IMPLICATIONS OF A GLOBAL SURVEY OF VENUSIAN IMPACT CRATERS

We present a global survey of the areal distribution, size-frequency d istribution, and morphometric properties of the venusian impact crater ing record. We explore the resurfacing history of Venus, crater degrad ation, ejecta emplacement, and cratering mechanics. The number of volc anically embayed and tectonically deformed craters from 0.5 to 1.0 km above mean planetary radius is disproportionately high for an otherwis e crater-deficient elevation range. More resurfacing occurred in this range, an elevation range dominated by volcanic rises, rifts, and coro nae, than elsewhere on Venus. Although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets, some cra ters appear particularly ''fresh'' because they have radar-bright floo rs, a radar-dark halo surrounding the ejecta blanket, and a west facin g parabola of low radar return; 20, 35, and 8%, respectively, of crate rs with diameters > 22.6 km have these features. Statistical correlati ons support the idea that such craters are younger than the remaining crater population. Characteristics of ejecta deposits for venusian cra ters change substantially with size, particularly at 20 km crater diam eter, which marks the transition at which the boundaries of ejecta bla nkets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens. We suggest that this transition may be simil ar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches. A diameter-independent variat ion 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 infrequent ly as isolated rays about smaller craters. The large distances seconda ry craters extend from the crater rim imply that the secondary-forming projectiles traveled in an atmosphere significantly altered by the im pact 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 i nterplanetary differences in gravity and crustal strength. The interpl anetary comparison indicates that Venus, the Moon, and Mercury appear to have stronger crusts than do Mars and Ganymede/Callisto. (C) 1994 A cademic Press, Inc.