摘要:

The Viking project launched two unmanned spacecraft to Mars in 1975 for scientific exploration with special emphasis on the search for life. Each spacecraft consisted of an orbiter and a lander. The landing sites were finally selected after the spacecraft were in orbit. Thirteen investigations were performed: three mapping experiments from the orbiter, one atmospheric investigation during the lander entry phase, eight experiments on the surface of the planet, and one using the spacecraft radio and radar systems. The experiments on the surface dealt principally with biology, chemistry, geology, and meteorology. Seventy‐eight scientists have participated in the 13 teams performing these experiments. This paper is a summary of the project and an introduction to the articles that follo

ISSN:0148-0227    

DOI:10.1029/JS082i028p03959

年代:1977

数据来源: WILEY

摘要:

The two Viking orbiters carried the two landers into orbit around Mars, observed the planet to certify the landing sites, released the landers for the landings, and subsequently served as telemetry relays for the lander data. In addition, they conducted scientific investigations using two cameras, an infrared radiometer for temperature measurements, an infrared spectrometer for water vapor measurements, and the radio communication system. The nature and extent of the orbiter observations have been influenced by the requirements for lander support, the capabilities of various orbiter subsystems, and the visibility of the planet from the orbits. All the orbiter scientific experiments are continuing.

ISSN:0148-0227    

DOI:10.1029/JS082i028p03971

年代:1977

数据来源: WILEY

摘要:

The plains to the south and southwest of Arsia Mons are composed of elongate lava flows, many of which appear to originate at a reentrant in the Arsia Mons edifice. Individual flows can be traced for up to 300 km. Their widths range from 5–6 km close to Arsia Mons to 40–60 km at distances greater than 400 km from the volcano. The age of the flows, as indicated by the number of superposed impact craters, increases systematically with increasing distance from Arsia Mons. The length of the flows varies with altitude of the vent: the higher the vent, the shorter the flow. The distribution of vents on Arsia Mons and Pavonis Mons suggests the presence of deep‐seated fractures trending NW‐SE and NE‐SW. Flows on the flanks of Olympus Mons extend far beyond the basal scarp, an indication of a much larger size for the volcano than was formerly believed. Around the basal scarp are numerous lobate features interpreted as landslides. Several types of flows are recognized on the flanks of Alba Patera. These include tube‐fed flows, sheet flows, and tube‐channel flows. The different types of flows are believed to indicate different eruption rates of a low‐viscosity lava similar to basalt in its Theolog

ISSN:0148-0227    

DOI:10.1029/JS082i028p03985

年代:1977

数据来源: WILEY

摘要:

Fluviatile and volcanic Martian channels, first discovered on Mariner 9 pictures, have been reexamined by using Viking orbital photography. The superior discrimination of the Viking photographs, resulting from clearer atmospheric conditions and an improved camera system, has permitted us to map additional channels and to estimate their relative ages, using a technique based on crater counting. Broad channels like the Ares and Tiu/Simud Valles are situated along the margin of the southern highlands near Chryse Planitia, the landing site of Viking 1. They originate in areas of collapsed terrain that may have been formed when subsurface water‐ice (permafrost) was melted by geothermal heat from deep‐seated volcanic centers. When permafrost melting reached an abrupt topographic slope, the interstitially stored meltwater ‘lakes’ were breached suddenly, releasing the great floods that modified the channels. The volume of material involved in the collapsed terrain is large enough to furnish the water calculated to have filled the broad channels. Conditions are reviewed for persistence of liquid water on Mars under present and more favorable pressures and temperatures. Sinuous channels of intermediate size, like the Ma'adim and Hrad Valles and other shorter, stubby channels, have multiple tributaries; in the limited coverage available, they appear to result from ‘spring sapping,’ with the underground permafrost meltwater emerging in box canyons at their heads. The widespread distribution of this type of channel makes their origin by local geothermal heating less likely; climatic warming may be required to explain their formation. The final fluviatile type, dendritic channel networks, has the widest areal distribution and appears to have been formed during at least two episodes. The filamentous channels in their source areas (often the rims of craters) seem to resemble terrestrial river systems; rainfall would seem to be required to form these features. All these channel types debouch onto lowland plains or crater floors, where they disappear in short distances; these abrupt terminations may have resulted from percolation and/or evaporation. Simple and complex lava channels are common; they originate at volcanic centers and are usually morphologically distinct from the aqueous channels. Three types of lava channels are recognized. The wide variation in crater densities implies varying channel ages. Water must have flowed on the Martian surface at many different times in the past, although this would be possible only with great difficulty under the present Martian thermal conditions. Based on a crater flux curve derived by Soderblom et al. (1974) the fluviatile channel ages vary from 3.5 to 0.5 Gy. Lava channel ages range from 3.5 Gy to an age too young to date by the crater counting technique (perhaps 200 m.y.). Methods for dating the channels and volcanic episodes are still insufficiently developed to determine whether episodes of volcanic heating and climatic change are coincident. It is possible that the large floods and volcanic eruptions might trigger a short ‘interglacial’ interval. Alternatively, the floods may be related to episodic volcanic activity, and the dendritic channels to rainfall that was associated with independent interglacial climatic episodes resulting from variations in solar output

