Wavelength dependence of polarization. XXXVIII. Analysis of ground-based observations of Venus.

The polarization of reflected sunlight is computed for a homogeneous model atmosphere of Venus as a function of location on the apparent planetary disk. The results are compared with available ground based observations for 1975 and 1977. The most striking character of these observations is the general distribution of the polarization which, rather than increasing towards the pole as would be expected for a uniform cloud cover, instead presents a gradient which is nearly null or inversed in both visible and ultraviolet light. The differences between the standard model and the measurements are quite reduced by adding a thin haze of submicrometric particles above the main usual cloud. Particle sizes of about 0.2 micron are convenient, with an optical thickness of a few percent, whatever the precise refractive index of the particles is.