A model of heat and mass transfer in a porous cometary nucleus based on a kinetic treatment of mass flow

The main aspect of this paper is to provide a synthesis between two major l ines of development in the understanding of mass and heat transfer in a vol atile porous medium. The first one is a macroscopic approach, where the med ium is considered as a continuum, and heat and mass transfer equations are solved under appropriate boundary conditions for temperature and gas pressu re (G. Steiner and N. I. Komle 1991, Planet. Space Sci. 39,507-513; Y. Mekl er etal. 1990, Astrophys. J. 356, 682-686; S. J. Espinasse et A 1991, Icaru s 92, 350-365), while the second one is a kinetic model, calculating gas fl ow in tubes under the assumption of a known temperature distribution (N. I. Komle and G. Dettleff 1991, Icarus 89,73-84; Yu. V. Skorov et al. 1999, Ic arus 140,173-188). We review briefly the main aspects of this previous work , and subsequently present a combined consistent model, which uses a macros copic heat transfer equation, but kinetic solutions for the gas flow. This new model was implemented as a numerical code and its performance is demons trated by a couple of example calculations. The main advantage of the new model in comparison to the macroscopic approa ch is the fact that it avoids specifying a boundary condition for gas press ure at the surface, because the emitted gas flux is found at any time with the aid of the kinetic calculation. The local balance of sublimation and co ndensation in the interior of the porous ice can be calculated more consist ently than is possible by macroscopic models only, because surface pressure and density develop in a "natural" way and no external boundary condition for the pressure must be imposed. We consider the development of temperatur e distribution and gas flux in ice samples in response to surface irradiati on. Both pure ice and ice covered by a dust mantle are studied. The results are compared with corresponding solutions obtained on the basis of a macro scopic model, and differences are discussed in detail. Finally, experimenta l data obtained from previous comet simulation experiments (KOSI and relate d laboratory experiments) are reconsidered. In particular, temperature prof iles and gas fluxes from the KOSI-9 experiment (E. Grun et al., 1993, J. Ge ophys. Res. 98, 15,091-15,104) are interpreted in terms of the kinetic appr oach. (C) 2001 Academic Press.