IMPACT CRATERING ON VENUS - PHYSICAL AND MECHANICAL MODELS

Results are presented on laboratory, theoretical and numerical simulations of various phenomena related to impact cratering processes specific to the Venusian environment. Such processes are influenced mainly by two factors: the high atmospheric density and the high surface temperature. Model results are presented for the influence of the atmosphere on high-velocity meteoroids, the influence of high-velocity meteoroids on the atmosphere, the influence of atmospheric shock waves on the Venusian surface, and the influence of high crustal temperature on impact melting and melt cooling. New modeling techniques based on the two-dimensional Free-Lagrangian method have been employed and show that an icy body of 1-km radius may reach the bottom of the Venusian atmosphere in a partially dispersed state and form an impact crater approximately 20 km in diameter. A significant amount of energy is transferred to the Venusian atmosphere during meteoroid transit, and the gas dynamic flow will be similar to a strong explosion. Modeling shows that a significant wake of low density will form behind the meteoroid and have an important influence on crater formation and ejecta distribution. According to our estimates, the boundary of the flow of dense gas may correspond to the region of hummocky ejecta deposition observed for Venusian impact craters. The collapse of the gas-rock debris column on the boundary between the low- and high-density regions may result in a turbiditelike flow outside the hummocky ejecta area. We carried out preliminary laboratory simulations of the interaction of meteoroid shock waves with the surface. Considerable surface disruption is possible, even under the trajectory of an oblique impact. The possibility of "back venting" caused by shock wave pressuring and depressuring in ground pores may provide an explanation for the origin of dark margins. The differences between Venus and Earth for impact melting and melt cooling were examined. Melt generation on Venus should be 20% to 40% greater than on Earth for the same crater diameter. Melt volumes should stay molten about an order of magnitude longer on Venus, leading to substantial melt flows and smooth crater floors.