AN ALTERNATIVE VENUS

Conventional interpretations assign Venus a young volcanotectonic landscape pocked only by sparse impact craters younger than perhaps 0.5 Ga. That Venus instead retains a landscape from late-stage planetary accretion is indicated by the saturation of much of its surface by variably modified small to giant ancient impact structures. Venus displays perhaps 5000 circular structures, typically rimmed depressions, with morphology expected for impact origins. The overwhelming consensus among venusian specialists is that among these only 1000 small “pristine” structures (of which half in fact are substantially modified) record impacts, and that all older circular structures are products, which have no terrestrial analogues, of plumes and other endogenic processes. (~1000 of the old structures are large “coronae” etc.; the rest have rims mostly <100 km in diameter and are conventionally disregarded.) The old structures are here argued to be mostly or entirely also of impact origin, and to date back to >3.9 Ga. Not only do the young “pristine” craters share with many of the old structures, throughout the size ranges of both, impact-diagnostic circular rims that enclose basins and are surrounded by radial aprons of debris flows, but all gradations in degradation and burial are represented between the least-modified obvious young impact structures and the most-modified ancient ones. Younger craters of the old family are cookie-cutter superimposed on older, not deflected as required by endogenic conjecture. Four of the best preserved old circular structures have topographic rims 800-2000 km in diameter, and if of impact origin must, by analogy with dated Imbrium Basin on the Moon, have formed by ~3.9 Ga. Most of the 4000 or so large and small old craters are of similar antiquity. Broad, low masses of impact melt reach 2000 km in diameter. Venusian plains appear to be of transient-ocean sediments deposited synchronously with late planetary accretion, and many craters of the older family may record submarine impacts. Early “pristine” craters formed in water-saturated sediments, subsequent greenhouse heating of which produced mud volcanoes and regional wrinkling. Plains sediments variably bury old craters and are compacted into them, providing strong evidence against the popular assumption of flood-basalt plains.