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 mu m, obtained from the occultation of the star 28 Sgr o n 3 July 1989 when the rings were near their maximum opening angle. The rad ial resolution of the data is similar to 18 km, as set by the angular diame ter of the star. Very few, if any, structural changes are observed in the r ings since the Voyager encounters in 1980/1981. Azimuthal variations in opt ical depth are restricted to the outermost part of the B Ring and to severa l previously known noncircular features. The mean optical depth in the here tofore unprobed central part of the B Ring is found to be 2.3 +/- 0.15 at 3 .9 mu m. Quantitative comparisons of the optical depths with those measured by the Voyager PPS stellar occultation experiment at 0.27 mu m do reveal s ystematic differences between the major ring regions. In the C Ring, we fin d tau(28Sgr)/tau(PPS) similar or equal to 1.4, while in the B Ring and oute r A Ring tau(28Sgr)/tau(PPS) similar or equal to 1.0. Only in the inner A R ing is tau(28Sgr) < tau(PPS) In agreement with previous studies at microwav e and optical wavelengths, we conclude that there is very little evidence i n the 28 Sgr data for a significant population of micrometer-sized particle s anywhere in the main rings. Most of the observed variation in tau(28Sgr)/ tau(PPS) can instead be attributed to variations in the particle size distr ibution in the rings in the centimeter to meter size range, which controls the angular width of the rings' forward-scattering cross section. This in t urn controls the effective extinction efficiency for Earth-based occultatio n 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. (C) 2000 Academic Press.