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  • 1
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    Comparison of observed and predicted gravity profiles over Aphrodite Terra, Venus (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Pioneer Venus orbiter (PVO) gravity profiles over Aphrodite Terra are compared with profiles predicted from models of thermal isostasy, mantle convection, and Airy compensation. Topography profiles across Aphrodite are compared to model spreading-ridge profiles in order to further assess this model. Airy compensation depths and convection-layer thicknesses are greater under eastern Aphrodite than western Aphrodite. Compensation depths in the east are greater than most estimates of lithospheric thickness, suggesting that this part of the ridge is dynamically supported. In parts of western Aphrodite, the spreading-ridge model gravity provides a better fit to the data than either Airy compensation or mantle convection. Best-fit spreading rates are between 0.3 and 1.6 cm/yr. Airy compensation and mantle convection cannot be distinguished in most places using only PVO data.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 301-315
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  • 2
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    An inversion of geoid and topography for mantle and crustal structure on Mars (1993)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: Mars has the largest amplitude geoid anomalies and surface topography known on the terrestrial planets. A number of prior studies have analyzed Martian gravity anomalies and topography in terms of isostasy and flexure of the crust and lithosphere. Other studies have emphasized the role of mantle convection in producing gravity anomalies and topography in some regions of Mars. Geoid and topography observations for simultaneous estimates of density anomalies in the crust and mantle of Mars are inverted. In performing this study, a recent degree 50 spherical harmonic expansion of the Martian gravity field (GMM-l) and a corresponding resolution expansion of the USGS Mars topography model are used. However, our analysis is restricted to harmonic degrees up to L equals 25, which are better determined than the higher harmonics. This provides a half-wavelength horizontal resolution of 425 km.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 2: G-M; p 797-798
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  • 3
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    Constraints on the depth and geometry of the magma chamber of the Olympus Mons Volcano, Mars (1990)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The summit caldera of the Olympus Mons volcano exhibits one of the clearest examples of tectonic processes associated with shield volcanism on Mars. The radial distance from the center of the transition from concentric ridges to concentric graben within the oldest crater provides a constraint on the geometry and depth of the subsurface magmatic reservoir at the time of subsidence. Here, researchers use this constraint to investigate the size, shape, and depth of the reservoir. Their approach consists of calculating radial surface stresses corresponding to the range of subsurface pressure distributions representing an evacuating magma chamber. They then compare stress patterns to the observed radial positions of concentric ridges and graben. The problem is solved by employing the finite element approach using the program TECTON.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Scientific Results of the NASA-Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles; p 321-322
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  • 4
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    Evolution of the Olympus Mons Caldera, Mars (1990)
    In:  CASI
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    Publication Date: 2013-08-31
    Description: Extensive high-resolution (15 to 20 m/pixel) coverage of Olympus Mons volcano permits the investigation of the sequence of events associated with the evolution of the nested summit caldera. The sequence of the intra-caldera events is well illustrated by image data collected on orbits 473S and 474S of Viking Orbiter 1. These data cover both the oldest and youngest portions of the caldera floor. The chronology inferred from the observations is presented which in turn can be interpreted in terms of the internal structure of the volcano (i.e., magma chamber depth and the existence of dikes).
