enormous floods on Mars-far greater than any known on Earth.


The time period in which the climatic conditions permitted liquid water to exist is uncertain because of the difficulty of precisely dating the channels. Most of the evidence, however, suggests that the more clement conditions prevailed very early, perhaps during Mars' first billion years, and that this period was followed by general global cooling. The present harsh conditions have probably existed for most of the planet's history.


Fretted channels occur mostly within the old, densely cratered terrain, especially at its boundaries with younger units. They lack features indicative of catastrophic flooding. The presence of tributaries and a decrease in channel size upstream also argue against formation by floods. The origin of the fretted channels is not known, but numerous features on the floors suggest that mass wasting may have played a significant role.


[33] Finely Channeled Old Cratered Terrain. The channels are concentrated on crater rims and tend to be approximately parallel, a few tens of kilometers long, with few tributaries. Such channels are typical of much of the heavily cratered terrain of Mars, but are rare in the sparsely cratered areas. [84A16-22; 23°S, 0°W]

Dendritic Channels in the Southern Highlands. These channels are deeply incised with relatively wide, undissected interfluves. Most terminate abruptly at their lower ends. [P-18115; 25°S, 10°W]


Dense Drainage Network in the Southern Highlands. Dendritic patterns like these suggest a fluvial origin and argue against alternatives, such as erosion by wind or lava. [63A09; 48°S, 98°W]

Outflow Channel Emerging from Chaotic Terrain. Oblique view, looking south, of the source region of an apparent flood. The channel starts full scale in a region of chaos enclosed by cliffs. Possible mechanisms for producing such a relation are rapid release of water from buried aquifers or the melting of ground ice by volcanism. [P-16983; 1°S, 43°W]

[35] Channels and Chaotic Terrain at the Source of Tiu Vallis. A 50 km wide channel emerges from chaotic terrain. It extends off the picture to the north, down the regional slope to Chryse Planitia 1000km away. [P19131; 5° S, 29° W]

Closeup of Part of Preceding Image. This frame shows Tiu Vallis (left) extending northwest from Hydaspis Chaos (right). Hydapsis Chaos is an elongated area of collapsed terrain almost 100 km wide, from which emerge the lineated bedforms and teardrop-shaped islands of Tiu Vallis. [83A37; 3°S, 27°W]



Part of Ares Vallis. (a) [above] This mosaic shows part of Ares Vallis incised in the heavily cratered upland. The channel is 25 km wide and about 1 km deep. Several layers, probably lava flows, are exposed in the walls of the channel. Many craters are present in the upland surface, but craters are few on the channel floor. (b) [right] A stereogram shows the origin of Ares Vallis in chaotic terrain. Channels from two large areas of chaos have merged into a single channel. Where flow from the lower chaos has merged into the channel, many streamlined forms are visible. [(a) 211-5238; 10° N, 24° W, (b) 451A03-10; 2° N, 19° W]


"Islands" near Chryse Planitia. Teardrop-shaped "islands" are shown at the mouth of Ares Vallis near the southern boundary of Chryse Planitia. Flow was from the south and apparently diverged around obstacles such as craters and low hills to form a sharp prow upstream and an elongate tail downstream. A shallow moat surrounds the entire island. Similar patterns on Earth have been formed by catastrophic floods, wind erosion, and glacial action. From top to bottom, the three large craters are named Lod, Bok, and Gold. [211-4987; 21°N, 31°W]


[39] Channels between Lunae Planum and Chryse Planitia. (a) Channels have been cut across old cratered terrain between the lava plains of Lunae Planum on the left and the plains of Chryse Planitia to the right. Three separate channel systems are visible, starting from the north: Vedra Vallis, Maumee Vallis, and Maja Vallis. Flow along the eastern edge of Lunae Planum converged to cut Maja Vallis. Numerous teardrop-shaped islands occur upstream of the main channel. Below the channel to the east (off the right side), the flow diverges across Chryse Planitia. (b) This stereogram shows Vedra and Maumee Valles between Lunae Planum and Chryse Planitia. Note that a branch of Vedra Vallis passes through Banh Crater. [(a) 211-5190, (b) 211-5419; 18°S, 55°W]

Upper Reaches of Maja Vallis. The surface of Lunae Planum is extremely scoured with long linear grooves and teardrop islands. Flow apparently converged on Maja Vallis from a wide area of Lunae Planum. [44A44; 17°S, 57°W]


Western Chryse Planitia. The west side of Chryse Planitia has been extensively sculpted by flow from Maja Vallis, which is situated just to the left of this mosaic. Flow diverged across the gently sloping plain of Chryse Planitia to form the sculpted features seen in this mosaic. Ridges, similar to those on the lunar maria, appear to have partly dammed or diverted flow to form a variety of scour patterns. [211-5015; 21°N, 49°W]

[41] Flow around Dromore Crater in Chryse Planitia. Flow was from the left and apparently ponded west of the mare ridge. It then cut gaps as it flowed over the low points in the ridge. Similar relations occur in the channeled scablands of Washington state. After crossing the ridge, the flow cut grooves in the Chryse Planitia floor as it flowed around Dromore, an old impact crater. [20A62; 20°N, 49°W]

[42-43] Bahram Vallis in Northern Lunae Planum. This channel has few tributaries, a flat floor, and cuspate walls, and resembles lunar sinuous rilles that are believed to be formed by lava flow. [211-5189; 20°N, 57°W]


Section of Valles Marineris. Each tributary on the southern wall of the canyon heads in a cirque-like feature and lacks a fine-scaled drainage network. The morphology suggests formation by ground water sapping rather than by surface run-off. Ground ice is a possible source for the water. [211-5158; 80°S, 85°W]


Flat-Floored Valley Northeast of Hellas. This valley is several kilometers and is cut into layered deposits that are clearly exposed in the valley walls. In some places, a channel is visible in the valley floor. Extensive debris fans surround many hills in the area and are probably formed by creeping of near-surface materials, perhaps aided by interstitial ice. [97A60-68; 43°S, 253°W]