Direct statistical simulation of the near-surface layers of a cometary atmosphere. II: A nonspherical nucleus

The study presents the results of numerical simulations of mass-transfer processes in the near-surface layer of the cometary nucleus and in the inner part of the cometary atmosphere, which is formed under the action of solar radiation. The gas-kinetic model of the inner part of the cometary atmosphere surrounding a spherical nucleus (Skorov et al., 2004) is extended to the case of a nonspherical nucleus with axial symmetry. After high-resolution images of comets 19P/Borrelly and Wild 2 have been obtained by Deep Space 1 and Stardust spacecraft, such an extension seems to be vital and important. The nucleus and the inner part of the coma are closely related to each other because of the permanent exchange of energy and mass; therefore, they are modeled consistently. As in the first part of our study, the boundary conditions at the inner boundary of the simulation domain, which are necessary for gas-kinetic simulations, were determined from the self-consistent model of heat and mass transfer in a porous cometary nucleus that was developed earlier by the authors. The model took into account the volumetric character of the radiation absorption in a porous sublimating medium, the kinetic regime of the transport of sublimation products in the pores, and the backward gas fluxes from the coma due to intermolecular collisions. We considered different models of the nucleus structure that determined the effective gas production. Using the direct simulation Monte Carlo method, we computed the two-dimensional gas flow from a heterogeneous nonspherical cometary nucleus. The simulations were performed using the SMILE software. The parallel computer implementation of the software made it possible to calculate the spatial structure of the gas flow for the entire circumnucleus zone.