Venus's evolution: A synthesis
Published:January 01, 2007
V.L. Hansen, D.A. Young, 2007. "Venus's evolution: A synthesis", Convergent Margin Terranes and Associated Regions: A Tribute to W.G. Ernst, M. Cloos, W.D. Carlson, M.C. Gilbert, J.G. Liou, S.S. Sorensen
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Perhaps the most surprising result of the NASA (National Aeronautics and Space Administration) Magellan mission to Venus was the preservation of ∼970 essentially pristine impact craters distributed in near-random fashion across the planet surface. The craters have been widely interpreted as evidence of near-global catastrophic volcanic resurfacing over 10–100 million years, ∼500 million years ago. This view of Venus permeates textbooks and popular science, and is rarely questioned. The view of a catastrophically resurfaced Venus emerged relatively early in Magellan mission data analysis. We revisit the question of impact crater distribution and implications for Venus's resurfacing and evolutionary processes using the wealth of observations that have emerged from ∼15 years of Magellan data analysis. The widely cited near-global catastrophic volcanic resurfacing hypothesis, although initially compelling, does not stand up to the rigors of detailed geologic mapping and analysis. A separate but related hypothesis, the global stratigraphy hypothesis, deems that catastrophic resurfacing involved the emplacement of 1–3 km thick stacks of lava flows, which buried preflood craters across ∼80% of Venus's surface. However, geologic mapping indicates that thin, rather than thick, flows cover hypothesized preflood surfaces. In addition, the ∼8% of the surface hypothesized as ancient preflood remnants in the global stratigraphy hypothesis, preserved in elevated plateaus, does not correlate spatially with Venus's oldest surfaces as indicated by impact crater density and crater morphology. Finally, extensive lowland regions, representative of the hypothesized flooded surface, correlate with some of the oldest surfaces on the planet, contrary to hypothesis predictions.
An alternative resurfacing hypothesis (the SPITTER hypothesis: Spatially Isolated Time-Transgressive Equilibrium Resurfacing), which combines aspects of previous hypotheses, calls for near-steady-state impact crater formation and destruction during a time of a globally thin lithosphere. Crater destruction occurred through time-integrated formation of numerous crustal plateaus, occurring in large local regions but punctuated in time and space. The SPITTER hypothesis does not depend on a particular mechanism of crustal plateau formation (whether by downwelling, plume, or impact-induced lava pond), but rather focuses on the elements common to all crustal plateau hypotheses. With a secular change to thick lithosphere, crater destruction processes could no longer operate, and the surface began to accumulate craters. Locally, young surfaces developed as a result of pronounced local volcanotectonic activity in the Beta-Atla-Themis and Lada regions. We call for a thorough reanalysis of Venus resurfacing hypotheses with vigilant consideration of stated and unstated assumptions. The view that is emerging from geologic mapping of the spectacular Magellan data seems to provide tantalizing evidence that Venus's surface records a rich and long history, rather than an abbreviated view of the last ∼500 million years as has been widely assumed.