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The Cassini cameras have captured the opposition effect in Saturn rings witha high radial resolution at phase angles down to 0.01o in the entire main ringsystem. We derive phase functions from 0.01 to 25 degrees of phase angle in theWide-Angle Camera clear filters with a central wavelength of 0.611microns andphase functions from 0.001 to 25 degrees of phase angle in the Narrow-Angle andWide-Angle Cameras color filters (from the blue 0.451 microns to the nearinfrared 0.752 microns). We characterize the morphology of the phase functionsof different features in the main rings. We find that the shape of the phasefunction is accurately represented by a logarithmic model (Bobrov 1970, inSurfaces and Interiors of Planets and Satellites, Academic, edited by A.Dollfus). For practical purposes, we also parametrize the phase curves by asimple linear-by-part model (Lumme and Irvine 1976, Astronomical Journal, 81,p865), which provides three morphological parameters : the amplitude and theHalf-Width at Half-Maximum (HWHM) of the surge, and the slope S of thelinear-part of the phase function at larger phase angles. Our analysisdemonstrates that all of these parameters show trends with the optical depth ofthe rings. These trends imply that the optical depth is a key-elementdetermining the physical properties which act on the opposition effect.Wavelength variations of the morphological parameters of the surge showimportant trends with the optical depth in the green filter (0.568 microns),which implies that grain size effects are maximum in this wavelength.

The opposition effect in Saturn rings seen by Cassini/ISS: 1. Morphology of phase curves