摘要:

Starting with the assumption that planetary surfaces are self‐affine (fractal) over the scales applicable to radar scattering, we derive various surface parameters, e.g., rms slopes and autocorrelation functions, and examine the implications for radar scattering models. The results of this work provide several new insights of interest to planetary geologists and others using radar to study surface features. First, the unidirectional slope histograms of self‐affine surfaces are Gaussian, and the adirectional slope histograms are Rayleigh. Normalization of the adirectional histogram by solid angle results in a Gaussian adirectional slope density function and therefore a Gaussian quasi‐specular angular scattering function. Next, the wavelength dependent behavior of surface roughness inferred from lunar radar observations is consistent with self‐affine topography. Finally, surface rms height measurements are functions of profile length. Therefore, when determining the applicability of the small perturbation model to a surface based on those measurements, it is necessary to consider the length of the profile with respect to the sampling wav

ISSN:0148-0227    

DOI:10.1029/95JE00664

年代:1995

数据来源: WILEY

摘要:

Previous studies of the effects of silicate phase transitions on mantle convection have tended to use numerical models in which the phase transitions have the same depths as found in the Earth's mantle, or in which a single transition is placed at a depth equal to half the layer thickness. This study presents a systematic investigation designed to examine the effects of an endothermic phase transition near the core‐mantle boundary of terrestrial type planets. The results indicate that a deep mantle endothermic phase change may trap the lower thermal boundary layer, allowing only one upwelling to penetrate the phase transition. In these situations the deep endothermic phase change excites a dipolar convection pattern. This mechanism may have a role in the degree one power observed in the distribution of the Earth's hotspots and the development of the Martian crustal dichotom

ISSN:0148-0227    

DOI:10.1029/95JE00710

年代:1995

数据来源: WILEY

摘要:

Mapping of Dione Regio, Venus, a regional highland 1200 by 2700 km in size with mean elevation of 6052 km located in the southern hemisphere to the NW of Lavinia Planitia, reveals a region characterized by early tectonically dominated structure and later widespread volcanism and centralized edifice construction. Small patches of complexly deformed terrain (tessera) and larger regions of plains characterized by a single direction of deformation (tectonized plains), preserved as inliers, are embayed by more recent widespread volcanic plains. Smaller‐scale plains resurfacing superposed these plains following a pervasive compressional event that lead to distributed wrinkle ridge formation. Four major volcanoes dominate the topographically irregular highland, each with similar early‐stage radar‐dark lava flow deposits but significantly different late‐stage eruptions. These late‐stage differences are manifested as dark, diffuse deposits (possibly pyroclastic in origin) and construction of a single steep‐sided dome at Ushas Mons, a small shield‐volcano field at Nepthys Mons, and bright diffuse deposits (possible pyroclastic deposits) with summit dome and pit formation at Innini and Hathor Montes. The presence at all of these volcanoes of radial lineaments and graben, interpreted to be related to near‐surface lateral dyke emplacement, and late‐stage summit constructional features indicate that shallow magma chambers occur at each edifice. Variations in the evolutionary history of and eruptions from these chambers may explain the differences observed in the deposits at these major edifices. It is also possible, however, that fundamental differences in the magma source region was a major factor in determining the appearance of the deposits. Of the several models considered for the formation of the region, secondary upwellings from a single large mantle plume or the near‐contemporaneous upwelling of multiple smaller plumes best explain the observed characteristics at the rise. A comparison to other volcanic highlands suggests that there are at least two classes of volcanic rises on Venus, an areally and topographically large type with significant volcanism and rift zone formation (like Beta Regio) and a smaller‐scale type with extensive volcanism but moderate rifting (like Western Eistla Regio). These two classes reflect a spectrum in size and vigor of mantle upwellings. The Dione rise is most like Western Eistla Regio in terms of areal extent, topographic elevation, an

ISSN:0148-0227    

DOI:10.1029/95JE00822

年代:1995

数据来源: WILEY

摘要:

Magellan (and Pioneer Venus Orbiter (PVO)) found high mountain terrains on Venus (∼2.5 km above 6051 km planetary radius) exhibit anomalously low radiothermal emissivity. This is thought to result from weathering of primary Venus rock, which at high altitudes produces a distinctive high‐dielectric‐constant mineral assemblage. Deviations from the nominal altitude‐emissivity pattern have been used as a crude chronometric tool with which to date Venusian landforms. This technique indicates Maat Mons (an unusually large shield volcano, standing 9.17 km above 6051 km planetary radius, at 2.1°N, 194.3°E) has undergone a “recent” episode of large‐scale volcanic activity; a deduction also implied morphologically. The present paper investigates whether a plinian eruption at Maat Mons could explain the enhanced concentrations of SO2gas in the upper atmosphere of Venus that were detected by the Pioneer Venus UV spectrometer. The results show for a minimum vent radius of 156 m, a minimum eruption temperature of 1200 K, and a magmatic volatile content of ∼5 wt%, a plinian eruption at the summit of Maat Mons can explain the anomalous concentration of SO2gas. This would mean Maat Mons is

ISSN:0148-0227    

DOI:10.1029/95JE00147

年代:1995

数据来源: WILEY

摘要:

Dendritic valley patterns in the equatorial highlands of Mars show evidence of internal drainage into restricted basins, which are interpreted to be floored with sedimentary fill. Based on crater frequency characteristics of six areas of enclosed basins, the origin of these intercrater plains fill units ranges from middle to late Noachian. In contrast, the age of modification of the same plains units derived from the frequency of fresh craters occupies a relatively narrow range centered on the Noachian/Hesperian boundary. In half the areas studied the timing of highlands and plains crater modification is consistent with a sedimentary origin for basin fill materials. The other plains units most likely consist of interlayered sedimentary and volcanic materials. Relations between the age of stability of these internally drained highland units and their elevation are not as distinct as prior studies suggested; a trend of decreasing age with decreasing elevation for the plains materials is not matched by similarly derived ages of the dissected highlands. Remapping and age dating of the dissected highlands and associated basins suggest that volcanic plains may be more extensive than those used in past models for magma and volatile evolution, and support local volcanism rather than a global‐scale magmatic head model for highlands plains formatio

ISSN:0148-0227    

DOI:10.1029/95JE00940

年代:1995

数据来源: WILEY

摘要:

Mars undergoes significant oscillations in its orbit, which will have a pronounced effect on its climate and, in particular, on the behavior of subsurface water ice. We explore and map the behavior of ice in the Martian near‐surface regolith over the past 1 m.y. using a diffusion and condensation model presented in an earlier paper, with two primary modifications to include orbitally induced variations in insolation and atmospheric water abundance. We find that the past behavior of ground ice differs significantly from that at the present epoch, primarily the result of high‐amplitude oscillations in obliquity (presently 25°). In midlatitude and equatorial regions, ground ice will condense from atmospheric water during times of higher obliquity, filling the top few meters of the regolith with significant amounts of ice. At an obliquity of 32°, ground ice becomes stable globally. During times of lower obliquity, ground ice will sublime and diffuse back into the atmosphere, dessicating the regolith to a depth of about 1 to 2 m equatorward of 60° to 70° latitude. In the high‐latitude regions these oscillations are considerably subdued. Below this depth of cyclic saturation and dessication a long‐term stability of ice exists in some geographic regions. We present a map of the distribution of ice expected at the present epoch. Cyclic exchange of water between the global regolith and polar regions will have significant implications for surface geology and the polar layer

ISSN:0148-0227    

DOI:10.1029/95JE01027

年代:1995

数据来源: WILEY