Implications from stereo-derived topography of Venusian impact craters

Using radargrammetry we have created high-resolution topographic maps of 74 Venusian craters, including all blight-floured craters over 12 km in diameter covered by Magellan stereo imagery, Our trend for rim-floor depths RF as a function of diameter D for bright-floored craters in the volcanic plains is RF = (0.345+/-0.05) D0.235+/-0.05, and for dark-floored craters in the plains, RF = (0.5+/-0.1) D0.035+/-0.07. Rim heights RH for bright-floored craters in the plains are RH = (0.06+/-0.03) D0.4+/-0.2 (D > 15 km), and for dark-floored craters in the plains, RH = (0.027+/-0.015) D0.4+/-0.2. These trends indicate that bright-floored craters 30 km in diameter are, on average, deeper than dark-floored craters by 180 m from rim to floor and have a 140 m higher rim, and at 90 km in diameter they are 380 m deeper from rim to floor and have a 220 m higher rim. The bright- and dark-floored populations ale different at a 99% confidence level for both rim-flour depths and rim heights. The interpretation most consistent with our data and previous work by others is that Venusian craters with radar-dark floors have been partially filled and had their ejecta blankets embayed by regional-scale lava flooding. When the topography is examined in conjunction with the imagery, it is clear that many dark-floored craters have been surrounded by lavas that rose neatly to the crater rim even though a substantial portion of the crater's ejecta blanket was retained. Most previous analyses of the resurfacing history of Venus have relied on past interpretations that only a small percentage of Venusian craters are embayed by exterior volcanism. Because most craters on Venus have dark floors, our data indicate that the majority of Venusian craters have been surrounded and partially filled by postimpact lavas, and consequently, those previous analyses may have significantly underestimated the amount of volcanism on the Venusian surface over the past few hundred million years. Rim-floor depths for Venusian craters are consistent with the inverse gravity trend observed for the terrestrial planets, and they are similar to 50% deeper than current estimates for complex craters on the Earth. Unlike the other terrestrial planets, neither terrain-floor depths nor central structure heights increase with increasing crater diameter. An interesting trend fur which we have no explanation is that on Venus, the Moon, Mars, and Ganymede, central peaks generally rise to within a constant elevation relative to the surrounding ten ain, but that elevation is lower on the Moon and Mars than on Venus and Ganymede.