Theoretical overview and modeling of the sodium and potassium atmospheres of the moon

Abstract

A general theoretical overview for the sources, sinks, gas-surface interactions, and transport dynamics of sodium and potassium in the exospheric atmosphere of the Moon is given. These four factors, which control the spatial distribution of these two alkali-group gases about the Moon, are incorporated in numerical models. The spatial nature and relative importance of the initial source atom atmosphere (which must be nonthermal to explain observational data) and the ambient (ballistic hopping) atom atmosphere are examined. The transport dynamics, atmospheric structure, and lunar escape of the nonthermal source atoms are time variable with season of the year and lunar phase because of their dependence on the radiation acceleration experienced by sodium and potassium atoms as they resonantly scatter solar photons. The dynamic transport time of fully thermally accommodated ambient atoms along the surface because of solar radiation acceleration (only several percent of surface 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 sodium observations of the sunward (D-1 + D-2) brightness profiles acquired near last quarter by Potter and Morgan (1988b), extending from the surface to an altitude of 1200 km, and near first quarter by Mendillo, Baumgardner, and Flynn (1991), extending in altitude from similar to 1430 to similar to 7000 km. The observations at larger altitudes could be fitted only for source atoms having a velocity distribution with a tail that is mildly nonthermal (like an similar to 1000 K Maxwell-Boltzmann distribution). For both the lower and higher altitude observations, a number of equally good fits were achieved for differing amounts of ambient atom atmosphere as determined by different combinations of (1) the shape of the velocity distribution for the lower speed source atoms and (2) the gas-surface sticking and thermal accommodation conditions for the ambient atoms. For cases considered here, the sodium flux for source atoms ranged for the Mendillo et al. (1991) observations from 3.5 x 10(5) atoms cm(-2) s(-1) for a dominant ambient atom atmosphere near the surface to 2.1 x 10(6) atoms cm(-2) s(-1) for no ambient atom atmosphere, while the flux values for the observations of Potter and Morgan (1988b) were similar to 40% lower. Solar wind sputtering appears to be a viable source atom mechanism for the sodium observations with photon-stimulated desorption also possible but highly uncertain, although micrometeoroid impact vaporization appears to have a source that is too smalt and too hot, with likely an incorrect angular distribution about the Moon.