1. Cassini UVIS Observations: Structure and History of Saturn’s Rings
Larry W. Esposito
LASP, University of Colorado
Fall AGU Meeting
13 December 2007

2. Significant Events from UVIS Occultations Show Aggregaates
13 events found in 44 cuts of F ring
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

3. Fluffy
Butterball

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

5. Are ancient rings possible? Regolith model for pollution:
Consider an infinite slab of depth, D
The regolith depth at time t: h(t)
For a moonlet or ring particle, D corresponds to the diameter.

6. Markov chain simulation matches analytic test case
For an impactor size distribution that is a power law of index 3, we can solve the differential equation for h(t), assuming all material is excavated:
h(t) = Hmax[1 - exp(-t/T0)]
Hmax = H1 amax
T0 =

7. Realistic case for Saturn
Use Cuzzi and Estrada (1998) impactor size distribution
Compare to Quaide and Oberbeck (1975) lunar regolith model
Our Markov chain model result gives depth within a factor of 2 of their values for 105 < t < 109 years

9. This implies young rings?
The fractional pollution of the regolith, fp, is given by
fp =
For meter -sized particles, fp is 0.01 in 108 years, a rough upper limit from ring observations at microwave

10. Estimating ring age from the volume pollution rate
For a ring system with surface mass density, , we have
fp(vol) =
So, fp(vol) =0.01 and  = 100g/cm2 also
gives t=108 years, consistent with CE98

11. Ring age
Ring particles of cm diameter should develop reflectance spectra showing 1% pollution in 107 years: this is a problem even if the rings formed a few hundred million years ago!
An alternate conclusion is that the ring mass is underestimated: then, continuing recycling could renew their composition, consistent with upper limits on non-icy material
Clumpy rings mean larger B ring mass, consistent with less pollution

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

14. Pollution Conclusions
Our stochastic regolith model shows the surfaces of cm-size ring particles could be 1% meteoritic after only 107 years
If rings recycle ejecta, the volume pollution rate is better: 1% meteoritic in 108 years for standard values of ring mass and impact flux
For larger mass or lower flux, the ring age could be proportionately greater: 109 years if B ring  = 1000 g/cm2

15. Numerical simulations show a spider-web structure
Numerical simulations show a spider-web structure. The ring opacity underestimates mass of the B ring!

16. 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!