SIMULATION OF GAS-FLOW IN A COMETARY KNUDSEN LAYER

A numerical model of the nonequilibrium innermost coma region in activ e comets is proposed. The presence of dust grains is neglected for a f irst approximation. We consider the Knudsen layer adjacent to the phas e boundary, where the velocity distribution function relaxes to the Ma xwell equilibrium distribution function and the gas macrocharacteristi cs vary severalfold. Analytical relations between the characteristics of the gas flow on the boundaries of this layer are examined. Three nu merical models of the nonequilibrium layer are then presented: (1) sub limation of the water ice from a plane homogeneous surface; (2) statio nary and non-stationary sublimation of a two-component gas mixture inc luding CO2; (3) sublimation of water ice through a porous dust mantle. The models are based on well known methods for the solution of rarefi ed gas dynamics problems: the Test Particle method and the Direct Simu lation Monte Carlo method with weights. The proposed weighting scheme allows to operate efficiently a model involving the same small number of particles for each component of the gas mixture independent of thei r real number densities, and to solve spatially inhomogeneous problems using the same number of test particles in each spatial cell during t he whole simulation for an arbitrary nonstationary distribution of the gas density. For case (I)we show that the emergent flow is,slightly s upersonic and that essentially complete thermalization occurs within s imilar to 10 mean free paths from the surface. For case (2) we find fo r stationary sublimation that both components attain a unique velocity of outflow but significantly different temperatures. For case(3)we de monstrate the influence of scattering from the mantle surface on the o utflow properties: such scattering increases the gas density and also leads to extra heating of the gas. (C) 1998 Elsevier Science Ltd. All rights reserved.