Physical Chemistry of a Heterogeneous Medium: Transport Processes in Comet Nuclei

We present a time-dependent transport model of a binary gas mixture through a porous comet nucleus. We critically discuss the modeling of this process in previous cometary models. We give the relationship between the time tc required to reach steady state and the permeability of the porous nucleus. It is shown that tc is very short (about a few seconds to a very few hours) compared to the rotation time (tR ~ a few hours to 1 day). We show qualitatively what the effect of the structure of the nucleus on the H2O-to-CO ratio could be for a comet approaching the Sun. We find that H2O/CO = f(P) (where P represents the total pressure) passes through a maximum. We use this to propose an indirect method to study the internal physical heterogeneity of comet nuclei by using the H2O-to-CO ratio. It is shown that our approach considerably improves the modeling of mass transport in porous comet nuclei.