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  • General Chemistry  (1.028)
  • Inorganic Chemistry  (693)
  • LUNAR AND PLANETARY EXPLORATION  (613)
  • Computational Chemistry and Molecular Modeling
  • 1990-1994  (2.737)
  • 1990  (2.737)
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  • 1
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    Uranus - The planet, rings and satellites (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The present overview of available data regarding Uranus and its planetary structures encompasses ground- and space-based observations of the planet with specific attention given to the interpretation of Voyager 2 data. A brief examination of historical observations is given which includes its discovery, position determination, and the related discoveries of Uranus' five large satellites and rings. The observational data preceding the Voyager 2 mission are reviewed in terms of the planetary interior, atmosphere, and magnetosphere. The Voyager 2 mission is given detailed treatment with descriptions of NASA's development and deployment of the spacecraft as well as detailed data from the Uranus encounter. Reference is given to structure and composition of Uranus' atmosphere, valid models of the interior, and wave-particle interactions in the magnetosphere. The structures of the rings and satellites are examined with reference to specific observational requirements from future missions.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
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  • 2
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    Mars Observer's global mapping mission (1990)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The objectives of the Mars Observer global mapping mission are to determine the global elemental and mineralogical character of the Martian surface material, define globally the topography and gravitational field of Mars, establish the nature of Mars's magnetic field, determine the time and space distribution, abundance, sources, and sinks of volatile Martian material and dust over a seasonal cycle, and explore the structure and aspects of the circulation of the Martian atmosphere. The experiments and instruments to be used in this mission are described, and the operations and data analysis are briefly considered.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: EOS (ISSN 0096-3941); 71; 1099
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  • 3
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    Small-scale plasma, magnetic, and neutral density fluctuations in the nightside Venus atmosphere (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The evolution of the Venus small-scale waves as they propagate into the nightsite is examined, and the small-scale structures are compared with the waves in the three components of the magnetic field, magnetic dip angle, and neutral density. It is demonstrated that the small-scale fluctuations evolve between the transterminator and antisolar regions. It is shown that atmospheric gravity waves may also be producing some of the fluctuations observed at longer wavelengths. The electron temperature and density are shown to be approximately 180 deg out of phase and exhibiting the highest correlation of any pair of variables. Waves in the electron and neutral densities are found to be correlated moderately on most orbits, while the average electron temperature is higher when the average magnetic field is more horizontal.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 4085-410
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  • 4
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    Comment on 'Update on terrestrial ages of Antarctic meteorites' by K. Nishiizumi, D. Elmore and P. W. Kubik (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Earth and Planetary Science Letters (ISSN 0012-821X); 99; 4, Se; 380-382;
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  • 5
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    Geochemistry and genesis of the angrites (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The major and trace elements were analyzed in samples of LEW86010, LEW87051, and Angra dos Reis angrites were analyzed using INAA and fused-bead EMP techniques. Results suggest that all three angrites originated on a volatile-depleted (Na-poor) but oxidized parent body or on several bodies. The combination of low Na and high FeO/MnO found in these samples suggests that the extreme Na depletion was caused by parent-body outgassing during differentiation, rather than by nebular processes. It was also found that the refractory-element abundances observed in LEW86010 and LEW87051 are related via olivine control; it is suggested that LEW86010 may be a residual melt from a LEW87051-like precursor or that LEW87051 formed from a LEW86010-like precursor via olivine accumulation. On the other hand, the Angra dos Reis displayed an unusual refractory element pattern suggesting that either the angrite parent body was heterogeneous or that Angra dos Reis was formed on a parent body different from that of the other two angrites.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 54; 3209-321
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  • 6
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    Zonal motion and structure in Jupiter's upper troposphere from Voyager infrared and imaging observations (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Global digital maps of Jupiter's upper-tropospheric temperature have been generated at the 270- and 150-mb pressure levels, together with IR cloud optical depths at 5 and 45 microns and the ammonia abundance near the 680-mb pressure level, on the basis of Voyager IRIS north-south mapping sequences. Attention is given to the 270-mb and 45 micron data; global digital maps are presented for violet and orange reflectivities. The dominant upper tropospheric thermal structures move at a rate far different from that of the cloud indicators, and are noted to remain stationary relative to the planet's bulk rotation. Strong stationary features are found at a zonal wavenumber of 9 near 15 deg N latitude and of 11 near 20 deg latitude.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 88; 39-72
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  • 7
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    Tidal disruption of solid bodies (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The problem of stress, strain, and breakup in solid satellites and stray bodies subject to tidal perturbations is presently addressed in view of three novel considerations. After presenting a new analytic solution for the stress tensor in a homogeneous and compressible elastic sphere, where the inclusion of compressibility alters stresses by several percent, realistic failure criteria are noted to demonstrate the general failure of such ductile bodies as iron meteoroids by plastic shear, while brittle ice bodies fail by either tensile or shear fracture. A reexamination of crack propagation after initial failure allows the diverse breakup criteria to be reconciled.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 88; 24-38
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  • 8
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    Cloud-tracked winds from Pioneer Venus OCPP images (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Results are presented on the analysis of the Pioneer Venus Orbiter Cloud Photopolarimeter (OCPP) UV images, covering 80-day periods from 1979 to 1985. The analysis provides a method for determining wind speeds at the cloud level, from the motions of cloud UV features. Data used for tracking the cloud feature motions are described along with the analysis method. The results identified four wave modes: a diurnal solar tide, a semidiurnal solar tide, a '4-day equatorial' wave, and a '5-day midlatitude' wave. The features of these wave modes are described.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of the Atmospheric Sciences (ISSN 0022-4928); 47; 2053-208
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  • 9
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    (Ar-39)-(Ar-40) dating of mesosiderites - Evidence for major parent body disruption less than 4 Ga ago (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The (Ar-39) (Ar-40) chronologies were determined for 14 different mesosiderites representing the full range of classification according to recrystallization, and these chronologies were compared with analogous data for other meteorite types and for lunar highland rocks. Results of Ar-Ar chronologies indicate the history of a degassing of Ar due to a major thermal event that occurred less than 3.9 Ga ago; this event is not the metal-silicate mixing event, which is known to have occurred earlier than 4.4 Ga ago. It is suggested that a major collisional disruption-reassembly event less than 3.9 Ga ago took place, leaving the metal-silicate breccias buried under tens of kilometers of rubble, where they cooled slowly through the Ar closure temperatures.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 54; 2549-256
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  • 10
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    Inter- and intra-group compositional variations in Apollo 15 pyroclastic green glass - An electron- and ion-microprobe study (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Results are presented on major- and trace-element abundance analyses of Apollo 15 pyroclastic green glasses from groups A, B, C, D, and E, carried out using electron- and ion-microprobe techniques. The diagrams depicting Sr, Zr, Ba, and Nd vs Co variations indicate the presence of a high-Co trend in groups A and D and a low-Co trend in groups B and C. Group-E glasses were found to be significantly enriched in Sr, relative to the other four glass groups. Chemical data of this study were integrated with previous data to evaluate various magmatic processes that have been proposed in the past to explain chemical variations in the lunar green glass. Results of calculations using a source mixing model suggest that the Apollo 15 green glasses represent multiple eruptive events from three chemically distinct but compositionally variable source regions.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 54; 2565-257
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  • 11
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    Energy sources for Triton's geyser-like plumes (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Four geyser-like plumes were discovered near Triton's south pole in areas now in permanent sunlight. Because Triton's southern hemisphere is nearing a maximum summer solstice, insolation as a driver or a trigger for Triton's geyser-like plumes is an attractive hypothesis. Trapping of solar radiation in a translucent, low-conductivity surface layer (in a solid-state greenhouse), which is subsequently released in the form of latent heat of sublimation, could provide the required energy. Both the classical solid-state greenhouse consisting of exponentially absorbed insolation in a gray, translucent layer of solid nitrogen, and the 'super' greenhouse consisting of a relatively transparent solid-nitrogen layer over an opaque, absorbing layer are plausible candidates. Geothermal heat may also play a part if assisted by the added energy input of seasonal cycles of insolation.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 250; 431-435
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  • 12
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    Temperature and thermal emissivity of the surface of Neptune's satellite Triton (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Analysis of the preliminary results from the Voyager mission to the Neptune system has provided the scientific community with several methods by which the temperature of Neptune's satellite Triton may be determined. If the 37.5 K surface temperature reported by several Voyager investigations is correct, then the photometry reported by the imaging experiment on Voyager requires that Triton's surface have a remarkably low emissivity. Such a low emissivity is not required in order to explain the photometry from the photopolarimeter experiment on Voyager. A low emissivity would be inconsistent with Triton having a rough surface at the about 100-micron scale as might be expected given the active renewal processes which appear to dominate Triton's surface.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 250; 429-431
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  • 13
