James A. Cutts

Preliminary mariner 9 report on the geology of Mars

Abstract Mariner 9 pictures indicate that the surface of Mars has been shaped by impact, volcanic, tectonic, erosional and depositional activity. The moonlike cratered terrain, identified as the dominant surface unit from the Mariner 6 and 7 flyby data, has proven to be less typical of Mars than previously believed, although extensive in the mid- and high-latitude regions of the southern hemisphere. Martian craters are highly modified but their size-frequency distribution and morphology suggest that most were formed by impact. Circular basins encompassed by rugged terrain and filled with smooth plains material are recognized. These structures, like the craters, are more modified than corresponding features on the Moon and they exercise a less dominant influence on the regional geology. Smooth plains with few visible craters fill the large basins and the floors of larger craters; they also occupy large parts of the northern hemisphere where the plains lap against higher landforms. The middle northern latitudes of Mars from 90 to 150† longitude contain at least four large shield volcanoes each of which is about twice as massive as the largest on Earth. Steep-sided domes with summit craters and large, fresh-appearing volcanic craters with smooth rims are also present in this region. Multiple flow structures, ridges with lobate flanks, chain craters, and sinuous rilles occur in all regions, suggesting widespread volcanism. Evidence for tectonic activity postdating formation of the cratered terrain and some of the plains units is abundant in the equatorial area from 0 to 120° longitude.Some regions exhibit a complex semiradial array of graben that suggest doming and stretching of the surface. Others contain intensity faulted terrain with broader, deeper graben separated by a complex mosaic of flat-topped blocks. An east-west-trending canyon system about 100–200 km wide and about 2500 km long extends through the Coprates-Eos region. The canyons have gullied walls indicative of extensive headward erosion since their initial formation. Regionally depressed areas called chaotic terrain consist of intricately broken and jumbled blocks and appear to result from breaking up and slumping of older geologic units. Compressional features have not been identified in any of the pictures analyzed to data. Plumose light and dark surface markings can be explained by eolian transport. Mariner 9 has thus revealed that Mars is a complex planet with its own distinctive geologic history and that it is less primitive than the Moon.

Geological framework of the south polar region of Mars.

Abstract The first 4 months of Mariner 9 photography of the south polar region are discussed. Three major geological units have been recognized, separated by erosional unconformities. From oldest to youngest they are: cratered terrain, pitted plains, and laminated terrain. The latter unit is unique in occurrence to the polar region, volatiles are probably involved in its origin, and may still be present within the laminated terrain as layered ice. The residual south polar cap has been observed to survive the disappearance of the thin annual CO2 frost deposit and to last virtually unchanged in outline through the southern summer. That exposed deposit is inferred to be composed of water-ice. The residual cap appears to lie at the apex of an unusual quasi-circular structure composed of laminated terrain; a similar structure also appears to exist near the north pole.

Stratigraphic relationships within Martian polar cap deposits

Abstract Deposits of layered ice and dust accumulate on the smooth and banded terrains of the north and south polar cap deposits, while erosion of equator-facing slopes of layered terrain expose these layers. Banded terrain occurs at exposures of irregular layers resulting from slight erosion of the feather edges of layers deposited on pole-facing trough walls. These deposits unconformably overlap onto adjacent layered terrain. Stripping of layers occurs in areas of strong, multidirectional wind, forming striped terrain . Local wind scour has also produced steep scarps with crescentic planform, often associated with dune fields positioned immediately downwind. Unconformities are common within layered deposits in areas of complex topography at the north pole. These indicate that episodes of deposition of 5–10 layers alternate with slight erosion of pole-facing trough walls. Troughs that strike north-south alternate between erosional widening and deepening and depositional infilling, creating multiple unconformities. Troughs near the equatorward limit of polar deposits increase in relief by deposition on adjacent smooth terrain and, locally, by erosional deepening. Troughs may migrate poleward, replacing the relief increase by depositional infilling when they near the poles.

North Polar Region of Mars: Imaging Results from Viking 2

During October 1976, the Viking 2 orbiter acquired approximately 700 high-resolution images of the north polar region of Mars. These images confirm the existence at the north pole of extensive layered deposits largely covered over with deposits of perennial ice. An unconformity within the layered deposits suggests a complex history of climate change during their time of deposition. A pole-girdling accumulation of dunes composed of very dark materials is revealed for the first time by the Viking cameras. The entire region is devoid of fresh impact craters. Rapid rates of erosion or deposition are implied. A scenario for polar geological evolution, involving two types of climate change, is proposed.

