A Spectroscopic Survey of Metallic Species Abundances in the Lunar Atmosphere

2 Abstract. We present the rst results of an ongoing eeort to search for new species in the lunar atmosphere. We discuss our observations in terms of the degree to which atomic metal abundances in the lunar atmosphere are stoichiometric, that is, proportional to surface abundances (as the Na:K ratio is). Na and K are the only atmospheric constituents to have been observed from Earth, but Apollo sample returns established that a variety of species are more abundant in the lunar surface than either Na or K Taylor 1982]. Simple stoichiometric arguments predict that relatively abundant lunar surface constituents such as Si, Al, Ca, Mg, Fe, and Ti should be more abundant in the lunar atmosphere than either Na or K. Using the 2.7 m coud e and the 2.1 m cassegrain echelle spectrographs at the University of Texas McDonald Observatory, we investigated this hypothesis by searching for solar resonant scattering lines of nine metallic species between 3700 and 9700 A. Spectra were taken 20 arcsec above the subsolar limb of the Moon near quarter phase on 30 July 1994 and 10-12 March 1995. Upper limits were obtained for the rst time for the abundant lunar surface species Si, Al, Ca, Fe, and Ti, as well as Ba and the alkalis Li, Rb, and Cs. In the cases of Si, Ca, Fe, and Ti, the derived upper limits are more than an order of magnitude lower than the simple stoichiometric model predicts. The upper limits for Li and Al are less constraining. Our Ba, Rb, and Cs upper limits lead us to conclude that those species are not stoichiometrically overabundant above our detection threshold in the atmosphere. We conclude that the stoichiometric Na:K ratio is peculiar in that the mechanism(s) that produce the lunar Na and K atmosphere somehow favor those atomic species over many more or comparably abundant lunar surface species.