Sputtering of sodium on Mercury.

The recent observation of a column density of neutral sodium vapor (~10(11) - 10(12) Na/cm(2)) associated with the solar- facing side of the planet Mercury has rekindled interest in the possible presence of atmospheric gases on that planet. Recent observations by Schneider et al. confirm the presence of Na above the solar-facing hemisphere. The sodium fluorescence is observed in the visible. Previous UV observations by Mariner 10 provided evidence for the presence of H and He in the atmosphere (~10-(11) and 10(12) atoms/cm(2) respectively (3)) and gave upper limits on other possible species (e.g., O, Ne, Ar and C).