On Venus impact basins: Viscous relaxation of topographic relief

Abstract

The question of whether impact basins dating from the time of heavy bombardment of the inner solar system are preserved on the surface of Venus is of paramount importance for an understanding of the geological processes that have shaped that planet. Two types of features have been suggested as candidates for impact basins on Venus: (1) large, quasi-circular topographic depressions and (2) large, quasi-circular regions of low radar backscatter surrounded by terrain of greater radar backscatter and generally displaying little topographic relief. As a guide to the expected character of an impact basin on Venus, we have calculated the viscous relaxation with time of the topographic relief of such a basin, under the assumption that the surface temperature of the planet has been at or near its presently high value since the time of basin formation. The viscous relaxation model includes the effects of a decrease in effective viscosity with depth and the partial to complete isostatic compensation of initial topography. We conclude that an impact basin several hundred kilometers or greater in diameter and 3 b.y. old or greater in age should have negligible topographic relief at present. This conclusion does not exclude the hypothesis that quasi-circular features of low radar backscatter but little topographic relief may be relaxed remnants of ancient impact basins. Large quasi-circular depressions on Venus, such as Atalanta Planitia, however, cannot exceed a few hundred million years in age; otherwise their initial topographic relief would be excessive. Such large depressions are thus likely to be geologically young and to have resulted from processes other than impact. An additional process must therefore have operated on Venus to create large, roughly circular areas of lowland terrain. One possibility is that these topographic features are analogs on Venus to terrestrial platform basins produced by lithospheric extension and thermal subsidence.

Elysium planitia, mars: Regional geology, volcanology, and evidence for volcano-ground ice interactions

“…Any model for the origin and evolution of the Tharsis province should also be applicable to the Elysium region. Geophysical data indicate that a broad positive free-air gravity anomaly is associated with the observed topographic rise of Elysium Planitia (Sjogren, 1979) from which Solomon and Head (1982) concluded that it is unlikely that a thermal mechanism is responsible for the origin of the Elysium Dome. Based on the spacing of preserved graben around Elysium Mans, Comer ef Ql.…”

Impact basin relaxation as a probe for the thermal history of Pluto

“…At this point, any remaining topography is effectively completely elastically supported. The result that low-degree (i.e., long-wavelength) topographies relax faster than high-degree (i.e., short-wavelength) topographies is consistent with previous studies [e.g., Solomon et al, 1982;Kamata et al, 2012] and is a consequence of two effects. First, and more important, a small basin is only sensitive to the shallow viscosity structure, which results in it experiencing a higher mean viscosity and thus a longer relaxation time.…”

“…A semianalytical solution for a viscous fluid model is given by Solomon et al [1982]. When the density is uniform, the surface topographic amplitude F of a viscous fluid layer overlying an inviscid fluid is given as…”

“…Depending on the thermal (and thus viscosity) structure, the topography of these basins will therefore relax over time. As a result, impact basins provide a probe of thermal histories of terrestrial planets and icy satellites [e.g., Parmentier and Head , ; Solomon et al , ; Thomas and Squyres , ; Dombard and McKinnon , ; Mohit and Phillips , ; Robuchon et al , ; Kamata et al , ; White et al , ]. Pluto, like most other icy planetary bodies, is likely to possess large impact basins.…”

Aeolian processes on Venus

“…Note that the total population has about three times the number of craters than the population of obvious impact craters. It is difficult to determine which of the models shown in Figure 4 is appropriate for Venus because: (a) crater 'relaxation' is not included (e.g., Cordell, 1981;Solomon et al, 1982); and (b) the production of small (~20 km ;Ivanov et al, 1986) craters is deficient due to atmospheric shielding. However, it would appear Figure 4a shows assumed cratering rate and obliteration rate histories.…”