The Geological History of Venus: Constraints from Buffered Crater Densities

Introduction: The density of impact craters is widely used in planetary science to study relative and absolute surface ages and the nature of resurfacing on planets. On Venus, the applicability of such methods is very limited due to the small total number of impact craters (about a thousand) [1-3], which is caused both by shielding with a thick atmosphere and active volcanic/tectonic resurfacing and renewal processes. The general spatial distribution of craters is only marginally distinguishable from purely random [4], which makes attempts at interpreting local crater density only marginally meaningful. The situation is significantly better, however, if we involve additional a-priori information in the crater density analysis. For example, if we outline some geological units on the basis of their morphology (not involving information about craters), crater populations on such units may provide important additional information [4,5]. Analysis of the Venus crater population has often used stochastic models of resurfacing. In this work we do not pursue this approach; we consider the deterministic geology and stochastic cratering. We take advantage of the recent global geological map of Venus by M. A. Ivanov [6]. Source data: The map [6] contains the following geomorphologic units (from locally older to younger): t, tessera; pdl, densely lineated plains dissected by numerous subparallel narrow and short lineaments; pr, ridged plains comprising elongated belts of ridges; mt, mountain belts around Lakshmi Planum; gb, plain material contemporaneous or predating regional plains and deformed by groove belts; psh, shield plains having numerous small volcanic edifices and locally predating regional plains; rp, regional plains deformed by wrinkle ridges; sc, shield clusters, morphologically similar to psh but occurring as small patches that postdate regional plains; ps, smooth plains of uniformly low radar brightness occurring near impact craters and at distinct volcanic centers; pl, lobate plains, fields of lava flows that typically are not deformed by tectonic structures and are associated with major volcanic centers; rz, rift zones. For each crater from the USGS crater database [3], M. I. registered unit(s) superposed by the crater and its continuous ejecta (that is units that predate the crater) and unit(s) that embay the crater (postdate it). This has been done with the Magellan radar mosaics (rather than with the map). Buffered crater density: Application of craters to geological timing is based on assumption that the impacts are well represented by a mathematical model of a Poisson process in space and time. Under this assumption, the crater density gives an estimate for the mean crater retention age of a unit. Craters on Venus are large and they often overlap several different units; typical patches of units are often comparable in size to craters. In this case, each crater has its own target area that depends on the crater size [e.g., 7]; the crater density Nu for unit u should be calculated as: