On Venus Impact Basins' Viscous Relaxation of Topographic Relief

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.