Observational Test for the Solar Wind Sputtering Origin of the Moon's Extended Sodium Atmosphere

We present observations of the lunar sodium atmosphere during four lunar eclipses between 1993 and 1997. With the Moon inside the magnetosphere, and therefore shielded from solar wind impact, we find its Na atmosphere to be comparable in abundance to cases near first and third quarter, implying that solar wind ion sputtering is not a significant source of the atmosphere. The atmosphere is azimuthally symmetric, and it extends beyond the field of view of our observations (∼12 Lunar radii). The average sodium atmospheric profile is best characterized by an r −1.4 radial power law, close to that for an entirely escaping atmosphere. The average extrapolated near-surface brightness of 1145 rayleighs is in agreement with the near-surface polar brightness seen at quarter Moon. This corresponds to a line-of-sight neutral column content of 1.4×10 9 Na atoms cm −2 above the limb and a density of ∼3 atoms cm −3 above the surface, decreasing as r −2.4 . We suggest that a blend of sources (15% micrometeor impact uniform over the surface and 85% photon-induced desorption dependent on solar-zenith angle over the sunlit hemisphere) could account for the observed extended sodium atmospheres.