Cassini Observations and Ring History Larry W. Esposito COSPAR Beijing 18 July 2006.
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Transcript of Cassini Observations and Ring History Larry W. Esposito COSPAR Beijing 18 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
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.
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
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• These short-live objects argue for
‘creeping’ growth of moonlets from ring particles and continuing recycling…
N1507015271 N1507099722
Bright arc and objectin 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 internalcollision (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
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
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 Inferred/observed age Implications OLDYOUNG RENEWEDNarrow ringlets in gaps months Variable during Cassini mission OK OKEmbedded moonlets millions of years Density shows accretion OK OK"Propeller" objects less than a million years Need better pix ? ? ?F ring clumps months Sizes not a collisional distrib OKF ring moonlets tens to millions of years OK OKCassini Div density waves 100,000 years Quickly ground to dust OK OKRing pollution (from color) A 1E7 - 1E8 years Expected more polluted than B OK OK B 1E8 - 1E9 years Meteoroid flux not so high? ? CColor/spectrum varies in A 1E6 - 1E7 years Ring composition not homogenized OKShepherd moons Breakup: 1E7 years OK OK
Momentum: 1E7 years No contradiction in ages!Self-gravity wakes days Particles continually collide; self OK OK OK
gravity enhances aggregation
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.
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!
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
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!
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
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
Observed Events
• Pywacket– In Alp Sco Egress– 200m wide– At 140552km from Saturn
• Mittens– In Alp Leo– 600m wide– 139917km from Saturn
Number of events observed, corrected by subtracting number detected in control regions. Searches with bins of 1, 5, 10.
.