Metallic species, oxygen and silicon in the lunar exosphere: Upper limits and prospects for LADEE measurements

The only species that have been so far detected in the lunar exosphere are Na, K, Ar, and He. However, models for the production and loss of species derived from the lunar regolith through micrometeoroid impact vaporization, sputtering, and photon-stimulated desorption, predict that a host of other species should exist in the lunar exosphere. Assuming that loss processes are limited to ballistic escape, photoionization, and recycling to the surface, we have computed column abundances and compared them to published upper limits for the Moon. Only for Ca do modeled abundances clearly exceed the available measurements. This result suggests the relevance of some loss processes that were not included in the model, such as the possibility of gas-to-solid phase condensation during micrometeoroid impacts or the formation of stable metallic oxides. Our simulations and the recalculation of efficiencies for resonant light scattering show that models for other species studied are not well constrained by existing measurements. This fact underlines the need for improved remote and in situ measurements of the lunar exosphere such as those planned by the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Our simulations of the LADEE neutral mass spectrometer and visible/ultraviolet spectrometer indicate that LADEE measurements promise to provide definitive observations or set stringent upper limits for all regolith-driven exospheric species. We predict that observations by LADEE will constrain assumed model parameters for the exosphere of the Moon.