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J. Harrington (Cornell), D. Deming (NASA/GSFC)
We model the plume flight and atmospheric response phases of the Shoemaker-Levy 9 impacts, and present synthetic impact-site images and lightcurves that are consistent with observations. The impacts are too complex to cover in a single model, involving hypersonic solid-gas interaction, ballistics, non-hydrostatic hydrodynamics, temperatures of 10 K - 40,000 K, and important interaction length scales from tens of meters to tens of thousands of kilometers. We thus break the event into five phases: comet fragment entry, entry response, plume flight, plume landing response, and dissipation. We initialize a ballistic Monte-Carlo model of the plume flight with the final velocity distribution of Zahnle and Mac Low's (1995, J. Geophys. Res. 100 16,885-16,894) entry-response model. The model ejects the plume into a cone with adjustble tilt and opening angles, and calculates where the mass, energy, and momentum land as a function of time. It also has parameterized material sliding, since the plumes were observed to slide after impact (Hammel et al. 1994, Science 267 1288-1296). The model produces the main features of the large observed impact sites, including the previously-unexplained expanding ring seen at 3.2 micron by McGregor et al. (1996, Icarus 121 361-388). As this is a ballistic model, the atmospheric waves are not reproduced. The model's output feeds a radiative-hydrodynamic model of the atmosphere based on the ZEUS 3D code of Stone and Norman (1992, Astrophys. J. Suppl. 80 753-790). This model produces synthetic lightcurves and explains the physics of the ``third precursors'', ``main event'', ``bounces'', ``flare'', and several spectroscopic observations, including hot CO. The maximum temperature produced is 2000 K. The pincipal cooling mechanism following the main event is adiabatic horizontal expansion, not radiation. Work funded by the NASA Planetary Atmospheres Program.