Impact Erosion of Planetary Atmospheres: Some Surprising Results

We have investigated by analytical and computational means the effect of Cretaceous–Tertiary (K/T) size impacts (5×1030erg, 9-km-radius bolide of 1019g) on terrestrial atmospheres. We have extended analytically the approximate solution due to A. S. Kompaneets (1960,Sov. Phys. Dokl. Engl. Transl.5, 46–48) for the blast wave obtained for atmospheric nuclear explosions (idealized to isothermal atmospheres) to ideal adiabatic atmospheres and to data-based models of the Earth's atmosphere. For the first time, we have been able to obtainanalyticallythe particle trajectories in an isothermal atmosphere. The outcome of this nonlinear analysis is that a massive impact (without the subsequent ejection of substantial mass) would only influence a column of ≈30-km radius in the Earth's atmosphere and that the shocked gas would be propelled up and against the column “wall,” but would not escape from the planet. We examined the validity of “hemispheric 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 finite difference scheme for multimaterial flow problems with large distortion and internal slip. In our CAVEAT calculations, the vapor cloud produced by 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 K/T size impact events, and it proved to be a robust tool. We explored a K/T size impact where the 9-km-radius bolide was vaporized and injected into the atmosphere and found no radial outflow in agreement with the analytic model but, instead, a 50-km-radius vertical column formed with only a small fraction of material reaching escape velocity—no more than about 7% of the vaporized bolide plus atmospheric mass will escape the gravitation of the Earth.