ISSN:0148-0227    

DOI:10.1029/JS082i028p04016

年代:1977

数据来源: WILEY

摘要:

The outgassing history of Mars and the prevailing temperature conditions suggest that ground ice may occur to depths of kilometers over large areas of the planet. The presence of permafrost is also indicated by several topographic features that resemble those found in periglacial regions of the earth. East of Hellas and in the Protonilus and Nilosyrtis regions there are features that resemble those formed on earth by gelifluction, the slow creep of near‐surface materials aided by freeze‐thaw of ground ice. In the south part of Chryse Planitia there are irregular depressions that resemble thermokarst features, and the pattern of tributaries to the equatorial canyons is suggestive of a sapping process that would result from the melting of ground ice. The morphology of ejecta around fresh Martian impact craters is distinctively different from that around lunar and Mercurian craters. Such differences could be ascribed to the presence of ground ice in the target materials. The convergence of these different observations supports permafrost conditions not only at present but also for much of the planet's hist

ISSN:0148-0227    

DOI:10.1029/JS082i028p04039

年代:1977

数据来源: WILEY

摘要:

Several types of Martian impact craters have been recognized. The most common type, the rampart crater, is distinctively different from lunar and Mercurian craters. It is typically surrounded by several layers of ejecta, each having a low ridge or escarpment at its outer edge. Outward flow of ejecta along the ground after ballistic deposition is suggested by flow lines around obstacles, the absence of ejecta on top and on the lee side of obstacles, and the large radial distance to which continuous ejecta is found. The peculiar flow characteristics of the ejecta around these craters are tentatively attributed to entrained gases or to contained water, either liquid or vapor, in the ejecta as a result of impact melting of ground ice. Ejecta of other craters lacks flow features but has a marked radial pattern; ejecta of still other craters has patterns that resemble those around lunar and Mercurian craters. The internal features of Martian craters, in general, resemble their lunar and Mercurian counterparts except that the transition from bowl shaped to flat floored takes place at about 5‐km diameter, a smaller size than is true for Mercury or the moo

ISSN:0148-0227    

DOI:10.1029/JS082i028p04055

年代:1977

数据来源: WILEY

摘要:

The Valles Marineris, an enormous canyon system spanning more than one quarter of the equatorial girth of Mars, exhibits in its landforms the consequences of uniquely Martian extensional tectonics and a variety of erosional and depositional processes. Reported here are new insights into the evolution of the canyon system and possible evidence for cyclical climate change from the equatorial region. Tectonic control appears to be the fundamental influence on canyon form and evolution, but the style or intensity of tectonism appears to be regionally variable. In the region of the west and central canyons, chains of elongate pits, graben, and the actual troughs are all inferred to be manifestations of pronounced north‐south and secondary east‐west crustal extension. It is proposed that this region of the Valles Marineris is made up of a large number of discrete elongate blocks which have shifted vertically and tilted in relation to one another. Depending on the geometry of this adjustment the surface layer subsides as a coherent block or collapses to form a chain of pits. In the eastern canyons and chaotic terrain the tectonics appear to follow a different pattern, as the crustal layer breaks up into large patches of equant blocks, seemingly reflecting a lesser amount of crustal extension. Diverse wall features and varied landslide morphologies in the canyons indicate that material has been transported from the walls of the troughs to the floors, contributing to their widening. The paucity of small impact craters inside the canyons suggests that this process spans a large part of Martian history, perhaps even up to the present day. Small, sharply defined scarps cut erosional features on many canyon walls, suggesting that the driving force for canyon enlargement continues to be downfaulting. This conclusion is supported by the absence of evidence for excavation by exogenic process. Many troughs are segmented into closed basins, precluding lateral transport of debris except by aeolian deflation, for which there is little evidence. Substantial accumulations of regularly layered sediments have been recognized on the floors of at least two unconnected canyons. Cyclical variations in sedimentation rates or conditions are implied. The stratigraphic relationships among various layered and unlayered canyon interior materials as well as dune fields require that substantial deposition and reerosion have taken place within some canyons since their formation. The pervasive influence of extensional tectonics in the Valles Marineris leads us to doubt that the decay of ground ice or volcanism needs to be invoked to explain collapse in the eastern canyons or chaotic terrain. We find no persuasive evidence for former fluvial episodes in the trough system proper, where most erosional features can be accounted for by mass wasting. There are suggestions, however, that cyclical climate change has played a role in the development of layered materials in the canyons in the same way that it has apparently controlled layering of deposits in the polar regions. Prolonged tectonic activity in the Valles Marineris, on the east flank of the Tharsis‐Syria Rise, may correlate with prolonged volcanism and tectonism in evidence on the west flank. The long‐lived dynamic process in the deep Martian interior suggested by this relationship may also be responsible for large gravity anomalies associated with t

