Wavelength Dependence of Polarization. XV. Observations of Venus

Observations were made of the linear polarization of sunlight scattered by Venus, using nine intermediate- bandwidth filters centered at wavelengths from 3400 to 9900 A. Routine coverage of polarization as a func- tion of phase angle is presented for the period of April 1959 through January 1968. Measurements were ob- tained from 70 to 1600 phase angle. Near quarter phase, the distribution of polarization across the planetary disk was observed as a function of wavelength. For the integrated disk, the electric vector position angle was found, within the errors of measurement, always to be either perpendicular or parallel to the plane of scatter- ing (mean angles 179?0 and 89?8, with standard deviations of 50) The amount of positive polarization in- creases with decreasing wavelength, at all but the largest phase angles. Large polarizations (+8%) occur in the ultraviolet at small phase angles (150). Near 900 phase angle the positive ultraviolet polarization has a somewhat uneven distribution over the disk, but shows a sharp general increase from the equator to the cusps (reaching as much as + 18%). But in the infrared the polarization is more negative at the cusps than at the equator. At 1170 phase angle the polarization at the equator is negative at all wavelengths, whereas at the cusps it is more negative at long wavelengths but is positive in the ultraviolet. These correlations are evidence for different mechanisms of scattering in the two spectral regions. The polarization of the disk shows temporal variations in the ultraviolet (with a time scale of months or years), but appears repetitive at longer wavelengths.