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 redistrib ution. The doubling-adding program generates a scattering function for a range of optical depth; interpolation is used to obtain the intensi ty at specific angles of incidence and scattering, and at a specific o ptical depth. Particular attention is paid to the influence of surface albedo, which is treated by the Hapke method. For comparison, an anal ytic approximation to Chandrasekhar's planetary problem is developed f or the thin-atmosphere limit and a Lambert albedo. For parameters typi cal of Mercury, about a fifth of the Na light is undistinguishable fro m 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 di sappear at certain an les.