High-resolution calculations of asteroid impacts into the venusian atmosphere

We present results from a number of 2D high-resolution hydrodynamical simul ations of asteroids striking the atmosphere of Venus. These cover a wide ra nge of impact parameters (velocity, size, and incidence angle), but the foc us is on 2-3 km diameter asteroids, as these are responsible for most of th e impact craters on Venus. Asteroids in this size range are disintegrated, ablated, and significantly decelerated by the atmosphere, yet they retain e nough impetus to make large craters when they meet the surface. We find that smaller impacters (diameter <1-2 km) are better described by a "pancaking" model in which the impactor is compressed and distorted, while for larger impacters (>2-3 km) fragmentation by mechanical ablation is pre ferred. The pancaking model has been modified to take into account effects of hydrodynamical instabilities. The general observation that most larger i mpacters disintegrate by shedding fragments generated from hydrodynamic ins tabilities spurs us to develop a simple heuristic model of the mechanical a blation of fragments based on the growth rates of Rayleigh-Taylor instabili ties. Although in principle the model has many free parameters, most of the se have little effect provided that they are chosen reasonably. In practice the range of model behavior can be described with one free parameter. The resulting model reproduces the mass and momentum fluxes rather well, doing so with reasonable values of all physical parameters. (C) 2000 Academic Pre ss.