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Cassini Observations and Ring History Larry W. Esposito UVIS Team Meeting 11 July 2006 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 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 • • • • 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 reassemble. ‘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 COLLISIONAL CASCADE USES UP RING MATERIAL TOO FAST! 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) 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! 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 • Evidence of ‘creeping’ growth of moonlets from ring particles and continuing recycling? 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) 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 Search Method • 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 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 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 Alp Sco • Spans 3 integrations • Also seen in VIMS data • At 140610.5 km • ~0.2 km wide “Pywacket” “Mitttens” • Starts at 139962 km • 21 integrations • Width: 0.6 km, and opaque Alp Leo Observed Events • Pywacket – In Alp Sco Egress – 200m wide – At 140552km from Saturn • Mittens – In Alp Leo – 600m wide – 139917km from Saturn Observed Events • 9 events • 30m to 600m wide Are these caused by structures like those we see in F ring? Figure from Tiscareno etal 2006 * Mittens: 600m 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, halving size in dt 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 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 RING AGE TRACEBILITY MATRIX Ring Feature Narrow ringlets in gaps Embedded moonlets "Propeller" objects F ring clumps F ring moonlets Cassini Div density waves Ring pollution (from color) A B C Color/spectrum varies in A Shepherd moons Self-gravity wakes Inferered/observed age months millions of years less than a million years months tens to millions of years 100,000 years Implications Variable during Cassini mission Density shows accretion Need better pix Sizes not a collisional distrib 1E7 - 1E8 years 1E8 - 1E9 years Expected more polluted than B Meteoroid flux not so high? 1E6 - 1E7 years Breakup: 1E7 years Momentum: 1E7 years days Ring composition not homogenized Quickly ground to dust No contradiction in ages! Particles continually collide; self gravity enhances aggregation OLD YOUNG RENEWED OK OK OK OK ? ? ? OK OK OK OK OK OK OK ? OK OK OK OK OK OK What do the processes imply? • Unidirectional size evolution (collisional cascade): Then the age of rings is nearly over! • Binary accretion is thwarted by collisions, tides: Larger objects must be recent shards • Creeping growth (lucky aggregations are established by compression/adhesion; melting/sintering; shaking/re-assembly): Rings will persist in an equilibrium distribution 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 reconciles youthful features (size, color, embedded moons) with ancient rings: rings will be around a long time! What’s Next? • Persistence of F ring objects: track in images? • A ring structures, events, color variations • Characterize aggregations from wakes to moonlets • Compare to Itokawa and other ‘rubble piles’ • Pollution models • ‘Creeping growth’ models Backup Slides Summary • Numerous features seen in RPX images • UVIS sees an opaque moonlet and many 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 reconciles youthful features (size, color, embedded moons) with ancient rings: rings will be around a long time! 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. 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 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) 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 . Number of events observed, corrected by subtracting number detected in control regions. Searches with bins of 1, 5, 10. Events compared to Barbara and Esposito 2002