Preliminary results from the Viking orbiter imaging experiment

During its first 30 orbits around Mars, the Viking orbiter took approximately 1000 photographic frames of the surface of Mars with resolutions that ranged from 100 meters to a little more than 1 kilometer. Most were of potential landing sites in Chryse Planitia and Cydonia and near Capri Chasma. Contiguous high-resolution coverage in these areas has led to an increased understanding of surface processes, particularly cratering, fluvial, and mass-wasting phenomena. Most of the surfaces examined appear relatively old, channel features abound, and a variety of features suggestive of permafrost have been identified. The ejecta patterns around large craters imply that fluid flow of ejecta occurred after ballistic deposition. Variable features in the photographed area appear to have changed little since observed 5 years ago from Mariner 9. A variety of atmospheric phenomena were observed, including diffuse morning hazes, both stationary and moving discrete white clouds, and wave clouds covering extensive areas.

Mariner 9 television reconnaissance of Mars and its satellites: Preliminary results

At orbit insertion on 14 November 1971 the Martian surface was largely obscured by a dust haze with an extinction optical depth that ranged from near unity in the south polar region to probably greater than 2 over most of the planet. The only features clearly visible were the south polar cap, one dark, spot in Nix Olympica, and three dark spots in the Tharsis region. During the third week the atmosphere began to clear and surface visibility improved, but contrasts remained a fraction of their normal value. Each of the dark spots that apparently protrude through most of the dust-filled atmosphere has a crater or crater complex in its center. The craters are rimless and have featureless floors that, in the crater complexes, are at different levels. The largest crater within the southernmost spot is approximately 100 kilometers wide. The craters apparently were formed by subsidence and resemble terrestrial calderas. The south polar cap has a regular margin, suggsting very flat topography. Two craters outside the cap have frost on their floors; an apparent crater rim within the cap is frost free, indicating preferentia loss of frost from elevated ground. If this is so then the curvilinear streaks, which were frost covered in 1969 and are now clear of frost, may be low-relief ridges. Closeup pictures of Phobos and Deimos show that Phobos is about 25 �5 by 21 �1 kilometers and Deimos is about 13.5 � 2 by 12.0 �0.5 kilometers. Both have irregular shapes and are highly cratered, with some craters showing raised rims. The satellites are dark objects with geometric albedos of 0.05.

Models of climate cycles recorded in Martian polar layered deposits

Abstract Layered strata observed in orbital images of polar sedimentary terrains on Mars have been hypothesized to contain information about recent variations in the Martian climate. Celestial mechanicians have determined that Mars has experienced large-amplitude quasi-periodic variations in obliquity and eccentricity: planetary meteorologists have inferred that the resulting changing patterns of solar heating must have had dramatic effects on Martian climate. Accordingly, it might be possible to link the known recent history of orbit/axial variations with the physical characteristics of the most recently deposited strate determined from orbital imagery. Our paper explores that possibility quantitatively for various models of the formation of the polar layered deposits and the layering developed within them. We modeled the formation of polar layered deposits during the past 10 my for two general classes of model. Uniform deposition rate (UDR) models assume that the major constituent of the layered deposits is deposited at a uniform rate and that layering is established by climate-induced variations in the concentration of a minor constituent. Climate-modulated deposition rate (CMDR) models assume that the deposition rate of the major constituent is controlled by climate and that layer boundaries correspond to intervals of nondeposition. In both models, abrupt changes in deposition regime occur when a function of the orbit/axial parameters crosses a threshold value. Detailed physical mechanisms for the control of sedimentation by orbit/axial variations were developed for two specific models of the CMDR type. In one model, the deposits are assumed to be primarily composed of dust; in the other, they are assumed to be predominantly water ice. We calculated the times at which abrupt changes in deposition regime occur and determined the relative thicknesses of successive layers which can be compared against layer thicknesses made from orbital imagery. In all these models, obliquity variations exert the dominant influence on the sequential variations in layer thickness, although eccentricity and precession index (longitude of perihelion) may take recognizable contributions as well. The layer patterns for the north and south polar deposits are identical for the dust model and only slightly different for the ice model, but this is at least partly a consequence of simplifying assumptions. The sequential variation in layer thickness is also very sensitive to those model parameters which control the fraction of time for which deposition occurs. When the climate threshold for deposition is low, deposition occurs a large fraction of the total time and characteristic patterns form with one thick layer and a number of much thinner ones. Such patterns would be easily recognized in photogeological measurements of layer thickness. When the threshold for deposition lies in the mid-range with respect to excursions of the climate function, deposition occurs approximately half of the time, and there is only slight modulation of layer thickness. When the threshold for deposition is high, deposition occurs only a small fraction of the total time, and groups of layers form which are separated form other groups by long periods of nondeposition. The stratigraphic unconformities that separate groupings of five to ten layers in the polar deposits may indicate that the Martian polar region has experienced such a depositional history. The feasibility of using these model results to verify the theory of climate control of polar stratigraphy on Mars and to gain additional insights into the significant processes is considered. The stratigraphy issues raised here are also explored in the context of recent work on the interpretation of stratigraphy of ocean margins. From this comparison there emerge some valuable insights into the stratigraphic effects of periods of nondeposition and the uncertaintiesin interpreting images of exposed polar strata on Mars.

