Simulation of gas flow in a cometary Knudsen layer

A numerical model of the nonequilibrium innermost coma region in active comets is proposed. The presence of dust grains is neglected for a first approximation. We consider the Knudsen layer adjacent to the phase boundary, where the velocity distribution function relaxes to the Maxwell equilibrium distribution function and the gas macrocharacteristics vary severalfold. Analytical relations between the characteristics of the gas flow on the boundaries of this layer are examined. Three numerical models of the nonequilibrium layer are then presented : (1) sublimation of the water ice from a plane homogeneous surface; (2) stationary and non-stationary sublimation of a two-component gas mixture including CO2; (3) sublimation of water ice through a porous dust mantle. The models are based on well known methods for the solution of rarefied gas dynamics problems : the Test Particle method and the Direct Simulation 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 their real number densities, and to solve spatially inhomogeneous problems using the same number of test particles in each spatial cell during the whole simulation for an arbitrary nonstationary distribution of the gas density. For case (1) we show that the emergent flow is slightly supersonic and that essentially complete thermalization occurs within ∼10 mean free paths from the surface. For case (2) we find for stationary sublimation that both components attain a unique velocity of outflow but significantly different temperatures. For case (3) we demonstrate the influence of scattering from the mantle surface on the outflow properties : such scattering increases the gas density and also leads to extra heating of the gas.