Inelastic collisions as the source of circumlunar sodium

Abstract

In 1988 Potter and Morgan [Science 241 (1988) 675] detected a sodium atmosphere above the limb of the sunlit lunar surface. There are three possible mechanisms which could generate a circumlunar sodium atmosphere. These are solar wind ion sputtering of the lunar surface by ∼ 1 keV protons and 4 keV alpha particles, micrometeoroid bombardment of the lunar surface, and photostimulated release of sodium by solar photons. Models of the lunar sodium atmosphere are presented and compared to the spatially extended sodium brightness observations of Mendillo et al. [Geophys. Res. Lett. 18 (1991) 2097]. The comparison strongly suggests that sodium from micrometeoroid bombardment or directly from ion sputtering cannot account for the observations. Instead the sort of sodium velocity distributions which are deduced from the observations appear to be most compatible with either a photostimulated desorption process or ion sputtered sodium which suffers some collisions within the porous surface of the moon before entering the atmosphere. Laboratory studies of the interaction of solar photons and solar wind ions with lunar type surfaces are required to verify this.