An HST search for magnesium in the lunar atmosphere

Abstract

In October 1996 we used the Hubble Space Telescope's Faint Object Spectrograph to make the first-ever mid-ultraviolet spectroscopic search for emissions from the lunar atmosphere. This spectrum revealed no emission lines, despite the fact that strong resonance emission transitions from the Al, Si, and Mg neutrals and from Mg+occur in the bandpass. We derive 5σ upper limits on the atmospheric abundances of each of these species and on OH (0-0) emission. The most constraining upper limit we obtained was for Mg, which we find to be depleted by a factor of at least 9 relative to model predictions using the known abundance of Mg in the lunar regolith. These findings reinforce the negative findings of our previous, ground-based search for neutral atoms in the lunar atmosphere (B. C. Flynn and S. A. Stern, 1996,Icarus124, 530-536) and suggest that Na and K may be rather unique in their ability to readily sputter from the surface as atomic neutrals. Other species may sputter away as ions or in molecular fragments. © 1997 Academic Press.