Prior to the Apollo era the lunar atmosphere was thought to be a collision-less, ballistic conduit for thermal evaporation that balances the inflow of solar wind ions with their loss as neutrals. However, the first atmospheric species to be identified on the moon was radiogenic argon-40, and its identification was the indirect result of a search for an explanation of implanted parentless argon-40 in Apollo 11 soil samples. Subsequently, a neutral mass spectrometer at the Apollo 17 site confirmed the presence of atmospheric argon-40 along with helium of both solar and radiogenic origin.
These data, although meagre, have guided research into the processes that control the escape of gases from the moon and other bodies that have surface-bounded exospheres. Briefly, the dynamic characteristics of the atmosphere are dominated by ballistic transport, and perturbed by radiation pressure, charge exchange, photo-ionization, and escape to earth orbit. However, the most important atmospheric processes are related to the surface physics of regolith grains, that is, diffusion, adsorption, desorption, and elastic scattering, which in turn are affected by the influences of orography and heat flow on regolith temperature. With the upcoming launch of the Lunar Atmosphere and Dust Environment Explorer mission, new light should be shed on these processes as well as on the much-expected existence of other constituents.