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  • 1975-1979  (16)
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  • 1
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    Planetary seismology and interiors (1979)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: This report briefly summarizes knowledge gained in the area of planetary seismology in the period 1969-1979. Attention is given to the seismic instruments, the seismic environment (noise, characteristics of seismic wave propagation, etc.), and the seismicity of the moon and Mars as determined by the Apollo missions and Viking Lander experiments, respectively. The models of internal structures of the terrestrial planets are discussed, with the earth used for reference.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 2
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    Moonquakes - Mechanisms and relation to tidal stresses (1977)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Observed features of moonquakes are combined with theoretical calculations of the tidal stresses to interpret the moonquake mechanisms. Tidal stresses, together with a postulated ambient tectonic stress, are sufficient to explain the depth, periodicity, and polarity reversal of moonquakes. Both of these stresses are small (on the order of 1 bar) and consistent with the small magnitudes of moonquakes.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 196; May 27
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  • 3
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    Seismology on Mars (1977)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: High-quality data (uncontaminated by lander or wind noise) obtained with a three-axis short-period seismometer operating on Mars in the Utopia Planitia region are analyzed. No large events have been detected during the first five months of operation covered in the present paper. This indicates that Mars is less seismically active than the earth. Winds, and therefore a seismic background, began to intrude into the nighttime hours, starting with sol 119 (sol is a Martian day). The seismic background correlates well with wind velocity, and is proportional to the square of the wind velocity, as is appropriate for turbulent flow. A local seismic event of a magnitude of 3 and a distance of 110 km was detected on sol 80. It is interpreted as a natural seismic event.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 82; Sept. 30
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  • 4
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    Variation in the number of meteoroid impacts on the moon with lunar phase (1975)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Data obtained with the Apollo 12 and 14 long-period seismometers in the period between December 1969 and January 1973 are used to determine the direction of approach and mass-distribution statistics of meteoroids in near-earth space. The total number of detected meteoroid impacts in this period is analyzed as a function of lunar phase with allowance for seismometer sensitivity and characteristics of lunar seismic-wave propagation. A logarithmic relation is derived which describes the mass-distribution statistics. It is concluded that most orbits for meteoroids with a mass in excess of 5 kg lie near the plane of the ecliptic with aphelia between 2 and 5 AU.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 2; July 197
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  • 5
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    Martian lithospheric thickness from elastic flexure theory (1978)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The thickness of the elastic lithosphere in the Tharsis region of Mars is estimated from effects due to the surface load of Olympus Mons. Deformation (vertical displacement) and stress are calculated using elastic flexure theory for a range of possible lithospheric thicknesses (T), modeling the lithosphere as a thin elastic shell and the interior as a Newtonian fluid. For T below 150 km, displacement and stress rise rapidly with decreasing thickness. For T near 100 km, deformation of the region surrounding the volcano would be clearly visible in the topography, and resulting tensional stresses exceeding 5 kbar should produce observable fracturing at the surface. In contrast, for T near 200 km deformation is minimal and the tensional stress, being less than a kilobar, would not result in extensive fracturing. Since significant deformation and fracturing are not observed, it is concluded that the Martian elastic lithosphere is at least 150 km in thickness. Seismic, tectonic, and gravity observations all suggest a thick Martian lithosphere as well.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 5; Nov. 197
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  • 6
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    Internal structure and properties of Mars (1977)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Theoretical physical models of the Martian interior are presented in the light of recent and revised data and constraints. These models include thermal evolution, densities, and seismic-wave velocities. The interior of Mars appears to be earthlike in many respects. Although thermal models indicate that Mars has passed its peak of evolution, it may still have an asthenosphere and may be moderately active tectonically. Mars has an Fe-FeS core with a radius of 1500-2000 km. The mantle is enriched in FeO with an olivine composition of about Fo75. Theoretically determined seismic-wave velocities are relatively well constrained in the mantle, with upper-mantle P-wave velocities ranging from 7.64 to 7.80 km/sec. However, there are wide variations in P-wave velocity in the core, dependent on composition. The shadow zone due to the core is larger than earth's.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus; 32; Sept
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  • 7
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    Elastic wave propagation in a highly scattering medium - A diffusion approach (1977)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The propagation of elastic waves in the moon, where the first seismograms were characterized by the presence of a long coda attributed to strongly scattered waves, is modeled with the aid of the time-dependent equation of radiative transfer. The average energy density as a function of time and space is described by the diffusion equation with linear dissipation on the assumption that all the energy present has been scattered many times and the time and distance scales of the problem are long compared to the scales of the scattering process. Ultrasonic experiments in the laboratory confirm the applicability of the formalism.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 8
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    The lunar interior - A summary report (1979)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The complete seismic data set collected by the Apollo network contains about 40 events which provide significant structural information on the lunar interior. The seismograms from these events yield a set of direct wave arrival times that constitutes the most reliable information on the seismic structure of the moon. Secondary data include possible reflected arrivals from crustal and mantle interfaces, an apparent shear wave shadow zone for surface events beginning at about 90 deg distance, and the shear wave amplitude decay with distance. Analysis of these data give well-constrained and stable average velocity values for the upper and lower mantle regions independent of most assumptions. The upper-lower mantle transition can begin no shallower than 400 km depth and may represent a compositional change although the effects of increased temperature cannot be ruled out.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Science Conference; Mar 19, 1979 - Mar 23, 1979; Houston, TX
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  • 9
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    The deep seismic structure of the moon (1977)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Data from 24 deep moonquakes are used to investigate the seismic structure of the lunar interior below the 300-500 km level. The deep moonquakes provide an uninterrupted ray-path coverage of the lunar mantle. Lower mantle seismic velocities are determined; the data suggest that moonquakes are confined to the near-side lower mantle. A compositional change rather than temperature effects is assumed to explain the upper mantle-lower mantle transition.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Science Conference; Mar 14, 1977 - Mar 18, 1977; Houston, TX
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  • 10
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    Seismic structure of the lunar mantle - An overview (1978)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The direct P and S wave arrival times from natural lunar seismic events are the most complete and reliable data set for determining the structure of the lunar mantle. A total of 40 events provide sufficiently well-observed arrivals to permit the extraction of structural information. Using this arrival time data set, the average velocities in a two-layered mantle with an assumed crustal structure (from Toksoz et al., 1974) have been obtained. Reflected phases arriving after direct S are then used to calculate the depth of the boundary between the two mantle layers, and to demonstrate that it is probably a complex transition zone approximately 80 km thick between 400 and 480 km depth. To determine velocity gradients in the upper mantle it is required that the model explain the pronounced decrease in shear wave amplitudes and accompanying delay in shear wave arrival times beyond a distance of about 90 deg. The final model is well-constrained.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Science Conference; Mar 13, 1978 - Mar 17, 1978; Houston, TX
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