Photometry and polarimetry of Saturn at 2640 and 7500 Å

Abstract

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.

Near-IR Spectrophotometry of Saturnian Aerosols—Meridional and Vertical Distribution

“…Owen (1969) pointed out that the haze is higher in the equatorial region than at midlatitudes. Since then, this has been confirmed by many model studies; see, for example, Apt and Singer (1982), West et al (1983), Tomasko and Doose (1984), , Tomasko (1992, 1993), and Ortiz et al (1996). Regarding the midlatitudes, we find that the haze top pressure is more or less constant in the southern hemisphere, whereas it increases (and the top's altitude thus decreases) with latitude in the northern hemisphere, reaching values as high as 120 mbar.…”

Flux and polarization signals of spatially inhomogeneous gaseous exoplanets

“…In particular the latter hazes are known to strongly polarize the incident sunlight, as observed at small phase angles from Earth (Schmid et al 2011), and at intermediate phase angles from spacecraft observations (see e.g. West et al 1983;Smith & Tomasko 1984).…”

Parameterized scattering matrices for small particles in planetary atmospheres

“…West et al [23] used a similar formulation, but speci"ed !b ( )/a ( ) at scattering angles corresponding to the phase angles at which Saturn was observed during the #yby of Voyager 2.…”

“…First, Smith and Tomasko [24] used two-term Henyey}Greenstein phase functions for a ( ) that were modi"ed between the scattering angles of 80 and 1403, to inspect the Pioneer photometric data of Jupiter for evidence of spherical particles (which they did not "nd). Second, West et al [23] used for Saturn, in addition to their approach outlined above, scattering matrices from Mie theory (pertaining to spherical particles), but they replaced the 1,2-and 2,1-elements with an expression based on Eq. (8).…”