AN HST SEARCH FOR MAGNESIUM IN THE LUNAR ATMOSPHERE

In October 1996 we used the Hubble Space Telescope's Faint Object Spec trograph to make the first-ever mid-ultraviolet spectroscopic search f or emissions from the lunar atmosphere. This spectrum revealed no emis sion lines, despite the fact that strong resonance emission transition s from the Al, Si, and Mg neutrals and from Mg+ occur in the bandpass. We derive 5 sigma 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 facto r of at least 9 relative to model predictions using the known abundanc e of Mg in the lunar regolith. These findings reinforce the negative f indings of our previous, ground-based search for neutral atoms in the lunar atmosphere (B. C. Flynn and S. A, Stern, 1996, Icarus 124, 530-5 36) and suggest that Na and K may be rather unique in their ability to readily sputter from the surface as atomic neutrals, Other species ma y sputter away as ions or in molecular fragments. (C) 1997 Academic Pr ess.