Escape of gases from the Moon

Contrary to expectations from the classical theory of an exosphere, we find that even the heaviest gases, for example the noble gases krypton and xenon, cannot be retained by the moon. Because of the ionizing effects of solar ultraviolet radiation, electric forces now become more important than gravitational forces. First we consider thermal escape from a thick atmosphere with a hot top, and then from a thin atmosphere accommodated to the temperature of the surface, but still not an exosphere. We calculate also the rate of escape of photoelectrons and deduce the escape rate of ions. The lifetime of an atmosphere containing all the krypton and xenon believed to have evolved since the origin of the moon is of the order of 800 and 50 years respectively. After the atmosphere has thinned to less than a mean-free path (i.e., an exosphere), yet another mechanism of escape of heavy gases exists. It seems likely that the moon as a whole is positively charged to a potential of about +20 volts, owing to the great intensity of solar ultraviolet radiation. Hence, in the vicinity of the moon there will exist a strong electrostatic field. Whenever a krypton or xenon atom is ionized while it is in flight within the screening length near the moon, the ion will be expelled by the electrostatic field. The lifetime turns out to be ∼1000 years and is somewhat increased by the effect of solar corpuscular streams. Hence we conclude that there is not much likelihood that light or heavy gases evolving from the moon will be retained by the moon. The only gas in the vicinity of the Moon can come from interplanetary space itself.