Oral, Friday, September 9, 2005, 2:00-3:30pm, Music Concert Hall

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*H. Salo (Univ. Oulu), R. G. French, C. McGhee (Wellesley C.), L. Dones (SwRI)*

\vskip 0.5cm Two major explanations for the opposition
brightening of Saturn's rings are i) the intrinsic
brightening of particles due to coherent backscattering, and
ii) the reduced mutual shadowing as the phase angle \alpha
arrow 0^{\}circ. Both mechanisms are likely to be
important but to what degree is currently unclear. Here we
utilize the extensive set of Hubble Space Telescope
observations for different elevation angles B and
wavelengths \lambda (Cuzzi et al. 2002, Icarus 158, 199)
to disentangle these contributions. We assume that the
intrinsic contribution is independent of B, so that the
B dependence of phase curves is due to mutual shadowing,
which must also act similarly for all \lambda's (unless
the particle albedo ~1 at the longer wavelengths, in
which case the multiple scattering would make the observed
effect somewhat weaker). Our study complements Poulet et al.
(2002, Icarus 158, 224) who used a subset of data for a
single B ~10^{\}circ. Since mutual shadowing depends
sensitively on volume density of the system, via particle
size distribution, optical depth and vertical thickness,
constraints can be obtained for these parameters. In
practice, we construct a grid of dynamical/photometric
simulation models, with the method of Salo and Karjalainen
(2003, Icarus 164, 428), Salo et al. (2004, Icarus 170, 70).

The observed opposition brightening is characterized by
OE=I(\alpha=0.5^{\}circ)/I(\alpha=6^{\}circ), which ratio
varies between 1.4-1.75 and 1.25-1.55 for F336W
(\lambda_{\rm eff}=334 nm) and F814W (\lambda_{\rm
eff}=794 nm) filters, respectively, depending on B and
the ring component. The choice of this phase angle range is
determined by the availability of data at all different
elevations - however, the brightening continues for \alpha
< 0.5^{\}circ (see French et al. this meeting). Most
importantly, the dependence on elevation angle is indeed
similar in all filters: for A and B rings
OE(B=4^{\}circ)/OE(B=26^{\}circ)~.2 while for the C ring
it is about 1.1. Based on our models, this suggests that
the width of the size distribution W=r_{max}/r_{min}>100
in C ring, whereas for A and B rings the best match is
obtained with W < 10 (cf. French and Nicholson 2000,
Icarus 145, 502). According to these models the intrinsic
contribution to OE in C ring is about 1.35 and 1.25
for F336W and F814W filters, respectively, and similarly
about 1.2 and 1.05 in B ring; thus most of B ring
opposition effect for \alpha> 0.5^{\}circ is attributed to
mutual shadowing (amounting to even 1.4 for B ~
4^{\}circ). The relative contributions of coherent
backscattering and mutual shadowing for \alpha < 0.5^{\}circ
is discussed by French et al. (this meeting).

This study is supported by the Academy of Finland.

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Bulletin of the American Astronomical Society, **37** #3

© 2004. The American Astronomical Soceity.