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.