ISSN:0148-0227    

DOI:10.1029/JB078i020p04007

年代:1973

数据来源: WILEY

摘要:

Mariner 9 acquired pictures of all of Mars at a resolution of 1–3 km; 1–2% of the planet is covered by pictures of 100‐ to 300‐meter resolution. From these data, preliminary 1:5,000,000 scale photomosaics have been made of the entire planet, and a 1:25,000,000 scale shaded relief map published. Geologic maps of the planet have also been made at a variety of scales. The more than 7300 pictures acquired indicate that Mars is more varied and dynamic than previously inferred. About one half of the surface consists of ancient cratered terrain; the largest circular feature, Hellas, is almost twice the size of the largest basin on the moon, Imbrium. The remainder of the surface is covered either by younger volcanic rocks and constructs that stand as much as 17km above the mean level or by extensive tracts of plains deposits, some of which are sedimentary in origin. The volcanic piles with summit calderas have fresh flank flows and appear to be geologically young. The great equatorial chasm or canyon system, comparable in size to the East African rift valley system, terminates in a complexly faulted plateau to the west and in large patches of chaotic terrain on the east. Large fluvial channels originate in this chaotic terrain possibly by melting of permafrost and appear to flow northward into the Chryse region. Other large sinuous channels with many tributaries have no such obvious source areas and many small dendritic channel networks abound in the equatorial regions and imply possible collection of rainfall. In addition, many small lava channels with distinctive characteristics are present like those on the moon and earth. Many of the basin floors are underlain by lava flows inferred to be basaltic from the form of the flows, ridges, and domes that characterize their surface. The polar regions are covered by glacio‐eolian layered sediments that appear to be still forming under the present climatic regime. Older, layered, somewhat different deposits are being eroded into large pits and troughs around the margins of the poles. A mantle of eolian debris presumably derived from these eroded circumpolar zones thins equatorward. Both eolian erosional features such as yardangs and depositional features such as dunes have been identified. Eolian erosion and deposition processes are currently active, as is seen by numerous changes in the albedo patterns that were monitored after the clearing of the planetwide dust storm. The largest planetary scale differences in crustal style are between the southern highlands, presumably underlain by less dense rocks, and the northern lowlands or ‘oceanic’ basins, underlain by more dense rocks. The greatest difference along the equator is between the high ‘continental’ block of the Tharsis ridge with its volcanoes aligned along its margin and the oceanic floor of the Amazonis basin in which the Nix Olympica v

ISSN:0148-0227    

DOI:10.1029/JB078i020p04009

年代:1973

数据来源: WILEY

摘要:

A geologic map of Mars has been constructed largely on the basis of photographic evidence. Four classes of units are recognized: (1) primitive cratered terrain, (2) sparsely cratered volcanic eolian plains, (3) circular radially symmetric volcanic constructs such as shield volcanoes, domes, and craters, and (4) tectonic erosional units such as chaotic and channel deposits. Grabens are the main structural features; compressional and strike slip features are almost completely absent. Most grabens are part of a set radial to the main volcanic area, Tharsis.

ISSN:0148-0227    

DOI:10.1029/JB078i020p04031

年代:1973

数据来源: WILEY

摘要:

Some large channels on Mars show features, notably bars and braiding, that indicate an origin by the action of running water. Smaller channels on steep slopes may have been produced by runoff of precipitation. Dendritic canyon systems suggest ground water sapping, which may have been an effective agent in cliff retreat generally. Extensive plains developed as cliffs retreated and, although modified by later wind action, may be regarded as relict landforms from a fluvial stage of Martian history.

ISSN:0148-0227    

DOI:10.1029/JB078i020p04037

年代:1973

数据来源: WILEY

摘要:

Two classes of volcanic features occur on Mars: sparsely cratered plains resembling the lunar maria and circular constructs such as shields, domes, and craters. They show a markedly nonuniform distribution, being largely but not exclusively confined to one hemisphere. Only analogs of terrestrial intraplate volcanic features are found; analogs of terrestrial subduction zone features are absent. The Mars shield volcanoes are larger than their terrestrial counterparts, because the Martian crust is fixed in relation to the mantle, allowing more time for shields to grow. Volcanic activity has occurred throughout all the decipherable history of the planet.

ISSN:0148-0227    

DOI:10.1029/JB078i020p04049

年代:1973

数据来源: WILEY

摘要:

An assemblage of huge parallel troughs, individually up to 200 km wide and hundreds of kilometers long, forms a belt 500 km wide extending for 2700 km E 15°S through the Tithonius Lacus‐Coprates region of equatorial Mars. Some troughs are closed depressions more than 3 km deep, and the maximum trough depth may approach 6 km. These troughs, heading just southeast of the huge Tharsis volcanic ridge, are transitional westward into a festoon ofU‐shaped hollows and eastward into chaotic and fretted terrain. Parallel linear chains of rimless pits and shallow graben on the adjacent cratered upland suggest structural control of trough development by fractures in the Martian crust. Trough walls show scarring by slides, slumps, andU‐shaped avalanche chutes. Some are dissected into narrow sharp anastomosing ridges reminiscent of badland topography. Dendritic tributaries extend 150 km into the bordering upland. These tributaries and the mass movements causing recession of trough walls are attributed to a sapping process possibly involving the evaporation or melting of exposed ground ice. Trough floors range from chaotically rough to rolling and subdued. They are not smoothly graded. In one instance layered materials, possibly 2 km thick, compose a dissected trough floor tableland. Most troughs may be partly filled by such deposits. The major problem of trough genesis involves disposal of about 2 × 106km3of material. Running water, solution, deflation, and ground ice deterioration all appear to have significant limitations as the sole or principal agent of trough formation. Subsidence caused by magma withdrawal to supply the extensive nearby volcanic fields or the spreading of crustal plates are both quantitatively adequate. Crustal spreading would have significant implications concerning the conditions and the behavior of the Martian interior, and the lack of any obvious subduction zones would imply planetary expansion. In our ignorance concerning Mars both magma withdrawal and crustal spreading merit continued consideration in respect to trough formation, with some favor to the former because of the obvious large‐scale Martian volcanism. Ground ice may have played a significant subsidiary role through sapping to produce extensive recession of tro

