Postimpact modification by volcanic or tectonic processes as the rule, not the exception, for Venusian craters

Abstract

We present three investigations that use the Venusian impact crater population to constrain the planet's resurfacing history. We evaluate stereo-derived topography for 91 Venusian craters that have a diameter (D) greater than 15 km. Craters with radar-bright floors have greater rim-floor depths and rim heights than craters with radar-dark floors. For the bright-floored craters rim-floor depths are dbf = 0.483 D 0.165 and rim heights are rhbf = 0.056 D0.483. Trends for dark-floored craters are ddf = 0.424 D0.108 and rhdf = 0.181 D-0.025. For a 60 km crater, this represents differences of 290 m in rim-floor depth and 240 m in rim height. We interpret these results to indicate that dark-floored craters have experienced postimpact volcanic embayment and filling. We examine the population of craters with D > 20 km that have radar-dark halos surrounding their continuous ejecta (114 craters). We find that a portion of the halo has been removed for almost all dark-floored craters, consistent with our interpretation that dark-floored craters have been affected by postimpact volcanism. Finally, we assessed geologic histories of 12 large impact structures with stereo coverage. All but one of these structures has experienced postimpact volcanism or tectonic deformation, often in multiple episodes. In summary, widespread volcanic and tectonic activity occurred throughout the time period of emplacement of the crater population. Postulated resurfacing histories that consider the majority of craters to be at the top of the stratigraphic column are invalid, and the mean surface age of Venus is young (∼150 My). Copyright 2011 by the American Geophysical Union.