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

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• 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

Alp Sco

• Spans 3 integrations

• Also seen in VIMS data

• At 140610.5 km

• ~0.2 km wide“Pywacket”

“Mitttens”

Alp Leo• Starts at 139962 km

• 21 integ-rations

• Width:

0.6 km,

and opaque

Observed Events• 9 events• 30m to

600m wide

Observed Events

Barbara and Esposito ‘02

q~2.5

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

* Mittens: 600m

Figure from Tiscareno etal 2006

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!

Backup Slides

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

Observed Events• 9 events• 30m to

600m wide

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