Navier-Stokes and direct Monte Carlo simulations of the circumnuclear coma II. Homogeneous, aspherical sources

The dayside near-nucleus comae formed by solar-driven sublimation from two different aspherical nuclei made of an homogeneous mixture of ice and dust are computed by (1) solving Navier-Stokes equations and (2) direct Monte Carlo simulations, for different nucleus sizes, heliocentric distances, and dust-to-ice mixing ratios. Excellent agreement between the two methods is found down to surprisingly low production rates: it is found that the limit of validity of the first method is not simply related to the coma rarefaction: a new dimensionless number is tentatively offered to characterize this limit. The present solutions show that the weak shocks always present in the fluid coma persist practically down to truly free-molecular conditions, excluding the observational discovery of a structureless coma. They also show that rarefied flow in the near-nucleus coma can have a quite complicated structure, in particular inside topographic depressions. As an example, coma recondensation on the sunlit flanks of a cavity was found to be possible. We compute, for the first time, a true collisionless coma and show that structures are still present in it but are confined to the immediate vicinity of the surface. Finally, we describe in detail the kinetic conditions in a rarefied water coma, i.e., the velocity distribution asymmetry and the rotational-translational nonequilibrium. The significance of the results for future missions to comets is outlined. (C) 2003 Elsevier Science (USA). All rights reserved.