Topography and stratigraphy of Martian polar layered deposits

Abstract Topography and stratigraphy of young sediments in the Martian north polar region are being studied from high-resolution Viking Orbiter images to test and constrain theories of terrain evolution driven by cyclical climate change. Several steepened slopes upon which layers are exposed in section were found to have local relief of 200 to 800 m and slopes ranging from 1 to 8°. First attempts to calculate average layer thicknesses from portions of sections yielded results ranging from 14 to 46 m, linearly correlated with slope. In some cases, under the assumption of short-range lateral continuity, a greater number of layers was inferred to be present than was actually resolved on a particular section. When apparent layer thickness are revised accordingly, the correlation with slope is destroyed. We conclude that images of the shallowest-sloping sections of layered deposits should yield the most stratigraphic information, and layers thinner than 14 m may occur; indeed, they may even be common or typical. Because of the sensitivity of summer frost distribution to local slope and other properties of exposed strata, interseasonal comparisons of images promise to enhance the capability to detect layers and differentiate them by material properties.

Photoclinometric determination of the topography of the Martian north polar cap

Abstract Photoclinometry is useful for the determination of topography for areas which have a uniform albedo. The technique is applied to early spring Viking images of the Martian north polar cap, taken when the surface was covered by a nearly uniform frost cover. Unlike earlier approaches, the topographic profiling can be used for surfaces with any photometric function, but the strike of the planetary surface relative to the illumination angle must be specified along the profile. The resultant profiles are relatively insensitive to misestimation of the photometric functin and slope orientation, but are quite sensitive to the assumed values of the reflectance of an equivalent level surface and the atmospheric opacity (if it is large).

Topography of Martian central volcanoes

Abstract New topographic maps of six large central volcanoes on Mars are presented and discussed. These features are Olympus Mons, Elysium Mons, Albor Tholus, Ceraunius Tholus, Uranius Tholus, and Uranius Patera. Olympus Mons has the general form of a terrestrial basaltic shield constructed almost entirely from lava flows; but with 20 to 23 km of relief it is far larger. Flank slopes average about 4°. A nominal density calculated from the shield volume and the local free-air gravity anomaly is so high that anomalously dense lithosphere probably underlies the shield. Uranius Patera is a similar feature of much lower present relief, about 2 km, but its lower flanks have been buried by later lava flood deposits. Elysium Mons has about 13 km of local relief and average slopes of 4.4°, not significantly steeper than those of Olympus Mons. Its upper flank slopes are significantly steeper than those of Olympus Mons. We suggest Elysium Mons is a shield volcano modified and steepened by a terminal phase of mixed volcanic activity. Alternatively, the volcano may be a composite cone. Albor Tholus is a partially buried 3-km-tall shield-like construct. Ceranius and Uranius Tholus are steeper cone-like features with relief of about 6 and 2 km, respectively. Slopes are within the normal range for terrestrial basaltic shields, however, and topographic and morphologic data indicate burial of lower flanks by plains forming lavas. These cones may be lava shield constructs modified by a terminal stage of explosive activity which created striking radial patterns of flank channels. Differences among these six volcanoes in flank slopes and surface morphology may be primarily consequences of different terminal phases of volcanic activity, which added little to the volume of any construct, and burial of shallow lower flanks by later geologic events. Additional topographic data for Olympus Mons, Arsia Mons, and Hadriaca Patera are described. The digital techniques used to extract topographiv data from Viking Orbiter stereo images are also described.