Robert P. Sharp

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.

Antarctic Ice Sheet: Stable Isotope Analyses of Byrd Station Cores and Interhemispheric Climatic Implications

Oxygen- and hydrogen-isotope analyses from the core hole through the Antarctic Ice Sheet at Byrd Station define temperature variations over more than 75,000 years. Synchronism between major climatic changes in Antarctica and the Northern Hemisphere is strongly indicated. The Wisconsin cold interval extended from 75,000 to 11,000 years ago. Three intra-Wisconsin warmer phases were all colder than pre- or post-Wisconsin times, which suggests that North American and Eurasian continental ice sheets did not disappear at any time during the Wisconsin.

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.

Pleistocene Ventifacts East of the Big Horn Mountains, Wyoming

Ancient ventifacts are abundant on gravel-mantled erosion surfaces along the eastern base of the Big Horn Mountains. Stones of varied lithologic characteristics and from less than 1 inch to 6 feet in diameter have wind-cut surfaces displaying the pitting, fluting, grooving, and luster that is characteristic of ventifacts. Faces making angles greater than 550 with the wind are pitted, and faces at lower angles are grooved and fluted. As many as twenty separate faces have been cut on a single stone. In this area such multiple faces cannot be explained by wind-splitting or by variable winds, so they are attributed to changes in stone position caused largely by congeliturbation (frost action) and wind scour. Ventifacts rest on erosion surfaces at four levels, 25-325 feet above stream grade. On the lower surfaces wind-cut stones are abundant and fresh, but on the higher surfaces they are sparse and deeply weathered. At least four separate periods of wind-cutting are indicated by this evidence. Cutting is attributed to sand picked up from barren flats and hurled against stones lying in the same environment. Conditions most favorable for wind-cutting prevailed during times of glaciation in the mountains, when winds were strong and floods of meltwater inundated parts of the piedmont, producing broad barren flats mantled by sand and gravel. Ventifacts on the lower erosion surfaces are probably Wisconsin, and those on the highest surfaces may be pre-Wisconsin. Pleistocene wind directions have been measured at twenty-nine separate localities by careful reference to wind-cut faces on large, presumably stable, boulders. The mean Pleistocene wind direction so determined is N. 290 W. Modern winds are also consistently from the northwest, and it appears that local orographic control was supreme in the Pleistocene as now. Neither glaciers in the Big Horn Mountains nor the continental ice sheet, 250 miles north, exerted much influence on local wind directions.

Periglacial Involutions in Northeastern Illinois

Certain beds of glaciofluvial sand, silt, and clay in the upper Illinois Valley are complexly deformed into variously shaped masses of silt and clay intruded into sand. Rounded forms and downward intrusions predominate. Individual structures are termed involutions, and layers of deformed beds are called involution layers-equivalents of the Brodelböden and Brodelzonen of German writers. Deformation has not been observed below a depth of 12 feet and may extend within 3 or 4 feet of the surface. Single involution layers are from a few inches to 6 feet thick. These involutions are attributed to intense differential freezing and thawing and to the development and melting of masses of ground ice. It is postulated that this occurred in a surficial layer above perennially frozen ground in an area peripheral to a glacier where periglacial (arctic) conditions prevailed. The involutions are believed to have been formed when the ice front lay 30-50 miles to the northeast and are dated as late Cary (late Middle Wisconsin).

Semiquantitative Differentiation of Glacial Moraines near Convict Lake, Sierra Nevada, California

Geometry, setting, and lithological constitution make Pleistocene moraines near Convict Lake well suited for development and testing of semiquantitative methods of temporal differentiation. The methods applied consist of surface-boulder frequency, weathering of granitic surface boulders, grain-size distribution in the upper 12 inches of these deposits, and color and pH variations in the upper 24r-36 inches. Weathering of granitic boulders is one of the more sensitive and consistent temporal indicators. It is evaluated on two bases: the preservation of remnants of original abrasion surfaces and the development of fretted forms by granular disintegration. On Tioga deposits in this area, an average approaching 25 per cent of all granitic surface boulders is judged unweathered on the abrasion basis and nearly 90 per cent on the fretted basis. On Tahoe deposits, the average values are about 3 and 50 per cent, respectively. Application of these techniques to moraines near Convict Lake is more than a calisthenic exercise. A huge, 1,000-foot-high lateral moraine extending east-northeast from Convict Lake has been attributed by earlier workers largely to the early Pleistocene Sherwin glaciation, and a massive lobate complex of loop and lateral moraines, due north of Convict Lake, has been regarded as a composite, late Pleistocene, Tahoe-Tioga feature. Semiquantitative data indicate that the lobate complex is the product of a single glaciation, the Tioga, and that the large lateral is primarily a Tahoe feature. Both are thus late Pleistocene and presumably younger than a nearby

Oxygen-Isotope Ratios in Antarctic Snow, Firn, and Ice

285,000 pm 30,000

Features of the Firn on Upper Seward Glacier St. Elias Mountains, Canada

year old rhyolitic mass. No Tenaya or Sherwin deposits are recognized near Convict Lake, and the semiquantitative methods used are not effective in denning the temporal position of the possibly still older McGee till.