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    Subsurface energy storage and transport for solar-powered geysers on Triton (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The location of active geyser-like eruptions and related features close to the current subsolar latitude on Triton suggests a solar energy source for these phenomena. Solid-state greenhouse calculations have shown that sunlight can generate substantially elevated subsurface temperatures. A variety of models for the storage of solar energy in a subgreenhouse layer and for the supply of gas and energy to a geyser are examined. 'Leaky greenhouse' models with only vertical gas transport are inconsistent with the observed upper limit on geyser radius of about 1.5 km. However, lateral transport of energy by gas flow in a porous N2 layer with a block size on the order of a meter can supply the required amount of gas to a source region about 1 km in radius. The decline of gas output to steady state may occur over a period comparable with the inferred active geyser lifetime of 5 earth years. The required subsurface permeability may be maintained by thermal fracturing of the residual N2 polar cap. A lower limit on geyser source radius of about 50 to 100 m predicted by a theory of negatively buoyant jets is not readily attained.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 250; 424-429
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  • 14
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    Triton's geyser-like plumes - Discovery and basic characterization (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: One model for the mechanism driving the plumes of the four active geyser-like eruptions observed by Voyager 2 on Triton is a heating up of nitrogen ice in a subsurface greenhouse environment, where nitrogen gas pressurized by solar heating explosively vents to the surface carrying clouds of ice and dark particles into the atmosphere. A temperature increase of less than 4 K above the ambient surface value of 38 + or - 3 K suffices to drive the plumes to 8-km altitude. Each eruption may last a year or more, over the course of which 0.1 cu km of ice is sublimed.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 250; 410-415
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  • 15
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    Magellan orbits Venus (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Various events surrounding Magellan's orbit of Venus are recounted. Significant events include the successful firing of a solid rocket motor while the spacecraft was behind Venus to transfer it from a solar-centered trajectory to an orbit around the planet. The spacecraft orbits Venus every 3.26 hours at a maximum altitude of 8500 km and minimum altitude of 291 km in an elliptical orbit. The successful August 16 test of the synthetic-aperture radar system is discussed, noting that it produced two strips, each about 20 km x 16,000 km, revealing details as small as 120 m. Two anomalies causing a delay in the start of mapping operations and subsequent breaks in the communication link with earth for 14.5 hours and 17.7 hours are discussed. Protective measures directed from the spacecraft's ROM during breach of contact are listed, and possible causes of the anomalies are suggested, such as solar activity or hardware or software elements, although the actual cause is not yet known.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Spaceflight (ISSN 0038-6340); 32; 334-337
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  • 16
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    Comparative dynamics of the ionospheres of Venus and Mars at large solar zenith angles (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Simple three-ion spectral models of the dynamics and ion chemistry in the upper ionospheres of Venus and Mars have been constructed by including two minor ions, H(+) and O2(+) for Venus and O(+) and CO2(+) for Mars, along with the major ion, O(+) for Venus and O2(+) for Mars. Horizontal flow velocities and ion densities have been calculated in each case and compared with available measured data. For Venus, the present calculations yield results in agreement with earlier finite-difference model studies and with experimental observations. Calculations for Mars are in agreement with the limited observational data obtained by the Viking landers.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Indian Journal of Radio and Space Physics (ISSN 0367-8393); 19; 88-102
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  • 17
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    Albedo and color maps of the Saturnian satellites (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The paper discusses the production of maps of the albedos and colors of Mimas, Enceladus, Tethys, Dione, and Rhea over the full range of their imaged surfaces. Voyager images were used to prepare maps of the normal reflectances and color ratios (0.58/0.41 micron) of these satelites.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 87; 339-357
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  • 18
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    Cosmogenic nuclides in extraterrestrial materials (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Research on cosmogenic nuclides in extraterrestrial materials is summarized. Methods for determining average production rates in meteorites are considered as well as the composition dependence of production rates. Also discussed are lunar meteorites, solar-gas-rich meteorites, presolar grains, and solar cosmic rays.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Reviews of Geophysics (ISSN 8755-1209); 28; 253-275
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  • 19
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    A model of Saturn's magnetic field based on all available data (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: A model of Saturn's magnetic field, called Saturn Pioneer Voyager (SPV), is developed on the basis of an analysis of three sets of data: those from the Pioneer 11, the Voyager 1, and the Voyager 2 magnetometers. It is shown that the SPV model fits the data observed between 1.3 and 8.0 Saturn radii from the planet's center with a 1.13 percent weighted rms average of the percent differences between the observed and modeled fields, which is substantially better than the fits yielded by any of the previous models.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 15257-15
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  • 20
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    Backscattering from frost on icy satellites in the outer solar system (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Two extreme models are presented of how frost and ice might be intermixed on a typical satellite surface: areal and intimate mixing. Applying such models to selected representative satellite data, it is found that the frost component of the surfaces of these outer satellites must itself be backscattering, unlike its terrestrial counterpart. The difference may arise because frost particles can have much more complex internal textures under the low-temperature and low-gravity conditions of the outer satellites than is the case on earth.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 347; 162-164
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  • 21
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    Ubiquitous interstellar diamond and SiC in primitive chondrites - Abundances reflect metamorphism (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: It is shown here that interstellar diamond and SiC were incorporated into all groups of chondrite meteorites. Abundances rapidly go to zero with increasing metamorphic grade, suggesting that metamorphic destruction is responsible for the apparent absence of these grains in most chondrites. In unmetamorphosed chondrites, abundances normalized to matrix content are similar for different classes. Diamond samples from chondrites of different classes have remarkably similar noble-gas constants and isotropic compositions, although constituent diamonds may have come from many sources. SiC seems to be more diverse, partly because grains are large enough to measure individually, but average characteristics seem to be similar from meteorite to meteorite. These observations suggest that various classes of chondritic meteorites sample the same solar system-wide reservoir of interstellar grains.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 347; 159-162
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  • 22
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    Vertical distribution of water in the atmosphere of Venus - A simple thermochemical explanation (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Several lines of evidence concerning the vertical abundance profile of water in the atmosphere of Venus lead to strikingly unusual distributions (the water vapor abundance decreases sharply in the immediate vicinity of the surface) or to serious conflicts in the profiles (different IR bands suggest water abundances that are discrepant by a factor of 2.5 to 10). These data sets can be reconciled if (1) water molecules associate with carbon dioxide and sulfur trioxide to make gaseous carbonic acid and sulfuric acid in the lower atmosphere, and (2) the discrepant 0.94-micrometer water measurements are due to gaseous sulfuric acid, requiring it to be a somewhat stronger absorber than water vapor in this wavelength region. A mean total water abundance of 50 + or - 20 parts/million and a near-surface free water vapor abundance of 10 + or - 4 parts/million are derived.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 249; 1273-127
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  • 23
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    Variations of Mars gravitational field and rotation due to seasonal CO2 exchange (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: About a quarter of the Martian atmospheric mass is exchanged between the atmosphere and the polar caps in the course of a Martian year: CO2 condenses to form (or add to) the polar caps in winter and sublimes into the atmosphere in summer. This paper studies the effect of this CO2 mass redistribution on Martian rotation and gravitational field. Two mechanisms are examined: (1) the waxing and waning of solid CO2 in the polar caps and (2) the geographical distribution of gaseous CO2 in the atmosphere. In particular, the net peak-to-peak changes in J2 and J3 over a Martian year are both found to be as much as about 6 x 10 to the -9th. A simulation suggests that these changes may be detected by the upcoming Mars Observer under favorable but realistic conditions.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14755-14
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  • 24
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    Atmospheric measurements at Mars via gamma ray spectroscopy (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: This paper describes three methods for measuring the Martian atmosphere using gamma ray spectroscopy. One method determines atmospheric thickness based on the energy-dependent differential attenuation of gamma-ray line pairs from a common element. Another makes a direct determination based on measurements of the line flux generated in the atmosphere, requires knowledge of the concentration of the component being used. The third, which makes use of a single line emitted from the surface where its flux can be established. The effects of stratigraphy on the differential attenuation method are studied, and calculations are reported which show that the measurement of atmospheric argon will be a sensitive indicator of the atmospheric fractionation accompanying CO2 precipitation in south polar regions.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14695-14
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  • 25
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    Infrared radiative transfer in the dust-free Martian atmosphere (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Two theoretical radiative transfer methods are used here to derive synthetic spectra of dust-free Martian atmospheres. One is a versatile, accurate, line-by-line model which completely resolves individual absorption lines to produce high-resolution spectra within gas absorption bands. The other is a fast Voigt quasi-random model which produces accurate results for spectral regions as narrow as 1/cm and is fast enough for routine use in broadband transmission calculations. Synthetic transmission and radiance spectra from these models for dust-free conditions in the Martian atmosphere are used to illustrate the radiative effects of each important absorbing gas and to provide a baseline for comparisons with dust atmosphere spectra.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14577-14
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  • 26
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    Thermal emission spectra of Mars (5.4-10.5 microns) - Evidence for sulfates, carbonates, and hydrates (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Spectra of the Martian thermal emission in the 5.4-10.5 micron region are reported. Emission features at 7.8 and 9.7 microns are attributed to surface silicates, and an emission feature at 6.1 micron is attributed to a molecular water component of the surface material. An absorption band at 8.7 micron and a possible one at 9.8 microns is attributed to sulfate or bisulfate anions probably located at a distorted crystalline site, and an absorption band at 6.7 microns is attributed to carbonate or bicarbonate anions located in a distorted crystalline site. Spectral simulations indicate that the sulfate- and carbonate-bearing minerals are contained in the same particles of airborne dust as the dominant silicate minerals, that the dust optical depth is about 0.6 at a reference wavelength of 0.3 micron over the area of the observed spots, and that sulfates and carbonates constitute 10-15 percent and 1-3 percent by volume of the airborne dust, respectively.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14595-14
