Cassini UVIS Observations and
the History of Saturn’s Rings
Larry W. Esposito, Joshua E. Colwell,
Glen R. Stewart
LASP, University of Colorado
24 August 2007
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 propeller objects
indicators of continuing accretion
• Autocovariance and varying transparency show
ephemeral aggregations
UVIS F ring occultations
• 38 star occultations cut F ring 44 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
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”
Butterball
Fluffy
Broad and narrow
features in gamma Arae
7
Feature 9
8
Figure from Tiscareno etal 2006
* Mittens: 600m
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!
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
larger, more compact objects that survive
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!
Relationship to planet formation
• Similar processes are likely occurring in
the other ring systems and in the
formation of planets around other stars.
• Particularly, such ‘creeping’ processes
could be important where more rapid
growth is frustrated, as in the asteroid
belt and the Kuiper belt