ANALYSIS OF TEMPORAL VARIATIONS OF THE POLARIZATION OF VENUS OBSERVEDBY PIONEER VENUS ORBITER

Time variations of the disk-averaged polarization of Venus at waveleng ths of 270, 365, 550, and 935 nm have been analyzed in terms of averag e haze optical thickness, cloud-top pressure, and cloud absorption opt ical thickness at 210 nm (presumably due to gaseous SO2 absorption). T he polarization data were deduced from observations made by the Pionee r Venus Orbiter between 1978 and 1990. Newton-Raphson iteration was us ed to derive the atmospheric parameter values from these observations. The required multiple scattering calculations were performed using th e adding/doubling method. The atmospheric model consists of a cloud la yer and a haze layer. The cloud and haze particles are composed of a c oncentrated sulfuric acid solution and have an effective radius of 1.0 5 mu m and 0.25 mu m, respectively. It was found that the disk-average d haze optical thickness, cloud-top pressure, and cloud absorption opt ical thickness at 270 nm varied irregularly between 1978 and 1990. No trends were identified for the cloud-top pressure and cloud absorption optical thickness at 270 nm, but we did deduce a decrease in haze opt ical thickness during this 12 year period. Specific numbers are: (1) a haze optical thickness at 365 nm of about 0.25 in 1978 and less than 0.1 in 1990, (2) a cloud-top pressure that varies between 5 and 20 mba r, and (3) a cloud absorption optical thickness that varies between 0. 0 and 4.0. Finally, our analysis revealed that the disk-averaged cloud -top pressure and haze optical thickness behaved anomalously in 1983.