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  • Weitere Quellen  (7)
  • ASTROPHYSICS  (5)
  • ASTRONOMY  (1)
  • Geophysics  (1)
  • 1
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    Dynamics of volatile delivery from outer to inner solar system (1994)
    In:  Other Sources
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    Publikationsdatum: 2019-01-25
    Beschreibung: Owen et al. propose that the Ar excess of Venus compared to Earth was caused by impact of a large icy planetesimal from the outer solar system, where temperatures were low enough for Ar to adhere to ice, about 30 K. A body of solar Ar/Si and C/H similar to Pluto and Triton less than 100 km diameter would suffice. However, direct delivery from the Uranus-Neptune zone to Venus would result in a very high approach velocity, causing erosion rather than accretion of volatiles. It would also be an extremely improbable event. Virtually all icy bodies scattered from the Uranus-Neptune zone to the terrestrial zone were strongly perturbed by Jupiter, but even then arrived at Venus (or Earth) at too-high velocities. What is needed is a 'deboost' of bodies scattered inward from Jupiter by Earth and Venus perturbations, prior to impact into Venus. This has low probability, since the influence radii of Venus and Earth for the high velocities are small compared to that of Jupiter for the lower velocities further out. It is desirable that this problem be examined by an integerator that allows for close approaches, such as developed by Levison and Duncan. However, it would be a long and expensive computation compared to explaining short-period comets to accomplish the third 'deboost' phase. But since close encounters dominate in such distant scattering, an Opik algorithm seems the appropriate reconnaissance tool.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Lunar and Planetary Inst., Conference on Deep Earth and Planetary Volatiles; p 21
    Format: text
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  • 2
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    Mechanical processess affecting differentiation of protolunar material (1977)
    In:  CASI
    Details
    Publikationsdatum: 2016-06-07
    Beschreibung: Mechanisms prior to lunar formation are sought to account for the loss of volatiles, the depletion of iron, and the enrichment of plagioclase. Some of the same mechanisms are necessary to account for achondritic, stony-iron, and iron meteorites. Collisions seem marginally capable of providing the heat to accomplish the differentiation into iron, magnesian silicates, and plagioclase. Once this differentiation is accomplished, the subsequent mechanical history should have been sufficient to sort material according to composition in the protolunar circumterrestrial cloud. Effects operating include the correlation of body size with mechanical strength; the lesser ability of the cloud to trap the larger, denser infalling bodies; the more rapid drawing into the Earth of the larger moonlets; and the higher energy orbits for dominantly plagioclase smaller pieces broken off by collision.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NASA, Washington The Soviet-Am. Conf. on Cosmochem. of the Moon and Planets, Pt. 2; p 805-813
    Format: application/pdf
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  • 3
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    Formation of the sun and its planets (1986)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: Aspects of star formation are considered, taking into account those observations and models which have a relation to the formation of the solar system. Attention is given to the galactic structure, the stellar properties, planetary indications, interstellar clouds, observations of forming stars, the conditions for cloud collapse, models of star formation, the modeling of secondary features, and aspects of dynamical evolution. Aspects of cosmochemistry are also explored. Questions of chronology are discussed along with nuclide variations, chemical variations among chondritic meteorites, and chemical variations among differentiated objects. A description of planet formation is provided, taking into consideration, gas-dust interaction, sun and accretion disk dynamics, planetesimal swarms, planet-disk interactions, planetary systems, terrestrial planet formation, and gaseous protoplanet contraction.
    Schlagwort(e): ASTROPHYSICS
    Format: text
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  • 4
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    Fragmentation in stellar collapse (1991)
    In:  CASI
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: A three dimensional Cartesian code was developed to model molecular cloud collapse and protostellar formation. The basic Eulerian scheme employs spatially second-order accurate, finite difference methods to advance the fluid variables in time. The hydrodynamic equations governing the collapse include the effects of self-gravity, rotation, and radiative transfer. Supplementary equations include Poisson's equation which relates the gravitational potential to the density distribution and a mean intensity equation derived with the Eddington approximation for radiative transfer. The gas pressures and internal energies are determined from equations of state derived for a molecular cloud composed of hydrogen, helium, and heavier elements.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NASA, Washington, Reports of Planetary Geology and Geophysics Program, 1990; p 420-421
    Format: application/pdf
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  • 5
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    A tidal theory for the origin of the solar nebula (1979)
    In:  Other Sources
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    Publikationsdatum: 2019-06-27
    Beschreibung: A model for the origin of the solar nebula is developed with attention to the significance of angular momentum considerations. Evidence that stars are born in groups rather than singly is examined. It is shown that protostars which are members of typical galactic clusters have some probability of undergoing a gravitational encounter with another star while they are collapsing. According to the model, these encounters impart disproportionate amounts of angular momentum to the later material to fall in toward already centrally condensed fragments. The amount of central condensation of a fragment is the overriding factor in determining its stability against destruction by tidal forces. The encounter also imparts angular momentum to matter that is still accreting onto the protosun.
    Schlagwort(e): ASTROPHYSICS
    Format: text
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  • 6
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    A gravitational kinetic theory for planetesimals (1980)
    In:  Other Sources
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    Publikationsdatum: 2019-06-28
    Beschreibung: An analytical theory is developed for the velocity evolution of nonaccreting planetesimal populations, based on the Boltzmann and Fokker-Planck equations. Adapting Shkarofsky's calculation of plasma viscosities, the rate of increase in random velocities due to gravitational encounters between planetesimals of equal mass is found to be one-third to one-half Safronov's result. Comparison with Wetherill's numerical experiments suggests that the Fokker-Planck equation underestimates the effectiveness of encounters and that Safronov's value is approximately correct. For populations of nonuniform sizes, the Fokker-Planck equation indicates an efficient redistribution of energy from the largest bodies to the smaller ones. By conserving angular momentum, the rate of radial spreading of orbits is also derived.
    Schlagwort(e): ASTRONOMY
    Materialart: Icarus; 44; Oct. 198
    Format: text
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  • 7
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    National Geodetic Satellite Program, Part II: University of California, Los Angeles (1977)
    In:  CASI
    Details
    Publikationsdatum: 2019-08-14
    Beschreibung: Satellite orbit analyses are presented which were undertaken for: (1) reasons of insight and economy; (2) obtaining geophysically interesting tesseral harmonics; (3) comparing effects of tracking station location error drag, radiation pressure, and luni-solar attraction to tesseral harmonic effects; and (4) combination of satellite and terrestrial data. The analyses were divided into the following phases: (1) MINITRACK interferometry; early Baker-Nunn camera directions; (2) late Baker-Nunn camera directions; and (3) combined Baker-Nunn camera and TRANET Doppler data.
    Schlagwort(e): Geophysics
    Materialart: National Geodetic Satellite Program, Part II: A Report Compiled and Edited for NASA by the AGU; 943-948
    Format: application/pdf
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