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    Origins of Marslike spectral and magnetic properties of a Hawaiian palagonitic soil (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Results are presented on spectral, Moessbauer, static magnetic, petrographic, and compositional data for a Hawaiian palagonitic soil from Mauna Kea (HWMK1). It was found that reflectivity spectra of size separates smaller than 20 microns resemble spectra for Martian bright regions, while spectra of larger size separates show characteristics in common with spectra for Martian dark regions. Data on the HWMK1 soil are consistent with the partitioning of iron among the following minerals: olivine, titanomagnetite, hematite, and a superparamagnetic colored ferric oxide. These mineralogies are heterogeneously distributed within the soil with respect to both particle type and soil-particle diameter. The strongly magnetic titanomagnetite is associated with black particles and is responsible for the magnetic nature of the soil, while the colored weakly magnetic superparamagnetic ferric oxide and minor hematite are associated with orange particles.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14427-14
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  • 28
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    Emplacement of lava flow fields - Application of terrestrial studies to Alba Patera, Mars (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The relationships obtained by Kilburn and Lopes (1988, 1990) for their models of lava flow fields were used to calculate the duration of the emplacement, the average discharge rates, and the average velocities of lavas for the Martian volcano Alba Patera, on which two distinct planimetric types of lava flow field were identified. The validity of using terrestrial analogues for investigating the emplacement conditions of the Alba Patera flow fields is examined. It is concluded that, at least for basaltic-basaltic andesitic compositions, the essential conditions of eruption from Alba Patera may have been similar to those currently observed on earth.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14383-14
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  • 29
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    Fault propagation folds induced by gravitational failure and slumping of the Central Costa Rica volcanic range - Implications for large terrestrial and Martian volcanic edifices (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Maps are presented that describe the compressional tectonic structures found at the base of the Central Costa Rica volcanic range (CCRVR), which comprise thrust faults and related fault propagation folds, only partly covered by syntectonic and posttectonic volcanoclastic deposits. Evidence is presented that these structures formed by gravitational failure and lumping of the flanks of the volcanic range. It is suggested that similar structures may be found at the toe of the southern flank of Kilauea volcano, Hawaii, and along the perimeter scarp of the Olympus Mons volcano on Mars.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14357-14
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  • 30
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    Flank tectonics of Martian volcanoes (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The origin of the numerous terraces on the flanks of the Olympus Mons volcano on Mars, seen on space images to be arranged in a roughly concentric pattern, is investigated. The images of the volcano show that the base of each terrace is marked by a modest but abrupt change in slope, suggesting that these terraces could be thrust faults caused by a compressional failure of the cone. The mechanism of faulting and the possible effect of the interior structure of Olympus Mons was investigated using a numerical model for elastic stresses within a Martian volcano, constructed for that purpose. Results of the analysis supports the view that the terraces on Olympus Mons, as well as on other three Martian volcanoes, including Ascraeus Mons, Arsia Mons, and Pavonis Mons are indeed thrust faults.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14345-14
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  • 31
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    The shallow structure of the Martian lithosphere in the vicinity of the ridged plains (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: On Mars, the most prominent assemblage of regularly spaced plain ridges occurs in the plains of the Coprates and Lunae Planum regions. In this paper, a number of tectonic models is developed and quantitatively evaluated, that relate the regular spacing of these ridges to the shallow internal structure of Mars. It is shown that models with both rigid and deformable megaregolith-basement interface conditions can provide solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials. Implications of the models for the structure and state of stress in the Tharsis region at the time of ridge formation are discussed.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14215-14
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  • 32
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    Speculations on the origin and evolution of the Utopia-Elysium lowlands of Mars (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: This paper proposes a qualitative model for the origin of the Utopia-Elysium northern lowlands on eastern Mars in terms of the long-term evolution of two large overlapping impact basins. The model, which is consistent with both the observed geologic constraints and more quantitative results obtained by numerical modeling of smaller (Orientale-size) impact basins, is shown to qualitatively account for the major topographic variation seen in the Utopia-Elysium region, including the overall 'lowness' of the area and localized depressions.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14203-14
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  • 33
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    Discontinuities in the shallow Martian crust at Lunae, Syria, and Sinai Plana (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Detailed photoclinometric data are presented for a variety of surface features (pits, troughs, wall valleys, and grabens) within three study areas in the western equatorial regions of Mars (Lunae, Syria, and Sinai Plana) that provide evidence for mechanical discontinuities within the shallow Martian crust in these regions. The data's relation to some of the previously proposed mechanical discontinuities within the Martian crust is discussed, and the geologic significance of these features is speculated upon.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14231-14
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  • 34
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    A new survey of multiring impact basins on Mars (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Literature data on Martian multiring impact basins with diameters greater than 500 km are summarized, and evidence is found for eight new such basins. The pattern of changes of basin morphology with increasing basin size suggests three subclasses of multiring basins: (1) multiring basins with diameters up to about 1850 km, which are characterized by the Orientale type concentric structure and cumulative frequency power law slope of -0.75; (2) the Argyre-type basins (with diameters between 1850 and 3600 km, defined by rugged concentric annnuli and a power law slope of nearly -2.0; and (3) the Chryse-type basins (with diameters greater than 3600 km), which exhibit multiple concentric rings and very shallow topographic profiles. Multiring basins are found in all parts of Mars, including the northern lowlands, Tharsis, and surrounding highlands, and are associated with much of the subsequent resurfacing of cratered terrain.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14175-14
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  • 35
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    Mars gravity field error analysis from simulated radio tracking of Mars Observer (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Results are presented on the analysis of the recovery of the Martian gravity field from tracking data in the presence of unmodeled error effects associated with different orbit orientations. The analysis was based on the mission plan for the Mars Observer (MO) radio tracking data from the Deep Space Network. From the analysis, a conservative estimate of the gravitational accuracy for the entire mission could be obtained. The results suggest that, because the atmospheric drag is the dominant error source, the spacecraft orbit could possibly be raised in altitude without a significant loss of gravitational signal. A change in altitude will also alleviate the large effects seen in the spectrum the satellite resonant orders.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 14155-14
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  • 36
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    Martian impact craters - Correlations of ejecta and interior morphologies with diameter, latitude, and terrain (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: An effort is made to establish the ability of a correlation between crater morphology and latitude, diameter, and terrain, to discriminate among the effects of impact energy, atmosphere, and subsurface volatiles in 3819 larger-than-8 km diameter craters distributed over the Martian surface. It is noted that changes in ejecta and interior morphology correlate with increases in crater diameter, and that while many of the interior structures exhibit distributions interpretable as terrain-dependent, central peak and peak ring interior morphologies exhibit minimal relationships with planetary properties.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 87; 156-179
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  • 37
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    A goal and strategy for human exploration of the moon and Mars (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Eventual settlement of the solar system, beginning with the moon and Mars, is proposed, and a strategy for the exploration of and initial settlement of the moon and Mars, based on the model of European settlement of the Americas, is discussed. Strategies suggest an allocation of functions between humans and telerobots to conduct the exploration and initial settlement.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Space Policy (ISSN 0265-9646); 6; 195-208
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  • 38
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    Large-scale patterns of eolian sediment transport on Venus - Predictions for Magellan (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The atmosphere of Venus is likely to exert a major influence on the surface. Arecibo and Venera 15/16 observations of the population of impact craters with bright halos give direct evidence of surface modification, on the scale of centimeters, to smooth the surface on time scales of 50 - 250 my. Both chemical and mechanical modifications probably occur. Diurnal winds in the planetary boundary layer can transport particles. Using boundary layer theory, including the effects of topography, surface stresses and resulting transport and deposition of sand-size particles are calculated. Regional slopes are sites of largest surface stresses. Sand will be generally transported downhill, although there is a preferential net transport from east to west, in the same direction as the atmospheric superrotation. It is predicted here that streaks may be seen in the Magellan radar images which will indicate directions of net eolian transport on the surface of Venus.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1365-136
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  • 39
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    Small domes on Venus - Probable analogs of Icelandic lava shields (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: On the basis of observed shapes and volumetric estimates, small, dome-like features on radar images of Venus are interpreted to be analogs of Icelandic lava-shield volcanoes. Morphometric data for Venusian domes in Aubele and Slyuta as well as measurements of representative dome volumes and areas from Tethus Regio are used to demonstrate that the characteristic aspect ratios and flank slopes of these features are consistent with a subclass of low Icelandic lava-shield volcanoes (LILS).