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Scientific Results of the NASA-Sponsored Study Project on Mars: Evolution of Volcanism, Tectonics, and Volatiles; p 225-226
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  • 5
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    New Mars free-air and Bouguer gravity: Correlation with topography, geology and large impact basins (1993)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: Free-air and Bouguer gravity anomalies from a 50x50 field (MGM635), derived at the Goddard Space Flight Center, with global topography, geology, and the distribution of large impact basins was compared. The free-air gravity anomalies were derived from re-analysis of Viking Orbiter and Mariner 9 tracking data and have a spatial resolution of 250-300 km. Bouguer anomalies were calculated using a 50x50 expansion of the current Mars topography and the GSFC degree 50 geoid as the equipotential reference surface. Rotational flattening was removed using a moment of inertia of 0.365 and the corrections from Table B2 of Sleep and Phillips. Crustal density and mean density were assumed to be 2.9 and 3.93 gm/cm(sup 3). The spherical harmonic topography used has zero mean elevation, and differs from the USGS maps by about 2 km. Comparisons with global geology use a simplified map with about 1/3 the number of units on the current maps. For correlation with impact basins, the recent compilation by Schultz and Frey was used.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F; p 513-514
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  • 6
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    Spectral analysis of the gravity and topography of Mars (1993)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: New spherical harmonic models of the gravity and topography of Mars place important constraints on the structure and dynamics of the interior. The gravity and topography models are significantly phase coherent for harmonic degrees n less than 30 (wavelengths greater than 700 km). Loss of coherence below that wavelength is presumably due to inadequacies of the models, rather than a change in behavior of the planet. The gravity/topography admittance reveals two very different spectral domains: for n greater than 4, a simple Airy compensation model, with mean depth of 100 km, faithfully represents the observed pattern; for degrees 2 and 3, the effective compensation depths are 1400 and 550 km, respectively, strongly arguing for dynamic compensation at those wavelengths. The gravity model has been derived from a reanalysis of the tracking data for Mariner 9 and the Viking Orbiters, The topography model was derived by harmonic analysis of the USGS digital elevation model of Mars. Before comparing gravity and topography for internal structure inferences, we must ensure that both are consistently referenced to a hydrostatic datum. For the gravity, this involves removal of hydrostatic components of the even degree zonal coefficients. For the topography, it involves adding the degree 4 equipotential reference surface, to get spherically referenced values, and then subtracting the full degree 50 equipotential. Variance spectra and phase coherence of orthometric heights and gravity anomalies are addressed.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F; p 105-106
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  • 7
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    Free-air and Bouguer gravity anomalies and the Martian crustal dichotomy (1993)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: Free-air and Bouguer gravity anomalies from a 50x50 field, derived from re-analysis of Viking Orbiter and Mariner 9 tracking data and using a 50x50 expansion of the current Mars topography and the GSFC degree 50 geoid as the equipotential reference surface, with the Martian crustal dichotomy are compared. The spherical harmonic topography used has zero mean elevation, and differs from the USGS maps by about 2 km. In this field the dichotomy boundary in eastern Mars lies mostly at -1 to -2 km elevation. Bouguer gravity anomalies are shown on a map of Noachian, Hesperian, and Amazonian age terrains, simplified from current geologic maps. The map is centered at 300 deg W to show the continuity of the dichotomy boundary. Contour interval is 100 mgals. Gravity and topography were compared along approximately 40 profiles oriented parallel to the dichotomy boundary topographic gradient, to determine how the geophysical character of the boundary changes along its length and what this implies for its origin and development.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F; p 511-512
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  • 8
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    The physical volcanology of Mars (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The physical volcanology of Mars is reviewed, with particular attention given to the diversity of volcanic landforms, the implied styles of eruption associated with the construction of these landforms, the inferred internal structure of the volcanoes, and the influence that the eruptions have had on the Martian environment (both local and global in scale). Volcanism in the central highlands appears to have been explosive in character, while most of the constructional activity in the northern plains was effusive. Highlands volcanism appears to be relatively old compared to that in the northern hemisphere. There is evidence for the existence of large magma chambers and very high effusion rate eruptions on Mars. Tectonic deformation associated with volcanic constructs is primarily a consequence of loading and magma transport, while deformation in the volcanic plains reflects stresses associated with Tharsis and major impact basins.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 424-452.
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  • 9
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    Constraints on the lithospheric structure of Venus from mechanical models and tectonic surface features (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The evidence for the extensional or compressional origins of some prominent Venusian surface features disclosed by radar images is discussed. Using simple models, the hypothesis that the observed length scales (10-20 km and 100-300 km) of deformations are controlled by dominant wavelengths arising from unstable compression or extension of the Venus lithosphere is tested. The results show that the existence of tectonic features that exhibit both length scales can be explained if, at the time of deformation, the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base, and an upper mantle that was stronger than or nearly comparable in strength to the upper crust.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 92; E541-E55
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  • 10
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    Planetary geosciences, 1989-1990 (1992)
    In:  CASI
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    Publication Date: 2019-08-27
    Description: NASA's Planetary Geosciences Programs (the Planetary Geology and Geophysics and the Planetary Material and Geochemistry Programs) provide support and an organizational framework for scientific research on solid bodies of the solar system. These research and analysis programs support scientific research aimed at increasing our understanding of the physical, chemical, and dynamic nature of the solid bodies of the solar system: the Moon, the terrestrial planets, the satellites of the outer planets, the rings, the asteroids, and the comets. This research is conducted using a variety of methods: laboratory experiments, theoretical approaches, data analysis, and Earth analog techniques. Through research supported by these programs, we are expanding our understanding of the origin and evolution of the solar system. This document is intended to provide an overview of the more significant scientific findings and discoveries made this year by scientists supported by the Planetary Geosciences Program. To a large degree, these results and discoveries are the measure of success of the programs.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA-SP-508 , NAS 1.21:508 , LC-91-33408
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