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NASA SP-441: VIKING ORBITER VIEWS OF MARS

- THE GREAT EQUATORIAL CANYONS -

[17] VALLES MARINERIS is composed of steep-walled canyons, individually measuring up to 9 km deep, 250 km wide, and 1000 long. They were named for Mariner 9, the Mars-orbiting spaces raft that took the first pictures of the canyons in 1971. The entire Valles Marineris system extends over 4000 km from west to east near the Martian equator, and its size dwarfs all similar terrestrial features except, perhaps, the 5000-km-long midocean rift system.

Pictures taken by the Viking orbiters show large areas of Valles Marineris at a resolution far better than that achieved by Mariner 9. The new features observed indicate that, although erosional landforms (such as landslide scars and deposits) and tributary canyons are common, faulting apparently has been the dominant factor in canyon development ..

Since discovery of Valles Marineris, the method of their formation has been a nagging puzzle. The canyons do not form a well-integrated d drainage system; some are completely closed depressions, and lateral transport by wind or water would be considerably impeded. Now, however, the new evidence of faulting suggests that most negative relief results from subsidence. Low, straight scarps, which apparently indicate downward subsidence of canyon floors along faults, cut across erosional features on many canyon walls. Similar scale faulting occurs on Earth: in East Africa the continental crust is in tension across large rift valleys. Erosion of the Valles Marineris walls apparently continued into the recent past, so the crustal tension causing the faulting within the canyons may also have been a relatively recent phenomenon.

Another exciting discovery resulting from Viking images is the presence e of thick layered deposits on the floors of several canyons. Layered rock is also visible in the canyon walls, and thus is part of the Martian crust predating the canyons. Some materials on the canyon floors are distinctive for the fine scale and regularity of their layering. Only climatic modulation of a sedimentary process seems adequate to explain them. Possibly, regular changes in the Martian climate, governed by known orbital variations, have controlled the level of dust storm activity and the rate of deposition of sediment from the atmosphere. Another theory is that some sections of Valles Marineris were sites of lakes in which layered sediments were deposited. Before Viking, regularly layered deposits were known only in the polar regions of Mars, and their creation may also be associated with cyclical climatic change.

Impact craters, which are so numerous on other Martian terrains, are scare e within Valles Marineris. They appear most frequently on smooth areas of canyon floor, and are possibly the tops of blocks down-faulted from the upland plain. Shallow pits, possibly eroded impact craters, are abundant in other places. Impact craters are probably scarce in the canyons because erosion and deposition by landslides and wind have been actively renewing interior surfaces. No evidence of flow of water has been found within Valles Marineris, although some channels on the adjacent upland are abruptly truncated by steep canyon walls.

[19] Global View of the Valles Marineris. As Viking 1 approached Mars in June, 1976, it recorded this color picture showing Valles Marineris stretching more than 4000 km across the face of Mars. North is toward the upper right, and it is the winter season in the southern hemisphere. The annual southern ice cap extends up to 45°S latitude, blanketing the Argyre basin, an impact crater 800 km across. [IPL, ID I 2038MBVI, VERS2; 30°S, 70°W]

[20] Western Valles Marineris. Because these canyons are poorly linked with one another, and their floors not a regularly graded slope, they could not have formed as water drainage features. The straight alignments of many canyon walls, and the faulting in several directions associated with Noctis Labyrinthus, combine to suggest that the Valles Marineris system is composed of great rift valleys formed on the surface of a dome. At the summit of the dome, near the labyrinth, the crust was stretched in all directions to form a network of fault-bounded valleys. On the flanks of the dome, the greatest strains were concentric about the summit, giving rise to a set of radial rift valleys. The inset shows names of individual canyons (chasma). [40A37-52; 13° S, 87° W]

[21] Eastern Valles Marineris. The broadest valleys merge with patches of chaotic terrain, apparently the product of collapse and erosion of ancient cratered terrain. Channels beginning at the margins of chaotic terrain extend northeastward onto Chryse Planitia, the region in which Viking Lander 1 is located. The relationship between Valles Marineris and the chaos is not well understood. Irregular collapse, to form the chaos, may reflect crustal stresses similar to those forming the rift valleys, but differing in orientation and complexity. It is also possible that the chaos formed during the catastrophic release of liquid water derived from artesian reservoirs or the melting of ground ice. [32A11-15; 9°S, 53° W]

[22] View of Coprates Chasma Looking East-Southeast. Interplay of faulting and erosion within Valles Marineris is apparent here. A low, steep, and generally straight scarp occurs at the foot of the north canyon wall. The scarp apparently results from downward offset of the flat canyon floor relative to the wall. Gullies on the canyon wall are truncated by the scarp and thus predate it. In contrast, large-scale landsliding from two great canyon wall alcoves postdates the latest down-faulting; landslide debris has buried the scarp for a distance of about 40 km. [58A89-91; 15° S, 60° W]

