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NASA SP-441: VIKING ORBITER VIEWS OF MARS

- POLAR REGIONS -

[125] THE APPEARANCE of the polar regions contrasts sharply with the rest of the planet, partly because of varying amounts of frost cover and partly because of some highly distinctive terrain not found elsewhere. Both poles have a cap of frozen carbon dioxide that advances and recedes with the seasons. In the north a small permanent residual cap left in midsummer is composed of water ice. The composition of the small residual cap left at the south pole is not known. The residual northern cap is substantially larger than the residual southern cap, so much so that the unique polar terrains of the north are rarely seen without some frost cover. The polar scenes are all from Viking Orbiter 2, which was placed in a high-inclination orbit specifically to view the poles. Because its periapsis was in the high northern latitudes, the highest resolution photographs are of the north.

The most distinctive geologic features of the polar regions are thick, layered deposits that cover much of the surface poleward from 80°. The layering is best seen where the frost has been preferentially removed such as on terraces and on walls of valleys within the deposits. The layers, which range in thickness from several tens of meters down to the resolution limit of the available photography, can be traced laterally for considerable distances. Unconformities occur but are relatively rare. In the north, the layers rest on sparsely cratered plains; in the south they rest on old cratered terrain. The layered terrain is almost completely devoid of impact craters. Either resurfacing by erosion or deposition is at a rate that is high compared with the impact rate, or the impact craters "heal" relatively quickly by flow or infilling.

The layered deposits arc believed to be accumulations of volatiles and wind-blown debris, with the layering caused by variations in the proportion and absolute amounts of these two components. If this interpretation is true, then the layered deposits preserve a partial record of the history of atmospheric activity, and hence climate, in the recent geologic past.

A vast belt of dunes, several hundred kilometers across, surrounds the layered terrain in the north. In some areas, the dunes form a nearly continuous sheet that almost completely masks the underlying topography. In other areas, particularly around large topographic features, the sheet is discontinuous and breaks up into strings of crescentic dunes or isolated forms. Dune fields of comparable continuity do not occur around the south pole, although numerous dark splotches on the surface in the high southern latitudes are probably local dune fields.

[126-127]

Photomosaic of North Pole. This photomosaic consists of some 300 Viking Orbiter 2 frames. Around the north pole, curving in huge arcs, are dark bands where polar frosts are absent. A giant ring of sand dunes surrounds the polar region between 80° and 70° N latitude. [211-5359]

[128] Remnant North Polar Cap Detail. This high-resolution, closeup view was made by combining three black-and-white images obtained through color filters. Above center in the picture is a giant cliff about 500 meters high. Layers averaging 50 meters in thickness are seen in the cliff face and surrounding areas, which are highlighted by occasional white patches of frost. The regularity of the layering suggests that it comes from periodic changes in the orbit of Mars - a relationship that, on Earth, may be at Ieast partially responsible for ice ages. These orbit changes may affect the frequency and intensity of global dust storms, in turn varying the amount of material available to form layered terrain. The cliff is apparently an erosional feature; the variety of scarps shows the complexity of erosion in the polar regions. Dune-like features (dark areas with a rippled textures), possibly formed from material eroded from the layered terrain, can be seen at the center and at the right of the picture. Just above the scarp, the polar ice layer is very thin and patchy; in other places it appears to be considerably thicker. The maximum thickness of the polar cap has not been determined. [75B52, 75B56, 75B58 (P-18459); 84° N, 237° W]

[129]

(a)
(b)

Layered Deposits Partially Covered by Frost near the North Pole. The light and dark pattern is caused largely by the presence or absence of frost. The layers are best exposed on southward facing slopes, which are generally without frost and hence are dark. Although sequences of layers can commonly be traced unbroken for considerable distances. breaks in the sequence do occur. This pair of pictures includes an example of an angular unconformity where one set of deposits truncates another at an angle. In (a), the unconformity is in the upper right center. In (b), an enlargement of the area with the unconformity, the fine scale layering of the sequence shows more clearly. [56B84; 80° N 339 °W ]

