Voyager 2 photopolarimeter observations of Titan

Abstract

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.

HST observations of the limb polarization of Titan

“…Titan full disk phase curves for intensity and polarization have been obtained by the Pioneer 11 (Tomasko & Smith 1982) and the Voyager 2 (West et al 1983) spacecrafts. The Pioneer 11 data were obtained in the B and R bands, covering phase angles between α = 28 • −96 • , whereas the Voyager 2 data were taken in the near UV at 264 nm and the near IR at 750 nm, covering phase angles α = 2.7 • −154 • .…”

“…Therefore, we derive the reflected-flux weighted geometric albedo A g,eff for the used filter pass bands (Table 3), using the spectrophotometry of McGrath et al (1998), Karkoschka (1998), andNegrão et al (2006); and we compare our model results for p(90 • ) with measurements obtained by the Pioneer 11 (Tomasko & Smith 1982) and the Voyager 2 (West et al 1983) spacecrafts.…”

“…The hazy atmosphere of Titan produces a very strong polarization signal over a wide wavelength range. At quadrature phase α ≈ 90 • , the fractional polarization of Titan from the UV to the R band is p ≈ 50%, as measured by the Pioneer 11 (Tomasko & Smith 1982) and Voyager 2 (West et al 1983) spacecrafts. Furthermore, thanks to the joint NASA-ESA Cassini-Huygens satellite mission, Titan's surface and atmospheric structure are known in great detail.…”

Parameterized scattering matrices for small particles in planetary atmospheres

“…Unless otherwise noted, a ( ) was parameterized by a one-or two-term Henyey}Greenstein function. As the degree of linear polarization as a function of phase angle for Titan hinted at particles with Rayleigh-like polarizing properties, Tomasko and Smith [13] and West et al [14] chose to use a`factor times Rayleigha approach for the 2,1-element, i.e.,…”

“…Employing terms like generalized single-scattering properties [13], synthetic phase matrix [14], and synthetic scattering matrix [15], investigators have previously adopted similar approaches for interpreting polarization measurements of the planet Saturn and its satellite Titan, without taking into account physical constraints that were later published. These approaches are brie#y reviewed in Section 3.…”