HST observations of azimuthal asymmetry in Saturn's rings

Abstract

From 378 Hubble Space Telescope WFPC2 images obtained between 1996–2004, we have measured the detailed nature of azimuthal brightness variations in Saturn's rings. The extensive geometric coverage, high spatial resolution ( ∼ 300 km px −1 ), and photometric precision of the UBVRI images have enabled us to determine the dependence of the asymmetry amplitude and longitude of minimum brightness on orbital radius, ring elevation, wavelength, solar phase angle, and solar longitude. We explore a suite of dynamical models of self-gravity wakes for two particle size distributions: a single size and a power law distribution spanning a decade in particle radius. From these N-body simulations, we calculate the resultant wake-driven brightness asymmetry for any given illumination and viewing geometry. The models reproduce many of the observed properties of the asymmetry, including the shape and location of the brightness minimum and the trends with ring elevation and solar longitude. They also account for the “tilt effect” in the A and B rings: the change in mean ring brightness with effective ring opening angle, | B eff | . The predicted asymmetry depends sensitively on dynamical ring particle properties such as the coefficient of restitution and internal mass density, and relatively weakly on photometric parameters such as albedo and scattering phase function. The asymmetry is strongest in the A ring, reaching a maximum amplitude A ∼ 25 % near a = 128 , 000 km . Here, the observations are well-matched by an internal particle density near 450 kg m −3 and a narrow particle size distribution. The B ring shows significant asymmetry (∼5%) in regions of relatively low optical depth ( τ ∼ 0.7 ). In the middle and outer B ring, where τ ≫ 1 , the asymmetry is much weaker ( ∼ 1 % ), and in the C ring, A < 0.5 % . The asymmetry diminishes near opposition and at shorter wavelengths, where the albedo of the ring particles is lower and multiple-scattering effects are diminished. The asymmetry amplitude varies strongly with ring elevation angle, reaching a peak near | B eff | = 10 ° in the A ring and at | B eff | = 15 – 20 ° in the B ring. These trends provide an estimate of the thickness of the self-gravity wakes responsible for the asymmetry. Local radial variations in the amplitude of the asymmetry within both the A and B rings are probably caused by regional differences in the particle size distribution.