Saturn's rings at true opposition

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 B-eff similar to -23 degrees, we measured the normalized ring reflectivity over broad regions of the A, B, and C rings as a 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 similar to 0.1 degrees at BVRI wavelengths, increasing to similar to 0.16 degrees and similar to 0.19 degrees 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, and thus these simulations are nearly wavelength independent. The interparticle shadow-hiding opposition surge increases in strength with ring optical depth and with broadened size distributions. Self-gravity produces wakes that somewhat complicate the picture, because mutual shadowing by wakes is strongly dependent on illumination and viewing geometry. The observed opposition surge in the rings is much stronger and narrower than that caused by interparticle shadowing. We examined regional variations in the opposition surge across the A, B, and C rings using linear-exponential model fits. Some of these are most easily explained on the basis of optical depth, volume filling factor, and the relative width of the particle size distribution. The opposition effect for the A, B, and C rings is substantially narrower than for two nearby icy satellites, Mimas and Enceladus, indicating that they have distinctly different surface properties.