Plumeless Venus has ancient impact-accretionary surface

Venus displays thousands of old circular structures, with topographic rims 5-2000 km in diameter, that have the morphology and cookie-cutter superposition required of impact craters and basins. Many structures have interior central or ring uplifts or broad, low volcanic constructs. Many are multiring. Old uplands are saturated with the structures, which there are variably eroded, whereas lowland structures are variably buried by sediments. The youngest include three of the largest (rim diameters 800 to 2000 km); analogy with dated Imbrium impact basin on the Moon indicates ages -3.90 Ga. Venus is argued here to preserve much of its surface of late-stage main planetary accretion. The Venus of conventional interpretation, by contrast, is currently inactive but was wholly resurfaced, mostly by plume-driven processes, no earlier than 1 Ga. This speculation is extrapolated from terrestrial conjectures, and rationalizes away voluminous contrary evidence from Venus itself The circular structures are assumed to be endogenic and young. Interpreters of early Venusian radar imagery accepted the possible impact origin and great age of the structures, but impact explanations were then replaced, almost without analysis, by plume conjectures. Nearly all specialists now assume that Venus has internal mobility comparable to the exaggerated mobility assumed for Earth, and that its only impact structures are “pristine” small to midsize craters and basins with an age of <1.0 Ga. (Ages to 3.9 Ga are advocated here.) The older circular structures are convenionally attributed to mantle plumes and upwellings that deformed crust and upper mantle from beneath, with or without lava extrusion. Transfer of plume conjecture to Venus from Earth has little merit. Terrestrial plume speculation is based on assumptions whose predictions have been consistently falsified. Not only do plumes probably not exist on Earth, but even the least-constrained attributions of geologic and tectonic features to them do not include circular structures that in any way resemble those of Venus. Conversely, Venusian speculations neither address nor account for circularity and superpositions. The hot-mobile-Venus assumption behind young-surface conjectures also is dubious. Venus’ lack of a magnetic field (core is likely solid), its positive correlation of topography and geoid (outer Venus is far stiffer than Earth), its origin close to the Sun (less volatiles, including potassium, so much less early radiogenic heat, less weakening volatiles, and higher solidus temperature), and other factors indicate Venus to be much less mobile than Earth. The “pristine” craters can be discriminated only arbitrarily from the best-preserved of the ancient circular structures. From those in turn, there are all gradations back to the deepest-eroded, or most-buried, structures of the old family. Broad, low volcanic constructs (unlike any terrestrial volcanoes) inside old impact basins likely are products of impact melts. Venusian lowlands are floored not by young lava plains but by ancient sediments, possibly including deposits in a transient ocean, derived from uplands by processes still poorly defined. The plains are speckled with mud volcanoes (not lava cones) that, like minor deformation of the sediments, are due to top-down heating by the evolving atmosphere. INTRODUCTION Much of the surface of Venus is saturated by ancient circular structures (Figures 1, 2, and succeeding illustrations) that appear to be impact craters and basins, for which lunar analogy requires ages $3.85 Ga. Venus preserves surfaces dating from late-stage main planetary accretion and, by comparison to Earth, is internally dead.