Spectra showing the sodium D lines above the Moon's bright limb (subsolar point) were obtained at first and last quarters, May 27 and June 7, 1988. The number density at the surface is 57¿20 atoms cm-3 and the scale height is 79¿8 km, compatible with the temperature of the surface. Comparison with the Mercurian density of 2.6¿104 atoms cm-3 reveals enigma. Despite the apparent general similarity of the surfaces, the sodium densities are in a ratio of ~400. The difference may be attributed to differences in source or loss mechanisms or to some fundamental compositional difference between the two planets. The possibility that the source is rapid diffusion of sodium through Mercury's crust and regolith is discussed as well as recent observational evidence that Mercury's regolith is of intermediate rock type and may be more sodium-rich than the Moon's. ¿ American Geophysical Union 1988