Stationary expansion of gas mixture from a spherical source into vacuum

Abstract

Sublimation under solar illumination of diverse ices composing the cometary nucleus forms multispecies gas coma. Depending on the heliocentric distance, the gas production of the nucleus varies in a broad range that results in circumnucleus flow regimes ranging from free-molecular to fluid. Therefore it is important to consider kinetic effects occurring in the gas and their influence on the flow and boundary conditions. We consider stationary flows of pure gases (H2O, CO2, CO) and their mixtures from a spherical surface. It is assumed that gas production of the surface is a consequence of sublimation within the surface layer. The detailed study of the structure and parameters of the gas outflow is based on the kinetic approach. The study considers flow conditions from fluid to free molecular. The results are numerically computed quantities which make the link between production rate, gas composition and surface temperature and: (1) velocities and abundances of gas species beyond the acceleration region; (2) mass, momentum and energy fluxes from the gas environment towards the surface; (3) parameters of the gas flow at the sonic surface circumscribing the source. Also, we define regions and conditions where the fluid approximation of the flow can be correctly applied. These data are necessary for definitions of parameters in: (1) simplified “Haser-type” models; and (2) advanced models of cometary activity which take into consideration the influence of gas environment on nucleus surface; and (3) coma models based on the fluid description of the gas flow.