1. Cassini Observations and the History of Saturn’s Rings Larry W. Esposito 12 October 2006

2. Ring structure and composition provide key evidence for ring origins and history Show active processes Provide timescale estimates Test evolution theories Allow predictions of ring future Give analogies for planet formation

3. Ring Structure Before spacecraft observations, an idealized view was possible, but no longer… Now: inclined and eccentric rings; waves and wavy edges; clumps and transient features Active processes show short lifetimes and indicate youthful, dynamic rings

4. VOYAGER, GALILEO AND CASSINI SHOW CLEAR RING - MOON CONNECTIONS Rings and moons are inter-mixed Moons sculpt, sweep up, and release ring material Moons are the parent bodies for new rings But youth cannot be taken at face value! All objects are likely transient, and may re- assemble.

5. Cassini observations show active ring system and short lifetimes Time variations in ring edges, D & F rings Inhomogeneities on multiple scales, with steep gradients seen by VIMS and UVIS: ballistic transport is evident, but has not gone to completion Density waves have fresher ice, dark haloes Low density in Cassini Division implies age of less than 10 5 years Under-dense moons and propeller objects indicators of continuing accretion Autocovariance and varying transparency show ephemeral aggregations

8. N1507015271 N1507099722 Bright arc and object in the F ring (2005 DOY276) Object could be 2004 S3 but is unlikely to be 2004 S6 Best candidate for external impact event (Showalter, 1998), or internal collision (Barbara & Esposito, 2002)

9. COLWELL AND ESPOSITO PROPOSED A ‘COLLISIONAL CASCADE’ FROM MOONS TO RINGS Big moons are the source for small moons Small moons are the source of rings Largest fragments shepherd the ring particles Rings and moons spread together, linked by resonances

10. COLLISIONAL CASCADE USES UP RING MATERIAL TOO FAST!

11. NEW MARKOV MODEL FOR THE COLLISIONAL CASCADE Improved by considering recycling Accretion in the Roche zone is possible if mass ratio large enough (Canup & Esposito 1995) Consider collective effects: nearby moons can shepherd and recapture fragments: shards of previous destruction are seeds of future accretion

12. MARKOV MODEL CONCLUSIONS Although individual rings and moons are ephemeral, ring/moon systems persist Ring systems go through a long quasi-static stage where their optical depth and number of parent bodies slowly declines Lifetimes are greatly extended!

13. UVIS F ring occultations 7 star occultations cut F ring 9 times Alp Sco shows 200m feature, also seen by VIMS This event used as test case to refine search algorithm (see Meinke talk, next) Alp Leo shows 600m moonlet Opaque event! This gives: 10 5 moonlets, optical depth 10 -3, consistent with predictions

14. VIMS and UVIS Alp Sco Egress occultation data are overplotted. The UVIS data curve is the one with higher spatial resolution. A multiplicative factor 17.24 ( = maximum of VIMS in region / max of UVIS) is used to scale the UVIS data. Pywacket, the event 10 km outside the F Ring core, is detected by both instruments.

15. “Mitttens”

16. Comparison to F ring model shows model deficient Barbara and Esposito ‘02

17. Are these caused by structures like those we see in F ring?

18. * Mittens: 600m Figure from Tiscareno etal 2006

19. Cassini results for ring evolution imply a broad age range Recent changes in D, F ring, Keeler gap Discovery of clumps and propeller objects Moonlets have mass density less than 1.0 Low density in Cassini division Ring heterogeneity at many scales Mostly pure ice composition

21. Ring History: Growth as a random walk This model emphasizes random events like fortunate orientation, compaction, local melting and annealing, collapse to spherical shape. Differs from solving accretion equation (which uses the accretion coefficient as the kernel of an integral equation) Instead, parameterize probabilities p,q for doubling or halving size in dt. States: size bins of factor 2. This gives a random walk in one dimension with reflecting boundaries.

22. Random Walk Conclusions Multiple collisions and random factors may invalidate standard accretion approach Slowly growing bodies could re-supply and re-cycle rings Key considerations: fortunate events (that is, melting, sintering, reorientation) create ‘hopeful monsters’ like in evolution of life

25. Numerical simulations show collisions and self-gravity effects

26. A plausible ring history Interactions between ring particles create temporary aggregations: wakes, clumps, moonlets Some grow through fortunate random events that compress, melt or rearrange their elements. Stronger, more compact objects would survive At equilibrium, disruption balances growth, producing a continuous size distribution, consistent with observations by UVIS, VIMS, RSS and ISS Growth rates require only doubling in 10 5 years Ongoing recycling resets clocks and reconciles youthful features (size, color, embedded moons) with ancient rings: rings will be around a long time!

27. What’s Next? Determine persistence of F ring objects: track them in images. Measure A ring structures, events, and color variations Characterize aggregations from wakes to moonlets: is this a continuum? Compare to Itokawa and other ‘rubble piles’ Run pollution models for color evolution Develop ‘creeping growth’ models

28. Summary Numerous features seen in RPX images UVIS sees an opaque moonlet and other events in 7 occultations: implies 10 5 F ring moonlets, roughly consistent with models Previous models did not distinguish between more or less transient objects: this was too simple, since all objects are transient Particle distribution can be maintained by balance between continuing accretion and disruption Ongoing recycling allows rings to be around a long time

29. Backup Slides

30. RING AGE TRACEABILITY MATRIX

32. MODEL PARAMETERS n steps in cascade, from moons to dust to gone… With probability p, move to next step (disruption) With probability q, return to start (sweep up by another moon) p + q = 1.

33. LIFETIMES This is an absorbing chain, with transient states, j= 1, …, n-1 We have one absorbing state, j=n We calculate the ring/moon lifetime as the mean time to absorption, starting from state j=1

34. EXPECTATION VALUES Lifetimes (steps): E 1 =(1-p n )/(p n q) ~n, for nq << 1 (linear) ~n 2, for nq ~ 1 (like diffusion) ~2 n+1 -2, for p=q=1/2 ~p -n, as q goes to 1 (indefinitely long)

35. EXAMPLE: F RING After parent body disruption, F ring reaches steady state where accretion and knockoff balance (Barbara and Esposito 2002) The ring material not re-collected is equivalent to ~6km moon; about 50 parent bodies coexist… Exponential decay would say half would be gone in 300 my. But, considering re-accretion, loss of parents is linear: as smaller particles ground down, they are replaced from parent bodies. The ring lifetime is indefinitely extended