ISSN:0148-0227    

DOI:10.1029/JB078i020p04063

年代:1973

数据来源: WILEY

摘要:

Fretted and chaotic terrains are members of a larger family of lowland terrains on the Martian surface. They have formed in equatorial and midnorthern latitudes, within areas reasonably proximate to the extensive volcanic fields of that region. Both are relatively recent developments. Fretted terrain is characterized by extensive, smooth, lowland plains separated from the old cratered upland by an abrupt escarpment of highly irregular planimetric configuration. Chaotic terrain features jumbled assemblages of large, irregular blocks occupying lowlands or depressions within the old cratered upland. Fretted terrain is thought to evolve by recession of a steep bounding escarpment, leaving a smooth lowland floor at a remarkably uniform level. Escarpment recession is speculatively attributed to undermining by evaporation of ground ice exposed within an escarpment face, or, under a different environment, by ground water emerging at its foot. The uniform floor level may reflect the original depth of frozen ground. Removal of debris shed by the receding escarpments could be by eolian deflation, subsequent to weathering by unknown processes to produce material of uniformly small grain size, or by fluvial transport under a vastly different climatic environment. Chaotic terrain is attributed primarily to localized collapse of the cratered upland owing to removal of subsurface material, either ground ice or magma. The ground ice hypothesis suffers from possible quantitative inadequacies, but evacuation of magma seems feasible because of the extensive volcanism in the northern hemisphere. Following collapse, ground ice sapping could have been effective in causing slumps and in modifying the collapsed blocks. Eventually the floors of some areas of chaotic terrain may have been smoothed to the condition of fretted terrain, as suggested by their intimate association. These terrains reflect significant activity on the Martian surface and within the Martian interior during relatively recent times. If liquid water has been the prime agent involved in developing fretted terrain, then it is a fossil feature and carries an implication of an enduring Martian environment vastly different from the environment at present.

ISSN:0148-0227    

DOI:10.1029/JB078i020p04073

年代:1973

数据来源: WILEY

摘要:

The Martian multiringed circular basins Argyre, Hellas, Libya, ‘Martian Schrödinger,’ Edom, and Iapygia resemble lunar basins of comparable size. There is a similar relation between basin size and number of rings; the larger the basin, the more rings it has and the wider their spacing. Spacing increases outward by a similar regular ratio. Features in the crater‐basin continuum apparently have multiple rings above about the same basin or crater diameter on both planets, 150–300 km. A ring at a given position relative to the basin center is morphologically similar in a Martian and a lunar basin. Radial structures are conspicuous around several Martian basins. Argyre is the large Martian basin most like large lunar basins; Hellas is least like them. Except in the young ‘Martian Schrödinger,’ ejecta from the basins and secondary impact craters have not been identified with certainty on Mars, probably because they are buried and modified and because of too high a sun illumination when photographed. The similarities between Martian and lunar basins are considered evidence for a common origin, proba

ISSN:0148-0227    

DOI:10.1029/JB078i020p04084

年代:1973

数据来源: WILEY

摘要:

Mars contains a range of crater types similar to the types found on the earth and on the moon and in varying states of preservation. Positive evidence of erosive loss of kilometer scale craters comes from the deficiency of craters on Mars relative to Phobos and Deimos. Although one hemisphere is dominated by large ancient craters, estimated to date back 3–4 aeons, the hemisphere centered on the volcanic region of Tharsis has a much younger surface attributed to volcanic activity about 0.3 aeon ago. The morphology and size distribution of craters in the latter region give very strong evidence that erosion and deposition rates on Mars have been much less in the last 0.6 aeon than in previous time. The major shield volcanoes and the south polar zone (south of −81° latitude) have surface ages of about 0.1 aeon for kilometer scale structures. The relative sequence of events described is more certain than the absolute ages, and the method of estimating absolute ages is revi

ISSN:0148-0227    

DOI:10.1029/JB078i020p04096

年代:1973

数据来源: WILEY

摘要:

Mariner 9 narrow angle pictures show that many areas on Mars are buried by debris mantles that later have been partially eroded. A survey of the present debris distributions shows that they are symmetrically disposed about the polar regions, extending poleward from the 30°N and 30°S latitudes. These blankets are inferred to represent eolian debris derived from circumpolar layered deposits; the process of equatorward redistribution of debris by wind action is probably still continuin

ISSN:0148-0227    

DOI:10.1029/JB078i020p04117

年代:1973

数据来源: WILEY