Engineering topics

Thomas, P. G.

Publications and source records attributed to Thomas, P. G..

6 records

NASA NTRS1985

Eolian Erosion on Mars: Three Scales

Formation and erosion of deposits on Mars with vertical scales of 100 to 1,000 meters and horizontal scales of 100 km may have occurred at rates very different from present ones. Evidence for the rates on small scales comes from observations of dust storms and photometry of wind streaks. These observations cover scales less than 1 cm vertically and time scales less than 10 years. Intermediate scale features such as yardangs and sand dunes are more difficult to evaluate, but for dunes, plausible rates of transport may be calculated. Net eolian action in some areas may be so low that in order to study the large features one needs to select those likely to have been modified more rapidly and that exhibit distinctive contrasts with surrounding areas. Best suited for such study are relatively soft deposits overlying more resistant substrates. Viking Orbiter images of four of the largest such areas were examined to test how the erosional features correlate with patterns attributable to winds, properties of deposits, or non-eolian erosion, and for constraints on erosion and transport rates.

Thomas, P. G.↗

NASA NTRS1985

Global Tectonics on Ganymede

Three large basin surroundings on Ganymede located on grooved terrains, which are supposed to be intensely tectonized areas were studied. All rectilinear morphological elements such as ridges, block edges; and parts of scarps were mapped. The geometric properties (grid pattern) characteristics were determined and a history for the formation of the Ganymede basins studied is proposed. Results indicate that the grooved terrains are very surficial layers, and that their formation does not significantly affect, disturb or rotate the basement. This is in agreement with the conclusion obtained from completely different data (crosscutting relationships between groove sets and their basements).

Thomas, P. G.↗

NASA NTRS1985

Tectonics of the Galileo Regio on Ganymede

The surface of Ganymede consists of dark cratered terrain, and groved terrain. The dark cratered terrains form polygonal units, the largest of which is Galileo Regio, the surface of which is transected by furrows, smooth floored valleys bounded by relatively sharp parallel ridges. The most apparent of them are grouped together and form an apparently arcuate system of subparallel furrows which was mapped using Voyager pictures and plotted on a map using a stereographic projection. With this kind of projection, the main furrow system is not arcuate, but rectilinear. Observations strongly suggest that the Galileo Regio furrow systems are not of impact origin and appear to be irrelevant to discussions about the basins' morphology or evolution of planetary lithosphere determined from multiring structures.

Thomas, P. G.↗

NASA NTRS1985

Importance of the Tectonic Motions on Ganymede

Voyager close up views of Ganymede reveal some provinces of old, cratered, dark terrains that are broken into polygons and cut through by light stripes of grooved terrains. The groove terrain unit and its relationships with the other units was studied at various scales because these features seem to relate at least to two main scales of processes. Minor displacements concerning only local features, i.e., large scale processes and major displacements concerning usually the major evolution of the surface, i.e., small scale processes. It appears that tectonic processes, at least in the studied areas, if playing a role in the formation of local or regional features, are very limited. The conclusions reached agree with those of Thomas et al. who show that post grooves basins reveal the presence of a primitive grid that is not disturbed by the grooved terrains.

Forni, O. P.↗

NASA NTRS1985

Tectonics of the Valhalla Structure on Callisto

The surficial morphology of Callisto is dominated by large concentric patterns of ridges, scarps, and furrows. The largest one, Valhalla, shows a smooth central area 350 km in radius, surrounded by concentric sinuous ridges which extend as far as 600km from the center. Beyond the ridged area, scarps and furrows are observed as far as 2000 km. The global azimuthal distribution of the scarps and furrows of the outer ring (beyond 700 km from the center) was analyzed. It is proposed that the local geometry and the global distribution of scarps and furrows around Valhalla are the results of the reactivation of an old pattern by the Valhalla event. The coincidence of these local directions of a preexisting pattern (NW-SE and NE-SW) with the directions of the grids observed on most bodies observed so far in the solar system would indicate that this local pattern is a part of a global calistean grid.

Thomas, P. G.↗

NASA NTRS1985

Dust devils on Mars

Large columns of dust have been discovered rising above plains on Mars. The storms are probably analogous to terrestrial dust devils, but their size indicates that they are more similar to tornadoes in intensity. They occur at locations where the soil has been strongly warmed by the Sun, and there the surface is smooth and fine grained. These are the same conditions that favor dust devils on Earth. Warm gas from the lowest atmospheric layer converges and rises in a thin column, with intense swirl developing at the edge of the column. In one area a mosaic of Viking images shows 97 vortices in a three day period. This represents a density of vortices of about one in each 900 square kilometers. Thus, these dust devils may be important in moving dust or starting over dust storms.

Thomas, P. G.↗