Clumps and Moonlets
in Saturn’s F Ring
Larry W. Esposito, Bonnie K. Meinke
and Joshua E. Colwell
LASP, University of Colorado
European Planetary Science Conference
23 August 2007
UVIS F ring occultations
• 38 star occultations cut F ring 44 times
• Alp Sco shows 200m feature, ‘Pywacket’, also
seen by VIMS
• This event used as test case to refine search
algorithm
• Alp Leo shows 600m moonlet: ‘Mittens’
• Opaque event! This gives: 105 moonlets, optical
depth 10-3 , consistent with predictions
Search Method
• Z Test seeks statistically significant events
• Persistence Test requires events features
have  > min
• Search tuned for Pywacket-like events
The Z Test
• Bin data by 5 signal measurements
• Compare baseline (running average) of stellar
signal to binned data
• Z= number of standard deviations away from the
baseline fluctuations are
• Flag bins with Z>Zmin
– Zmin is chosen so 1 flag would occur by chance in set,
for Poisson-distributed statistical fluctuations
The Persistence Test
• Reexamine points flagged from Z test
• Extract events where τpeak ≥ τpywacket
• Ring particle collision rate is proportional to
opacity  escape time of particles in
aggregations by collisional diffusion is
proportional to opacity squared  more
opaque events are longer lived (Shu and
Stewart 1985)
Significant Events from
Occultations
• 13 events found in 44 cuts
• Widths: 27m to 9km
• Pywacket must be elongated: alpha Sco B
offset by 600m from alpha Sco A
• At least one, and perhaps several may be
opaque
• Consistent with predictions by Cuzzi and
Burns and by Barbara and Esposito
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”
alpha Leo: Mittens
‘Mittens’ closeup
Butterball
Fluffy
Broad and narrow
features in gamma Arae
7
Feature 9
8
zeta Oph
theta Arae 41
126 Tauri 8
126 Tauri 8
126 Tau 8
126 Tau 8
chi Cen 39
beta Per 42
alpha Sco UVIS and VIMS
Alpha Sco 29
Numerical simulations show collisions and self-gravity
effects will create transient elongated trailing structures.
Ring History Model:
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
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
• 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!
Conclusions
• Significant features seen in F ring
• 1 confirmed feature, Pywacket - this is real!
• Opaque feature(s) consistent with moonlet
prediction of Cuzzi and Burns (1988)
• Barbara and Esposito (2002) argued that
the larger bodies accrete within F ring
• Recycling of ring material allows rings to
be ancient, although varying with time