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1381-138
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  • 40
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    Geologic evidence of hotspot activity of Venus - Predictions for Magellan (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A number of distinctive types of geologic features have been identified on Venus that are interpreted to be related to thermal plumes including domal rises, coronae, and major composite shield volcanoes. The basic characteristics of these features as well as their distribution are documented. The three types of features have related morphologies and are interpreted to represent a continuum of features formed by mantle plumes at scales from 100s to over 1000 km. The Artemis structure, located in Aphrodite Terra, is proposed to be a large corona. If crustal spreading processes are operating on Venus, hotspot features should form chains on the surface as seen in terrestrial ocean basins. On the basis of current data on hotspot-related feature distribution on Venus, no clear evidence exists for hotspot chains. The complete distribution of hotspot features in Magellan data will be used to understand better the relationship between interior processes and surface features, as well as to provide a test for the crustal spreading hypothesis.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1377-138
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  • 41
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    Radar characteristics of the equatorial plains of Venus from Goldstone observations - Implications for interpretation of Magellan data (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Based on analyses of recent high resolution observations of the approximately 1000 km diameter Heng-O ring structure by the Goldstone Solar System Radar, it is predicted that the structure is a large corona. Calibrated values of radar backscatter coefficient indicate lateral variations of a factor of 4 in Fresnel reflectivity in the vicinity of Heng-O, as well as in other areas of the plains, at scales of tens of km. It is predicted that areas with very low reflectivity values, which occur in a variety of geological settings, are soil-dominated, with bulk densities as low as 1.5 g/cu cm.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1357-136
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  • 42
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    Titan and other icy satellites - Dielectric properties of constituent materials and implications for radar sounding (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Radar sounding observations of Titan and the other outer solar system icy satellites have been conducted in order to ascertain the materials present, their dielectric properties, and the degree of radar signal penetration. Deep penetration is possible at long wavelengths, while strong reflections may occur due to pockets or layers of H2O-NH3 solution. Attention is given to the case of Titan, using models to calculate radar reflection S/N ratios; the models involve H2O ice covered by a 100-m thick layer of organic sediments that are overlain by an ocean of liquid CH4-C2H6-N2. At 13.6-cm wavelength, a greater-than-10 dB S/N is computed from the ocean-sediment and sediment-ice interfaces, suggesting that the geology of Titan may be investigated by such a sounding mode despite deep ocean coverage of the satellite.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 86; 336-354
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  • 43
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    The deep atmosphere of Venus revealed by high-resolution nightside spectra (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The first high-resolution spectroscopic observations of the night side of Venus obtained in two narrow spectral windows centered at 1.74 and 2.3 microns, where the nightside is anomalously bright, are reported. Absorption features from CO2, CO, H2O, HDO, HCl, HF, and COS are detected, and there are a number of unidentified features. A preliminary analysis indicates that the observed radiation is thermal emission from atmospheric layers in the eight-bar pressure region for the 2.3 micron window and even deeper at 1.7 micron. The derived CO and H2O abundances agree with in situ Pioneer measurements in the deep troposphere and are consistent with the high deuterium enrichment inferred from Pioneer data. The first measurements of HCl and HF below the clouds are reported along with the first firm detection of COS.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 345; 508-511
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  • 44
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    Mars ionopause during solar minimum - A lesson from Venus (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The ion densities measured by the Viking landers (Hanson et al., 1977) do not show an abrupt falloff with height, giving the false impression that Mars has no ionopause. On the basis of knowledge gained from the solar wind interaction at Venus during solar minimum, it is demonstrated that the observed O2 profile above about 160 km on Mars is a distributed photodynamical ionosphere and can produce an ionopause at around 325 km, similar to that observed on Venus during solar minimum. It is concluded that the solar wind interacts directly with the Mars ionosphere, suggesting that the planet does not have an intrinsic magnetic field of any consequence.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 8265-826
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  • 45
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    A method for estimating eruption rates of planetary lava flows (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The eruption rates of single-lobed planetary lava flows are presently accounted for by a model for radiative losses from a partially crusted hotter core, assuming thermal characteristics similar to terrestrial flows. Eruption rates are within this framework expressible as functions of (1) exposed core area fraction, (2) initial core temperature, (3) core temperature upon cessation of flow, (3) crust thickness, and (4) the lava's density and heat capacity.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 85; 512-515
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  • 46
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    CH4 nonlocal thermodynamic equilibrium in the atmospheres of the giant planets (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The present consideration of methane's vibrational relaxation in the Giant Planets' upper stratospheres employs a model for the thermalization of solar energy absorbed under non-LTE conditions, at the 1.7, 2.3. and 3.3 micron-centered CH4 band groups. On this basis, a range of model atmospheres was produced, reflecting current uncertainties concerning CH4 collisional excitation rates. At 0.1 mbar, all of the non-LTE models are within 2 K of the LTE reference models; the extreme non-LTE models steadily depart from reference LTE ones, however, and reach about + or - 20 K at 0,1 mbar.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 85; 355-379
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  • 47
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    Titan's stratospheric temperatures - A case for dynamical inertia? (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Voyager IRIS spectral radiances in the nu4-band of CH4 for the Titan atmosphere exhibit a hemispheric asymmetry. While asymmetry in the meridional distribution of opacity about the equator cannot be discounted, attention is given to the need for angular momentum transport concurrent with seasonally varying temperatures in the Titan stratosphere, which would maintain the cyclostrophic thermal wind relation between zonal winds and temperatures. The adiabatic heating and cooling associated with these motions can produce the observed temperature asymmetry.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 85; 346-354
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  • 48
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    The extraordinary radar echoes from Europa, Ganymede, and Callisto - A geological perspective (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Electromagnetic echo-scattering models, empirical and theoretical considerations on regolith formation, and ice physics, are presently invoked in a geologic treatment of the anomalous radar properties of the icy Galilean satellites. These are held to arise because of the electrical differences between ice and silicates, and the absence of icy regoliths' ice-density structures in silicate regoliths. Icy regoliths may uniquely smooth out discontinuities between solid ejecta fragments and more porous surroundings, forming refraction-scattering 'lenses'; high-order multiple scatterings will thereby become more likely responsible for the echoes than low-order scattering.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 85; 335-345
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  • 49
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    On the effect of X rays on the color of elemental sulfur - Implications for Jupiter's satellite Io (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Absorption features arose at 0.42 and 0.52 microns when spectral reflectance measurements were conducted for elemental sulfur allotrope mixtures produced by the exposure of the cyclooctal S(8) allotrope of sulfur to X-rays; while these features due to the presence of short-chain molecules are stable at low temperatures, they fade upon warming of the sample. The hypothesis that X-rays in the Jovian environment would cause other allotropic forms of sulfur to revert to S(8) is contradicted by the fact that these features are not present in S(8) at the same temperature that had not been exposed to X-rays.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 85; 326-334
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  • 50
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    Internal structure of Io and the global distribution of its topography (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A global topography is presently calculated for two multilayer Io models in which dissipation occurs in a viscous asthenosphere and a solid mantle: (1) a 'thermal swell' model, in which topography and heat flow are positively correlated, and (2) a 'differentiated lithosphere' model, in which topography and heat flow are negatively correlated. Both the polar topography and the hypsometric distribution of elevations in the differentiated lithosphere model are better matched with observations than the thermal swell model. The shift of the equatorial basin-swell pattern indicates a recent zonal rotation of about 25 deg for Io's lithosphere.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 85; 309-325
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  • 51
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    TV observations of Phobos - First results (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: From February to March, 1989, Phobos-2 acquired 37 TV images of Phobos at distances from 200 to 1100 km. These images will be used to update the figure model and topographic and geological maps of Phobos, its spectral characteristics and scattering law, the surface composition and texture, and the parameters of the orbital motion and forced libration.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Pis'ma v Astronomicheskii Zhurnal (ISSN 0320-0108); 16; 378-388
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  • 52
