Interaction of the surfaces of the Moon and Mercury with their exospheric atmospheres

The atmospheres of the moon and Mercury are controlled entirely by gas atom‐surface interaction. Model calculations describing the steady state atmospheres have all been based on the assumption that the atmospheric particle source is a ‘saturated’ adsorbed surface layer of gas. We suggest that this is in disagreement with what is known of the physics of gas‐surface interaction. On the assumption that interaction is with solid atoms bound in a lattice structure, most collisions with the light atmospheric particles are free‐free, not free‐bound. The observational and theoretical evidence indicates that energy accommodation per collision for He and H on the moon and Mercury is dominated by first‐order interaction terms and is generally less than 0.1 per collision. As a result, estimated thermal escape is drastically reduced for the light atoms on both bodies. We consequently require much reduced solar wind collection efficiencies for protons and α particles. It is suggested that some peculiarities observed in the Mercury He and H atmospheres could be explained by the nature of the gas‐surface coupling. The formation of H2 may not be efficient as a surface or subsurface phenomenon, and the H/H2 atmospheric ratio may be large.