The global hydrogen Lyman alpha, helium (584 angstroms), and molecular hydrogen band emissions from Saturn are qualitatively similar to those of Jupiter, but the Saturn observations emphasize that the H(2) band excitation mechanism is closely related to the solar flux. Auroras occur near 80 degrees latitude, suggesting Earth-like magnetotail activity, quite different from the dominant Io plasma torus mechanism at Jupiter. No ion emissions have been detected from the magnetosphere of Saturn, but the rings have a hydrogen atmosphere; atomic hydrogen is also present in a torus between 8 and 25 Saturn radii. Nitrogen emission excited by particles has been detected in the Titan dayglow and bright limb scans. Enhancement of the nitrogen emission is observed in the region of interaction between Titan's atmosphere and the corotating plasma in Saturn's plasmasphere. No particle-excited emission has been detected from the dark atmosphere of Titan. The absorption profile of the atmosphere determined by the solar occultation experiment, combined with constraints from the dayglow observations and temperature information, indicate that N(2) is the dominant species. A double layer structure has been detected above Titan's limb. One of the layers may be related to visible layers in the images of Titan.
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.
We have reduced and tabulated photometry and polarimetry data at 2640 and 7500 Å observed by the Voyager 2 photopolarimeter experiment. Spatially resolved limb‐to‐terminator scans across Saturn's Equatorial Zone from 12° to 68° phase angle provide information on the altitude distribution of UV absorbing hazes and the phase function and polarizing properties of stratospheric and tropospheric aerosols. Limb‐to‐terminator scans across the northern hemisphere at 10° phase angle are used to study altitude variations of the tropospheric cloud at several latitudes. For the Equatorial Zone we find (1) the UV photometry and polarimetry are best fit by Rayleigh's phase matrix. (2) A stratospheric haze of small particles is allowed as long as the optical depth is near unity or less and the center of the haze layer is in the 30‐ to 70‐mbar region. A diffuse haze fits better than a thin layer, and the aerosol/gas mixing ratio diminishes above 10 mbar. The vertical distribution and optical depth of the haze differ significantly from models proposed by others. To be in agreement with ground based and other spacecraft data, the haze optical depth is about 0.4 at 2640 Å and decreases by a factor of 10 or more at 6400 Å. If the haze aerosol scattering properties are similar to those for spheres with mean radius 0.1 μm, their imaginary refractive index is 0.4 or larger at 2640 Å and the total column density above the tropopause is 109 cm−2. (3) UV contrasts between belts and zones are interpreted as altitude variations in the top of the tropospheric cloud. The altitudes derived here for three latitudes agree with altitudes derived from ground‐based methane band studies and analyses of polarization from Pioneer 11. A high altitude absorber is abundant in the polar regions. (4) At 7500 Å, the phase function of tropospheric aerosols in the Equatorial Zone is described by a synthetic two‐term Henyey‐Greenstein function with g1 = 0.54 ± 0.11, g2 = −0.47 ± 0.08, ƒ = 0.87 ± 0.03, and ω = 0.986. The single scattering albedo in the North Equatorial Belt is ω = 0.967. The Equatorial Zone tropospheric aerosols are positively polarizing at all the phase angles of our observations.
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