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D. G. Korycansky, K. J. Zahnle, M.-M. Mac Low (AAS)
We compare high-resolution 2D and 3D numerical hydrocode simulations of asteroids striking the atmosphere of Venus. Our focus is on aerobraking and its effect on the size of impact craters. We consider impacts both by spheres and by the real asteroid 4769 Castalia, a severely nonspherical body in a Venus-crossing orbit. We compute mass and momentum fluxes as functions of altitude as global measures of the asteroid's progress. We find that, on average, the 2D and 3D simulations are in broad agreement over how quickly an asteroid slows down, but that the scatter about the average is much larger for the 2D models than for the 3D models. The 2D models appear to be strongly susceptible to the ``butterfly effect'', in which tiny changes in initial conditions (e.g., 0.05% change in the impact velocity) produce quite different chaotic evolutions. By contrast the global properties of the 3D models appear more reproducible despite seemingly large differences in initial conditions. We argue that this difference between 2D and 3D models has its root in the greater geometrical constraints present in any 2D model, and in particular in the conservation of enstrophy in 2D that forces energy to pool in large-scale structures. It is the interaction of these artificial large-scale structures that causes slightly different 2D models to diverge so greatly. These constraints do not apply in 3D and large scale structures are not observed to form. A one-parameter modified pancake model reproduces the crater-forming potential of the 3D Castalias quite well.
This work was supported by NASA's Exobiology and Planetary Atmospheres Programs. Image rendering was done using the resources of UCSC Vizualizaton Lab. M-MML is partially supported by a CAREER fellowship from the US NSF. This work was partially supported by the National Computational Science Alliance, utilizing the NCSA SGI/CRAY Power Challenge array at the University of Illinois, Urbana-Champaign.