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  • LUNAR AND PLANETARY EXPLORATION  (1,076)
  • 1990-1994  (1,076)
  • 1992  (1,076)
  • 1
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    Gravity modeling of Mars and Venus at NASA/GSFC (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The paper describes the results from the development of improved high-resolution gravity models of Mars and Venus carried out at Goddard Space Flight Center, together with the solution design and the tracking data employed. These models are spherical harmonic expansions complete to degrees 50 and 36 for Mars and Venus, respectively. Both models result in improved orbital computation accuracies over previously available models, and in improved resolution of geophysical features. The paper also discusses new developments anticipated in the future.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: From Mars to Greenland: Charting gravity with space and airborne instruments - Fields, tides, methods, results (A93-55951 24-46); p. 11-27.
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  • 2
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    Properties of the gravity fields of terrestrial planets (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The properties of the gravity fields of the earth, Mars, and Venus, as expressed by spherical harmonic coefficients, are examined, using the harmonic expansions of the respective planetary topographies reported by Balmino et al. (1973), Bills and Ferrari (1978), and Bills and Kobrick (1985). The items examined include the spectral magnitudes and slopes of the gravity coefficients; the correlations between gravity and topography; and the correlations among different gravity harmonics, expressed by axiality and angularity. It was found that Venus differs from the other two planets in its great apparent depths of compensation, indicating a tectonics dominated by a stiff upper mantle. In addition, Venus has less activity deep in the mantle than do earth or Mars. Mars is marked by large gravity irregularities, as well as by their axial symmetry on a global scale. Although earth is probably the most peculiar planet, spherical harmonics do not bring out its varied characteristics. It is clearly a more active planet than Venus, with activity deep in the mantle. The lower magnitude of its higher harmonics is considered to be due to water recycled to the upper mantle.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: From Mars to Greenland: Charting gravity with space and airborne instruments - Fields, tides, methods, results (A93-55951 24-46); p. 1-10.
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  • 3
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    Vacuum melting and mechanical testing of simulated lunar glasses (1992)
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    Publication Date: 2019-08-28
    Description: Lunar silicate glasses may possess superior mechanical properties compared to terrestrial glasses because the anhydrous lunar environment should prevent hydrolytic weakening of the strong Si-O bonds. This hypothesis was tested by melting, solidifying, and determining the fracture toughness of simulated mare and highlands composition glasses in a high vacuum chamber. The fracture toughness, K(IC), of the resulting glasses was obtained via microindentation techniques. K(IC) increased as the testing environment was changed from air to a vacuum of 10 exp -7 torr. However, this increase in toughness may not result solely from a reduction in the hydrolytic weakening effect; the vacuum-melting process produced both the formation of spinel crystallites on the surfaces of the glass samples and significant changes in the compositions which may have contributed to the improved K(IC).
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Engineering, construction, and operations in space III: Space '92; Proceedings of the 3rd International Conference, Denver, CO, May 31-June 4, 1992. Vol. 2 (A93-41976 17-12); p. 1219-1231.
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  • 4
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    On the origin of the prograde rotation of the planets (1992)
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    Publication Date: 2019-08-28
    Description: A series of analytic and numerical calculations of the systematic component of angular momentum accretion is described. Wide ranges of planetesimal eccentricities and planetary radii are considered. Numerical simulations using a Rayleigh distribution of eH values show that very little prograde rotation is produced in a disk with a realistically broad range of planetesimal eccentricities. It is hypothesized that the observed spin rates may result from nonuniformities in the disk of planetesimals, specifically, a partial gap in planetesimal semimajor axes around that of the planet. This would yield an overabundance of impactors from the edges of the planet's accretion zone. Bodies from this region produce strongly prograde rotation at most values of eH.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Astronomicheskii Vestnik (ISSN 0320-930X); 26; 6; p. 54-78.
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  • 5
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    The Martian dust cycle (1992)
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    Publication Date: 2019-08-28
    Description: Martian dust plays a key role in determining the current climate of Mars, and is suspected of having had a major influence on the evolution of the surface and the history of climatic conditions on the planet. From spacecraft and ground-based observations, some understanding has been acquired of the styles and frequency of dust motion in the atmosphere and at the surface, of atmospheric dust loading, dust particle properties, and their variability. Limited constraints have also been derived for the volumes and ages of some potential dust reservoirs on the surface. These data are adequate to frame questions about the seasonal and long-term cycles of dust of Mars. Additional data are needed to place quantitative constraints on components of these cycles, on their possible linkage to one another, and to draw conclusions from the dust budget about the climate history of Mars.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 1017-1053.