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    Charge-coupled device spectra of asteroids. I - Near-earth and 3:1 resonance asteroids (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Main-belt asteroids and cometary nuclei are both frequently suggested sources of the relatively short-lived near-earth asteroids (NEAs). The asteroidal origin hypothesis is studied by obtaining new high-quality reflectivity spectra of 19 NEAs and 23 asteroids near the 3:1 resonance, and by performing a systematic comparison of these two groups of spectra. Nonparametric statistical tests are used to check for consistency between the color distributions of the two populations. The tests indicate that the difference between the two distributions is generally not significant at the 3 sigma level. However, this conclusion is dependent on the (unknown) size distribution of the NEAs. The observations are compatible with the hypothesis that the NEAs are derived from the 3:1 resonance region, but do not imply that this is the only possible source region.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Astronomical Journal (ISSN 0004-6256); 99; 1985-201
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    Mars - Change in axial tilt due to climate? (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The average tilt of Mars' equator with respect to its orbital plane may have increased significantly over the age of the solar system. Obliquity oscillations might have induced changes in the climate, which altered the mass distribution and hence the solar torque on the planet. Viscous deformation attributable to loading by the large polar caps expected at low obliquity may have induced secular changes in the axial tilt. Earth-like effective viscosities can account for virtually the entire present obliquity of 24.4 degrees. Thus the present average tilt of Mars may not be primordial.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 248; 720
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    Degradation of the lunar vacuum by a moon base (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Several industrial processes requiring high and ultra-high vacuum similar to the lunar vacuum are outlined. The effects of a 20-person lunar base and a 250-person industrial facility on this vacuum are discussed. It is shown that exhaust from transport spacecraft and leakage from the habitat will be comparable to the daytime gas pressure for the 20-person base, and will degrade the vacuum to the range of 2 x 10 to the -9th torr for the use of 250-person facility. This will result in replacing the mostly nonreactive gases hydrogen, helium, and neon with more reactive gases containing carbon and oxygen. This vacuum is still good enough to perform many important vacuum processes such as plasma-deposition of amorphous silicon for solar cells, but processes such as molecular beam epitaxy or locating an intersecting beam accelerator on the moon will require additional vacuum pumping.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Acta Astronautica (ISSN 0094-5765); 21; 183-187
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    The dynamics of the ionosphere of Mars at large solar zenith angles (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A model of the ion chemistry and dynamics of the ionosphere of Mars at large solar zenith angles has been constructed using techniques previously developed for Venus. It is found that a substantial nighttime ionosphere should occur even when the ionopause height is as low as 250 km. At low altitudes the interaction of the plasma flow with the neutral atmosphere appears to result in considerable 'noise' in the flow patterns. Transterminator plasma velocities are smaller than for Venus, although they may exceed the local sonic speed. The effect of a nocturnal source of suprathermal electrons is also discussed.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 6263-626
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    Petrogenesis of mesosiderites. I - Origin of mafic lithologies and comparison with basaltic achondrites (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: New petrologic and trace element data on basaltic and gabbroic clasts in mesosiderites and basaltic achondrites, combined with existing petrologic and trace element data, have served as a basis for interpretation of the petrogenesis of mesosiderite clasts. Compared with the basaltic achondrites, the mesosiderite basaltic and gabbroic clasts contain more abundant modal tridymite and merrilite, and more commonly contain augite as a late magmatic phase. Their pyroxenes tend to be more MgO-rich, and have lower Fe/Mn ratios which are positively correlated with the Fe/Mg ratio. Some of the mesosiderite basaltic and gabbroic clasts contain xenocrystic plagioclase. The basaltic clasts commonly show superchondritic Eu/Sm ratios and slight LREE depletions. The cumulate gabbro clasts are extremely depleted in LIL, have very high Eu/Sm ratios and high Lu/Sm ratios. All of these features can be explained by a model in which the mafic lithologies of mesosiderites were formed by remelting of a mixed basalt-cumulate gabbro-metal source region near the parent body surface.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 54; 1165-117
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    Matrix effects for reflectivity spectra of dispersed nanophase (superparamagnetic) hematite with application to Martian spectral data (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The effect of the matrix on the reflectivity spectra of nanophase (superparamagnetic) hematite (np-Hm) dispersed within the matrix was investigated in four series of powder samples containing np-Hm dispersed within discrete powder particles (of two size ranges) of silica gel and activated alumina. The spectral data show that matrix effects are large. Samples with the same Fe2O3 content can have np-Hm absorption edges characterized by very different positions and curvature and slope indices, while samples with equivalent absorption edges can have very different Fe2O3 concentrations. Thus, quantitative relationships between the positions of ferric absorption edges and Fe2O3 concentrations are unreliable without knowledge of matrix properties of the system. It is shown that it was possible to match the Fe2O3 concentration, magnetic properties, and spectral data for Martian surface material with a laboratory mixture whose only ferric-bearing phase was hematite.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 5101-510
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    Influence of the solar wind/interplanetary medium on Saturnian kilometric radiation (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Previous studies on the periodicities of the Saturnian kilometric radiation (SKR) suggested a considerable solar wind influence on the occurrence of SKR, so it was obvious to investigate the relationship between parameters of the solar wind/interplanetary medium and this Saturnian radio component. Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the external control of SKR. Out of the examined quantities known to be important in controlling magnetospheric processes this investigation yielded a dominance of the solar wind momentum, ram pressure and kinetic energy flux, in stimulating SKR and controlling its activity and emitted energy, and confirmed the results of the Voyager 1 analysis.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geomagnetism and Geoelectricity (ISSN 0022-1392); 42; 12, 1
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    Thermospheric gravity waves - Observations and interpretation using the transfer function model (TFM) (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: This paper presents some numerical experiments performed with the TFM to study the various wave components excited in the auroral regions that propagate through the thermosphere and lower atmosphere, and to demonstrate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements, and gravity waves seen in the thermosphere of Venus are discussed. Gravity waves are prominent in the terrestrial thermosphere polar region and can be excited by perturbations in Joule heating and Lorentz force due to magnetospheric processes. Observations from the Dynamics Explorer-2 satellite are used to illustrate the complexity of the phenomenon and to review the TFM that is utilized.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Space Science Reviews (ISSN 0038-6308); 54; 297-375
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    A comparative study of Venus and Mars - Upper atmospheres, ionospheres and solar wind interactions (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The neutral atmospheres of Mars and Venus are discussed. A comparative study is presented of the upper atmospheres, ionospheres, and solar wind interactions of these two planets. The review is mainly concerned with the region about 100 km above the surface of the planets.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Indian Journal of Radio and Space Physics (ISSN 0367-8393); 19; 444-465
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    The global budget of impact-derived sediments on Venus (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The sedimentary cycle of Venus is examined, with specific attention to the contribution from impact events. The observed record of impact craters on the surface of Venus can be used to calculate the contribution of fine materials generated by impact processes to the global sedimentary cycle. Ejecta volume calculations carried out on the basis of various crater population scenarios suggest that the maximum global ejecta layer thickness over the past 1 Gyr is about 1-2 mm. This would translate to a global accumulation rate of only 0.002 to 0.01 micron per year. Moreover, most of this material could never have been available for global transport; it is projected that over 90 percent of the ejecta would be deposited within 1-3 radii of the crater rim, and only 0.02-0.05 percent could be available as fine dust for global sedimentary cycle. Therefore, the global layer thickness of transportable impact-derived dust can be no more than a few mm over the past 1 Gyr.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Earth, Moon, and Planets (ISSN 0167-9295); 50-51; 175-190
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    In situ measurement of interstellar silicon carbide in two CM chondrite meteorites (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The first in situ observations of isotopically anomalous SiC are reported. The grains were found by X-ray mapping of polished sections of two chondritic meteorites, Cold Bokkeveld and Murchison. Ion microprobe measurements showed C-13 enrichments, delta C-13, from 199 to 2800 per mil, proving that the grains are indigenous. SiC grains are revealed only as isolated matrix particles, ruling out the possibility that SiC grains were brought into the solar system as inclusions in larger grains, which protected them from destruction in the solar nebula. Several of the SiC grains are cracked, suggesting that the etching treatment may result in size distributions biased toward smaller grains.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 348; 715-717
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    Discovery of hotspots on Io using disk-resolved infrared imaging (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: First results are presented using two new techniques for ground-based observation of Io's hotspots. An IR array camera was used to obtain direct IR images of Io with resolution better than 0.5 arcsec, so that more than one hotspot is seen on Io in Jupiter eclipse. The camera was also used to make the first observations of the Jupiter occultation of the hotspots. These new techniques have revealed and located at least three hotspots and will now permit routine ground-based monitoring of the locations, temperatures, and sizes of multiple hotspots on Io.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 348; 618-621
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    A simple inertial model for Neptune's zonal circulation (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Voyager imaging observations of zonal cloud-tracked winds on Neptune revealed a strongly subrotational equatorial jet with a speed approaching 500 m/s and generally decreasing retrograde motion toward the poles. The wind data are interpreted with a speculative but revealingly simple model based on steady gradient flow balance and an assumed global homogenization of potential vorticity for shallow layer motion. The prescribed model flow profile relates the equatorial velocity to the mid-latitude shear, in reasonable agreement with the available data, and implies a global horizontal deformation scale L(D) of about 3000 km.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 2269-227
