THEORETICAL OVERVIEW AND MODELING OF THE SODIUM AND POTASSIUM ATMOSPHERES OF THE MOON

A general theoretical overview for the sources, sinks, gas-surface int eractions, and transport dynamics of sodium and potassium in the exosp heric atmosphere of the Moon is given. These four factors, which contr ol the spatial distribution of these two alkali-group gases about the Moon, are incorporated in numerical models. The spatial nature and rel ative importance of the initial source atom atmosphere (which must be nonthermal to explain observational data) and the ambient (ballistic h opping) atom atmosphere are examined. The transport dynamics, atmosphe ric structure, and lunar escape of the nonthermal source atoms are tim e variable with season of the year and lunar phase because of their de pendence on the radiation acceleration experienced by sodium and potas sium atoms as they resonantly scatter solar photons. The dynamic trans port time of fully thermally accommodated ambient atoms along the surf ace because of solar radiation acceleration (only several percent of s urface gravity) is larger than the photoionization lifetimes and hence unimportant in determining the local density, although for potassium the situation is borderline. The sodium model was applied to analyze s odium observations of the sunward (D-1 + D-2) brightness profiles acqu ired near last quarter by Potter and Morgan (1988b), extending from th e surface to an altitude of 1200 km, and near first quarter by Mendill o, Baumgardner, and Flynn (1991), extending in altitude from similar t o 1430 to similar to 7000 km. The observations at larger altitudes cou ld be fitted only for source atoms having a velocity distribution with a tail that is mildly nonthermal (like an similar to 1000 K Maxwell-B oltzmann distribution). For both the lower and higher altitude observa tions, a number of equally good fits were achieved for differing amoun ts of ambient atom atmosphere as determined by different combinations of (1) the shape of the velocity distribution for the lower speed sour ce atoms and (2) the gas-surface sticking and thermal accommodation co nditions for the ambient atoms. For cases considered here, the sodium flux for source atoms ranged for the Mendillo et al. (1991) observatio ns from 3.5 x 10(5) atoms cm(-2) s(-1) for a dominant ambient atom atm osphere near the surface to 2.1 x 10(6) atoms cm(-2) s(-1) for no ambi ent atom atmosphere, while the flux values for the observations of Pot ter and Morgan (1988b) were similar to 40% lower. Solar wind sputterin g appears to be a viable source atom mechanism for the sodium observat ions with photon-stimulated desorption also possible but highly uncert ain, although micrometeoroid impact vaporization appears to have a sou rce that is too smalt and too hot, with likely an incorrect angular di stribution about the Moon.