Photometry and polarimetry of Saturn's rings from Pioneer Saturn

Abstract

We present a profile of the average normal optical depth for Saturn's rings between 1.22 and 2.35 Rs. In the A and B rings, horizontal inhomogeneities make these values deceptive. A thinner component of the B ring with optical depth less than 0.08 covers ½% to 4% of its surface area. In the A ring the more transparent component has optical depth τs >0.10 and covers more than 7% of its area. These thinner parts of the rings would rarely be apparent from earth‐based observations. The particles in the C ring are larger than 15µ and different from those in the B and A rings. The C ring is either homogeneous with high albedo (in the red) and forward scattering phase functions or shows a gradient in albedo with distance from Saturn. In the latter case the red albedo ranges from at the inner edge of ring C to at the outer edge, and the phase function is both moderately forward and backward scattering. Polarimetry of Saturn's rings provides only an upper limit for the B ring polarization, p <15%, which is consistent with predictions from ground‐based data. In the outer A ring, the polarization is negative p(blue) = −7±2%; p(red) = −1±2%. Azimuthal variations are seen on the darkside of the A ring with a peak‐to‐trough ratio of 1.25. The position angle of the maximum is close to that seen from earth. No variations are seen in the B and C rings with an upper limit of 6%. The measured color of the A ring supports the many particle thick model of Saturn's rings.