Preliminary results of polarimetry and photometry of Titan at large phase angles from Pioneer 11

The imaging photopolarimeter (IPP) aboard the Pioneer 11 spacecraft measured the linear polarization of the integrated disk of Titan in red and blue light at a variety of phase angles from 15° to 97°. The large polarization (54%) measured in blue light at 90° phase constrains the size of the aerosols near the top of Titan's atmosphere to have radii smaller than about 0.09 µm if they have a refractive index of 2.0. The polarization at 90° phase in red light is smaller (41%) and implies that the optical thickness of the layer of small aerosols is about 0.6 above an effectively depolarizing surface. The shape of the polarization versus phase curve in blue light suggests increasing particle size with increasing depth into the atmosphere. The limb darkening of Titan was measured at 28° phase by using the imaging mode of the IPP. The limb darkening observed in both colors is reasonably consistent with that given by the scattering models derived from the polarization observations.