Saturn's Rings I. Optical Depth Profiles from the 28 Sgr Occultation

We present optical depth profiles for Saturn's rings at wavelengths of 0.9, 2.1, 3.3, and 3.9 μm, obtained from the occultation of the star 28 Sgr on 3 July 1989 when the rings were near their maximum opening angle. The radial resolution of the data is ∼18 km, as set by the angular diameter of the star. Very few, if any, structural changes are observed in the rings since the Voyager encounters in 1980/1981. Azimuthal variations in optical depth are restricted to the outermost part of the B Ring and to several previously known noncircular features. The mean optical depth in the heretofore unprobed central part of the B Ring is found to be 2.3±0.15 at 3.9 μm. Quantitative comparisons of the optical depths with those measured by the Voyager PPS stellar occultation experiment at 0.27 μm do reveal systematic differences between the major ring regions. In the C Ring, we find τ _{28 Sgr}/τ _PPS≃1.4, while in the B Ring and outer A Ring τ _{28 Sgr}/τ _PPS≃1.0. Only in the inner A Ring is τ _{28 Sgr}<τ _PPS. In agreement with previous studies at microwave and optical wavelengths, we conclude that there is very little evidence in the 28 Sgr data for a significant population of micrometer-sized particles anywhere in the main rings. Most of the observed variation in τ _{28 Sgr}/τ _PPS can instead be attributed to variations in the particle size distribution in the rings in the centimeter to meter size range, which controls the angular width of the rings' forward-scattering cross section. This in turn controls the effective extinction efficiency for Earth-based occultation experiments, and thus the measured optical depth. In a companion paper we use these data to derive power-law models of the size distribution in each major ring region.