[130] A Dune Field in Borealis Chasma. Dark dune-forming materials appear to have been transported away from the pole in a curving stream extending from the top of this frame. They arc accumulated in an approximately triangular dune mass that occupies the center of the mosaic. The sinuous ridges in the dune mass rotate in a clockwise direction through an angle of approximately 45° from the northern to the southern margin. The discontinuous dark texture on the right side arises from partial dune cover. Perennial ice is visible near the top of the frame and associated with the crater near tile bottom of the frame. The bright patch near center right may be a cloud. [58B21- 34; 48° N, 52° W]

[131]

Sand Dunes at the Rim of the North Polar Cap. The dunes form a sharp-edged, dark band near the bottom of this image. Martian sand is dark, unlike Earth sands which are usually light colored. This shows the minerals in Martian rocks most resistant to erosion are the dark ones. The center of the image shows a flat desert region. At the upper right are a region of mottled terrain of unknown origin, a strip of layered terrain (its layering clearly visible in this view). and a pinkish-white region of polar frost. [IPL, ID:I2398AX; 81° N, 83° W]

[132]

(a)

[133] Widespread North Polar Edge Sand Dune Fields. (a) Dunes here have a consistent trend (approximately north-south) with minor sinuosity, branching and merging. Vague circular forms are probably buried craters, and bright spots within the ridges are ice deposits. (b) Dunes with much more variation in direction also occur; a shorter wavelength and greater sinuosity appear in this dune field which adjoins and in places appears to be mantled by frost deposits. Vague circular forms again are probably buried craters. The bright patches of ice near the upper left are associated with a distinct change in the dune pattern, possibly indicating that the deposits of ice preceded the development of the present dune pattern. (c) Transition from a transverse ridge structure to isolated linear and equant dunes. [(a) 59B32; 81° N, 141° W, (b) 58BO1; 80° N, 120 ° W, (c) 58B28; 78°N, 50° W]

(b)

(c)

[134] Photomosaic of South Pole. At the left of this photomosaic is the remnant south polar cap of Mars. Until recently, evidence suggested that its composition was water ice like the remnant north polar cap. New temperature measurements, however, suggest that it may be carbon dioxide ice. Extending from beneath the polar cap to the bottom of the frame are large, lobate expanses of glacioaeolian deposits with wind- scoured surfaces. At their northern margins, these deposits overlap and partially fill a number of craters. They also mantle the entire southern wall of a huge impact basin, 800 km in diameter, which is approximately at the center of the photomosaic. The unburied d part of the basin rim, or rampart, forms a mountainous, semicircular arc, with plains in the interior and a rugged landscape of large craters stretching to the north. At the smallest scales, the polar terrains exhibit mysterious patterns and textures which can possibly be attributed to volcanic and wind action, and to cyclical climate change. [383B04-75, 211-554 ]

[135]

Layered Materials Unconformably on Cratered Terrain near the South Pole. Layered material with a smooth, uncratered surface partly covers a 40-km diameter crater in the upper half of the picture. Strings of secondary craters around the larger crater are also transected by the layered deposits. [383B50; 81°S, 271° W]

[136]

Secondary Craters in Layered Materials Close to the South Pole. Layered deposits are shown in the lower half of the picture but have been eroded away in the upper half to form a low scarp to the north which is illuminated by the Sun. Numerous secondary craters occur in the layered deposits around a partly eroded crater. The relations suggest that the crater formed after the layered deposits but before the erosional episode that formed the north-facing scarp. Part of the remnant cap is visible in the lower left. [421B79; 85° S, 352° W]

Sinuous Ridges on the South Polar Plains. The origin of these ridges is unclear. They branch and rejoin like river channels, and somewhat resemble terrestrial eskers (ridges formed by deposits from subglacial rivers), but a volcanic or tectonic origin is more likely here. Similar features occur elsewhere on the planet, such as on the floor of Argyre. [421B53; 78° S, 40° W]

[137]

Pitted terrain near the South Pole. Some areas peripheral to the layered deposits at the south pole appear to be deeply etched with numerous irregularly shaped depressions inset into a formerly planar surface. The depressions may form by collapse after melting of ground ice or, alternately, they may be simply deflation hollows formed by removal of material by the wind. Similar features do not occur at the north pole. [390B90; 77°S, 74°W]