Engineering topics
Publications and source records attributed to Squyres, Steven W..
The morphology and morphometry of a number of coronae and related features on Venus are discussed with reference to new data from the Magellan spacecraft. The specific features discussed are concentrated in the portion of Venus imaged during about the first three months of Magellan mapping. Sequences of events and basic geophysical processes involved in the formation of these features are inferred.
As far as we know, Earth is the only planet in our solar system that supports life. It is natural, therefore, that our understanding of life as a planetary phenomenon is based upon Earth-like planets. There are environments in the solar system where liquid water, commonly believed to be a prerequisite for biological activity, may exist in a distinctly non-Earth-like environment. One such location is Europa, one of the Galilean satellites of Jupiter. The possibility that liquid water exists on Europa presents us with some interesting exobiological implications concerning the potential of the satellite to support life. Topics include the following: an ocean on Europa; thermal evolution of Europa; Europa's three models; exobiological implications; early conditions of Europa; low-temperature abiotic chemistry; possibility of the emergence of life on Europa; prerequisites for the habitability of Europa; energy sources for biosynthesis and metabolic activity; habitability of Europa by anaerobic life; and habitability by aerobic life.
A quantitative analysis of error sources in 1D planetary photoclinometry is presented. The technique is affected by error sources arising from the spacecraft image, the planetary body, and the scan line orientation. Slope errors are calculated for each of these sources, using examples of Voyager imaging of Ganymede and Viking orbiting imaging of Mars. Slope errors are investigated for a variety of viewing and lighting geometries, slope angles, and slope orientations. The results are broken down into nonsystematic and systematic errors. Derivations that allow the calculation of photoclinometric slope errors for any photometric function are presented, and the implications of these results for 2D photoclinometric techniques are discussed.
Thermal stress histories of the Saturnian and Uranian satellites are investigated. To this end, the thermal evolution of an icy satellite subjected to accretional and radiogenic heating, thermal conduction, and solid-state convection is modeled, and changes in the internal stress that occur during satellite evolution are examined. Results show that internal temperature changes that occur during normal evolution of many of the satellites of Saturn and Uranus can be expected to generate large extensional stresses in the satellites' outer regions. These stresses arise from three sources: (1) radiogenic warming, causing thermal expansion of materials in the satellite's deep interior; (2) radiogenic warming in larger satellites that can induce a phase transition from ice II to ice I and to produce a volume increase in the deep interior; and (3) accretional heating depositing heat in the satellite'e outer regions.
High resolution radar images from the Magellan spacecraft have revealed the first details of the morphology of the Lavinia Planitia region of Venus. Lavinia is a broad lowland over 2000 km across, centered at about 45 deg S latitude, 345 deg E longitude. Herein, the tectonic evolution of Lavinia is discussed, and its possible relationship to processes operating in the planet's interior. The discussion is restricted to the region from 37.3 to 52.6 deg S latitude and from about 340 to 0 deg E longitude. One of the most interesting characteristics of Lavinia is that the entire region possesses a regional tectonic framework of striking regularity. Lavinia is also transected by a complex pattern of belts of intense tectonic deformation known as ridge belts. Despite the gross topographic similarity of all of the ridge belts in Lavinia, they exhibit two rather distinct styles of near surface deformation. One is composed of sets of broad, arch-like ridges rising above the surrounding plains. In the other type, obvious fold-like ridges are rare to absent in the radar images. Both type show evidence for small amounts of shear distributed across the belts.
Coronae and ovoids on Venus were first identified in Venera 15/16 data. They are distinctive and apparently unique to the planet, and may be important indicators of processes operating in the Venusian mantle. Magellan images have provided the first high resolution views of coronae and ovoid morphology. Herein, the general geologic character is described of coronae and ovoids, and some inferences are drawn about their geologic evolution. Coronae are circular to elongate features surrounded by an annulus of deformational features, with a relatively raised or indistinct topographic signature and, commonly, a peripheral trough or moat. Ovoids are circular to elongate features other than coronae with either positive or negative topographic signatures, associated with tectonic deformation and volcanism. The relationship of these two geologic features to each other and to Venusian geology is briefly discussed.
