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J. A. Burns, M.M. Hedman, M.S. Tiscareno, P.D. Nicholson, B. J. Streetman (Cornell U.), J. E. Colwell (U. Colorado), M. R. Showalter (SETI Inst.), C. D. Murray (Queen Mary C.), J. N. Cuzzi (NASA-Ames), C.C. Porco (Space Science Inst.), Cassini ISS Team
Cassini has discovered significant and puzzling variability in three faint ringlets that reside within the Encke Gap, an opening in Saturn's outer A ring, which is home to the 25-km moon Pan. The radial spans of tadpole orbits, horseshoe region, and zone of chaos due to Pan would be about 1, 25 and 250 km, respectively. The most prominent ringlets lie: i.) at 133,485 km near the gap's inner edge (133,420 km); ii) in Pan's orbit at 133,581 km; and iii) at 133,720 km near the gap's outer edge (133,740 km). The full-width half-maximum dimensions are tens of km. The brightnesses of these ringlets vary irregularly with longitude by more than an order of magnitude; thus, at a given longitude, any of the three can have the strongest signal. The brightest parts of the ringlets commonly correspond to regions that have radial kinks (amplitude /sim5 km). In the central ringlet, one cluster of luminous patches stays /sim50/circ ahead of Pan, near its L4 triangular point. Throughout the past year, some other bright clumps originally seen 0/circ -20/circ ahead of Pan have drifted toward the moon where they vanished; since these clumps generally lie slightly outwards of Pan's orbit, we suspect that these particles are moving on the outer loops of horseshoe paths. Simulations of ringlet interactions in 3-D elliptical 3-body models (ring particle, Pan, Saturn) find that horseshoeing clumps disintegrate after passing Pan We have seen smooth sinusoidal waves on the inner ringlet, generated by Pan's perturbations. If sizable moonlets other than Pan populated the gap, we should expect to see similar wiggles elsewhere and, as yet, do not. Stellar occultations viewed by VIMS and UVIS determine the ringlets's optical depths. We have sufficient phase coverage and good spectral coverage to constrain the particle size distributions. Funded by Cassini Project.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.