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  • LUNAR AND PLANETARY EXPLORATION  (25)
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  • 1
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    Seismic energy release of the moon (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Lunar seismicity is investigated by calculating various source parameters for a number of shallow and deep-focus moonquakes. The seismic moment, seismic energy release, annual seismic energy release, stress drop, and body-wave magnitude are determined for the largest shallow moonquakes and for large deep-focus events. It is found that the shallow events dominate the lunar seismic energy release, that tidal dissipation may account for the energy release by the deep-focus events, and that the stress drops for the deep-focus events are comparable to or smaller than the calculated tidal stresses. A comparison of the results with terrestrial data indicates that the seismic characteristics of a planet are controlled more by tectonic style and state than by the relative magnitude of the driving forces.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 86; Jan. 10
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  • 2
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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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  • 3
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    Seismic codas on the earth and the moon - A comparison (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The seismic codas, representing that part of the seismogram following the arrival of the surface waves or the direct S waves, characteristic of the earth and moon are compared with regard to the implications of coda characteristics for scattering and attenuation. Scattering models based on single S-S scatter theory, in which seismic energy in the coda is assumed to be S waves that have undergone only one scattering without conversion, and on diffusion theory, which assumed that energy in the coda has been scattered many times, are examined, and situations where the single-scattering and multiple-scattering theories are applicable are distinguished by the ratio of attenuation distance to the scattering mean free path. Values of the attenuation distance and the mean free path derived from coda studies for the earth and moon are compared, and it is found that for the frequency range 0.5-10 Hz, diffusion scattering is important in lunar codas, while at frequencies from 1 to 25 Hz single scattering is important in terrestrial codas. It is pointed out that attenuation acts to eliminate scattering paths much longer than the attenuation length. The observed differences between terrestrial and lunar codas are thus attributed to lesser attenuation and greater scattering on the moon.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Physics of the Earth and Planetary Interiors; 26; Sept
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  • 4
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    Structure of the lunar crust at highland site Apollo Station 16 (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The seismic crustal structure of the moon is known in the region of Oceanus Procellarum from the analysis of artificial impact data. To extend this knowledge we have used data from natural lunar seismic events to search for secondary seismic wave arrivals in the form of peg-leg multiples caused by reflections at crustal interfaces and converted arrivals caused by refractions at crustal interfaces. A polarization filter has been applied to the data to enhance the rectilinear particle motion expected for the onset of these body wave arrivals in the scattered coda of lunar seismograms. The results of this work tentatively indicate that the highland site at station 16 has a 75 km thick crust with an intermediate 20 km interface, compared to the 60 km crust and 20 km interface in Oceanus Procellarum, a mare region. Since the 20 km upper crust appears to exist at both highland and mare sites, it probably does not represent a mare basalt layer but rather a more general feature of the crust. Crustal thickness may partially control elevation by isostasy.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 8; Jan. 198
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  • 5
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    Thermal evolution of Venus (1984)
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    Publication Date: 2019-06-28
    Description: A modification of the Boussinesq fluid assumption is the basis of the present theory of three-dimensional and finite amplitude convection in a viscous spherical shell with temperature- and pressure-dependent physical parameters. The theory is applied to the definition of thermal evolution models for Venus which emphasize the effects of certain physical parameters on thermal evolution, rather than the specific thermal history of the planet. It is suggested that a significant portion of the present temperature in the mantle and surface heat flux of Venus is due to the decay of a high temperature that was established in the planet at the completion of its core formation, and that Venus has been highly convective over the course of its history, until about 0.5 Ga ago.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Physics of the Earth and Planetary Interiors (ISSN 0031-9201); 34; 232-250
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  • 6
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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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  • 7
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    Seismology on Mars (1977)
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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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  • 8
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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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  • 9
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    Lunar seismology - The internal structure of the moon (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: It is pointed out that seismology has provided the most detailed information concerning the structure and state of the earth's interior. Beginning in 1969, seismometers were landed on the moon by the Apollo missions, providing the first opportunity to attempt similar studies on another planetary body. In September 1977 the operation of these instruments was terminated. A description is presented of the internal structure of the moon, as determined from the obtained lunar seismic data. The analysis of the lunar data is approached in a systematic fashion, using appropriate techniques to minimize the number of necessary assumptions, extract the maximum amount of structural information, and determine its reliability. The completed lunar seismic network consists of four stations located at the landing sites of Apollo missions 12, 14, 15, and 16. Attention is given to crustal structure, the structure of the lunar mantle, the attenuating region, and the core.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 86; June 10
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  • 10
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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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