Interpretation of the Variation of Polarization over the Disk of Venus

Abstract

The polarization of reflected sunlight is computed for model atmospheres of Venus as a function of location on the apparent planetary disk. The calculations are for both homogeneous (uniformly mixed) and layered models, as required to investigate the vertical distribution of particles. The results are compared with available observations, which are few in number and of poor spatial resolution. The results for the homogeneous and layered models are also integrated over the planet and compared with whole-disk observations. It is shown that the Rayleigh scattering observed in the polarization of Venus originates primarily from within the visible clouds, rather than from above the clouds. The photon mean free path is ?5 km at the 50 mb pressure level, which is well within the visible clouds. Thus the visible ?clouds? are actually a very diffuse hazy region. This visible cloud layer extends at least up to the level where the pressure is ?10 mb. The results indicate that the atmosphere behaves, for this type of observation, more nearly as the so-called ?homogeneous model? than as the ?reflecting layer model.? However, there is some indication in the data that the turbidity (ratio of cloud particle opacity to Rayleigh opacity) increases with depth into the atmosphere. This conclusion receives stronger support from a comparison of particle number densities obtained from the polarization data (?30 particles cm?3 at the 50 mb level) with the number densities obtained from other observations which refer on the average to higher and lower levels in the atmosphere.