Age and Evolution of Saturn’s Rings

Age and Evolution of Saturn’s Rings

Larry W. Esposito, Bonnie K. Meinke, Nicole Albers and Miodrag Sremcevic

LASPThird 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!

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

FromAlbers etal2012

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

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) 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]

Predator-Prey model of Moon-triggered Accretion?

V2

M

Phase plane trajectory

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 containboth new andancient 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.