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Age and Evolution of Saturn’s Rings. Larry W. Esposito, Bonnie K. Meinke, Nicole Albers and Miodrag Sremcevic LASP Third Moscow Symposium 11 October 2012. Key Cassini Observations.
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Age and Evolution of Saturn’s Rings Larry W. Esposito, Bonnie K. Meinke, Nicole Albers and Miodrag Sremcevic LASP Third Moscow Symposium 11 October 2012
Key Cassini Observations • High resolution images of propellers, straw, embedded moons, F ring objects. These show aggregation in the rings! • Occ’s confirm structure, self-gravity wakes, B ring propellers, ghosts. More evidence for aggregates. • Equinox images: embedded objects • Key question: Are Saturn’s rings young or old?
Sub-km structure seen in wavelet analysis varies with longitude • Wavelet analysis from multiple UVIS occultations is co-added to give a significance estimate • For the B ring edge, the significance of features with sizes 200-2000m shows maxima at 90 and 270 degrees ahead of Mimas. Same distance where objects are seen in the images!
Edges also show structure • Some of this can be explained by multiple modes • Other sharp features appear stochastic, likely caused by local aggregates
From Albers etal 2012
F ring Observations • 27 significant features in F ring: ‘Kittens’ from 22m to 3.7km, likely they are elongated and transient • They have weak correlation to Prometheus, may evolve into moonlets
Buerleetal 2010: Bright spots cast shadows.
Predator-Prey Equations for Ring Clumping M= ∫ n(m) m2 dm / <M>; Vrel2= ∫ n(m) Vrel2dm / N dM/dt= M/Tacc – Vrel2/vth2 M/Tcoll [accretion] [fragmentation/erosion] dVrel2/dt= -(1-ε2)Vrel2/Tcoll + (M/M0)2 Vesc2/Tstir [dissipation] [gravitational stirring] - A0 cos(ωt) [forcing by streamline crowding]
Rare accretion can renew rings • Solid aggregates are persistent , like the absorbing states in a Markov chain • Even low transition probabilities can populate the states: e.g., 10-9 per collision to an absorbing state • These aggregates can renew the rings: • shield their interiors from meteoritic dust pollution • release pristine material when disrupted by an external impact
Analogy: Coast Redwoods 1 in 104 seeds grows to a tree!
Like Beijing, rings contain both new and ancient structures!
Conclusions • Although rings show youthful features, this may imply renewal instead • Changes since Voyager and abundant embedded objects indicate accretion • A predator-prey model shows how moons can trigger accretion today • Are rings young or old? Yes, maybe both! • Key measurement: Ring mass in 2017.