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  • 1
    Publication Date: 2011-08-19
    Description: The impact flux history of the Saturn system has been modelled, and absolute ages estimated. The model assumes that the general history of impact flux was similar to that of the inner solar system and that the dominant source of material in the postaccretionary phases was external to the Saturn system. Population I and II craters represent the synheavy and postheavy bombardment periods, respectively. Observed crater densities are representative of the actual ages and do not appear to be saturation-equilibrium surfaces. It is concluded that some of the satellites have geologic histories which extend to about 3.5 b.y. Significant endogenic resurfacing has occurred on several satellite surfaces, which indicates that thermal conditions and material compositions are more complicated than might have been anticipated prior to Voyager encounters.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 90; 2029-203
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  • 2
    Publication Date: 2019-06-27
    Description: It is shown that Mercury's surface has only 70% as many large craters (of at least 200 km in diameter) as the moon. The density of Mercurian impact craters having diameters over 400 km is 30% of that of the moon, and for craters with diameters between 400 and 700 km, Mercurian density is 21% of that of the moon. The size-frequency distribution curve of Mercury is the same as the lunar cumulative -2 slope. The Mercurian curve, however, lies well below the 10% surface saturation level of the lunar curve. This may indicate that the old, heavily-cratered Mercurian terrain is not presently in a state of cratering equilibrium. The differences in crater and basin densities observed between Mercury and the moon may be functions of crater-production rates or of different crustal histories. The total isostatic compensation of impact craters having diameters of about 800 km suggests that the average viscosity of the Mercurian crust during approximately the past 4 eons was the same as that of the moon.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 3
    Publication Date: 2019-06-28
    Description: The reported evaluation of Martian channel characteristics is based on Viking photographs taken from July 1976 to February 1977. The wide variation in crater densities shown by the considered Martian channels strongly implies widely differing ages for both fluviatile and lava channels. Attention is given to age determination methodology, a description of channels and implications for channel formation, surface water under present Martian conditions, surface water under more favorable Martian conditions in the past, channel parameter estimates, and volcanic channels.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research; 82; 28; 4016-4038
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  • 4
    Publication Date: 2019-06-27
    Description: In order to determine the origin of Cayley-type lunar light plains, their physical properties, distribution, and relative ages are examined from Apollo orbital and Lunar Orbiter photographs. The distribution and apparent age of the plains deposits and data on highly feldspathic breccias indicate that these superficial materials are neither locally derived nor part of the Imbrium ejecta. The existence of a planar facies of continuous ejecta at Orientale and in the ejecta blankets of small craters is demonstrated. The data and interpretation presented support the hypothesis that the surface and near-surface materials of some light plains, including those at the Apollo 16 site, are at least partly composed of ejecta from the Orientale basin and that the materials of many rugged areas, such as the Descartes highlands, are overlain by similar material. The possibility that some Cayley-type plains may have a different origin is not excluded.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: U.S. Geological Survey; vol. 3
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  • 5
    Publication Date: 2019-07-13
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Science Conference; Mar 17, 1975 - Mar 21, 1975; Houston, TX
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  • 6
    Publication Date: 2019-07-13
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Science Conference; Mar 14, 1977 - Mar 18, 1977; Houston, TX
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  • 7
    Publication Date: 2019-07-13
    Description: Unique features of Imbrium lava flows are their thickness (10-30 + m) and lengths (up to 400 km for phase-III, and up to 600 km for phase-II) when compared along later Imbrium and Eratosthenian units. They are distinct by virtue of their inferred short-lived (on the order of days) and extremely rapid rates of effusion. It is shown that there are numerous other basalt eruptives within the young blue western maria. The emplacement of these flows was by complex multilayering and intertonguing of individual flow units with thicknesses less than 10 m. They are generally restricted to the near vicinity of multiple vent sources.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Science Conference; Mar 15, 1976 - Mar 19, 1976; Houston, TX
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  • 8
    Publication Date: 2019-07-13
    Description: A map dividing the lunar nearside maria into six units each of uniform age was compiled from Lunar Orbiter IV photographs. Relative ages were determined using a crater morphology technique. Relative ages were then converted to absolute ages by comparison to a curve calibrated from Apollo and Luna sample data; three of the six units, including the two youngest (about 3.1 and 2.6 b.y. old), were not sampled by Apollo or Luna. Vast regions in the western maria are covered by these young flows. The old flows (about 3.75 b.y. in age) are found to occur mostly in the eastern maria. A comparison of the age units derived from these data with those units derived from other remote-sensing data provide good agreement in the location of unit boundaries. These data also indicate that the emplacement history of mare basalt types is more complex than was earlier thought.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Science Conference; Mar 15, 1976 - Mar 19, 1976; Houston, TX
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  • 9
    Publication Date: 2019-08-13
    Description: Images of the Saturn system acquired by Voyager 2 in its encounter in August 1981 are presented and information gained from the imagery on the atmosphere, satellites, and rings of Saturn is discussed. The images have shown the Saturn atmosphere to contain persistent oval clouds similar to those of Jupiter, and small irregular features indicative of a pattern of zonal winds that is symmetric about the equator and appears to extend to great depths. The atmosphere of Titan is found to possess an upper haze layer above the main haze, a hemispherical brightness asymmetry, and a dark band in the north polar region. Other satellite observations included relatively high resolution coverage of Enceladus and Tethys, moderate resolution views of Hyperion and Iapetus, and the first observations of Phoebe, as well as views of the eight minor satellites associated with the orbits of Dione, Tethys and Mimas, and the F and A rings. Images of various ring structures were obtained, most notably the birth of a spoke and variable ringlet structures in the outer B ring. The observations may be used to deduce the collisional and thermal history of the rings and satellites.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science; 215; Jan. 29
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  • 10
    Publication Date: 2019-08-14
    Description: Neptune's atmosphere is revealed by Voyager 2 images to contain clouds of methane ice above a lower deck of hydrogen sulfide or ammonia ices, and to be dominated by an anticyclonic storm system designated the 'Great Dark Spot'; this bears both similarities and differences to the Great Red Spot of Jupiter. Like the rings of Uranus, those of Neptune are composed of very dark, but in addition very dusty, material. Six new regular satellites have been discovered whose radii range from 25 to 200 km. Triton is noted to be a differentiated body showing evidence of early surface-melting episodes. At least two active plumes are found on Triton, which may be driven by solar heating.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science (ISSN 0036-8075); 246; 1422-144
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