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Cassini UV Imaging Spectrograph Observations Show Active Saturn Rings

This research study analyzes the implications of UVIS observations from the Cassini spacecraft on the active Saturn rings, including the loss of mass, water abundance, and particle properties. It also discusses the processes involved in the collisional cascade from moons to rings and the potential lifetimes of ring systems.

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Cassini UV Imaging Spectrograph Observations Show Active Saturn Rings

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

  2. 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.

  3. 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

  4. 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

  5. A Ring Particle Properties

  6. RING HISTORY • Large scale variation consistent with meteoritic pollution of initially pure ice • Variations over scales of 1000 - 3000 km too narrow to be explained by ballistic transport over the age of rings • One or more 20-km moonlets shattered in last 10 -100 million years could do it • Same future result from disrupting Pan

  7. A Ring Particle Properties

  8. 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

  9. 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

  10. 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

  11. COLLISIONAL CASCADE

  12. 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)

  13. 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.

  14. 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

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

  16. 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

  17. 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

  18. 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)

  19. How big a moonlet, when?

  20. 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

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