Saturn’s rings at true opposition

ABSTRACT. On 2005 January 14, the Saturn system was observed at true opposition with the planetary camera of the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. This was the culmination of nearly a decade of similar UBVRI observations, yielding a uniform set of over 400 high spatial resolution and photometrically accurate radial profiles of the rings that spans the full range of ring inclinations and solar phase angles accessible from the Earth. Using a subset of these images at similar effective ring opening angles, we measured the normalized ring reflectivity over broad regions of the A, B, and C rings as aB ∼ 23 I/Feff function of solar phase angle and wavelength. There is a strong surge in brightness near opposition. To measure the width and amplitude of the opposition effect, we fitted two models to the observations: a simple four-parameter linear-exponential model and a more complex model (B. Hapke) that incorporates a wavelength-dependent description of the coherent backscatter opposition effect, as well as the shadow-hiding opposition effect by a particulate surface. From fits to the linear-exponential model, the half-width at half-maximum for the rings is ∼0.1 at BVRI wavelengths, increasing to ∼0.16 and ∼0.19 for the A and B rings, respectively, in the U filter (338 nm). To assess the contribution of the shadowing of ring particles by each other to the opposition surge, we used Monte Carlo simulations of dynamical models of the rings for a variety of optical depths and particle size distributions, both with and without self-gravity. Multiple scattering is very weak at low phase angles, an