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    Field-independent source localization of Neptune's radio bursts (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: During the Voyager 2 encounter with Neptune, a narrowbanded bursty radio component was observed between 500 and 1326 kHz by the Planetary Radio Astronomy instrument. Based on the emission occurrence pattern, the radio source has been localized without the explicit use of the Neptunian offset-tilted dipole magnetic field model, which is accurate only at distances greater than 4 R(N) (Neptune radii) from the planet. Only assumptions based upon the general nature of radio wave propagation in planetary magnetospheres were used. A number of different candidate radial positions were sampled. For example, at 1.5 R(N), the derived source location was positioned only about 10 deg from the south magnetic pole. The radiation from this source was beamed into a cone of 77.5 + or - 6.3 deg half-angle that was tilted about 10 deg from the radial direction to the north-northeast. At other sampled radial positions, similar source locations were obtained. Due to its proximity to the south magnetic pole, the kilometric emission radio source is believed to be associated with an active auroral region, similar in nature to those found at earth and Saturn.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 19143-19
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    The surface of Venus (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: NASA's Magellan spacecraft first revealed the true nature of Venus's surface when it began to transmit photos in September 1990, and it continues its planned mission to map the whole surface of the planet. The very sharp images obtained and processed to date leave the overall impression that Venus is a dynamic world that has been shaped by processes fundamentally similar to those that have taken place on earth, but often with dramatically different results. Magellan's synthetic aperture radar reveals details as small as 120 meters across, one tenth the size of those detected previously. Venus will be mapped eight times over the next five years, which will improve the resolution and fill in missing areas. A number of images are provided along with explanatory descriptions.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Scientific American (ISSN 0036-8733); 263; 60-65
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    Isotope mass fractionation during evaporation of Mg2SiO4 (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Synthetic forsterite (Mg2SiO4) was partially evaporated in vacuum for various durations and at different temperatures. The residual charges obtained when molten Mg2SiO4 was evaporated to 12 percent of its initial mass were enriched in heavy isotopes by about 20, 30, and 15 per mil/amu for O, Mg, and Si, respectively, whereas solid forsterite evaporated to a similar residual mass fraction showed negligible fractionations. These results imply that calcium and aluminum-rich refractory inclusions in carbonaceous chondrites must have been at least partially molten in the primordial solar nebula if the observed large mass fractionation effects were caused by evaporation processes in the nebula.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 347; 655-658
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    Xenon fractionation in porous planetesimals (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The distinctively fractionated Xe on Mars and earth may have its root in a common source from which both planets accreted. Beginning with Ozima and Nakazawa's (1980) hypothesis that terrestrial Xe fractionation was caused by gravitational separation of adsorbed solar nebular gases inside large porous planetesimals, it is pointed out that Xe would have been trapped as the planetesimal grew and pores were squeezed shut by lithostatic pressure. It is shown that enough fractionated Xe to supply the earth could have been trapped this way. The degree of fractionation is controlled by the lithostatic pressure at the pore-closing front and so would have been roughly the same for all large planetesimals. The predicted degree of fractionation agrees well with that preserved in terrestrial and Martian Xe. Relative to Xe, this source is strongly depleted in other noble gases. In contrast to the original Ozima and Nakazawa hypothesis, the present hypothesis predicts the observed fractionation, and it allows planetary accretion to occur after the dissipation of the solar nebula.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 54; 2577-258
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    Surface and airborne evidence for plumes and winds on Triton (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Aeolian features on Triton that were imaged during the Voyager Mission have been grouped. The term 'aeolian feature' is broadly defined as features produced by or blown by the wind, including surface and airborne materials. Observations of the latitudinal distributions of the features probably associated with current activity (known plumes, crescent streaks, fixed terminator clouds, and limb haze with overshoot) all occur from latitude -37 deg to latitude -62 deg. Likely indicators of previous activity (dark surface streaks) occur from latitude -5 deg to -70 deg, but are most abundant from -15 deg to -45 deg, generally north of currently active features. Those indicators which give information on wind direction and speed have been measured. Wind direction is a function of altitude. The predominant direction of the surface wind streaks is found to be between 40 deg and 80 deg measured clockwise from north. The average orientation of streaks in the northeast quadrant is 59 deg. Winds at 1- to 3-kilometer altitude are eastward, while those at more than 8 kilometers blow west.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Science (ISSN 0036-8075); 250; 421-424
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    Global color and albedo variations on Triton (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Global multispectral mosaics of Triton have been produced from Voyager approach images; six spectral units are defined and mapped. The margin of the south polar cap (SPC) is scalloped and ranges in latitude from + 10 deg to -30 deg. A bright fringe is closely associated with the cap's margin; form it, diffuse bright rays extend north-northeast for hundreds of kilometers. Thus, the rays may consist of fringe materials that were redistributed by northward-going Coriolis-deflected winds. From 1977 to 1989, Triton's full-disk spectrum changed from markedly red and UV-dark to nearly neutral white and UV-bright. This spectral change can be explained by new deposition of nitrogen frost over both the northern hemisphere and parts of a formerly redder SPC. Frost deposition in the southern hemisphere during southern summer is possible over relatively high albedo areas of the cap (Stansberry et al., 1990), which helps to explain the apparent stability of the unexpectedly large SPC and the presence of the bright fringe.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1765-176
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    Observations of Neptunian rings by Voyager Photopolarimeter Experiment (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The Voyager Photopolarimeter Experiment detected the Neptunian rings 1989N1R and possibly 1989N2R. These rings were also photographed by the Voyager imaging cameras in August 1989. The firm detection of 1989N1R has an equivalent depth (the product of the radial width and mean normal optical depth) of 0.77 + or - 0.13 km, while a less certain detection of 1989N2R has an equivalent depth of 0.7 km + or - 0.2 km. Several statistical techniques were used to search for additional material in the Neptune ring system, and none was identified at a high confidence level.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1745-174
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    Nitrogen ion clusters in Triton's atmosphere (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The nitrogen ion chemistry that controls Triton's ionospheric composition and may be responsible for the hazes detected by the Voyager spacecraft are discussed. In particular, it is shown that nitrogen cluster ion formation should readily occur in Triton's thin cold atmosphere. The very low temperatures of Triton's atmosphere imply that these clustered ions can nucleate into solid nitrogen particles, creating the extended visible hazes. A model based on the chemical kinetics of nitrogen ions predicts a dense ionosphere at 200-400 km, as detected by Voyager radio occultation measurements.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1725-172
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    Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The theory of mass fractionation by hydrogen is presently extended to atmospheres in which hydrogen is not the major constituent. This theoretical framework is applied to three different cases. In the first, it is shown that the fractionation of terrestrial atmospheric neon with respect to mantle neon is explainable as a consequence of diffusion-limited hydrogen escape from a steam atmosphere toward the end of the accretion process. In the second, the anomalously high Ar-38/Ar-36 ratio of Mars is shown to be due to hydrodynamic fractionation by a vigorously escaping and very pure hydrogen wind. In the last case, it is speculated that the currently high Martian D/H ratio emerged during the hydrodynamic escape phase which fractionated Ar.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 84; 503-527
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    Ice sublimation and rheology - Implications for the Martian polar layered deposits (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: If the sublimation and creep of water ice are important processes in the Martian polar layered deposits, ice-rich scenario formation and evolution schemes must invoke a mechanism for the inhibition of sublimation, such as a dust layer derived from the residue of the sublimating deposits. This layer could be of the order of 1 m in thickness. If the deposits are ice-rich, flows of more than 1 km should have occurred. It is noted that the dust particles in question may be cemented by such ice that may be present, but that impurities may also have served to cement dust particles together even in the absence of ice.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 84; 352-361
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    Search techniques for near-earth asteroids (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Knowledge of the near-earth asteroids (Apollo, Amor, and Aten groups) has increased enormously over the last 10 to 15 years. This has been due in large part to the success of programs that have systematically searched for these objects. These programs have been motivated by the apparent relationships of the near-earth asteroids to terrestrial impact cratering, meteorites, and comets, and their relative accessibility for asteroid missions. Discovery of new near-earth asteroids is fundamental to all other studies, from theoretical modeling of their populations to the determination of their physical characteristics by various remote-sensing techniques. The methods that have been used to find these objects are reviewed, and ways in which the search for near-earth asteroids can be expanded are discussed.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Vistas in Astronomy (ISSN 0083-6656); 33; 21-37
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    Voyager 2's encounter with Neptune (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The results of Voyager 2 observations of Neptune are reviewed. Observations of Neptune's Great Dark Spot, rotation atmosphere, magnetic field, rings, and satellites are discussed. Also, observations of Triton are considered, noting the presence of geyser activity on the satellite. Several photographs of features on both Neptune and Triton are presented.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Vistas in Astronomy (ISSN 0083-6656); 33; 1-20
