Cassini Observations and the
History of Saturn’s Rings
Larry W. Esposito
12 October 2006
Ring structure and composition
provide key evidence for ring
origins and history
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Show active processes
Provide timescale estimates
Test evolution theories
Allow predictions of ring future
Give analogies for planet formation
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
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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.
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 105 years
• Under-dense moons and propeller objects
indicators of continuing accretion
• Autocovariance and varying transparency show
ephemeral aggregations
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)
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
• 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
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!
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: 105 moonlets, optical
depth 10-3 , consistent with predictions
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.
“Mitttens”
Comparison to F ring model
shows model deficient
Barbara
and
Esposito
‘02
Are these caused by structures like those we see in F ring?
Figure from Tiscareno etal 2006
* Mittens: 600m
Cassini results for ring evolution
imply a broad age range
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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
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.
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
Numerical simulations show collisions and self-gravity effects
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 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 allows rings to be around a
long time
Backup Slides
RING AGE TRACEABILITY 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
Inferred/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
OLDYOUNG RENEWED
OK OK
OK OK
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OK
OK OK
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OK
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OK OK
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