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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=rmax/rmin>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.