Age and Evolution of Saturn’s Rings
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Age and Evolution of Saturn’s Rings
Larry W. Esposito, Bonnie K. Meinke, Nicole Albers and Miodrag Sremcevic
LASPThird Moscow Symposium
11 October 2012
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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?
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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!
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We identify this structure as temporary clumps
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Edges also show structure
• Some of this can be explained by multiple modes
• Other sharp features appear stochastic, likely caused by local aggregates
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FromAlbers etal2012
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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
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New Features
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I Gatti di Roma: temporary features in an ancient structure
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We identify our ‘kittens’ as transient clumps
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Prometheus excites F ring structures
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Buerle etal 2010:Bright spots cast shadows.
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Predator-Prey Equations for Ring Clumping
M= ∫ n(m) m2 dm / <M>; Vrel
2= ∫ n(m) Vrel2 dm / N
dM/dt= M/Tacc – Vrel2/vth
2 M/Tcoll
[accretion] [fragmentation/erosion]
dVrel2/dt= -(1-ε2)Vrel
2/Tcoll + (M/M0)2 Vesc2/Tstir
[dissipation] [gravitational stirring]
- A0 cos(ωt) [forcing by streamline crowding]
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Predator-Prey model of Moon-triggered Accretion?
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V2
M
Phase plane trajectory
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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
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Analogy: Coast Redwoods
1 in 104 seeds grows to a tree!
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Like Beijing, rings containboth new andancient structures!
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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.