Clouds of Venus: Evidence for Their Nature

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Abstract
The linear polarization of sunlight multiply scattered by the atmosphere and cloud particles of Venus has been computed and compared with observations over the wavelength range from the ultraviolet to the infrared region. The following properties of the visible cloud layer are derived: the refractive index of the cloud particles is 1.45 ± 0.02 at a wavelength of 0.55 micron, and there is an indication of a slight decrease in the value from the ultraviolet to the near-infrared region; the mean particle radius is very near 1 micron, and most of the particles are spherical; the cloud layer occurs high in the atmosphere where the pressure is about 50 millibars (equivalent to an altitude of approximately 20 kilometers on the earth). The results for the index of refraction eliminate the possibility that the visible clouds are composed of pure water or ice.