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[107] THE MARTIAN SURFACE, has been subjected to a wide variety of processes, collectively termed gradation, throughout its geological history. The net effect of gradation is to bring planetary surfaces to a common level by eroding topographically high areas and filling in low areas by deposition. Thus, gradation involves the weathering, erosion, transportation, and deposition of surface materials by wind, water (frozen or liquid), and gravity.


Even before spacecraft were sent to Mars, telescopic observations showed that dust storms are common, and it was speculated that these storms could alter the surface. When Mariner 9 arrived at Mars, a major dust storm had obscured the surface of the planet. After the dust storm cleared, the Mariner cameras revealed a wide variety of landforms related to aeolian (wind-related) processes, including dune fields, yardangs, and shifting albedo patterns consisting of light and dark streaks. The Viking orbiters and landers have provided much additional information on both aeolian processes and landforms.


In the tenuous atmosphere of Mars, much stronger winds than those on Earth arc required to pick up particles and set them into motion. Winds of some 150 kph are estimated as minimum for initiation of particle movement. Viking orbiter pictures show several areas in which storms seem to originate; these areas include Daedalia, Hellas, and Syrtis Major, which also display: numerous "streaks" associated with craters. Streaks appear to be zones in which fine-grained particles arc, redistributed in response, to wind patterns generated around craters and other landforms.


Some areas on Mars appear to be zones of deposition for windblown particles, as evidenced by enormous dune fields. These areas include the north polar region, the floor of the large impact basin, Hellas, and the floors of other smaller impact craters. The most spectacular of the dune fields, those at the north pole, are discussed in the section Polar Regions.


Wind-eroded features include yardangs and grooves etched in some plains. Because the atmosphere is very thin the wind speeds needed to move particles are much higher on Mars than on Earth, so that the grains travel much faster once set into motion. Consequently, when they strike other particles and bedrock surfaces, they have a greater erosion capability than they would have on Earth.


Mass wasting is the downslope movement of materials, primarily caused by gravity, and is seen as landslides, avalanches, and soil creep. Its c effectiveness is controlled by factors like cohesion of the material, steepness of slope, gravity, and the presence of lubricants such as liquids and volatiles. Mariner 9 and Viking pictures show many features that can be attributed to [108] mass wasting. Mass wasting along the walls of Valles Marineris has produced some of the most spectacular landslides observed anywhere.


Surface and near-surface processes that occur in the vicinity of former and existing ice regions are referred to as periglacial processes. Although periglacial features and related phenomena have not been positively identified on Mars, it is reasonable to expect them in view of the low temperatures and the probable existence of subsurface ice in some regions. "Etch" pits, polygonal ground, and rock "glaciers" are among the features observed from orbit that may be related to periglacial processes on Mars.



Sand Dunes and Landslides in Valles Marineris. A 40-km-long field of sand dunes (dark area in lower left) and a massive avalanche (middle of mosaic) are seen here on the floor of Gangis Chasma, one of the branches of Valles Marineris. In this region, the walls have been modified by landslides. Debris flows are numerous, as are jumbled masses of debris below the cliffs. Wind may be an effective agent in removing debris that has slumped into the canyon. The canyon thus enlarges itself by the combined processes of slumping and wind excavation. [P16941; 7°S, 45°W]



Details of Valles Marineris Sand Dunes. An enlargement of the dune field on the preceding picture is presented here to show individual dunes about 500 meters across. The wind appears to have been blowing from the west and leading dunes to the east appear to climb the canyon wall. [P16950; 7°S, 45°W]

Landslide in Noctis Labyrinthus. This landslide mass completely fills the floor of the canyon. The canyons in this area appear to be graben that resulted from crustal extension with subsequent widening and modification by landslides. [46A19-22; 10°S, 96°W]



Small Dune Field in Kaiser Crater. Craters and other topographic depressions are natural traps for windblown sediments. The crater shown here is typical of many that have been photographed from orbit. [94A42; 46°S, 339° W]



Part of the Dune Field in the North Polar Region. The dune field covers an area of at least 3500 km2 and is composed of barchan (crescent-shaped) dunes. In the area shown here, the dunes s are aligned in ridges that appear to be transverse to the prevailing wind. From the relation of the dune field to the crater at the bottom of the picture, the prevailing winds se e m to be from the west (left side of picture). [59B65; 76° N, 88° W]

