Coupled gas-surface chemistry in the precursor of the early solar nebula

Abstract

We have set up a master-equation to model chemical reactions on the surface of dust grains. Our master-equation accounts for adsorption processes on the dust-phase, desorption into the gas-phase and mutual reactions between surface species forming a gas-phase or a dust-phase product. A decoupling approach resulting in a set of multi-dimensional master-equations is made to avoid the combinatorial explosion in the number of needed states if there are more than three surface species involved. The system of differential equations for the dust-phase is automatically set- up using a cut-off-condition on the maximum number of particles for each species to be considered. We used the desorption of either dust-species or gaseous products of the surface species to couple the surface phase to the gas-phase. According to their respective abundances gaseous species that adsorb onto the surface will contribute to the surface-population on the grain. The abundances of important molecules for the cometary chemistry such as H2O, CO, CO2 which are also of prebiotic importance will be calculated.