The opposition effect in Saturn’s main rings as seen by Cassini ISS: 1. Morphology of phase functions and dependence on the local optical depth

Abstract

During the Prime mission (2004–2008), the Cassini cameras captured the opposition effect in Saturn’s main rings with a fine radial resolution at extremely small phase angles. Phase functions were extracted for 211 different radial locations in the main ring system (135 in the broadband clear filter of the Wide Angle Camera at 635nm, 76 in four narrowband color filters (at the respective central wavelengths 440, 568, 650, and 752nm for the Narrow Angle Camera and 460, 567, 649, and 742nm for the Wide Angle Camera). The morphology of the observed opposition surges is fitted with a simple linear model in order to derive three useful parameters – the amplitude A, the width of the surge HWHM, and the slope S at larger phase angles – throughout the whole main ring system. In agreement with previous studies, the opposition surge is found to vary substantially across the ring system: whereas values of A and HWHM are nearly constant across the A and B Rings, they show strong local variations for the C Ring and the Cassini Division. Thanks to the high spatial resolution of Cassini images, the opposition surges are obtained for very narrow regions, allowing us to study the effect of the local normal optical depth (τ) on the surge. For τ>0.7, A(τ) and HWHM(τ) are nearly constant (1.3° and 0.25°, respectively) across the whole ring system. For τ<0.7, A(τ) and HWHM(τ) exhibit clear decreasing trends for the A, B and C Rings only. Conversely, points from the Cassini Division show a very large dispersion in both A and HWHM, with no visible trend. The slope S is found to be an increasing function of the optical depth across the full ring system. A trend S∝τ0.5 is observed, in qualitative agreement with a geometrical description of the shadow hiding effect. No strong trend of A, HWHM and S with the wavelength of observation was found. We also tabulate the fit parameters for the opposition surge of 139 regions in the clear filter for future comparison with numerical simulations and photometric models.