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  • 1
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    Magellan - Electrical and physical properties of Venus' surface (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Magellan probes Venus'surface by 12.6-cm-wavelength vertical and oblique radar scattering and measures microwave thermal emission. Emissivity and root-mean-square slope maps between 330 deg and 30 deg E and 90 deg N and 80 deg S are dissimilar, although some local features are exceptions. Inferred surface emissivities typically are 0.85, but vary from 0.35 at Maxwell to 0.95 northeast of Gula Mons and other locations. Lowest emissivities appear in topographically high areas; this relation suggests that a phase change or differences in chemical weathering occur at about 6055-kilometer radius. Initial results indicate that there are significant variations in the surface scattering function.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science (ISSN 0036-8075); 252; 265-270
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  • 2
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    Magellan - Mission summary (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The Magellan radar mapping mission is in the process of producing a global, high-resolution image and altimetry data set of Venus. Despite initial communications problems, few data gaps have occurred. Analysis of Magellan data is in the initial stages. The radar system data are of high quality, and the planned performance is being achieved in terms of spatial resolution and geometric and radiometric accuracy. Image performance exceeds expectations, and the image quality and mosaickability are extremely good. Future plans for the mission include obtaining gravity data, filling gaps in the initial map, and conducting special studies with the radar.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Science (ISSN 0036-8075); 252; 247-249
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  • 3
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    Pancakelike domes on Venus (1992)
    In:  Other Sources
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    Publication Date: 2019-01-25
    Description: The shape of seven large domes on the plains of Venus, with volumes between 100 and 1000 cu km, is compared with that of an axisymmetric gravity current spreading over a rigid horizontal surface. Both the altimetric profiles and the horizontal projection of the line of intersection of domes on the SAR images agree well with the theoretical similarity solution for a newtonian fluid, but not with the shape calculated for a rigid-plastic rheology, nor with that for a static model with a strong skin. As a viscous current spreads, it generates an isotropic strain rate tensor whose magnitude is independent of radius. Such a flow can account for the randomly oriented cracks that are uniformly distributed on the surface of the domes. The stress induced by the flow in the plains material below is obtained, and is probably large enough to produce the short radial cracks in the surface of the plains beyond the domes. The viscosity of the domes can be estimated from their thermal time constants if spreading is possible only when the fluid is hot, and lies between 10(exp 14) and 10(exp 17) Pa s. Laboratory experiments show that such viscosities correspond to temperatures of 610 - 690 C in dry rhyolitic magmas. These temperatures agree with laboratory measurements of the solidus temperature of wet rhyolite. These results show that the development of the domes can be understood using simple fluid dynamical ideas, and that the magmas involved can be produced by wet melting at depths below 10 km, followed by eruption and degassing.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Papers Presented to the International Colloquium on Venus; p 70
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  • 4
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    Venus surface radiothermal emission as observed by Magellan (1992)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Radiothermal emission results obtained by the Magellan spacecraft during its first 8-month cycle of data taking, corresponding to one full rotation of Venus, are reported. The new Magellan data confirm the existence of regions with extremely low values of radiothermal emission. Possible mechanisms to explain the anomalous phenomena are an interaction with a single interface between the atmosphere and a high-dielectric permittivity medium versus volume scattering from the interior of a low-loss medium. The discrimination between these mechanisms requires careful observation of the polarized state of the radiothermal emission and that of the radar scattering.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; E8 A; 13
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  • 5
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    Venus topography and kilometer-scale slopes (1992)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: During the first 8 months of the Magellan mission, the radar altimeter has made some three million measurements of the surface of Venus covering the latitude range from 85 deg N to 80 deg S. Methods involving range correlation, Doppler filtering, multiburst summation, and range migration are used to focus the observations and to achieve high surface resolution. Results are presented as maps of the global distribution of topography, meter-scale roughness, and power reflection coefficient. The results are similar to those reported in previous experiments (surface heights exhibit a unimodal distribution with more than 80 percent of the surface lying within 1 km of the 6051.84-km mean radius) but the higher resolution of the Magellan altimeter has disclosed several surprisingly steep features, e.g., the southwest face of the Maxwell Montes, the southern face of the Danu Montes, and the chasmata to the east of Thetis Regio, where average kilometer-scale slopes of greater than 30 deg are not uncommon. This conclusion is corroborated by close inspection of synthetic aperture radar imagery.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; E8 A; 13
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  • 6
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    Venus - Surface electromagnetic properties (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Pioneer Venus high-angle radar imaging data are used here to correct values of the specular power reflection coefficient for the effects of diffusely scattering small-scale surface roughness. A value of 5.0 + or - 0.9 is found for the average dielectric permittivity of the rolling plains and lowlands, suggesting that most of the Venus surface is overlain by at most only a few cm of soil or dust. The unexpectedly high values of power reflection coefficient are confirmed by observations of correspondingly low values of radiothermal emission. Thus there can be little doubt that most of the highlands contain significant amounts of a conducting mineral near the surface.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 93; 14881-14
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  • 7
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    Alphonsus crater - Floor fracture and dark-mantle deposit distribution from new 3.0-cm radar images (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: The lunar crater Alphonsus is characterized by numerous fractures or graben, and by endogenic dark-halo craters. Existing maps of fractures from analysis of lunar photography may be biased by the east-west solar illumination. This paper presents new high-resolution, dual-polarization 3.0-cm wavelength radar images of Alphonsus with radar illumination from northerly directions, and uses these data to better map the locations of both the graben and a variety of dark-mantle deposits. The distribution of fractures, and several graben which cut the crater floor and central ridge, are cited as possible evidence for simultaneous, post-Imbrium uplift of both structures. Some of the endogenic dark halo deposits are more extensive in depolarized radar images than in photographs; these extensions are attributed in some cases to more distant emplacement of pyroclastic material, and in others to fortuitous connections with smoother, less cratered portions of the Alphonsus floor.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 2137-214
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