The global resurfacing of Venus

Abstract

The impact cratering record on Venus is unique among the terrestrial planets. Fully 84% of the craters are in pristine condition, and only 12% are fractured. Remarkably, only 2.5% of the craters and crater‐related features are embayed by lava, although intense volcanism and tectonism have affected the entire planet. Furthermore, the spatial and hypsometric distribution of the craters is consistent with a completely random one, including stochastic variations. Monte Carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed, or unobserved nonrandom crater distributions for resurfacing areas between 0.03% and 100% of the planet's surface. These models also are not consistent with the number and nonrandom distribution of volcanoes, and the nonrandom distribution of embayed and heavily fractured craters. The constraints imposed by the cratering record strongly indicate that Venus experienced a global resurfacing event about 300 m.y. ago followed by a dramatic reduction of volcanism and tectonism. This global resurfacing event ended abruptly (<10 m.y.). The present crater population has accumulated since then and remains largely intact. Thermal history models suggest that similar global resurfacing events probably occurred episodically in the past. The tesserae statistically have the same average surface age (crater retention age) as the rest of the planet, but they probably represent older rock units deformed by earlier episodes of global resurfacing. Although they largely survived the latest global resurfacing event, their surfaces were severely deformed by it. Monte Carlo simulations indicate that only about 4%–6% of the planet has been volcanically resurfaced since the global event, and that the lava production rate has been no more than 0.01–0.15 km 3 /yr during this time. This rate is significantly less than the current rate of intraplate volcanism on Earth (0.33–0.5 km 3 /yr). Most of Venus' recent volcanism occurs in the Beta‐Atla‐Themis region, and most of the recent tectonism is associated with the major global‐scale tectonic disruption zones that lie within and connect the equatorial highlands. The approximately 33% of the planet's surface bounded by latitudes 30°N and 30°S, longitudes 60° and 300°E contains twice as many heavily fractured craters and 1.4 times more lava‐embayed craters as the planetary average. This region includes most of the major tectonic belts in the equatorial region. Because the craters are indistinguishable from a statistically random distribution, both spatially and hypsometrically, this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m.y. However, these craters are simply fractured and/or embayed, and very few have been subjected to complete tectonic disruption, complete burial, and subsequent removal from the surface, as was the case during the global resurfacing event. We show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record. The effects of recent geologic activity are much less than those of the earlier global resurfacing event, when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity. Episodic regional resurfacing events that had global effects also occurred on Earth (e.g., the mid‐Cretaceous superplume) and probably on Mars. On Mars they may have triggered the catastrophic releases of water that formed the outflow channels.