Cassini Observations and Ring History

1. Cassini Observations and Ring History Larry W. Esposito COSPAR Beijing 18 July 2006

2. 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 has not gone to completion • Density waves have fresher ice, dark haloes • Low density in Cassini Division implies age of less than 105 years • Under-dense moons and propellers indicate continuing accretion • Autocovariance from occultations and varying transparency show ephemeral aggregations

6. 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.

8. 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

9. COLLISIONAL CASCADE USES UP RING MATERIAL TOO FAST!

10. NEW MARKOV MODEL FOR THE COLLISIONAL CASCADE • Improve by considering recycling • Consider collective effects: nearby moons can shepherd and recapture fragments • Accretion in the Roche zone is possible if mass ratio large enough (Canup & Esposito 1995)

11. 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. Now we see them :F ring clumps and moonlets • F ring objects are abundant • RPX images and movies show numerous objects • UVIS sees 9 events, including opaque object 600m across • These short-live objects argue for ‘creeping’ growth of moonlets from ring particles and continuing recycling…

14. Bright arc and object in the F ring (2005 DOY276) N1507015271 N1507099722 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)

15. 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 • Alp Leo shows 600m moonlet • Opaque event! This gives: 105 moonlets, optical depth 10-3 , consistent with predictions

16. Calculate standard deviation of each data point Determine baseline for F ring Assume normal distribution Flag statistically significant points: Zmin so that 1 event by chance in each occ Testing unocculted stars gives control, expected number from pure chance  = √DN Baseline (Bsln) = 80 point running mean Z = (DN – Bsln)/ Flagged events are Zmin from Bsln Search Method

17. Persistence test • Ring particle collision rate is proportional to opacity (Shu and Stewart 1985) • Number of collisions needed to escape from an aggregate is proportional to opacity squared • Lifetime against diffusion is the ratio, which increases as opacity increases: the more opaque events are thus more persistent

18. Applying the persistence test Reexamine points flagged from Z test • Extract events where opacity greater than Pywacket • Particles in such aggregations must collide multiple times each orbit ---> structure persists for some number of orbits

19. Alp Sco • Spans 3 integrations • Also seen in VIMS data • At 140610.5 km • ~0.2 km wide “Pywacket”

20. “Mitttens”

21. Alp Leo • Starts at 139962 km • 21 integ-rations • Width: 0.6 km, and opaque

22. Observed Events • 9 events • 30m to 600m wide

23. Observed Events q~2.5 Barbara and Esposito ‘02

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

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

26. Ring History:Model accretion as a random walk • This model emphasizes random events like fortunate orientation, local melting and annealing, collapse to spherical shape • Differs from solving accretion equation, which involves “accretion coefficient” with indices for accreting mass bins • Instead, parameterize probabilities p,q for doubling or halving size in dt

27. Random Walk Results • Solve for irreducible distribution • For power-law size distribution with index -3 • p/q = 2 • Mass loss rate: 4 x 1012 g/year • dt > 105 years to maintain distribution against shattering of largest objects by external impacts • For a clump or temporary aggregation with 103 collisions/year: 108 interactions to double in mass! • This ‘creeping’ growth is below the resolution of N-body and statistical calculations

28. 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

32. RING AGE TRACEBILITY MATRIX

33. What do the processes imply? • If unidirectional size evolution (collisional cascade): Then the age of rings is nearly over! • If binary accretion is thwarted by collisions, tides: Larger objects must be recent shards • If creeping growth (lucky aggregations are established by compression/adhesion; melting/sintering; shaking/re-assembly): Rings will persist with an equilibrium distribution.

34. 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 • At equilibrium, disruption balances growth, producing a power law size distribution, consistent with observations by UVIS, VIMS, radio and ISS • Growth rates require only doubling in 105 years • Ongoing recycling resets clocks and reconciles youthful features (size, color, embedded moons) with ancient rings: rings will be around a long time!

35. 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

36. Summary • Numerous features seen in RPX images • UVIS sees an opaque moonlet and other events in 7 occultations: implies 105 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 implies rings will be around a long time!

37. Backup Slides

38. Inferred lifetimes are too short for recent creation of entire rings • Are some rings more recent than Australopithecines, not to mention dinosaurs? • Small shepherds have short destruction lifetimes, and it is not surprising to find them near rings • Low density moons in A ring gaps show accretion happens now • B ring not as big a problem: it has longer timescales, more mass

39. 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.

40. 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

41. EXPECTATION VALUES Lifetimes (steps): E1=(1-pn)/(pnq) ~n, for nq << 1 (linear) ~n2, for nq ~ 1 (like diffusion) ~2n+1-2, for p=q=1/2 ~p-n, as q goes to 1 (indefinitely long)

42. 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

43. Observed Events • Pywacket • In Alp Sco Egress • 200m wide • At 140552km from Saturn • Mittens • In Alp Leo • 600m wide • 139917km from Saturn

44. Observed Events • 9 events • 30m to 600m wide

45. . Number of events observed, corrected by subtracting number detected in control regions. Searches with bins of 1, 5, 10.

46. Events compared to Barbara and Esposito 2002