Larry W. Esposito, Bonnie K. Meinke and Joshua E. Colwell LASP, University of Colorado

1. Clumps and Moonlets in Saturn’s F Ring Larry W. Esposito, Bonnie K. Meinke and Joshua E. Colwell LASP, University of Colorado 12 June 2007

2. UVIS F ring occultations • 30 star occultations cut F ring 35 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 • 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 avg) 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 normally-distributed statistical fluctuations

5. The Persistence Test • Reexamine points flagged from Z test • Extract events where τpeak ≥τpywacket • Opacity proportional to ring particle collision rate  escape of particles in aggregations is proportional to opacity squared  more opaque events are more persistent (Shu and Stewart 1985)

6. Significant Events from Occultations • 9 events found in 35 cuts • Widths: 27m to 9km • Pywacket must be elongated: alpha Sco B offset by 600m from alpha Sco A • 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. 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.

8. “Mitttens”

9. Fluffy Butterball

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

11. Figure from Tiscareno etal 2006 * Mittens: 600m

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

13. 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!

14. Conclusions • Statistically significant features in F Ring, unlikely 9 observations are random • 1 confirmed feature, Pywacket - this is real! • Opaque feature(s) consistent with moonlet prediction of Cuzzi and Burns (1988), Barbara and Esposito (2002)