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  • 1
    facet.materialart.
    Unknown
    In:  Other Sources
    Publication Date: 2019-01-25
    Description: The differences of the two planets in dynamical characteristics and inert gas abundances require major differences of formation. There probably was an impact into the Earth much greater than any into Venus. The resulting heat pulse would have caused more rapid and thorough outgassing of the Earth, lending to an ocean retaining water. Water is the key to the differences in evolution between the planets. A most important consequence was less effective recycling of lithosphere on Venus, leading to a thick global crust which suppressed plate tectonics. Stratification is more pronounced in Venus, but there must remain sufficient heat sources at depth for convective support of the high plateaus.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst. Terrest. Planets: Comp. Planetology; p 16
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  • 2
    Publication Date: 2013-08-31
    Description: Two of the most important constraints are known from Pioneer Venus data: the lack of a system of spreading rises, indicating distributed deformation rather than plate tectonics; and the high gravity/topography ratio, indicating the absence of an asthenosphere. In addition, the high depth/diameter ratios of craters on Venus indicate that Venus probably has no more crust than Earth. The problems of the character of tectonics and crustal formation and recycling are closely coupled. Venus appears to lack a recycling mechanism as effective as subduction, but may also have a low rate of crustal differentiation because of a mantle convection pattern that is more distributed, less concentrated, than Earth's. Distributed convection, coupled with the nonlinear dependence of volcanism on heat flow, would lead to much less magmatism, despite only moderately less heat flow, compared to Earth. The plausible reason for this difference in convective style is the absence of water in the upper mantle of Venus. We have applied finite element modeling to problems of the interaction of mantle convection and crust on Venus. The main emphasis has been on the tectonic evolution of Ishtar Terra, as the consequence of convergent mantle flow. The early stage evolution is primarily mechanical, with crust being piled up on the down-stream side. Then the downflow migrates away from the center. In the later stages, after more than 100 m.y., thermal effects develop due to the insulating influence of the thickened crust. An important feature of this modeling is the entrainment of some crustal material in downflows. An important general theme in both convergent and divergent flows is that of mixing vs. stratification. Models of multicomponent solid-state flow obtain that lower-density crustal material can be entrained and recycled, provided that the ration of low-density to high-density material is small enough (as in subducted slabs on Earth). The same considerations should apply in upflows; a small percent of partial melt may be carried along with its matrix and never escape to the surface. Models that assume melt automatically rising to the crust and no entrainment or other mechanism of recycling lower-density material obtain oscillatory behavior, because it takes a long time for heat to build up enough to overcome a Mg-rich low-density residuum. However, these models develop much thicker crust than consistent with estimates from crater depth/diameter ratios.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Papers Presented to the International Colloquium on Venus; p 55-56
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  • 3
    Publication Date: 2013-08-31
    Description: The science objectives of Pioneer Venus and future investigations of the planet are discussed. Concepts and payloads for proposed missions and the supporting research and technology required to obtain the desired measurements from space and Earth-based observations are examined, as well as mission priorities and schedules.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: JPL-PUB-77-51-VOL-3 , NASA-CR-155187
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  • 4
    Publication Date: 2019-01-25
    Description: In models of lunar origin by great impact, attention is usually paid to the hydrodynamic expansion resulting from the great amount of thermal energy. However, the source of this disruption is inevitably significant in a close approach between large bodies, it is to be expected that significant departures from simple hyperbolic orbits would occur even before impact. These departures could arise from mechanical effects, and hence purely mechanical models are worth pursuing. The most interesting results obtained for approach offsets are small multiples of the planet radius and approach velocities of a few kilometers/second. In an interaction between Mars and Earth sized protoplanets, most of the material ends in collision, but a few percent end in elliptic orbits and a few percent escape. Another model considered is an offset collision, arising from a wide range of approach velocities and offsets.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Planetary Inst. Conf. on the Origin of the Moon; p 59
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  • 5
    Publication Date: 2019-01-25
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: HIG-CONTRIB-1244-VOL-2 , Hawaii Univ. Contrib. of the Hawaii Inst. of Geophys., Vol. 2; 27 p
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  • 6
    Publication Date: 2011-08-16
    Description: When Jupiter was on the order of three to ten earth masses in size, there undoubtedly was a considerably larger mass of condensed matter in its zone, since Jupiter would have perturbed most of it to other parts of the solar system. Monte Carlo studies indicate a significant portion would have crossed the earth's orbit. If the earth and moon had not yet fully formed, the probability of earth-zone planetesimals being hit by this Jupiter-scattered material was high. Further Monte Carlo models of these collisions and their products indicate a significant portion of matter was heated to melting, even if less than 5% of the relative kinetic energy went into heat. The models include capture probabilities by an embryo earth and a protolunar swarm. Because heat energy is correlated with comminution energy, and because the capture probability of the swarm is mass-dependent while embryo's is not, the protolunar material suffered much higher heating on the average than did the proto-earth material.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus; 25; May 1975
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  • 7
    facet.materialart.
