IMPACT CRATERS AND VENUS RESURFACING HISTORY

Venusian impact crater size-frequency distributions, locations, and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism. An atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km, the size-frequency distribution is close to the atmosphere-free case. With this result, and assuming that the surface records a crater production population (a catastrophic resurfacing model, CRM), an age of cessation of rapid resurfacing of approximately 500 Ma is obtained. Crater locations are widely dispersed across Venus and the hypothesis that they are completely spatially random (CSR) cannot be rejected. However, craters that show embayment by plains materials or modification by throughgoing faults (i.e., tectonized) are preferentially found in areas with relatively few craters overall. The primary region where these modified craters are found is the Aphrodite volcanotectonic zone, extending from Ovda Regio on the west to the region east of Atla Regio. These results, together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy, imply that a range of surface ages are recorded by the impact crater population; e.g., the Aphrodite zone is relatively young. An end-member model (equilibrium resurfacing model, ERM) was developed to quantify resurfacing scenarios. In the ERM, Venus has been resurfacing at an average rate of approximately 1 km2 yr-1. However, the CRM and ERM are idealized end-member representations of possible resurfacing histories. For both models, the resurfacing rate can be expressed as the product of resurfacing patch area a (normalized by planetary surface area) and the frequency omega of resurfacing events. Numerical simulations of resurfacing showed that there are two solution branches that satisfy the CSR constraint: a < 0.0003 (4-degrees diameter circle ) and a > 0.1 (74-degrees diameter circle). The former range corresponds to resurfacing diameters smaller than the average intercrater distance, whereas the latter is associated with large, infrequent events, resurfacing 10% of the planet every 50 Ma to 100% every 500 Ma. The observed fraction of embayed and tectonized craters further constrains values of a and only values near 0.0003 are admissible. The resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs, in any given geological episode, in only a limited number. of regions on the planet.