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  • 11
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    Flooding of Smooth Terrain on Ganymede by Low-Viscosity Aqueous Lavas: Direct Evidence from VGR-GLL Stereo Synergism (2000)
    In:  Other Sources
    Details
    Publication Date: 2017-10-02
    Description: Voyager-Galileo stereo images of Ganymede reveal that smooth bright terrains are depressed at least 700 m compared to older terrains, and that less deformed terrains are more depressed. These observations are consistent with flooding of bright terrain by low-viscosity lavas.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Lunar and Planetary Science XXXI; LPI-Contrib-1000
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  • 12
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    Geology of icy satellites (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The geology of the major icy satellites of Jupiter, Saturn, Uranus, and Neptune is discussed in terms of the four major processes that shape icy satellite surfaces: impact cratering, volcanism, tectonism, and interactions with planetary magnetospheres and solar radiation. The role of these processes in creating the differences that exist among the satellites, in particular the orderly progression of geological properties in the Jovian satellites, is emphasized. Important questions left open after the Voyager missions are summarized.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 13
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    Impacts in Ice: Commissions of Solar System Crater Populations (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-08
    Description: The classical phases of cratering are compression, excavation, and modification. Recent theory, however, supports and alternative sequence of physical events: (1) coupling - energy and momentum are transferred, and the cratering flow field is set up; (2) power-law-growth the flow field expands, with transient cavity dimensions expressible as a simple power law function of time; and (3) late phase - some combination of gravity, strength, and viscosity limits cavity growth, initially causing a deviation from power-law behavior and eventually determining the size and shape of the final (transient) crater. The dominant mechanism that limits cavity growth defines a cratering regime and a single variable, the coupling parameter, determines the scaling laws in the various regimes. The coupling parameter is intermediate in dimensionality between energy and momentum and its exact form is determined. For simple materials with rate and scale independent strength, the transition crater diameter between the strength and gravity regimes is, within reasonable bounds, independent of velocity. Even the smallest craters observed by Voyager appear to be gravity dominated.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 180-182
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  • 14
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    Three-layer Generic Ganymedes: Structure and Evolution (1985)
    In:  CASI
    Details
    Publication Date: 2014-10-09
    Description: Accurate, three layered structural models were generated for Ganymede and Callisto. Three layered satellites consist of a rock core, a region of mixed rock and ice, and an outer shell of pure ice. This structure would result from either accretional melting of the outermost region of the satellite or differentiation subsequent to localized melting. A completely undifferentiated ice rock satellite possesses only the single, top boundary layer and, in this case, melting is also relatively difficult to initiate; this suggests that, if accretional melting results in a small degree of differentiation initially, subsequent melting may readily occur as the satellite warms due to radiogenic heating. Alternatively, if accretion is not accompanied by a small degree of differentiation, it may prove too difficult to initiate nonaccretional melting and the satellite might remain undifferentiated indefinitely.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 519-521
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  • 15
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    A reappraisal of Darwin's fission hypothesis and a possible limit to the primordial angular momentum of the Earth (1984)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: G. H. Darwin proposed that the primordial Earth may have rotated fast enough that the solar tidal period was nearly resonant with the fundamental free oscillation period of a fluid Earth and that a large and unstable tidal oscillation split off to become the moon. Jeffreys argumented that dissipation during resonance would be sufficient to prevent such an unstable oscillation greater than the tidal frequency (period - 2.68 hr). It is considered that solar tides have extracted angular momentum from the Earth-Moon system over 4.5 b.y. The correspondence of the primordial tidal and resonant frequencies is nearly exact. (The effect of central condensation of the proto earth is to increase both frequencies by a similar amount, though the resonance is not precisely known. This result, was unknown to Darwin or Jeffreys. The effects of resonance were evaluated. The resonance is likely to be too damped for fission. This argument is more general than Jeffreys', who considered friction between the oscillating mantle and a rigid core. It is argued that the fact that Q must be so great for fission that equilibrium can not be maintained; the fluid proto Earth passes so quickly through resonance that maximum amplitude is not reached. It is suggested that solar resonant tides acted as a brake on the spin of the primordial partially molten Earth. Certain proposed origins for the Moon do not necessarily involve addition of substantial amounts of angular momentum to the Earth-Moon system. The primordial Earth-Moon system may have had nearly the same angular momentum as it has today.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Planetary Inst. Conf. on the Origin of the Moon; p 34
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  • 16
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    Large Impact Features on Saturn's Middle-sized Icy Satellites: Global Image Mosaics and Topography (2003)
    In:  CASI
    Details
    Publication Date: 2017-10-02
    Description: With the approach of Cassini to the Saturn system, attention naturally focuses on the planet, its rings and Titan, but the Saturn system is also populated by a number of smaller satellites. The seven middle-sized icy satellites, along with those of Uranus, (between 400 and 1500 km wide) are distinctly different geophysically and geologically from their much larger Galilean-class brethren [e.g., 1]. Topographic mapping of these bodies is a critical part of understanding their geologic evolution. Here we describe our recent efforts to map the topography of these satellites using Voyager data.
