1. Cassini UV Imaging Spectrograph Observations Show Active Saturn Rings
Larry W. Esposito
Joshua E. Colwell
LASP, University of Colorado
16 December 2004

6. Implications of UVIS observations
The rise and fall in abundance of OI between Dec 25, 2003 and May 13, 2004 amounts to ~500 Mkg of mass apparently lost from the system in this period. Mean inferred loss rate of bulge in mass during 2 months is ~4 X1027 atoms s-1.
Total mass of OI + OH in system is ~2200. Mkg.
The estimated micron sized particles in the E-ring involved in the Mimas – Tethys region is 600 Mkg.

7. Problems with E ring moonlet collision explanation
Use up E ring (and parents) too rapidly
Very unlikely that such a rare event occurred during Cassini approach
No parent bodies larger than 1-2 km seen yet by Cassini cameras
Even a cascade requires a rare event in last 25 years

8. UVIS RESULTS FOR MAIN RINGS
We summed all SOI spectra from same distance to produce radial profile and color rendition
UV spectra show water abundance increasing outward to a peak in outer A ring
Rings A and B more icy than ring C and Cassini Division, consistent with VIMS
More structure than seen in Voyager and HST images of A ring

13. A Ring Particle Properties

14. RING HISTORY
Large scale variation consistent with meteoritic pollution of initially pure ice
Variations over scales of km too narrow to be explained by ballistic transport over the age of rings
One or more 20-km moonlets shattered in last million years could do it
Same future result from disrupting Pan

15. A Ring Particle Properties

16. Planned pollution calculations
Determine a quantitative pollution fraction f from spectrum
Write a transport equation for f: collisional diffusion and ballistic transport
Use regolith models and physical parameters to match radial spectral variations
Compare results with evolution models

17. YOUTHFUL RINGS: DESTRUCTIVE PROCESSES ACT QUICKLY
Grinding and sputtering
Spreading and momentum transfer to small moons
Darkening from meteoroid bombardment
Ring ages: 107 to 109 years

20. 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
Small moons caught in resonances with larger moons: this slows linked evolution

21. COLLISIONAL CASCADE

22. MARKOV MODEL FOR THE COLLISIONAL CASCADE
Improve by considering recycling
Collective effects: nearby moons can shepherd and recapture fragments
Accretion in the Roche zone is possible if mass ratio large enough (Canup & Esposito 1995)

23. MODEL PARAMETERS
n steps in cascade, from moons to dust to gone…
With probability p, move to next step (disruption)
With probability q, return to start (sweep up by another moon)
p + q = 1.

24. LIFETIMES
This is an absorbing chain, with transient states, j= 1, …, n-1
We have one absorbing state, j=n
We calculate the ring/moon lifetime as the mean time to absorption, starting from state j=1

25. EXPECTATION VALUES E1=(1-pn)/(pnq) Lifetimes (steps):
~n, for nq << (linear)
~n2, for nq ~ (like diffusion)
~2n+1-2, for p=q=1/2
~p-n, as q goes to 1 (indefinitely long)

26. EXAMPLE: F RING
After parent body disruption, F ring reaches steady state where accretion and knockoff balance (Barbara and Esposito 2002)
The ring material not re-collected is equivalent to ~6km moon; about 50 parent bodies coexist…
Exponential decay would say half would be gone in 300 my.
Considering re-accretion, loss of parents is linear: as smaller particles ground down, they are replaced from parent bodies. The ring lifetime is indefinitely extended

28. 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
Below some threshold, recycling declines and the rings are rapidly lost

29. CAUTIONS
Some rings are too close for much recycling: Uranus and Neptune rings may require flatter strength distribution (Colwell et al)
Momentum transfers and moon radial evolution still a problem: do chaotic interactions and linkup to larger moons solve this? (Goldreich and Rappaport)

30. How big a moonlet, when?

31. UVIS data are consistent with active ring/moon recycling
Oxygen fluctuations require replacement of E ring grains from parent bodies
Radial spectral variations in A ring require multiple reservoirs and recent release of purer ice
20 km embedded moonlets have right lifetimes and mass, given estimated diffusion rates