The influence of collisions and thermal escape in Callisto's atmosphere

A molecular kinetic method is applied to simulate intermolecular collisions and thermal escape in single and multi-component Callisto-like atmospheres. The 1D atmospheres at noon and midnight are composed of radiolytically produced volatiles (CO2, O2, H2) which are assumed to permeate the regolith and thermally desorb. The escaping H2 is shown to have a critical effect on the atmosphere as it is the dominant collision partner, it cools the background gases producing non-isothermal profiles, and its diffusion through the O2 and CO2 affects the vertical structure. Therefore, at the densities inferred from observations, molecular collisions must be included when describing the structure of and escape from Callisto's atmosphere.