Grooves are the dominant structural features on Ganymede. While single grooves are found in many areas, it is somewhat more common to find them grouped together in groove sets (groupings of grooves with common structural trends). It is generally believed that the grooves are extensional features. Their underlying geologic nature cannot be determined from Voyager images, however. It appears likely that they are grabens, but the images are insufficient to rule out the possibility that they are modified extension fractures or some kind of ductile necking features. The oldest materials within the Nun Sulci and Perrine quadrangles occur within the cratered dark materials unit, as is generally the case for all of Ganymede. This is the most widespread of the dark units mapped in these two quadrangles. Crater densities within cratered dark materials are consistent with an age on the order of several billion years, and thus cratered dark terrain probably represents crust that has survived from the end of the primordial intense bombardment phase of solar system history. The brief geologic history is greatly oversimplified as a result of the poor resolution and unfavorable viewing geometry of the images covering the area.
Dark and light terrain materials in the Perrine and Nun Sulci quadrangles are divided into nine map units, four dark, and five light. These are placed in time-stratigraphic sequence primarily by means of embayment and cross-cutting relationships. Dark terrain is generally more heavily cratered and thus older that light terrain but, at least in these quadrangles, crater densities are not reliable indicators of relative ages among the four dark material units. The four mapped material units within dark terrain are: cratered dark materials (dc), grooved dark materials (dg), transitional dark materials (di), and dark materials, undivided (d). The five mapped units within light terrain are: intermediate light materials (li), grooved light materials (lg), irregularly grooved light materials (lgl), smooth light materials (ls), and light materials, undivided.
Viking orbiter images are presently used to calculate approximate volumes for the inflow valleys of the ancient cratered terrain of Mars; a sediment-transport model is then used to conservatively estimate the amount of water required for the removal of this volume of debris from the valleys. The results obtained for four basins with well-developed inflow networks indicate basin sediment thicknesses of the order of tens to hundreds of meters. The calculations further suggest that the quantity of water required to transport the sediment is greater than that which could be produced by a single discharge of the associated aquifer, unless the material of the Martian highlands was very fine-grained and noncohesive to depths of hundreds of meters.
A model is presented for the mechanism of the formation of crater palimpsests (circular features of very little relief) on Ganymede, that resolves some of the inconsistencies present in previous hypotheses of the palimpsest formation. It is suggested that palimpsest formation may be a result of cratering during a period of vigorous convection within the satellite early in the Ganymede's history. The morphology of palimpsests is reviewed and an updated compilation of palimpsests distribution and dimensions is obtained on the basis of final Voyager data. Calculations are performed that show that palimpsest formation is expected when large impacts take place in an icy lithosphere that is underlain by a zone of warm buoyant vigorously convecting material. The implications of the model for the thermal history of Ganymede is discussed.
The origin of the numerous terraces on the flanks of the Olympus Mons volcano on Mars, seen on space images to be arranged in a roughly concentric pattern, is investigated. The images of the volcano show that the base of each terrace is marked by a modest but abrupt change in slope, suggesting that these terraces could be thrust faults caused by a compressional failure of the cone. The mechanism of faulting and the possible effect of the interior structure of Olympus Mons was investigated using a numerical model for elastic stresses within a Martian volcano, constructed for that purpose. Results of the analysis supports the view that the terraces on Olympus Mons, as well as on other three Martian volcanoes, including Ascraeus Mons, Arsia Mons, and Pavonis Mons are indeed thrust faults.
