An alternative Venus

Conventional interpretations assign Venus a volcanotectonic surface, younger than 1 Ga, pocked only by 1000 small impact craters. These craters, however, are superimposed on a landscape widely saturated with thousands of older, and variably modified, small to giant circular structures, which typically are rimmed depressions with the morphology expected for impact origins. Conventional analyses assign to a fraction of the most distinct old structures origins by plumes, diapirs, and other endogenic processes, and ignore the rest. The old structures have no analogues, in their venusian consensus endogenic terms, on Earth or elsewhere in the solar system, and are here argued to be of impact origin instead. The 1000 undisputed young “pristine” craters (a misnomer, for more than half of them are substantially modified) share with many of the old structures impact-diagnostic circular rims that enclose basins and that are surrounded by radial aprons of debris-flow ejecta, but conventional analyses explain the impact-compatible morphology of the old structures as coincidental products of endogenic uplifts complicated by magmatism. A continuum of increasing degradation, burial, and superposition connects the younger and truly pristine young impact structures with the most modified of the ancient structures. Younger craters of the ancient family are superimposed on older ones in impact-definitive cookie-cutter bites and are not deflected as required by endogenic conjectures. Four of the best-preserved of the pre-“pristine” circular structures are huge, with rimcrests 800–2000 km in diameter, and if indeed of impact origin, must have formed, by analogy with lunar dating, no later than 3.8 Ga. Much of the venusian plains is seen in topography to be saturated with overlapping 100–600 km circular structures, almost all of which are disregarded in conventional accounts. Several dozen larger ancient plains basins reach 2500 km in diameter, are themselves saturated with midsize impact structures, and may date back even to 4.4 Ga. Giant viscously spread “tessera plateaus” of impact melt also reach 2500 km in diameter; the youngest are little modified and are comparable in age, as calibrated by superimposed “pristine” impact structures, to the least modified of the giant impact basins, but the oldest are greatly modified and bombarded. The broad, low “volcanoes” of Venus formed within some of the larger of the ancient rimmed structures, resemble no modern volcanic complexes on Earth, and may be products of the collapse and spread of impact-fluidized central uplifts. Venusian plains are saturated with impact structures formed as transient-ocean sediments were deposited. The variable burial of, and compaction into, old craters by plains fill is incompatible with the popular contrary inference of flood basalt plains. Early “pristine” craters were formed in water-saturated sediments, subsequent greenhouse desiccation of which 879 *E-mail: whamilto@mines.edu. Hamilton, W.B. , 2007, An alternative Venus, in Foulger, G.R. , and Jurdy, D.M. , eds., Plates, plumes, and planetary processes: Geological Society of America Special Paper 430, p. 879–911, doi: 10.1130/2007.2430(41). For permission to copy, contact editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved. produced regional cracking and wrinkling of the plains and superabundant mud volcanoes (“shields”). The minimal internal planetary mobility indicated by this analysis is compatible with geophysical evidence. The history of the surface of Venus resembles that of Mars, not Earth.