AIRBURST ORIGIN OF DARK SHADOWS ON VENUS

A simple analytic model for the catastrophic disruption and deceleration of impactors in a thick atmosphere is used to (1) reproduce observed Venusian cratering statistics and (2) generate radar-dark disks by the impact of atmospheric shock waves with the surface. When used as input to Monte Carlo simulations of Venusian cratering, the model nicely reproduces the observed low diameter cutoff. Venusian craters are found to be more consistent with an asteroidal rather than a cometary source. The radar-dark ''shadows'' of the title are surface features, usually circular, that have been attributed to airbursting impactors. A typical craterless airburst is the equivalent of a approximately 10(6) megaton explosion. The airburst is treated as a massive, extended explosion using a thin-shell, isobaric cavity approximation. The strong atmospheric shock waves excited by the airburst are then coupled to surface rock using the usual impedance matching conditions. Peak shock pressures experienced by surface rock typically exceed 0.2 GPa for distances 15-30 km from ground zero (the place on the surface immediately beneath the site of the airburst), and 1 GPa for 10-20 km. These high shock pressures are felt to considerable depth, often more than a kilometer. Beneath the airburst the shock could reduce surface rocks to fine rubble, while at greater distance the weaker shock would leave fields of broken blocks, perhaps in part accounting for radar-bright halos that often surround the dark shadows.