6,536 research outputs found
Informal names for surface features in the Apollo 15 landing area
Informal names and descriptions of lunar topographic features in Apollo 15 landing regio
Some geologic observations concerning lunar geophysical models
The distribution of lunar geologic units in space and time and their mode of origin were considered since they provided significant data which bear on a number of current problems in lunar geophysics. Observations and problems were discussed which deal with the characterization of the upper 25 km of the lunar crust, the tectonic style of the crust, the formation of mascons within major basins, analysis of lunar magnetic anomalies, and the history of the lunar crust
Derivation of topographic feature names in the Apollo 15 landing region
Location and derivation of nomenclature for lunar topographic features in Apollo 15 landing regio
Significant achievements in the planetary geology program, 1975 - 1976
Developments in planetology research as reported at the 1976 NASA Planetology Program Principal Investigators' meeting are summarized. Topics range from solar system evolution, comparative planetology, and geologic processes to techniques and instrument development for future exploration
The distribution and modes of occurrence of impact melt at lunar craters
Numerous studies of the returned lunar samples as well as geologic and remote-sensing investigations have emphasized the importance of impact melts on the surface of the Moon. Information concerning the distribution and relative volumes is important for (1) an improved understanding of cratering processes, (2) kinetic energy estimates and energy partitioning studies, (3) the proper interpretation of melt-bearing lunar samples, and (4) comparative planetology studies. The identification of major flows of fluidized material associated with impact craters on the surface of Venus has increased interest in impact melt flows on the other terrestrial planets. For a number of years, we have been investigating the distribution, modes of occurrence, and relative and absolute amounts of impact melt associated with lunar craters as well as the manner in which melt volumes vary as a function of crater size, morphology, and target characteristics. The results of this effort are presented
Rift systems on Venus: An assessment of mechanical and thermal models
The formation and distribution of major tectonic features on Venus are closely linked to the dominant mechanism of lithospheric heat loss. Among the most spectacular and extensive of the major tectonic features on Venus are the Chasmata, deep linear valleys generally interpreted to be the products of lithospheric extension and rifting. Systems of chasmata and related features can be traced along several tectonic zones up to 20,000 km in linear extent. Mechanical and thermal models for terrestrial continental-rifting are applied to the rift systems of Venus. The models are tested against known topographic and tectonic characteristics of Venus chasmata as well as independent information on the physical properties of the Venus crust and lithosphere
Microscopic basis for pattern formation and anomalous transport in two-dimensional active gels
Active gels are a class of biologically-relevant material containing embedded
agents that spontaneously generate forces acting on a sparse filament network.
In vitro experiments of protein filaments and molecular motors have revealed a
range of non- equilibrium pattern formation resulting from motor motion along
filament tracks, and there are a number of hydrodynamic models purporting to
describe such systems. Here we present results of extensive simulations
designed to elucidate the microscopic basis underpinning macroscopic flow in
active gels. Our numerical scheme includes thermal fluctuations in filament
positions, excluded volume interactions, and filament elasticity in the form of
bending and stretching modes. Motors are represented individually as bipolar
springs governed by rate-based rules for attachment, detachment and
unidirectional motion of motor heads along the filament contour. We
systematically vary motor density and speed, and uncover parameter regions
corresponding to unusual statics and dynamics which overlap but do not
coincide. The anomalous statics arise at high motor densities and take the form
of end-bound localized filament bundles for rapid motors, and extended clusters
exhibiting enhanced small-wavenumber density fluctuations and power-law
cluster-size distributions for slow, processive motors. Anomalous dynamics
arise for slow, processive motors over a range of motor densities, and are most
evident as superdiffusive mass transport, which we argue is the consequence of
a form of effective self-propulsion resulting from the polar coupling between
motors and filaments.Comment: 14 pages, 17 figures. Minor clarifications and updated/additional
references. To appear in Soft Matte
Thermal buoyancy on Venus: Preliminary results of finite element modeling
Enhanced surface temperatures and a thinner lithosphere on Venus relative to Earth have been cited as leading to increased lithospheric buoyancy. This would limit or prevent subduction on Venus and favor the construction of thickened crust through underthrusting. In order to evaluate the conditions distinguishing between underthrusting and subduction, we have modeled the thermal and buoyancy consequences of the subduction end member. This study considers the fate of a slab from the time it starts to subduct, but bypasses the question of subduction initiation. Thermal changes in slabs subducting into a mantle having a range of initial geotherms are used to predict density changes and thus their overall buoyancy. Finite element modeling is then applied in a first approximation of the assessment of the relative rates of subduction as compared to the buoyant rise of the slab through a viscous mantle
Oxidized basalts on the surface of Venus: Compositional implications of measured spectral properties
Venera Lander reflectance data are compared with high temperature spectra of the same basaltic materials. The dark, flat unoxidized basalts are still inconsistent with the Venera data in the near-infrared. Basaltic material with a ferric component, however, would satisfy both the increase in reflectance beyond 0.7 microns as well as the dark, relatively colorless character in the visible. Therefore, it is concluded that besaltic surfaces of Venus represented by these measurements either contain minerals with uncommon characteristics, or, more likely, are relatively oxidized
Significant achievements in the Planetary Geology Program
Developments reported at a meeting of principal investigators for NASA's planetology geology program are summarized. Topics covered include: constraints on solar system formation; asteriods, comets, and satellites; constraints on planetary interiors; volatiles and regoliths; instrument development techniques; planetary cartography; geological and geochemical constraints on planetary evolution; fluvial processes and channel formation; volcanic processes; Eolian processes; radar studies of planetary surfaces; cratering as a process, landform, and dating method; and the Tharsis region of Mars. Activities at a planetary geology field conference on Eolian processes are reported and techniques recommended for the presentation and analysis of crater size-frequency data are included
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