Deau, E., Charnoz, S., Dones, L., Brahic, A., Porco, C. (2006). "Sunshine on the rings: The opposition effect seen at high resolution with CASSINI-ISS" American Astronomical Society, DPS meeting #38, #51.01.


Abstract
In June 2005, the CASSINI-ISS camera system captured the opposition effect in the rings of Saturn. The opposition effect is characterized by anon-linear increase of brightness when the phase angle is near zero. With the ISS images, we have observed the brightness surge of the rings atphase angles as small as 0.001 degrees, far smaller than the Sun's angular diameter of 0.05 degrees at Saturn. Our data set shows opposition surgesfor features in the A, B, C, and F rings and the Cassini Division with unprecedented radial resolution. Phase functions from 0 to 10 degrees werederived in order to study the source of the opposition effect. Two different physical processes have been invoked to explain the oppositioneffect -- shadow hiding and coherent-backscattering -- but their respective roles are still a matter of debate because of the paucity ofground-based data at phase angles less than 0.3 degrees. We fit the derived phase curves with different models, including the empiricallinear-exponential function of Kaasalainen et al., and Shkuratov and Hapke models. We find correlations between some output parameters of thesemodels and relevant physical parameters of the rings, thus constraining the nature and optical properties of the rings.