Photometric Properties of the Surface of Io and Their Influence on Line Formation in the Atmosphere

We give a quantitative theory of line formation in an atmosphere above a surface with backscattering properties. Sufficiently high spatial and spectral resolution spectra of resonance lines in Io region A can yield data on the surface scattering properties as well as the number density of scattering molecules. We discuss maeroseopically homogeneous models of scattering from the surface of Io and conclude that multiple reflection from crystal facets is the most likely cause for the observed geometric albedo and phase variation.