Analysis of linear polarization data obtained by the Pioneer Venus Orbiter Cloud Photopolarimeter experiment indicates that the visible clouds at low and mid‐latitudes are composed predominantly of 1 µm radius H2SO4 droplets, an identification made previously by using earth‐based observations. Mixed within and extending above this main visible cloud is an extensive haze of submicron‐sized particles. These haze particles have a refractive index of 1.45±0.04 at λ ≃550 nm, an effective radius of 0.23±0.04 µm, and a size distribution with an effective variance of 0.18±0.1. The polarization of the bright regions poleward of about 55° latitude is produced almost entirely by this submicron haze. The submicron haze has been found to exhibit large spatial and temporal variations. In January 1979 the haze vertical optical thickness in the polar region at λ=365 nm was τh ∼0.8 above the main cloud of l‐µm particles. By comparison, the optical thickness of the haze above the main cloud at low latitudes was typically 1 order of magnitude smaller, τh ∼0.06, however haze mixed within the cloud contributed ∼18% of the total scattering cross section per unit volume at λ=365 nm. More recent observations indicate that there are major changes of the haze on time scales of hundreds of days (e.g., the optical thickness of the polar cap haze is smaller by a factor of 2–3 in October 1979 than in January 1979). Substantial diurnal variations exist at low latitudes, with a greater amount of haze near the morning terminator than near the noon meridian. The global distribution of haze will be monitored during the extended Pioneer Venus mission to permit analysis of long time scale variations as well as correlation with characteristics of the atmospheric dynamics deduced from ultraviolet images of the cloud tops.

The Voyager 2 photopolarimeter was reprogrammed prior to the August 1981 Saturn encounter to perform orthogonal-polarization, two-color measurements on Saturn, Titan, and the rings. Saturn's atmosphere has ultraviolet limb brightening in the mid-latitudes and pronounced polar darkening north of 65 degrees N. Titan's opaque atmosphere shows strong positive polarization at all phase angles (2.7 degrees to 154 degrees ), and no single-size spherical particle model appears to fit the data. A single radial stellar occultation of the darkened, shadowed rings indicated a ring thickness of less than 200 meters at several locations and clear evidence for density waves caused by satellite resonances. Multiple, very narrow strands of material were found in the Encke division and within the brightest single strand of the F ring.

Observations of Titan's whole disk polarization at 2460 and 7500 Å are presented and analyzed in terms of model scattering atmospheres. If the Titan aerosols are spherical or nearly spherical, no single combination of refractive index and size distribution is able to fit data at both wavelengths. However, a vertically inhomogeneous distribution suggested by Tomasko and Smith (1980), characterized by a size gradient with altitude, fits the data at 2640 Å moderately well but must be modified at intermediate and large optical depths to fit the 7500‐Å data. Results for synthetic phase functions indicate that the single scattering polarization must be 70% or larger in the UV and 78% or larger in the near‐IR at 90° phase angle, depending on the phase function. If the correct phase function is similar to that for 0.5‐μm‐radius spheres, the UV single‐scattered polarization must be 84% and the near‐IR single‐scattered polarization must be over 90%. Such large polarizations are impossible for 0.5‐μm‐radius spheres but may be possible for nonspherical particles with effective radii near 0.5 µm, although the existence of nonspherical particles with the scattering properties required by these and other observations has not been demonstrated.

On August 25, 1981, the Voyager 2 photopolarimeter system observed a stellar occultation by Saturn's rings. We present a brief description of this experiment along with details of the data reduction. The occultation results are given in tabular and graphical form at a resolution of 60 km. Histograms of the frequency of optical depth show dominantly unimodal distributions in each of the classical ring elements. The frequency distribution of the entire ring system shows three modes at τ ≈ 0.08, τ ≈ 0.5, and τ ≳ 2.50.

An imaging photopolarimeter aboard Pioneer 11, including a 2.5-centimeter telescope, was used for 2 weeks continuously in August and September 1979 for imaging, photometry, and polarimetry observations of Saturn, its rings, and Titan. A new ring of optical depth < 2 x 10(-3) was discovered at 2.33 Saturn radii and is provisionally named the F ring; it is separated from the A ring by the provisionally named Pioneer division. A division between the B and C rings, a gap near the center of the Cassini division, and detail in the A, B, and C rings have been seen; the nomenclature of divisions and gaps is redefined. The width of the Encke gap is 876 +/- 35 kilometers. The intensity profile and colors are given for the light transmitted by the rings. A mean particle size less, similar 15 meters is indicated; this estimate is model-dependent. The D ring was not seen in any viewing geometry and its existence is doubtful. A satellite, 1979 S 1, was found at 2.53 +/- 0.01 Saturn radii; the same object was observed approximately 16 hours later by other experiments on Pioneer 11. The equatorial radius of Saturn is 60,000 +/- 500 kilometers, and the ratio of the polar to the equatorial radius is 0.912 +/- 0.006. A sample of polarimetric data is compared with models of the vertical structure of Saturn's atmosphere. The variation of the polarization from the center of the disk to the limb in blue light at 88 degrees phase indicates that the density of cloud particles decreases as a function of altitude with a scale height about one-fourth that of the gas. The pressure level at which an optical depth of 1 is reached in the clouds depends on the single-scattering polarizing properties of the clouds; a value similar to that found for the Jovian clouds yields an optical depth of 1 at about 750 millibars.