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