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    Identification of lunar rock types and search for polar ice by gamma ray spectroscopy (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: This paper examines the possibility of mapping the surface composition of the moon from an orbiting spin-stabilized spacecraft, using gamma ray spectroscopy and a cooled germanium solid-state device as a detector. A design for accommodating the germanium detector gamma ray spectrometer was devised, and the detection sensitivity was applied to typical lunar-rock compositions. For sets comprising nine highland and 16 mare types, the most useful elements were found to be Mg, Al, K, Ti, Fe, U, and Th. An analysis of the expected instrument response to the gamma ray and neutron fluxes of water ice indicated that a neutron mode added to the spectrometer will be more sensitive than the gamma ray mode to the possible presence of polar ice. It was calculated that, with a pair of selected neutron absorbers and a model which provides that 2.5 percent of the area above 75-deg latitude is occupied by trapping sites, the instrument will provide a 1-yr mission detection limit of 0.056 percent H2O by weight for each polar region.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 449-460
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    The 2.5-5.0 micron spectra of Io - Evidence for H2S and H2O frozen in SO2 (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The present analysis of IR spectra of Io in the 2.5-5.0 micron region with laboratory data on plausible surface ices gives attention to four unidentified features exhibiting spatial and temporal band strength variations; these fall close to the fundamental stretching modes in H2S and H2O, respectively. It is suggested that the 3.85- and 3.91-micron bands in the Io spectra can be accounted for by the absorption of the S-H stretching vibration in H2S clusters and isolated molecules in an SO2-dominated ice. The observations resemble the transmission spectra of SO2 ices that contain about 3 percent H2S and 0.1 percent H2O.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 83; 66-82
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  • 79
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    Upward ion flow in ionospheric holes on Venus (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The nature of ion flow within two ionospheric holes on the nightside of Venus was investigated using ion composition measurements made by the ion mass spectrometer on the Pioneer Venus Orbiter. A comparison of the altitude profiles of the observed ion densities with those expected under diffusive equilibrium conditions indicates that the major ions O(+), NO(+), and O2(+), as well as the minor ions H(+) and He(+) flow upward and away from Venus along the axes of the holes. This result agrees with a quantitative evaluation of the ion flow speeds appearing in expressions derived from the equations for conservation of mass and momentum of the ions and electrons. The analysis shows that all ion species flow upward in the holes because the upward force produced by the plasma pressure gradient exceeds all downward forces. However, the nature of the ion source required to maintain such flow is not known.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 31-37
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  • 80
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    Results for Titan's atmosphere from its occultation of 28 Sagittarii (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The occultation of the bright K giant star 28 Sgr by Titan was observed from three stations in the Mediterranean area, and the resulting data set is examined here. Average mesospheric temperatures of about 180 K are derived, with evidence for lateral and vertical atmospheric inhomogeneities on scales ranging from about 10-1000 km. The results are consistent with published models of Titan's atmosphere.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 343; 353-355
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  • 81
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    Solar-cycle changes in the thermal structure of the Venus dayside ionosphere (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Vertical profiles of electron density in the dayside ionosphere of Venus, measured by the Pioneer Venus radio occultation experiment, averaged in bins of solar zenith angle, have been used to examine the behavior of the ionospheric thermal structure with the solar cycle. It was found that the plasma scale height decreased by a factor ranging from 1.5 at 190 km to about 3.0 at 300 km altitude as the solar input went from maximum to minimum. If the assumption of diffusive equilibrium holds above h = 180 km for both the times of solar maximum and solar minimum, similar decreases in the plasma temperature and, consequently, the electron and ion temperatures, are inferred. If, however, diffusive equilibrium conditions do not apply at the time of solar activity minimum, perhaps due to the effects of enhanced solar wind interaction, no information on the plasma temperatures is contained in the data on the topside electron density structure.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Advances in Space Research (ISSN 0273-1177); 10; 5, 19
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  • 82
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    Response of Venus exospheric temperature measured by neutral mass spectrometer to solar EUV flux measured by Langmuir probe on the Pioneer Venus orbiter (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The photoelectron current from the Pioneer Venus Langmuir probe has provided measurements of the total flux of solar EUV photons at Venus since 1979. The neutral oxygen scale height measured by the orbiter neutral mass spectrometer has permitted the exospheric temperature to be derived during the same mission. In this paper, the EUV observations are used to examine the response of exospheric temperature to changes in solar activity, primarily those related to solar rotation. It is found that the dayside exospheric temperature quite faithfully tracks variations in the EUV flux. Comparison is also made with the earth-based solar activity index F10.7 adjusted to the position of Venus. This index varied from 142 to 249 flux units during the period of measurements. The exospheric temperature is better correlated with EUV flux than with the 10.7-cm solar radio flux.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 1091-109
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  • 83
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    Upper limit set for level of lightning activity on Titan (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Because optically thick cloud and haze layers prevent lightning detection at optical wavelength on Titan, a search was conducted for lightning-radiated signals (spherics) at radio wavelengths using the planetary radioastronomy instrument aboard Voyager 1. Given the maximum ionosphere density of about 3000/cu cm, lightning spherics should be detectable above an observing frequency of 500 kHz. Since no evidence for spherics is found, an upper limit to the total energy per flash in Titan lightning of about 10 to the 6th J, or about 1000 times weaker than that of typical terrestrial lightning, is inferred.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 343; 442-444
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  • 84
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    Ignition of global wildfires at the Cretaceous/Tertiary boundary (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The recent discovery of an apparently global soot layer at the Cretaceous/Tertiary boundary indicates that global wildfires were somehow ignited by the impact of a comet or asteroid. It is shown here that the thermal radiation produced by the ballistic reentry of ejecta condensed from the vapor plume of the impact could have increased the global radiation flux by factors of 50 to 150 times the solar input for periods ranging from one to several hours. This great increase in thermal radiation may have been responsible for the ignition of global wildfires, as well as having deleterious effects on unprotected animal life.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 343; 251-254
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  • 85
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    Isotopic Hg in an Allende carbon-rich residue (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A carbon-rich residue from Allende subjected to stepwise heating yielded two isotopically resolvable types of Hg: one with an (Hg-196)/(Hg-202) concentration ratio the same as terrestrial (monitor) Hg; the other enriched in Hg-196 relative to Hg-202 by about 60 percent. Hg with the 202 isotope enriched relative to 196, as is found in bulk Allende, was not observed. Whether the result of mass fractionation or nucleosynthesis, the distinct types of Hg entered different carrier phases and were not thermally mobilized since the accretion of the Allende parent body.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Meteoritics (ISSN 0026-1114); 25; 345
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  • 86
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    Volatile compounds in shergottite and nakhlite meteorites (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Shergottites ALHA77005, EETA79001, and Shergotty, and the nakhlite Nakhla are all found to contain oxidized sulfur (sulfate) of preterrestrial origin, In EETA79001, which is composed of three different lithologies, the degree of sulfur oxidation increased from A to C. Significant bulk carbonate was confirmed in Nakhla and trace carbonate was confirmed for EETA79001, all of which seems to be preterrestrial in origin. Chlorine in shergottites and Nakhla is found to covary with oxidized sulfur, whereas carbonate and sulfate are related inversely. Sulfate and carbonate in shergottites and Nakhla were probably formed by aqueous geochemistry in a highly oxidizing environment sometime near the late-stage histories of the rocks that are now represented as meteorites.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Meteoritics (ISSN 0026-1114); 25; 281-289
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  • 87
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    Recent flood lavas in the Elysium region of Mars (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A volcanic origin is presently suggested for the 'Cerberus Formation' region of smooth plains in the southeastern Elysium region of Mars, on the basis of its surface morphology, lobate edges, vents, and an embayment relation of the unit with adjacent, older units. The low viscosity lavas that filled a topographic depression in southeastern Elysium subsequently flowed into western Amazonic Planitia via channels formed by an earlier fluvial episode. A young, upper Amazonian dating is indicated by crater frequencies and stratigraphic relations, implying that large-scale eruptions of low-viscosity lava were still possible late in Martian history.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 88; 465-490
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  • 88
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    Dynamic crystallization study of barred olivine chondrules (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The 'classic' barred olivine (BO) texture was defined by Donaldson (1976) as a single-plate dendrite which shares the entire chondrule with the remaining glass or subsequent crystallization products. This paper describes experiments in which BO textures were produced experimentally from two starting compositions: one modeled after the average BO chondrule composition in L-chondrites determined by Weisberg (1987), and the other after an olivine-rich composition (CH-1) studied by Lofgren (1989). It is shown that most of the textural features observed in natural BO chondrules are replicated in the experimentally produced chondrules. Several new constraints on models for the chondrule-forming process are proposed.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geochimica et Cosmochimica Acta (ISSN 0016-7037); 54; 3537-355
