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Stellar occultation studies of Saturn's rings with the Hubble Space Telescope

Recent determinations of Saturn's ring-plane pole and radius scale incorporated subsets of currently available data (Elliot, et al. 1993. Astron. J. 106, 2544-2572; French, et al. 1993. Icarus 103, 163-214; Hubbard, et al. 1993. Icarus 103, 215-234). We present a solution for Saturn's pole and radius scale using the following occultation data: (i) the 1991 occultation of GSC6323-01396, observed with the Hubble Space Telescope, (ii) the 1989 occultation of 28 Sgr, observed from 11 Earth-based sites, (iii) the 1981 occultation of 8 Sco, observed with the photopolarimeter on Voyager 2, and (iv) the 1980 occultation of the Voyager I radio science signal, received at Earth. To these data, we fit a solarsystem barycentric vector geometric model that includes in-track errors for Voyager (in the form of clock offsets), general relativistic bending by an oblate planet, and pole precession. Since the magnitude of errors in ring-plane radii calculated from this geometric model varies significantly across features, we employ a weighting scheme that assigns higher weights to those features with lower rms residuals. From these model fits, we find a ring plane pole position of ap = 40.59287 ± 0.00470 degrees, p = 83.53833 ± 0.00022 degrees at the Voyager 1 epoch, consistent with the results of all above-mentioned works. We search for inclined rings and find none that are statistically significant. We also find one new feature that is probably non-circular: the inner edge of the C ring, feature 44. This feature appears to be freely precessing (due to Saturn's non-spherical gravity field). In modeling previously determined non-circular features, we find that models for the Titan and Maxwell ringlets agree well with previous models (Porco et al. 1984. Icarus 60, 1-16). The center and inner edge of the 1.470 RS ringlet appear circular, while the outer edge is best fit by a Prometheus 2:1 Lindblad resonant model. The outer edge of the 1.495 RS ringlet is described by a superposition of Mimas 3:1 Lindblad resonance model and a freely precessing model. These two ringlets were previously thought to be purely freely precessing (Porco & Nicholson, 1987. Icarus 72, 437-467), but now it appears that this is not the case. Despite the proximity of the inner edge of the 1.990 RS ringlet to the Pandora