[23] Evolution of Canyons by Landsliding. This mosaic of Viking Orbiter 1 pictures was taken looking southward toward the junction of Gangis and Capri Chasmae. Rugged d terrain consisting of numerous tilted blocks occurs within alcoves in the can! on wall. Extending from the alcoves are thin blankets of material with fan-like patterns of surface striations. Where two patterns intersect, one thin lobe clearly overlies the other and cuts off its striations. These features are giant landslide deposits that formed when sections of canyon rim collapsed. The broad thin lobes of material apparently flowed at high velocity from the bases of the collapsing masses. The mechanism by which the surface striations formed is not well understood. although similar features have been observed on terrestrial landslides. Groups of hills, similar to chaotic terrain, and sand dunes contribute to a varied canyon floor landscape. Layering in the upland rocks is evident near the top of the south canyon wall within the landslide alcoves. [14A29-32; 9°S, 42°W]

Stereogram of Ius Chasma. The south can! canyon wall here is incised by several large, branching tributary canyons that appear to have developed headward along joints or zones of weakness in upland rocks. It has been suggested that these canyons developed through a sapping process, in which the melting of ground ice caused the upland to collapse. The floor of the main canyon between A and B appears to have been fractured and down faulted below the level of the mouths of the tributary canyons. Layering can be seen in several places on the canyon walls (C and D ). [Left 66A07-09, Right 63A40: 7°S, 85°W]

		[24] Noctis Labyrinthus [left]. The origin of the canyon by faulting is most apparent in Noctis Labyrinthus at the western end of Valles Marineris. Many canyons have a classic graben form, with the upland plain surface preserved on the valley floor. Other canyons are more irregular in form and have rough floor terrains, evidently the consequence of landsliding and the puzzling process of pit formation. In places it appears that surface materials have sifted downward into a gaping hole in the subsurface. The inset shows a slope covered with light albedo dunes and several small landslide lobes. [46A13-28, 47A17-28, 48A21-28, 49A22-28, 50A14-28, Inset 62A64; 7°S, 100°W]

		[25] Stereogram of Central Tithonium Chasma [below]. This section of Tithonium Chasma is about 6 km deep. Overlapping landslide lobes cover the canyon floor and scarps that bound a rift valley within the canyon. On the south canyon wall, distinct bright and dark horizontal stripes are probably outcrops of layered rocks. Parallel chains of pits and graben mark the upland surface to the south. [Left 57A45, Right 64A22; 5°S, 85°W]

[26] Large Albedo Contrasts and Relief in Tithonium Chasma. Large contrasts in the brightness of surface materials can so confuse perception of depth in single frames that stereoscopic imaging is necessary to interpret surface features To the left is a long, narrow rift valley about 5 km deep. North of this valley the canyon floor is 1 to 2 km higher and irregularly mottled. A mountain with finely gullied flanks rises about 2 km from the canyon floor. This mountain is representative of many plateaus, ridges, and hills on the floors of broader canyons. They differ drastically from canyon wall materials in their patterns of erosion. Many are composed of materials with a distinct horizontal layering. [Left 44A27, Right 63A63; 5°S, 65°W]

West Candor Chasma. Here the canyon floor is entirely covered with eroded, layered materials. Layering is most prominent at A, B and C. An offset spur (D) and a low, steep scarp (EF) along the western walls of the canyon may have been formed by faults trending north to south across the main Valles Marineris. The tributary canyon at G seems to have developed by two processes; subsidence of a block of crust (graben), and irregular collapse into a string of pits. [65A25, 57; 66A17-27; 7°S, 75°W]

[27] Candor and Ophir Chasmae. Large plateaus (A, B), formed at least in part of regularly layered materials (as at A), rise from the floor of Candor and extend across the gap between Candor and Ophir Chasmae. At C, plateau materials apparently were deposited upon an eroded spur of the canyon wall and are now themselves being eroded away. Streamlines ridges and grooves in Ophir Chasma are probably wind sculpted. [66A23-30; 5° S, 73° W]

		[28] Plateaus between Ophir Chasma and Candor Chasma. These enormous, streamlined plateaus bridge the gap between the two canyons, possibly indicating wind erosion on a very large scale. Alternately, it has been suggested that a lake (or sea) once existed in the north section of Ophir Chasma until the canyon wall was breached southward to unleash an enormous flood. The dark material on the canyon floors is probably a wind deposit, as dune-like forms are visible in other images. [IPL ID, IV2515CGX2, I2515DGX2; 5°S, 73°W]

		[29] Layered Material in Juventae Chasma. A ridge of very uniformly layered light and dark materials rises from the floor of Juventae Chasma. Cyclical changes in sedimentation, perhaps modulated by climate, seem the most probable explanation for their origin. [85A15-17; 5°S, 62°W]