Sarantos, Menelaos - The Na and K content of the Moon's exosphere is limited by the impact vaporization rate

On the basis of scale height considerations, the Na exosphere of the Moon is believed to be provided mainly by photon-stimulated desorption (PSD). Recent measurements of Na ions from the Kaguya ion spectrometer demonstrated that the sodium atmosphere exhibits a dawn-dusk asymmetry and does not peak at the subsolar point as expected from a photon source acting on an unlimited reservoir. These measurements implied that the reservoir for exospheric Na is rapidly depleted on the dayside and must be replenished on the nightside by exospheric migration and/or micrometeoroid impacts. In this view of the exosphere-surface system, impacts vaporize sodium from the subsoil, and the recycled portion  of the gas ejecta can be reintroduced to the exosphere by PSD, which acts only on the topmost Angstrom of a grain. Monte Carlo transport simulations of the finite reservoir of Na on the surface performed here demonstrate that the range of  impact vaporization rates in the literature suffice to provide the observed exosphere (~70 Na cm-3 near the subsolar point). Besides this proof of concept, three new results are suggested by these simulations. One, the dependence of exospheric sodium with latitude derived from ground based telescopes and Kaguya can be explained by this scheme only if the impact vapor is concentrated at the equator and drops off rapidly at higher latitudes. Two, the location of the peak, 10 degrees off the subsolar point as inferred from Kaguya data, constrains the PSD yield. Third, for a species like K that clearly has a non-uniform distribution on the lunar surface, the simulations demonstrate that if impact vaporization limits the PSD rate, the resulting exosphere  will exhibit a periodic variation within each lunation. Therefore, it is possible to probe with Kaguya and LADEE measurements the unknown distribution of Na in the lunar subsoil.