The Phase Curve of Venus and the Nature of its Clouds

Abstract

Theoretical models of the Venus cloud layer are compared with observations in the U, B and V spectral regions. It is found that the models are sensitive to the detailed scattering properties of the particles. A model of a terrestrial type cloud containing spherical water droplets or ice particles with radii distributed around 4 ? provides good agreement with the observed phase curve of Venus, superior to that obtained in previously published calculations. There is a small disagreement with the observations at low phase angles, suggesting the particles may have a slightly higher index of refraction than for water. However, observations are sparse and uncertain at these angles and improved data are needed to resolve this point. The comparison with observations leads to the following conclusions: the particles in the cloud layer must be of micron-size or larger, and are highly transparent; highly reflective but opaque particles are excluded; and scattering properties of the cloud particles on Venus resemble those of water droplets, ice particles, or particles of transparent minerals such as quartz.