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