RESONANCE SCATTERING BY MERCURIAN SODIUM

The albedo of Mercury's sodium atmosphere is computed by the method of Yelle & Wallace, in the approximation of complete frequency redistribution. The doubling-adding program generates a scattering function for a range of optical depth; interpolation is used to obtain the intensity at specific angles of incidence and scattering, and at a specific optical depth. Particular attention is paid to the influence of surface albedo, which is treated by the Hapke method. For comparison, an analytic approximation to Chandrasekhar's planetary problem is developed for the thin-atmosphere limit and a Lambert albedo. For parameters typical of Mercury, about a fifth of the Na light is undistinguishable from outside the atmosphere, because it is hidden in a depression in the continuum caused by the Na extinction. A brief discussion is given of Io, whose surface albedo is so large that the lines can essentially disappear at certain an les.