    Keywords: Lunar and Planetary Science and Exploration
    Type: Lunar and Planetary Science XXXIV; LPI-Contrib-1156
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  • 17
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    On the Origin of Triton and Pluto (1985)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Lyttleton's (1936) hypothesis that Triton and Pluto originated as adjacent prograde satellites of Neptune is evaluated, and it is shown that with the presently accepted masses of Triton and Pluto-Charon, the momentum and energy exchange required to sell Triton on a retrograde orbit is impossible. The Pluto-Charon system could not have acquired its present angular momentum state during an ejection event unless a physical collision was involved, which is quite unlikely. The simplest hypothesis is that Triton and Pluto are independent representatives of large outer solar system planetesimals. Triton is simply captured, with spectacular consequences that include runaway melting of interior ices and release to the surface of clathrated CH4, CO, and N2. Condensed remnants of this protoatmosphere could account for features in Triton's unique spectrum.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 64-66
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  • 18
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    Dark Halo Craters and the Thickness of Grooved Terrain on Ganymede (1985)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Dark halo craters on grooved terrain on Ganymede represent potential probes of the subsurface geology. These craters are surrounded by a broad, diffuse low albedo annulus, or halo and have apparently lost the bright rim and ray deposits associated with very young craters. The recognition of dark halo craters, almost all greater than 12 km in diameter, indicates that material darker and redder than grooved terrain material forms a stratigraphy horizon at about 1 km depth the present surface of grooved terrain in Uruk Sulcus. This material is most likely downdropped cratered terrain material, which only larger craters have excavated into. This is most consistent with extensional tectonic models for grooved terrain formation whereby the lithosphere is stretched and blocks of ancient cratered terrain are downdropped along bounding faults, and subsequently resurfaced by a shallow layer of relatively clean icy material. Evidence for the preservation of cratered terrain material at shallow depth beneath grooved terrain material poses great difficulties for alternative grooved and smoothed terrain formation mechanisms.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA, Washington Repts. of Planetary Geol. and Geophys. Program, 1984; p 27-29
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  • 19
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    Evolution of planetary lithospheres - Evidence from multiringed structures on Ganymede and Callisto (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The thickness and viscosity of a planetary lithosphere increase with time as the mantle cools, with a thicker lithosphere leading to the formation of one (or very few) irregular normal faults concentric to the crater. Since a gravity wave or tsunami induced by impact into a liquid mantle would result in both radial and concentric extension features, which are not observed in the case of the large impact structures on Ganymede and Callisto, an alternative mechanism is proposed in which the varying ice/silicate ratios, tectonic histories, and erosional mechanisms of the two bodies are considered to explain the subtle differences in thin lithosphere ring morphology between Ganymede and Callisto. It is concluded that the present lithosphere thickness of Ganymede is too great to permit the development of any rings.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 20
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    Dark halo craters and the thickness of grooved terrain on Ganymede (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: On the basis of a morphological analysis of recent Voyager images of the dark halo craters and grooved terrain in the Uruk Sulcus region on Ganymede, it is proposed that dark halos result from the incorporation of cratered terrain material into crater ejecta. In Uruk Sulcus, only craters greater than about 12 km in diameter have halos, indicating that the cratered material there forms a stratigraphic horizon buried in the grooved terrain. A depth of excavation of cratered terrains of 1.0 to 1.6 km is calculated. It is shown that the estimated depth is consistent with current theoretical models of grooved terrain, where relatively clean ice is emplaced over downdropped blocks of cratered terrain. Several Voyager 2 images of the Uruk Sulcus region on Ganymede are provided.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research, Supplement (ISSN 0148-0227); 90; C775-C78
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