IMPLICATIONS OF A GLOBAL SURVEY OF VENUSIAN IMPACT CRATERS

We present a global survey of the areal distribution, size-frequency distribution, and morphometric properties of the venusian impact cratering record. We explore the resurfacing history of Venus, crater degradation, ejecta emplacement, and cratering mechanics. The number of volcanically embayed and tectonically deformed craters from 0.5 to 1.0 km above mean planetary radius is disproportionately high for an otherwise crater-deficient elevation range. More resurfacing occurred in this range, an elevation range dominated by volcanic rises, rifts, and coronae, than elsewhere on Venus. Although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets, some craters appear particularly ''fresh'' because they have radar-bright floors, a radar-dark halo surrounding the ejecta blanket, and a west facing parabola of low radar return; 20, 35, and 8%, respectively, of craters with diameters > 22.6 km have these features. Statistical correlations support the idea that such craters are younger than the remaining crater population. Characteristics of ejecta deposits for venusian craters change substantially with size, particularly at 20 km crater diameter, which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens. We suggest that this transition may be similar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches. A diameter-independent variation in extent of the ejecta deposits is not related to impact angle or target properties but may be related to the shape, distribution, and dispersion of impacting fragments. Secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters. The large distances secondary craters extend from the crater rim imply that the secondary-forming projectiles traveled in an atmosphere significantly altered by the impact process. Comparison of complex craters found on Venus with those of other planets gave results that were consistent with the idea that interplanetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength. The interplanetary comparison indicates that Venus, the Moon, and Mercury appear to have stronger crusts than do Mars and Ganymede/Callisto. (C) 1994 Academic Press, Inc.