Barchan Dunes at Edge of North Polar Cap. This figure shows the well-defined lines of individual barchan dunes. The wind direction is from left to right. [58B22; 75° N, 53° W]


"Etched" Terrain in Southern Chryse Planitia. This etched terrain shows light-toned, angular depressions in southern Chryse Planitia in the area where Tiu Vallis empties into the Chryse basin. The etching process that removed the dark plains material may be the result of cavitation or plucking during active channel formation or wind deflation. Many small, volcano-like features occur in this region. The arrow points to one of these features, a low mound with a summit crater. This feature (also discussed in the Volcanoes section) lies on a sinuous line of unknown origin; the line may be the trace of a fracture or possibly a dike. [211-4990; 19° N, 35° W]



Northern Contact of Chryse Planitia. Chryse Planitia "plateau", the mottled light surface at the bottom, is shown at its contact with the darker plains. Irregular pits on the plateau (lower right) suggest formation by collapse: the scalloped scarp of the plateau seems to result from scarp retreat am] the connection of' the irregular pits. The morphology of the pits and scarp resembles thermokarst features on Earth that result from the melting of ground ice and the subsequent settling of the ground. [211-4994: 23° N, 36° W]



Concentric Flow Features at the Foot of Olympus Mons Scarp. These flow features are more like those typically developped on avalanches and landslides. The unit on which they occur is probably material formed by landsliding on the scarp front. This process may have played a major part in developing the scarp around the volcano. [48B04; 23°N, 138° W]


Mosaic of the Nilosyrtis Region. This is a transitional zone between an ancient cratered terrain to the south (bottom) and sparsely cratered terrain to the north. In many of the low-lying areas there are sub-parallel ridges and grooves that suggest creep of near-surface materials. They resemble terrestrial features where near-surface materials flow en masse very slowly, aided by the freeze and thaw of interstitial ice water frozen between layers of ground materials. This is additional evidence suggesting the presence of ground ice in the near surface materials of Mars. [P-18086; 34°N, 290°W]


Flow Structures in Ancient Cratered Terrain East of Hellas. Mass- wasting structures around positive features extend up to 20 km from the source. The aprons are not composed of discrete lobate flows, as would be expected if they were formed by landslides, nor are they talus deposits close to the angle of repose; surface slopes are probably less than 10°. Instead, these features may be the result of slow creep of debris containing interstitial ice. [97A62; 41° S, 257° W]

Chaotic Terrain North of Elysium. The plains of the south (lower half of this mosaic) appear to have partly collapsed and then eroded so that only isolated remnants remain. Collapse may have occurred as a result of removal of subsurface ice. A process of planation appears to have removed materials down to a specific depth and created a new planar surface at that depth. It is unclear what the erosive mechanism was or where the material went. [211-5274; 33° N, 213° W]

[118-119] Contrasting Terrain West of Deuteronilus Mensae. (a) The smooth areas shown may be either debris mantles or remnants of older terrain. In the textured areas, the linear markings may mark the position of former escarpments- the outline of smooth areas. [52A31-44; 44°N, 352°W]


Contrasting Terrain West of Deuteronilus Mensae. (b) A view is shown of part of the Cydonia region of Mars, a 65-km-long remnant of the same plateau unit shown in (a). [26A72; 45°N, 7°W]



Striped Ground. (a) Geometric markings resembling contour plowing in the Cydonia region are seen, and consist of low ridges and valleys about l km from crest to crest. 'The features may mark successive positions of the retreat of an escarpment during removal of a plateau or mantling unit. (b) In this high resolution image of striped ground similar to that in (a), the parallel markings are caused by low ridges and, less commonly, shallow depressions. [(a) P- 17599; 46° N, 350° W, (b) 11B01; 50° N, 289° W]



Highly Textured Eroded Surface. The upper half of this image shows a layer of relatively erodable material that is being sculpted d and swept away by the wind. In the lower left a more resistant older surface has been exposed which is dominated by small hills and sinuous narrow ridges. The hill at the bottom may be of volcanic origin. The narrow ridges are especially puzzling. It has been suggested that they may be dikes but their extensive continuity and ridge-like surface forms argue against this. An alternative, but weaker, hypothesis is that they may be esters. [72 4A22;2° S, 210° W ]