Polarimetry and photometry of Saturn from Pioneer 11: Observations and constraints on the distribution and properties of cloud and aerosol particles

Abstract

The Imaging Photopolarimeter (IPP) experiment aboard the Pioneer spacecraft measured the linear polarization of red and blue sunlight scattered from Saturn's atmosphere at phase angles from 9 to 150°. This paper presents the observations, discusses their reduction, and summarizes the reduced data. Detailed tables are given for a bright zone (7°S–11°S), a darker belt (15°S–17°S) and a north-south scan. In blue light the minimum polarization on the disk rises from near zero at small phase to a maximum of > 10% near 100° phase in the zone (> 20% in the belt) and decreases toward zero at larger phase, suggestive of Rayleigh scattering. In red light the polarization is small and positive (maximum electric vector perpendicular to scattering plane) at small and large phase, and negative (maximum electric vector parallel to scattering phase) at intermediate phase angles in both regions, suggesting dominance by relatively large cloud particles. In both colors the polarization at all latitudes increases steeply from the center of the map toward both limb and terminator at phase angles near 90°. The data in the belt and zone have been compared with multiple scattering models containing a single type of cloud particle with various single-scattering phase matrices and vertical distributions in Rayleigh scattering gas. Unless there is some minimum optical depth of positively polarizing material (such as the gas) above a negatively polarizing or nonpolarizing cloud, the steep increase in polarization toward the limb is not reproduced. If the gas optical depth above the cloud top is too great, the cloud particles must be extremely negatively polarizing to compensate—unlike aboratory measurements of ammonia crystals. Models in which the cloud top ( τ = 0.4 level) is located at a pressure level of 150 ± 50 mb in the zone and 270 ± 80 mb in the belt fit the blue data with a relatively nonpolarizing base cloud. These cloud-top locations are consistent with scattering models in methane absorption bands for a CH 4/H 2 mixing ratio of 2.5 × 10 −3. Assuming the single-scattering properties of the cloud particles are similar at other locations, the variation in cloud-top pressure has been derived for latitudes from 25°S to 55°N from the blue polarimetry. Near the equator, the cloud top rises to ∼150 mb, while at latitudes > 20° the cloud top falls to pressures as great as ∼400 mb. This variation in cloud height is also seen qualitatively in the methane band data, and the higher cloud top at equatorial latitudes coincides with the high-speed equatorial jet seen by Voyager. The steep increase in polarization toward the limb in red light requires an additional optical depth of ∼0.02 of small, highly polarizing aerosols (probably photochemically produced) above both the belt and zone clouds. If these aerosols have radii of ∼0.1 μm, they do not strongly affect the blue polarimetry, while they can provide an absorption optical depth of a few tenths at pressure levels of 70 mb or less as required by Voyager observations in the ultraviolet.