Noble gases in ancient asteroidal atmospheres

The atmospheres of rocky asteroids are unlikely to have ever been anything more than tenuous exospheres. However, it is possible that the densities of radiogenic heavy noble gases might have once been high enough to have implanted observable quantities in the regoliths that became meteorites. Analytical and numerical results suggest that acceleration of photoions by the solar wind motional field is a significant loss process for Xe on asteroids about 200 km in radius or larger, if the Xe is thermalized by its interactions with the surface. For Ar, photoion acceleration can only become important for asteroids nearly 500 km in radius. Thus photoion acceleration, previously invoked for lunar samples, could be responsible for excess fission-produced Xe found associated with solar wind Xe in howardite meteorites. -from Author