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  • 1
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    A large-amplitude rotational wave in the Venusian ionosheath (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Results are reported of a preliminary analysis of a large-amplitude rotational wave in the Venusian ionosheath. The results are based on an analysis of the Pioneer Venus Orbiter plasma and magnetic field observations from six orbits in the first (1979) tail season. This wave appears to be the standing super-Alfvenic wing. It is located within the shocked plasma flow outside the boundary of the tail. The rotation of the magnetic field by about 90 deg across the wave occurs through several successive cycles making the wave similar to the group velocity wing consisting of phase velocity waves. The transition through the wave is accompanied by the vector change of the plasma velocity with the magnitude of the plasma velocity jump comparable to the vector jump of the Alfven velocity. The observed super-Alfvenic wing appears to originate upstream and closer to the planet, possibly near the upper boundary of the magnetic barrier on the dayside.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 87-99
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  • 2
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    On the negative polarization of light scattered by subwavelength regolithic grains (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Recent laboratory polarimetric measurements of light scattering by layers of alumina grains of different size are used to examine the relevance of the Wolff & Dollfus (1990) theory of negative polarization, which has been developed to describe light scattering by regolithic grains of arbitrary size. It is demonstrated that, while reproducing satisfactorily the main polarization characteristics of light scattering by grains much greater than the wavelength, this theory does not describe the negative polarization of light measured by Geake & Geake (1990) for subwavelength-sized grains. The smallest grains exhibit a polarization feature that may be called the 'polarization opposition effect'. This feature requires an alternative explanation and may be due to the so-called weak localization of photons in discrete random media.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 260; 3; p. 550-552.
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  • 3
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    Radar determination of Mars surface properties (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Radar studies of Mars have provided measurements of surface texture on scales of centimeters to hundreds of meters and measurements of surface material properties. Texture (rms surface tilts and estimates of small-scale roughness) may be inferred from dispersion and/or polarization of the radar echo; material properties (reflectivity of dielectric constant) are derived from echo strength. Mars is a diverse target; depending on location, rms surface tilts have been found to vary over the range 0.25 to 10 deg while reflectivity covers at least 3 to 13 percent. Plains units are the most variable, having both the smoothest and roughest surfaces, cratered terrain can be considered predictable and 'average' by comparison. Recent data identify scattering by small structures (perhaps rocks on or near the surface) as playing a more important role than previously recognized. Scattering by the residual ice cap near Mars' south pole is particularly unusual. The present state of radar surface studies is summarized.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: In: Mars (A93-27852 09-91); p. 652-685.
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  • 4
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    A physical model of Titan's aerosols (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A modeling effort is presented for the nature of the stratospheric haze on Titan, under several simplifying assumptions; chief among these is that the aerosols in question are of a single composition, and involatile. It is further assumed that a one-dimensional model is capable of simulating the general characteristics of the aerosol. It is suggested in this light that the detached haze on Titan may be a manifestation of organized, Hadley-type motions above 300 km altitude, with vertical velocities of 1 cm/sec. The hemispherical asymmetry of the visible albedo may be due to organized vertical motions within the upper 150-200 km of the haze.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Icarus (ISSN 0019-1035); 95; 24-53
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  • 5
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    Ulysses radio occultation observations of the Io plasma torus during the Jupiter encounter (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Radio signals from Ulysses were used to probe the Io plasma torus (IPT) shortly after the spacecraft's closest approach to Jupiter. The frequencies of the two downlinks at S-band (2.3 gigahertz) and X-band (8.4 gigahertz) were recorded, differenced, and integrated in order to derive the columnar electron density of the IPT. The measurements agree qualitatively with contemporary models of the IPT based on Voyager data, but significant differences are apparent as well. The overall level of the IPT electron density is approximately the same as the prediction, implying that the amount of gas (or plasma) injected from Io is similar to that observed during the Voyager era. On the other hand, the IPT seems to be less extended out of the centrifugal equator, implying a smaller plasma temperature than predicted.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science (ISSN 0036-8075); 257; 5076,; 1531-153
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  • 6
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    Reflection and emission properties on Venus - Alpha Regio (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Procedures developed from Hagfors' (1964) scattering model have been used with success to extract various properties (reflectivity, emissivity, slope, etc.) from both Pioneer Venus and Magellan data. However, under certain circumstances, such as physically rough areas or areas of nonhomogeneous composition, Hagfors' model may lead to serious errors in the interpretation of radar and radiometry data. The Magellan-derived surface properties are examined for some areas of below average emissivity in Alpha Regio and in an area to the east of Alpha, Crater Stuart, in order to determine if Hagfors' model is applicable. Methods of classical multiple scattering theory are also applied to show that enhanced radar backscattering and depressed thermal emission can be expected with a matrix of silicic or mafic rocks embedded in low loss soils.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; E8, A; 13
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  • 7
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    Neptune's non-thermal radio emissions - Phenomenology and source locations (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: During the inbound and the outbound leg of Voyager 2's encounter with Neptune, the Planetary Radio Astronomy (PRA) experiment aboard the spacecraft detected short radio bursts at frequencies within the range of about 500-1300 kHz, and broad-banded smoothly varying emission patterns within the frequency range from about 40-800 kHz. Both emissions can be described in terms of a period of 16.1 hours determining Neptune's rotation period. Furthermore, just near closest approach, a narrow-banded smoothly varying radio component was observed occurring between 600 and 800 kHz. After giving a brief overview about some general characteristics of Neptune's nonthermal radio emission, the source locations of Neptune's emission components are determined, using an offset tilted dipole model for Neptune's magnetic field. Assuming that the emission originates near the electron gyrofrequency a geometrical beaming model is developed in order to fit the observed emission episodes.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Kleinheubacher Berichte (ISSN 0343-5725); 35; 821-830
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  • 8
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    Mexican site for K/T impact crater? (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Research throughout the Caribbean suggests that the geophysical anomalies in the Yucatan first noted by Penfield and Camargo (1981) and called the Chicxulub crater could be the site of the impact purported to have caused the K/T extinctions. A semicircular ring of sink holes, known locally as cenotes, which correlates with the geophysical anomalies has been identified, and it is argued that the origin of the cenote ring is related to postimpact subsidence of the Chicxulub crater rim. If there is indeed a crater, the region within the cenote ring corresponds to its floor and the crater rim diameter is probably larger than 200 km. If confirmed as a site of impact, the Chicxulub crater would be the largest terrestrial impact crater known, which is consistent with the uniqueness of the K/T global catastrophe.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Nature (ISSN 0028-0836); 351; 105
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  • 9
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    Influence of the solar wind/interplanetary medium on Saturnian kilometric radiation (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geomagnetism and Geoelectricity (ISSN 0022-1392); 42; 12, 1
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  • 10
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    In situ measurement of interstellar silicon carbide in two CM chondrite meteorites (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: 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.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Nature (ISSN 0028-0836); 348; 715-717
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