ISSN:0148-0227    

DOI:10.1029/JS082i028p04067

年代:1977

数据来源: WILEY

摘要:

On July 20, 1976, Viking 1 made the first successful landing on Mars in Chryse Planitia, a plains covered basin in the northern hemisphere. Viking orbiter pictures reveal more surface detail of the area and show the basin to be more complex than was seen on Mariner 9 images. The plains consist of areas with smooth and relatively uniform surfaces with prominent lunarlike mare ridges, mesas and plateaus, surfaces that appear to be ‘etched,’ fields of knobs, low shields that may be volcanic, and vast areas that have been subjected to channel‐forming processes. At least two sets of channels, originating from distant sources, terminate in Chryse Planitia. The four major units of the basin are basal hilly and cratered terrain, plateau material which can be divided into upper and lower units, lower smooth plains, and upper smooth plains. There is no evidence for the origin of the basin. Deposition of the plateau‐forming material to the east followed the period of bombardment, and since that time the history of the basin includes eruptions of flood lavas, channel formation, and deposition with at least two channel‐forming events. The later history of the basin includes possible local volcanic events, etching, and aeolian

ISSN:0148-0227    

DOI:10.1029/JS082i028p04093

年代:1977

数据来源: WILEY

摘要:

During the Viking mission, three broad areas of the northern plains were investigated as possible landing sites for Viking lander 2. We present a geological map of the area designated the B1 landing site in Cydonia centered on 45°N latitude, 4°W longitude. Viking imagery of this area has given detailed coverage, allowing the northern plains to be examined in more detail over wider areas than was possible from Mariner 9. Some plains areas with polygonal fracture patterns are considered to be pediments in ancient southern hemisphere rocks. The fracture patterns predate at least some of the younger northern plains material. Several northern plains units are distinguished as well as surface textures of unknown origin. Viking pictures provide good data for future studies of stratigraphy and surface process in this region and others like it in northern latitude

ISSN:0148-0227    

DOI:10.1029/JS082i028p04111

年代:1977

数据来源: WILEY

摘要:

The Viking primary mission, principally covering the northern summer, has provided observations of a season previously unstudied from close range. The morning formation stage of the clouds, apparently orographic, associated with the Tharsis Montes and Olympus Mons, shows northwest slope clouds developing in late morning, at which time, discrete clouds, indicative of convection in a ˜6‐km layer near the surface, are formed over much of the elevated terrain. Low‐level morning condensate clouds, or fogs, are associated with the canyons of Labyrinthus Noctis at this season. In Memnonia, at only 15°S, CO2frost condenses on the surface at night over a large area. The southern winter polar cap reaches 40°S, having an irregular margin about 10° in width. Frost deposits within the cap are nonuniform and patchy. Discrete clouds have been observed at mid‐latitudes in the south reaching 50‐km altitude. One example of a local dust storm has been detected, but generally, the season has been one of comparative calm, having a relatively clear atmosphere in the south and dust and condensate hazes i

ISSN:0148-0227    

DOI:10.1029/JS082i028p04121

年代:1977

数据来源: WILEY