Impact erosion of planetary atmospheres: Some surprising results

We have investigated by analytical and computational means the effect of Cr etaceous-Tertiary (K/T) size impacts (5 x 10(30) erg, 9-km-radius bolide of 10(19) g) on terrestrial atmospheres. We have extended analytically the ap proximate solution due to A. S, Kompaneets (1960, Sov. Phys, Dokl, Engl. Tr ansl, 5, 46-48) for the blast wave obtained for atmospheric nuclear explosi ons (idealized to isothermal atmospheres) to ideal adiabatic atmospheres an d to data-based models of the Earth's atmosphere. For the first time, we ha ve been able to obtain analytically the particle trajectories in an isother mal atmosphere. The outcome of this nonlinear analysis is that a massive im pact (without the subsequent ejection of substantial mass) would only influ ence a column of approximate to 30-km radius in the Earth's atmosphere and that the shocked gas would be propelled up and against the column "wall," b ut would not escape from the planet. We examined the validity of "hemispher ic blowoff," the hypothesis that all material in a hemisphere lying above a plane tangent to the point of impact radially accelerated outward and, if sufficiently energetic, would also be ejected. We adapted and used a state- of-the-art code (CAVEAT), a hybrid Los Alamos-Sandia Lagrangian-Eulerian fi nite difference scheme for multimaterial flow problems with large distortio n and internal slip. In our CAVEAT calculations, the vapor cloud produced b y the impact produces a shock that is orders of magnitude stronger than any previous use of such codes. We developed new methods to test the accuracy and convergence of CAVEAT for KIT size impact events, and it proved to be a robust tool. We explored a KIT size impact where the 9-km-radius bolide wa s vaporized and injected into the atmosphere and found no radial outflow in agreement with the analytic model but, instead, a 50-km-radius vertical co lumn formed with only a small fraction of material reaching escape velocity -no more than about 7% of the vaporized bolide plus atmospheric mass will e scape the gravitation of the Earth. (C) 1999 Academic Press.