Stochastic theory of molecule formation on dust

The formation of interstellar molecules on the surfaces of dust grains is c alculated in the framework of stochastic reaction kinetics. The master equa tion and the state transition probabilities are defined, and the growth of grain mantles following accretion and reaction of gas-phase species is comp uted. The results are compared to the observed composition and structure of interstellar ices. The differences between this approach to gas-grain kine tics and previous work is discussed, and possible extensions of the theory are outlined.