Age and Evolution of Saturns Rings Larry W

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 -2000 m shows maxima at 90 and 270 degrees ahead of Mimas. Same distance where objects are seen in the images!

We identify this structure as temporary clumps

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 22 m to 3. 7 km, likely they are elongated and transient • They have weak correlation to Prometheus, may evolve into moonlets

New Features

I Gatti di Roma: temporary features in an ancient structure

We identify our ‘kittens’ as transient clumps

Prometheus excites F ring structures

Buerle etal 2010: Bright spots cast shadows.

Predator-Prey Equations for Ring Clumping M= ∫ n(m) m 2 dm / <M>; Vrel 2= ∫ n(m) Vrel 2 dm / N d. M/dt= M/Tacc – Vrel 2/vth 2 M/Tcoll [accretion] [fragmentation/erosion] d. Vrel 2/dt= -(1 -ε 2)Vrel 2/Tcoll + (M/M 0)2 Vesc 2/Tstir [dissipation] - A 0 cos(ωt) [gravitational stirring] [forcing by streamline crowding]

Predator-Prey model of Moon-triggered Accretion?

Phase plane trajectory V 2 M

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.