37th DPS Meeting, 4-9 September 2005
Session 66 Planetary Rings III
Oral, Friday, September 9, 2005, 2:00-3:30pm, Music Concert Hall

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[66.08] HST Observations of the Opposition Effect in Saturn's Rings

R. G. French (Wellesley College), L. Dones (SwRI), H. Salo (U. Oulu), C. A. McGhee (Wellesley College), A. Verbiscer (U. Virginia)

The opposition surge of Saturn's rings has been measured from a long-term, uniform series of observations with the Hubble Space Telescope's (HST) Wide Field and Planetary Camera 2 (WFPC2). Extending from 1996--2005, the measurements spanned nearly the full range of ring opening (3\circ<|B|<26\circ) and solar phase (0\circ<\alpha<6\circ) angles accessible from the Earth, using UBVRI filters. The rings were observed 3--7 times per year, concentrating on near opposition. The spatial resolution of the Planetary Camera (PC), ~q300 km/pixel, enabled us to measure radial variations in the opposition surge in the main rings. During a particularly favorable alignment of the Sun, Earth, and Saturn in January 2005, we observed the rings at exact opposition, with the minimum phase angle limited only by the angular semidiameter of the Sun as seen from Saturn (~q0.02\circ), accounting for solar limb-darkening. The opposition surge is quite strong throughout the entire ring system, with a narrow spike near exact opposition: [I/F(0\circ)]/[I/F(0.5\circ)]=1.4 for the C ring, and 1.3 for the A and B rings, with a weak wavelength dependence. The opposition surge results from a combination of mutual interparticle shadowing and intrinsic brightening (for example, due to coherent backscatter or regolith shadowing). From photometric models that account for particle size distributions, ring volume filling factors, and multiple scattering (Salo et al., this meeting), we find that the mutual shadowing effect saturates at \alpha<0.5\circ, and we attribute the sharp core of the opposition surge to intrinsic brightening. For example, for B=23\circ, between \alpha=0\circ and 6\circ, the mutual shadowing contribution to the opposition surge [I/F(0\circ)]/[I/F(6\circ)] is 1.1 in the C ring and 1.23 in the B ring; the intrinsic brightening is 1.78--1.8 in the C ring and 1.39--1.46 in the B ring, decreasing slightly with increased wavelength.

The author(s) of this abstract have provided an email address for comments about the abstract: rfrench@wellesley.edu

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