David H. Scott

Latent outflow activity for western Tharsis, Mars: Significant flood record exposed

Observations permitted by the newly acquired Mars Observer Laser Altimeter data have revealed a system of gigantic valleys northwest of the huge Martian shield volcano, Arsia Mons, in the western hemisphere of Mars (northwestern slope valleys (NSVs)). These features, which generally correspond spatially to gravity lows, are obscured by veneers of materials including volcanic lava flows, air fall deposits, and eolian materials. Geologic investigations of the Tharsis region suggest that the system of gigantic valleys predates the construction of Arsia Mons and its extensive associated lava flows of mainly late Hesperian and Amazonian age and coincides stratigraphically with the early development of the outflow channels that debouch into Chryse Planitia. Similar to the previously identified outflow channels, which issued tremendous volumes of water into topographic lows such as Chryse Planitia, the NSVs potentially represent flooding of immense magnitude and, as such, a source of water for a northern plains ocean.

Mars: Paleostratigraphic restoration of buried surfaces in Tharsis Montes

Volcanism in the Tharsis province of Mars occurred in several different areas and was generally continuous without large time intervals between eruptive episodes. Major lava flow units are numerous and extensive, but relatively thin. In many places, impact craters on buried surfaces project above younger flows that overlie them. A new application of crater dating methods has been developed to aid in the identification of these buried surfaces and to determine their lateral extent. The technique is especially adaptable to the Tharsis region where the stratigraphic succession of major flow units has been established by detailed geologic mapping. Knowledge of the overall stratigraphy allows correlations to be made between known and unknown surfaces by comparing their crater frequencies at diameters large enough to insure their recognition on the buried unit. The method has been applied to aid in the restoration of buried rock units and to construct a series of paleostratigraphic maps showing the sequence of major eruptive events in the Tharsis region.

Martian paleolakes and waterways: Exobiological implications

The problems of how warm and wet Mars once was and when climate transitions may have occurred are not well understood. Mars may have had an early environment similar to Earths that was conductive to the ermergence of life. In addition, increasing geologic evidence indicates that water, upon which terrestrial life depends, has been present on Mars throughout its history. This evidence suggests that life could have developed not only on early Mars but also over longer periods of time in longer lasting, more clement local environments. Indications of past or present life most likely would be found in areas where liquid water existed in sufficient quantities to provide for the needs of biological systems. We suggest that paleolakes may have provided such environments. Unlike the case on Earth, this record of the origin and evolution of life has probably not been erased by extensive deformation of the Martian surface. Our work has identified eleven prospective areas where large lacustrine basins may once have existed. These areas are important for future biological, geological, and climatological investigations.

Global geologic mapping of Mars: The western equatorial region

Abstract Global geologic mapping of Mars was originally accomplished following acquisition of orbital spacecraft images from the Mariner 9 mission. The mapping program represented a joint enterprise by the U.S. Geological Survey and other planetary scientists from universities in the United States and Europe. Many of the Mariner photographs had low resolution or poor albedo contrast caused by atmospheric haze and high-sun angles. Some of the early geologic maps reflect these deficiencies in their poor discrimination and subdivision of rock units. New geologic maps made from higher resolution and better quality Viking images also represent a cooperative effort, by geologists from the U.S. Geological Survey, Arizona State University, and the University of London. This second series of global maps consists of three parts: 1) western equatorial region, 2) eastern equatorial region, and 3) north and south polar regions. These maps, at 1:15 million scale, show more than 60 individual rock-stratigraphic units assigned to three Martian time-stratigraphic systems. The first completed map of the series covers the western equatorial region of Mars. Accompanying the map is a description of the sequence and distribution of major tectonic, volcanic, and fluvial episodes as recorded in the stratigraphic record.

Mars, highlands-lowlands: Viking contributions to mariner relative age studies

Stratigraphic relations between lowland plains and highlands, two major types of Martian geologic-terrain units, were not directly distinguishable on Mariner-9 images. Morphologic characteristics and crater densities suggested that the lava plains beneath their eolian cover were younger than adjacent highland rocks, which form a plateau bounded in many places by highly dissected escarpments. Alternatively, the lowland plains could be the older unit and represent a broad erosional surface exhumed by southward retreat of the highlands along their frontal scarp. Viking photos across five areas of the highland-lowland boundary, however, tend to confirm the younger age of the plains-forming lava flows. A time interval of several hundred million years probably occurred between the retreat of the highland scarp and its latest embayment by lava extrusions in the lowlands.

The geologic setting of the Luna 16 landing site

Abstract The Luna 16 landing site is similar in its geologic setting to Apollos 11 and 12. All three sites are located on basaltic mare fill which occurs mostly within multi-ring basins formed by impact earlier in the moons history. A regolith developed by impact bombardment is present at each of these sites. The regolith is composed mostly of locally derived volcanic material, but also contains exotic fine fragments that have been ballistically transported into the landing sites by large impact events which formed craters such as Langrenus and Copernicus. These exotic fragments probably consist mostly of earlier reworked multi-ring basin debris and, although not directly traceable to individual sources, they do represent a good statistical sample of the composition of most of the premare terrac regions.

Moon-Mercury: Relative preservation states of secondary craters

Abstract Geologic mapping of the Kuiper quadrangle of Mercury and other geologic studies of the planet indicate that secondary craters are much better preserved than those on the moon around primary craters of similar size and morphology. Among the oldest recognized secondary craters on the moon associated with craters 100 km across or less are those of Posidonius, Atlas and Plato; these craters have been dated as middle to late Imbrian in age. Many craters on Mercury with dimensions, morphologies and superposed crater densities similar to these lunar craters have fields and clusters of fresher appearing secondary craters. The apparent differences between secondary-crater morphology and parent crater may be due in part to: (1) rapid isostatic adjustment of the parent crater; (2) different impact fluxes between the two planets; and (or) (3) to the greater concentration of Mercurian secondaries around impact areas, thereby accentuating crater forms. Another factor which may contribute to the better state of preservation of Mercurian secondaries relative to the moon is the difference in crater ejecta velocities on both bodies. These velocities have been calculated for fields of secondary craters at about equal ranges from lunar and Mercurian parent craters. Results show that ejection velocities of material producing most of the secondary craters are rather low (