Radar sounding observations of Titan and the other outer solar system icy satellites have been conducted in order to ascertain the materials present, their dielectric properties, and the degree of radar signal penetration. Deep penetration is possible at long wavelengths, while strong reflections may occur due to pockets or layers of H2O-NH3 solution. Attention is given to the case of Titan, using models to calculate radar reflection S/N ratios; the models involve H2O ice covered by a 100-m thick layer of organic sediments that are overlain by an ocean of liquid CH4-C2H6-N2. At 13.6-cm wavelength, a greater-than-10 dB S/N is computed from the ocean-sediment and sediment-ice interfaces, suggesting that the geology of Titan may be investigated by such a sounding mode despite deep ocean coverage of the satellite.
Taking the heat-transport physics of ice-covered lakes in the Dry Valleys of Antarctica as a model, it is presently suggested that liquid water lakes could have persisted for significant periods under protective ice covers in the Valles Marineris depressions of Mars. Calculations of ground ice thermodynamic stability in a Martian setting indicate that they may exist close to the surface at high latitudes, but are able to persist near the equator only at substantial depths. Such Martian landforms as terrain-softening are attributable to the creep of the Martian regolith under the influence of ground-ice deformation; FEM modeling of the flow process implies terrain-softening to be a near-surface phenomenon.
The problem of relaxation of very large impact craters on icy satellites is addressed and the extent to which such studies can help place constraints on the nature of such satellite interiors is investigated. Very general calculations aimed at understanding the nature of relaxation of large impact structures, including the directions, relaxation velocities, and stress levels, are presented. The dependence of relaxation on such factors as silicate core size and viscosity gradients in the ice is examined. The general results are used to address whether comparing the current morphology of impact basins to estimates of their original shape will yield an understanding of the thermal and mechanical structure of the interiors of the icy satellites. It is found that the relaxation rates derived from models of satellite interiors can provide constraints on viscous layer thicknesses. High thermal gradients can permit substantial relaxation even in thin viscous layers. Finally, the constraints on the internal structure of Tethys arising from the extremely relaxed state of the Odysseus basin and the existence of Ithaca Chasma are discussed.
Crater distributions on Voyager images of Saturn's satellites are analyzed and models of relative cratering rates on these bodies are developed on the basis of orbital dynamics. A history of satellite bombardment, disruption, and resurfacing in the Saturn system is constructed, concentrating on Rhea. It is suggested that the surface of Rhea has approached saturation equilibrium for craters with diameters up to 32 km. Also, the disruption probabilities of Saturn's inner moons are estimated.
Voyager images of the Uranian satellites Ariel and Miranda show flow features with morphologies indicating that ice has been extruded to the satellites' surfaces in the solid state. These images provide the first observational evidence for solid-state ice volcanism in the solar system. Topographic profiles have been measured across a number of flow features on Ariel. With a simple model of extrusion, spreading, and cooling of a viscous flow, the initial viscosity of the flow material is found to have been no more than about 10 to the 16th poise, far lower than expected for H2O ice at the ambient surface temperatures in the Uranian system. Sharply reduced viscosities may have resulted from incorporation of ices like NH3 or CH4 in the Uranian satellites.
A model developed specifically for accretional heating of small icy satellites of massive planets was used to investigate the effects of various parameters on the accretional heating of the Saturn and Uranus satellites. It was shown that accretional heating is completely dominated by cisplanetary impactors, with substantial deposition of heat occurring well below a satellite's surface. Three parameters that have large uncertainties were found to affect accretional temperature profile: the Safronov parameter for cisplanetary impactors, the fraction of impactor energy deposited below the surface, and the exponent in the impactor size-distribution equation. For most satellites, the depth of maximum heating and the maximum temperature reached have smooth positive dependences on satellite size. Accretional heating of these satellites is insufficient to melt H2O ice, but, if NH3 or CH4 are present, accretional heating may produce a warm buoyant mobile zone tens of kilometers below the surface.
Martian magnetic history; quarantine issues; surface modifying processes; climate and atmosphere; sampling sites and strategies; and life sciences were among the topics discussed.