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  • 89
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    A close-up view of Triton (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Triton, the only large moon in the solar system with a retrograde motion, is investigated. The moon rotates about Neptune every 5.88 days and its annual cycle lasts 165 years. The orbit of Triton is 355,000 km from Neptune and it is inclined 23 deg relative to Neptune's equator. The precession of its orbital plane causes complications in its seasonal progression. Triton has a radius of 1353 km and a density of 2.07 gm/cu cm. Triton is believed to have a core of rock surrounded by water ice and a surface veneer of methane and nitrogen ice. The bright haze in its atmosphere could be small grains of particulates. Triton's surface features suggest that the moon should have remained molten until about 1 billion years ago. In order to explain the active geyser-like plumes observed near the subsolar latitude of about 50 deg south, various mechanisms are suggested including explosive escape of nitrogen gas, surface winds, and buoyancy of warmer gas. Voyager 2, which left Neptune and Triton in August 1989 and is now moving out of the solar system, is expected to provide the first glimpses of interstellar material.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Earth in Space (ISSN 1040-3124); 3; 10-14
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  • 90
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    Coherent backscatter and the radar characteristics of outer planet satellites (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: It is presently suggested that the diffuse component-dominated high radar reflectivities and large polarization of Europa, Ganymede, and Callisto could be due to the illumination by a collimated source of a weakly absorbing particulate medium in which wavelength-scaled scatterers are separated by distances somewhat larger than the wavelength. A plausible medium could be a regolith composed of voids and/or silicate rocks imbedded in an icy matrix; multiply-scattered parts of the wavefront traversing the same path in opposite directions combine coherently in the backscatter direction to generate increased intensity, and the enhancement is different for the two components of polarized reflected radiation.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 88; 407-417
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  • 91
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    Venus - Improved spin vector from Goldstone radar observations (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Twenty-one radar images of the surface of Venus obtained between 1972 and 1982 have been compared to follow the motion of 28 distinct features. The delays and Doppler shifts of these features with respect to the subradar points on the various days were used in a nonlinear least-squares parameter estimation algorithm to refine the period and pole orientation of Venus. The period is found to be -243.022 + or - 0.003 days; the right ascension and declination of the pole are: alpha = 272.816 + or - 0.14 deg; beta = 67.218 + or - 0.05 deg (B1950). These standard error estimates attempt to include effects of systematic errors.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Astronomical Journal (ISSN 0004-6256); 100; 1369-137
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  • 92
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    Magellan radar to reveal secrets of enshrouded Venus (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Imaging Venus with a synthetic aperture radar (SAR) with 70 percent global coverage at 1-km optical line-pair resolution to provide a detailed global characterization of the volcanic land-forms on Venus by an integration of image data with altimetry is discussed. The Magellan radar system uses navigation predictions to preset the radar data collection parameters. The data are collected in such a way as to preserve the Doppler signature of surface elements and later they are transmitted to the earth for processing into high-resolution radar images. To maintain high accuracy, a complex on-board filter algorithm allows the altitude control logic to respond only to a narrow range of expected photon intensity levels and only to signals that occur within a small predicted interval of time. Each mapping pass images a swath of the planet that varies in width from 20 to 25 km. Since the orbital plane of the spacecraft remains fixed in the inertial space, the slow rotation of Venus continually brings new areas into view of the spacecraft.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Earth in Space (ISSN 1040-3124); 3; 10-14
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  • 93
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    Spectral geometric albedo and bolometric Bond albedo of Neptune's satellite Triton from Voyager observations (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The spectral geometric albedo and the bolometric Bond albedo of Triton are calculated using data from the Voyager spacecraft photopolarimeter and science experiments. The geometric albedo is not inconsistent with the presence of a weak absorption feature in Triton's spectrum near 0.75 micron. The bolometric Bond albedo (0.65) is consistent with the 38 K daytime surface temperature of Triton. The results are also in agreement with the 37.5 K temperature of nitrogen at an infrared basal pressure of 14 microbar.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1761-176
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  • 94
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    Post Voyager comparisons of the interiors of Uranus and Neptune (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The recent Voyager flyby of Uranus and Neptune has provided refined values for the gravitational moments and rotation periods of those planets. Using these new parameters, models of the interiors of these planets show that their density distributions are very similar. This lends support to the conjecture that their compositions are similar as well. The models are indeed consistent with such a conjecture. The difference in the internal heat sources of these two planets may be due to the fact that heat transport from the interior of Uranus is inhibited by a statically stable interior.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1737-174
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    The atmosphere of Neptune - Results of radio occultation measurements with the Voyager 2 spacecraft (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: This paper presents the vertical temperature and composition profiles of Neptune's troposphere and stratosphere, covering an altitude of 250 km, obtained from radio tracking data that were acquired during Voyager-2's occultation by Neptune, which began near 62 deg N planetographic latitude and ended near 45 deg S latitude. In the computations, the He/H2 abundance ratio 15/85 was adapted, which is consistent with solar abundance estimates and with recent results from Uranus. It was assumed that aerosols and heavier gases such as CH4, NH3, H2S, and H2O have a negligible effect on the microwave refractivity above the 0.5 bar pressure level.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 1733-173
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    Triton's streaks as windblown dust (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: Explanations for the surface streaks observed by Voyager 2 on Triton's southern hemisphere are discussed. It is shown that, despite Triton's tenuous atmosphere, low-cohesion dust trains with diameters of about 5 micron or less may be carried into suspension by aeolian surface shear stress, given expected geostrophic wind speeds of about 10 m/s. For geyser-like erupting dust plumes, it is shown that dust-settling time scales and expected wind velocities can produce streaks with length scales in good agreement with those of the streaks. Thus, both geyserlike eruptions or direct lifting by surface winds appear to be viable mechanisms for the origin of the streaks.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Nature (ISSN 0028-0836); 346; 546-548
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  • 97
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    Young volcanic deposits in the Valles Marineris, Mars? (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The emplacement of a sequence of deposits covering the chasma floors of the central Valles Marineris of Mars, ranging from thin dust to several km in thickness, appears to have been the last major formational event. These deposits fall into the categories of (1) dark patches lying along faults, (2) light-colored deposits locally associated with craters, and (3) mottled, rugged deposits. It is presently suggested that these materials may be of volcanic origin, in view of their low albedo, their association with faults and possible volcanic features, and their embayment relations and lobate margins. If the suggestion is true, then the occasionally explosive volcanism in question was associated with rifting in the Valles Marineris, and may be as young as the late Tharsis volcanism.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Icarus (ISSN 0019-1035); 86; 476-509
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    Voyager 2's encounter with the gas giants (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The findings made by Voyager 2 as it passed by the gas giants are reviewed. The unexpected problems that had to be overcome during the mission, including receiver failure and scan platform problems, are described. The scientific instrumentation aboard Voyager is briefly examined, and the major types of Voyager scientific investigations and principal investigators are listed. The scientific findings regarding the atmospheres, thermal properties, electromagnetic environments, ring systems, satellite surfaces, body properties, and bulk properties are examined.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Physics Today (ISSN 0031-9228); 43; 40-47
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    The Magellan Venus radar mapping mission (1990)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: The NASA Magellan Venus Radar Mapper spacecraft, which will be placed into orbit around Venus on August 10, 1990, is described and its mission is discussed. The orbiter's 12-cm wavelength, multimode radar system is examined and the applications of its modes are addressed. In the SAR mode, it can image most of the Venus surface at a resolution of better than 300 m, approaching 120 m over more than half the planet. In the altimeter mode, the radar will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter. In the radiometer mode, the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 K, at a resolution of 2 K. The nature of the data products and the archiving plans are also considered.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 95; 8339-835
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    Directional variations in thermal emission from geologic surfaces (1990)
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    Publikationsdatum: 2011-08-19
    Beschreibung: The directional emission properties of geologic surfaces were investigated using a ground-based, hand-held infrared radiometer and thermistor probe. Field sites involved surfaces ranging from smooth playa and sand surfaces to a very rough aa lava flow. Large directional variations in thermal emission were found; they result from the presence of surface roughness at large scales producing spatial variations in kinetic temperature and at small scales producing emissivity variations. These variations are important in remotely determining surface structure and understanding surface energy balance and emission spectra.
    Schlagwort(e): LUNAR AND PLANETARY EXPLORATION
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 17; 985-988
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