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Detection of Free Unstable Modes and Massive Bodies in Saturn's Outer B Ring

Wednesday, May 12, 2010
Spitale (CICLOPS/Space Science Institute)

Voyager images and Cassini occultation data have previously shown that the behavior of the outer edge of Saturn's massive B ring is determined only in part by the 2:1 inner Lindblad resonance with Mimas. In Cassini images of this region, we find, in addition to the wavenumber-2 forced distortion, evidence for unforced self-excited wavenumber-3, wavenumber-2, and wavenumber-1 normal modes, the first observations to suggest substantial wave amplification in Saturn's broad rings. Analysis of the observed waves yields an average surface mass density of ∼ 44 g cm-2 for the outer 250 km of the ring. Interference between the forced and free wavenumber-2 modes yields a single wavenumber-2 pattern that varies in amplitude and orientation with a characteristic period of ∼ 5.5 years. We also find localized disturbances, including 3.5-km-tall vertical structures, that move with speeds near the expected Keplerian rate for that region of the rings, which may be caused by embedded massive bodies. The presence of such bodies is supported by the direct discovery of a moonlet ∼ 0.3 km wide near the ring's edge.

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