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M.S. Tiscareno, J.A. Burns, M.M. Hedman (Cornell), J.N. Spitale, C.C. Porco (SSI), C.D. Murray (QMUL), Cassini Imaging Team
Cassini has observed an unexpectedly complex array of radial variations in the edges of the Encke Gap and the Keeler Gap. Each of these gaps in Saturn's rings (both located near the outer edge of the A Ring) is inhabited and maintained by a known moon (Pan, and the newly-discovered S/2005~S1, respectively).
In the Encke Gap edges, the wavelength in some regions does not remain constant but varies smoothly in a repeating pattern. Additionally, the shape of the waveform deviates significantly from sinusoidal, with amplitudes that sometimes grow and sometimes decay. Wavy edges are ubiquitous in the Encke Gap, covering 360\circ of longitude on both sides. This confirms Voyager indications that particle collisions and streamline crossing are not efficient at damping them (timescales \gtrsim 10 months), and negates the simplistic assumption that ring particles enter a new encounter with no memory of the previous one.
The situation is quite different in the Keeler Gap edges, where only ~3 wavecycles appear before undergoing a transition, in which the edge recedes from the gap's center by ~5 km, and the waveform takes on a turbulent appearance, lacking any sharp edge. The waveforms from the Keeler moon then die out completely, only several degrees of longitude away from the moon. Yet the Keeler Gap edges are hardly quiescent away from the moon. Sharp-edged asymmetric features dubbed ``wisps'', ~1 km in amplitude, are seen in abundance on the outer edge. Most spectacularly, the radial location of the inner edge varies quasi-periodically over a range of ~15 km. The dominant frequency of this variation is consistent with the influence of the nearby 32:31 resonance (ILR) with Prometheus, though other frequencies also exist. Thus, the total width of the Keeler Gap varies by a factor of nearly 50%, from 32 to 47 km.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.