1. 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

2. 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

3. Search Method Z Test seeks statistically significant events
Persistence Test requires events features have  > min
Search tuned for Pywacket-like events

4. 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
StDev = sqrt(number in bin)
Baseline: used IDL SMOOTH
Assumed statistical fluctuations are normally distributed, found deviation in single bin, from Taylor ‘intro to error analysis’

5. 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)
Shu and Stewart for optical depth req.

6. 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

7. VIMS and UVIS Alp Sco Egress occultation data are overplotted
VIMS and UVIS Alp Sco Egress occultation data are overplotted. The UVIS data curve is the one with higher spatial resolution. A multiplicative factor ( = 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.

8. “Mitttens”

9. alpha Leo: Mittens

10. ‘Mittens’ closeup

11. Fluffy
Butterball

12. Broad and narrow
features in gamma Arae 7 8
Feature 9

13. zeta Oph

14. theta Arae 41

15. 126 Tauri 8

16. 126 Tauri 8

17. 126 Tau 8

18. 126 Tau 8

19. chi Cen 39

20. beta Per 42

21. alpha Sco UVIS and VIMS

22. Alpha Sco 29

26. Numerical simulations show collisions and self-gravity
effects will create transient elongated trailing structures.

27. 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.

28. 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

29. 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!

30. 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