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 ec lipses between 1993 and 1997. With the Moon inside the magnetosphere, and t herefore shielded from solar wind impact, we find its Na atmosphere to be c omparable in abundance to cases near first and third quarter, implying that solar wind ion sputtering is not a significant source of the atmosphere. T he atmosphere is azimuthally symmetric, and it extends beyond the field of view of our observations (similar to 12 Lunar radii). The average sodium at mospheric profile is best characterized by an r(-1.4) radial power law clos e to that for an entirely escaping atmosphere. The average extrapolated nea r-surface brightness of 1145 rayleighs is in agreement with the near-surfac e polar brightness seen at quarter Moon. This corresponds to a line-of-sigh t neutral column content of 1.4 x 10(9) Na atoms cm(-2) above the limb and a density of similar to 3 atoms cm(-3) above the surface, decreasing as r(- 2.4). We suggest that a blend of sources (15% micrometeor impact uniform ov er the surface and 85% photon-induced desorption dependent on solar-zenith angle over the sunlit hemisphere) could account for the observed extended s odium atmospheres. (C) 1999 Academic Press.