Initiation of Run-Out Flows on Venus by Oblique Impacts

Abstract

A hypervelocity oblique impact results in a downrange-moving vapor cloud, a significant fraction of which is derived from the projectile. Since the vapor cloud expands to great extent and becomes very tenuous quickly on a planet with a thin or no atmosphere, it does not leave a well-defined geologic expression. The thick atmosphere of Venus, however, is sufficient to contain such a rapidly expanding vapor cloud. As a result of atmospheric interactions, impact vapor condenses and contributes to run-out flows around craters on Venus. Previous results of both laboratory experiments and simple semi-analytical calculations indicate that an impact-vapor origin can account for the morphology of run-out flows on Venus most consistently. However, the detailed dynamics and geologic record of downrange-moving impact vapor clouds in Venus's atmosphere are not understood quantitatively. To approach these problems, we carried out two-dimensional hydrocode calculations. Parametric studies of these hydrocode calculations yield simple scaling laws for both the total downrange travel distance and the final temperature of impact vapor clouds under conditions on Venus. Under typical impact conditions, impact vapor clouds travel downrange more than a crater radius prior to the completion of crater formation. Furthermore, the scaling law for the total travel distance is compared with observations for the downrange offset of the source regions of run-out flows around oblique craters. The results of this comparison suggest that energy/momentum-partitioning processes other than pure shock coupling may play important roles in hypervelocity impact at planetary scales. The results of hydrocode calculations also indicate that the terminal temperature of the impact vapor is close to the condensation temperatures of silicates, suggesting that two scenarios are possible for expected range of impact conditions: 1. Impact vapor condenses and forms run-out flows. 2. Impact vapor fails to condense and leaves no run-out flows. Consequently, natural variation in impact angle, velocity, and projectile composition may account for partial occurrence of run-out flows around impact craters on Venus.