Atmospheric effects on cratering on Venus

A paraboloidal bow shock model is developed in order to estimate the surface distribution of gas shock-induced modifications surrounding Venusian impact craters. We apply two-dimensional oblique shock dynamics to describe a three-dimensional paraboloidal-shaped bow shock impinging upon an assumed incompressible Venusian surface. The effects of the hypersonic atmospheric shock acting on the Venusian surface are considered in terms of induced maximum gas pressure, density, particle velocity, and temperature, for varying angles and velocities of impact. The maximum boulder size that can be saltated by the shock wave induced gas flow and the degree of mutual collision of the surface materials are also considered. The present calculations quantitatively predict the areal extent of the gas shock perturbed surface for normal and oblique impact as a function of impact angle and velocity, and radii of impactors. For a 1-kin radius stony meteorite impacting normally at 20 kin/s, the radius of the disturbed area extends •10-17 times the 3-5 kin crater radius. The perturbed surface affects the surface radar properties, and the present results can provide an explanation of the wide "dark/bright halos" surrounding some of the Venusian impact craters observed via Magellan imagery. For example, a •050-km radius bright halo surrounding a •20-km dark halo is observed around the 3.1-kin radius crater located at 16.5 ø north latitude and 334.4 ø longitude. The average value of the radar backscatter cross section of the -•20-km radius dark halo indicates that •50-cm-thick layer of porous lithologic material is superimposed upon an assumed undisturbed basement rock surface. The bright halo indicates that the surface roughness in this region is •30 % greater than that of the surrounding original surface. These features can be induced by atmospheric shock waves. The present model can relate the observed crater halo radii to the impact parameters, such as projectile radius and density, and the impact velocity and angle.