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K. J. Zahnle (NASA Ames), D. G. Korycansky (UC Santa Cruz)
We analyze detailed high resolution 3-D numerical computations of asteroids and comets striking Venus and Titan. Our purpose is to use the numerical results to calibrate the efficacy of atmospheric filtering. In this sense the numerical experiments are used analogously to laboratory scale impact cratering experiments. Our approach is to fit our numerical results to simple analytic expressions; we then ask how these simple analytic expressions can arise from more fundamental considerations. We find that the numerical events are dominated by mechanical ablation rather than by aerodynamic drag. The effects of ablation are reasonably well described by the pancake model, although the pancake itself exists only as shedded shards. The peak of the (differential) crater size-number distribution appears to be robustly determined. The numbers, sizes, and characteristics of the smallest craters to be found on Venus (or Titan) are more problematic. It appears that small craters are mostly made by high velocity fragments or swarms of fragments. These remnants may retain less than 1% of the bolide's original mass yet retain 50% of its cosmic velocity. A surprising consequence of ablation's dominance over drag is that all small craters on Venus are made by asteroids that, at the top of the atmosphere, were all about the same critical size. Thus the number of small craters is determined almost entirely by the details of the aerodynamic interactions, and is independent of the size-number distribution of the impacting population.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.