    Unknown
    In:  Other Sources
    Publication Date: 2011-08-16
    Description: A model of the accumulation process of a satellite about an accreting planet is proposed in order to explain the difference in relative size between the moon and the outer planets' satellites. The parameter that most strongly affects the final mass ratio of the pair in this model is the time at which the secondary embryo is introduced. Thus the difference between terrestrial and outer planet satellite systems is easily understood in terms of the differences in Q (the specific dissipation function of the particles in the circumplanetary cloud) between these planets. The high Q of the outer planets does not allow a satellite embryo to survive a significant portion of the accretion process; therefore only small bodies formed very late in the accumulation of the planet remain as satellites.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 8
    Publication Date: 2011-08-19
    Description: The formation of the Earth, was mainly from sizeable bodies: perhaps moon sized. Models of interaction among small planetesimals which take into account only close encounters all lead to the formation of moon sized objects, thus leading to several 100 in the inner solar system. Longer term interactions, such as secular resonance sweepings, are needed to get these planetesimals together to form the observed terrestrial bodies. After the accumulation of the Earth, during which core formation certainly occurred, further impacts probably influenced the locations of rifting centers in the system of mantle convection and crustal differentiation. They may have affected craton stabilization by promoting lateral heterogeneity, but had little influence on the key problem of early recycling of sial.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst. Workshop on the Early Earth: The Interval from Accretion to the Older Archean; p 45-47
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  • 9
    Publication Date: 2011-08-17
    Description: Improved estimates of impact energy partitioning are combined with models of planetesimal size distribution and planetary growth to infer the early thermal evolutions of the earth and moon. Binary accretion models of the moon which allow for enhancement of velocities by proximity of the earth do not get hot enough for appreciable melting unless a planetesimal mass distribution starting at a rather high value (earth mass/20 or more) is assumed. This melting occurs deeper than is inferred from petrological and thermotectonic data; hence the results favor formation of the moon as a consequence of a great impact (or great impacts) into the earth.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 84; Mar. 10
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  • 10
    facet.materialart.
    Unknown
    In:  Other Sources
    Publication Date: 2011-08-16
    Description: The ratio between the mass of condensed matter in Mercury's nebular zone and the final mass of condensed matter in the planet is calculated, and the result suggests that condensed matter equal to more than 10 times the planet's mass was lost from that zone. Five hypotheses to account for this loss are considered: (1) the temperature in Mercury's zone was appreciably higher than the suggested value of 1400 K, (2) the excess was expelled by solar mass outflow, (3) the excess was dragged down by the sun during its contraction, (4) the excess was knocked out by Jupiter-perturbed planetesimals, and (5) the excess was knocked out by earth- and Venus-perturbed planetesimals. The plausibility of each hypothesis is examined, and it is concluded that only planetesimal scattering by earth and Venus appears to be ruled out
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus; 28; Aug. 197
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