Observations of potassium in the tenuous lunar atmosphere

Spectra showing the potassium D1 emission line above the Moon's bright limb (subsolar point) were obtained at first quarter the night of 29 April 1989. The number density at the surface is 9.5 ± 1 atoms cm^-3. Linear regression analyses find equally good fits to a one term model with a scale height of 80 km and corresponding temperature of 600 K, or a two term model with a thermal component accommodated to the sub-solar surface temperature (T=395 K, scale height = 52 km; nt=9.6 K atoms cm^-2 sec^-1) and with a nonthermal component (represented as T=2500 K, scale height = 329 km; n_nt= 1.1 K atoms cm^-2 sec^-1). In either case there is a large nonthermal component and a deficiency of atoms equilibrated to the surface temperature. The calculated thermalization rate of the nonthermal component through encounters with the lunar surface gives a source strength for the thermal component a factor of 7 greater than loss by photoionization. A possible explanation for the low thermalized population is the existence of a fast and efficient way of depleting the thermal component and supplying the nonthermal by momentary adsorption on the surface and subsequent photo-desorption by the considerable visible solar irradiance. Also important, this plausible mechanism for the redistribution of thermal and nonthermal components may completely mask the original velocity distribution of the source population.