Impact cratering is strongly affected by the presence of an atmosphere. Our solar system offers four relevant targets: Venus, Titan, Earth, and Mars. Our greatest concern is with the Earth, but Venus is the best subject to study, because its atmosphere is about 100 times thicker than the Earth's, and the surface of Venus is randomly peppered with a thousand craters, most of which are apparently little altered since their creation. Thus Venus provides the ideal testbed for theories of atmospheric permeability to stray cosmic bodies - there is both strong atmospheric interaction and enough craters to provide ground truth to calibrate results.
In this study, numerous two-dimensional (2-D) high-resolution hydrodynamical simulations of asteroids striking the atmosphere of Venus were performed. The computations used ZEUS, a grid-based Eulerian hydro-code designed to model the behavior of gases in astrophysical situations. The numerical experiments address a wide range of impact parameters (velocity, size, and incidence angle), but the focus is on 1-, 2-, and 3-kilometer-diameter asteroids, because asteroids of these sizes are responsible for most of the impact craters on Venus. Asteroids in this size range disintegrate, ablate, and decelerate in the atmosphere, yet retain enough impetus to make large craters when they strike the ground. Smaller impactors usually explode in the atmosphere without cratering the surface.
In the simulations, the impactor is broken up by aerodynamic forces generated by the rapid deceleration of the bolide and the shearing flow that develops around it. This results in a complicated and turbulent flow at high Mach number, featuring a broad range of exponentially growing unstable waves. The simulations are sensitive to small differences (both physical and computational) in the initial conditions of the computation. It is found that the shape, resolution, velocity, or other details of the impact can strongly influence which wavelengths grow first, and how quickly. The evolution of each impact is unique, highly chaotic, and sensitively dependent on details of the initial conditions. Atmospheric permeability thus becomes somewhat probabilistic. One lumpy object might fail to reach the surface, while another object identical except for different lumps might leave a 10-kilometer crater. The impact process is chaotic at some level; this study concentrated on extracting robust and useful results from the welter of detail that emerges from the numerical hydro-code simulations. The sensitivity of the computational results to seemingly innocuous and inconsequential differences in the model appears to be a real, physically based characteristic of the impact process, generated by the nonlinear development of the hydrodynamical instabilities. The chaotic character of the impact process adds extra scatter, as it were, to the distribution of results that would already exist because of variations in the parameters of incoming impactors, such as shape, impact velocity, etc.
Because most of the larger impactors disintegrate by shedding fragments generated from hydrodynamic instabilities, a simple heuristic model of the mechanical ablation of fragments was developed, based on the growth rates of Rayleigh-Taylor instabilities. In practice, the range of model behavior can be
described with one free parameter. This "ablation" model supplements the more traditional "pancake" model that treats the impactor as a single hydrodynamically deforming body. The two models have different and somewhat overlapping realms of validity. The key distinction between large and small impactors is that compression waves can cross the smaller impactor before the hydrodynamic instabilities mature, thus involving the whole object in the hydrodynamics. By contrast, the larger impactor can have its front face stripped off before the trailing hemisphere is noticeably distorted. For Venus, the pancake model generally works better for impactors smaller than 1-2-kilometer diameter, and the ablation model generally works better for impactors larger than 2-3 kilometers.
Point of Contact: K. Zahnle
(650) 604-0840
kzahnle@mail.arc.nasa.gov
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