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H. Salo, R. Karjalainen (Univ. of Oulu)
The scattering properties of rings with volume density in the range D=0.001 - 0.4 are studied by Monte Carlo ray tracing simulations. The main goal is to check whether the moderately flattened ring models predicted by dynamical simulations (central plane D > 0.1) are consistent with the basic ground-based observations of Saturn's rings, including the opposition brightening and the B-ring tilt effect. Previously the opposition effect has been explained either by mutual shadowing among well separated particles (D about 0.02, Lumme et al. Icarus 53, 174, 1983) or by intrinsic opposition peak of particle surfaces, caused e.g. by coherent backscattering (Mishchenko ApJ 411, 351, 1993). The tilt effect has been interpreted to indicate multiple scattering with particle albedo near unity (Lumme and Irvine AJ 81, 865, 1976).
According to our simulations dense rings are brighter than those with D=0, roughly by a factor of 1+2D. For extended size distribution, dynamical simulations indicate that smallest particles typically occupy a several times thicker layer than the largest ones. Thus dynamically dense systems may have a low volume density envelope formed by small particles, which makes their photometric behavior sensitive to the viewing elevation. Especially, this can account for the tilt-effect, as dense and thus bright central parts of the ring become better visible for larger elevation. Since multiple scattering is not involved, the explanation works also for albedo well below unity. Similarly, the observed about 1 degree wide opposition peak can arise due to mutual shadowing among the small particles. Inclusion of non-zero D also helps to model some Voyager-observations of A-ring (Dones et al. Icarus 105, 184, 1993), including the simultaneous fit of both low and high-phase brightness. Differences in local size distribution and thus on the effective D may also account for the contrast reversal in resonances sites.
This study is supported by the Academy of Finland and the Väisälä Foundation.
The author(s) of this abstract have provided an email address for comments about the abstract: heikki.salo@oulu.fi