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  • 6
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    Aeronomy of the current Martian atmosphere (1992)
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    Publication Date: 2019-08-28
    Description: The thermal structure of the Martian atmosphere, which varies diurnally, seasonally and episodically, is discussed. The atomic oxygen airglow at 1304 A is used to determine the density of atomic oxygen, and the 1216-A Lyman-alpha line is used to calculate the density of atomic hydrogen and, when coupled with the temperature measurement, the escape flux of atomic hydrogen. The most intense airglow is the IR atmospheric band of O2 at 1.27 micron that results from the photodissociation of ozone. The escape mechanism for atomic hydrogen is thermal, or Jeans, escape, while the atomic oxygen escape is caused by a nonthermal process, namely, the dissociative recombination of O2(+). The ratio of deuterium to hydrogen is enriched by a factor of 6. Three-dimensional models of the Mars thermospheric circulation show that planetary rotation has a significant effect on the wind, composition, and temperature structure.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 1054-1089.
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  • 7
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    The seasonal behavior of water on Mars (1992)
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    Publication Date: 2019-08-28
    Description: The current understanding of the seasonal water cycle on Mars is summarized. A brief history of the observations of water vapor in the Martian atmosphere and an outline of the seasonal water cycle are presented. Attention is given to the possible exchange of atmospheric water with nonatmospheric reservoirs of water, the role of transport of water through the atmosphere either as vapor or as condensate via the atmospheric circulation, and inferences about the behavior of the seasonal water cycle at other epochs, when the solar forcing would have been different from that at the present due to Mars having had different orbital elements. Measurements and analyses which are to play an important role in understanding the seasonal water cycle are summarized.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 969-1016.
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  • 8
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    The seasonal cycle of carbon dioxide on Mars (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Results of Viking investigations relevant to the CO2 cycle on Mars are presented, and the extensive modeling efforts directed towards understanding this cycle and its couplings to the seasonal cycles of water and dust are reviewed. It is found that winter condensation is suppressed either because the solid CO2 deposits are inefficient radiators or because of the scattering effects of CO2 clouds. With regard to the qualitative difference between the spring regression curves and between the wind systems for the two seasonal caps, it is suggested that the greater amount of aerosols in the atmosphere during northern winter results in a greater proportion of atmospheric condensation there than in the south. The seasonal pressure curves obtained by the Viking Landers during four Martian years are remarkably similar. It is suggested that the CO2 cycle is not sensitive to atmospheric dust and the meteorological variations accompanying global storms or that some subtle cancellation between different mechanisms suppresses variations.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 934-968.
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  • 9
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    Polar deposits of Mars (1992)
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    Publication Date: 2019-08-28
    Description: Layered deposits unique to the poles of Mars indicate long-term winter deposition of ice and sediment modulated by periodic climate change. The layered deposits contain at least two kinds of nonvolatile materials: one that is similar to the bright red dust raised in Martian dust storms, and one similar to dark sand dune materials observed at low and mild latitudes. The ice fraction of the deposits remains unknown. Because the present polar dust and ice budgets are poorly constrained, correlation of layer thicknesses with specific astronomical cycles remains very uncertain. Layered deposits in both polar regions appear to have been slightly more widespread in the past. Other sediments near the poles, and the estimated ages of the layered deposits, suggest that the polar sedimentary regime has evolved over the geologic history of Mars. Acquisition of good data on the vertical sequence of layer characteristics and better crater retention ages should allow revealing comparisons with astronomically driven cycles and secular atmospheric changes.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 767-795.
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  • 10
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    Dynamics of the atmosphere of Mars (1992)
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    Publication Date: 2019-08-28
    Description: The current understanding of the Martian atmospheric circulation and boundary layer is described. The available meteorological data and the first-order constraints that can be derived from them are reviewed. Pertinent aspects of atmospheric radiation on Mars are briefly presented; their main features are the short radiative time constraints on Mars, as compared to earth, and the important role of suspended dust in providing a potent thermal drive for the atmosphere. The current understanding of the Martian atmospheric circulation and its various components - the zonal-mean, zonal, and meridional flows, stationary and traveling planetary waves, atmospheric thermal tides, topographic wind systems, free modes, and gravity waves - is examined. Present-day knowledge of the PBL on Mars is reviewed, with particular attention given to the Viking Orbiter and Lander observations relating to the winds required for raising dust from the surface. The theory of the origin and decay of great dust storms on Mars is reviewed.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 835-933.
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