Discovery of Sodium and Potassium Vapor in the Atmosphere of the Moon

Abstract

Spectra of the region just above the bright limb of the Moon show weak emission features that are attributed to resonant scattering of sunlight from sodium and potassium vapor in the lunar atmosphere. The maximum omnidirectional emission flux above the bright limb is 3.8 ± 0.4 kilorayleighs for sodium and 1.8 ± 0.4 kilorayleighs for potassium. The zenith column densities above the subsolar point are estimated to be 8 ± 3 × 10 8 atoms cm -2 for sodium and 1.4 ± 0.3 × 10 8 atoms cm -2 for potassium. Corresponding surface densities are 67 ± 12 atoms cm -3 and 15 ± 3 atoms cm -3 , respectively. The scale height for the sodium atmosphere is 120 ± 42 kilometers, and for potassium 90 ± 20 kilometers, which implies that the effective temperature of the sodium and potassium is close to the lunar surface temperature. The sodium density at the south polar region was found to be similar to that at the subsolar point, indicating widespread distribution of sodium vapor over the lunar surface. The ratio of the density of sodium to the density of potassium is (6 ± 3) to 1, which is close to the sodium to potassium ratio in the lunar surface, suggesting that the atmosphere originates from the vaporization of surface minerals.