NOBLE-GASES IN ANCIENT ASTEROIDAL ATMOSPHERES

The atmospheres of rocky asteroids are unlikely to have ever been anyt hing more than tenuous exospheres. However, it is possible that the de nsities of radiogenic heavy noble gases might have once been high enou gh to have implanted observable quantities in the regoliths that becam e meteorites. For this to have happened, a significant fraction of the se species must have been photoionized and accelerated by the electrom agnetic fields associated with the solar wind, rather than escaping th ermally. Then, some fraction of the photoions would be accelerated int o the asteroid's surface. Analytical and numerical results presented h ere 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, pre viously invoked for lunar samples, could be responsible for excess fis sion-produced Xe found associated with solar wind Xe in howardite mete orites. The lack of such Xe in other types of meteorites may reflect e ither smaller parent bodies or later times of regolith exposure. Simil arly, the failure to observe solar-wind-associated radiogenic Ar-40 in meteorites is consistent with the much smaller likelihood that Ar wil l be photoionized.