Regolith Growth and Darkening of Saturn’s Ring Particles Larry W. Esposito Joshua P. Elliott LASP, University of Colorado 15 December 2008

Are Saturn’s Rings Young or Old? Voyager found active processes and short inferred lifetimes: we concluded the rings were created recently It is highly unlikely a comet or moon as big as Mimas was shattered recently to produce Saturn’s rings; Are we very fortunate? Cassini observations show a range of ages, some even shorter… and even more massive rings!

Key Cassini Observations Changes since Voyager and even since SOI F ring clumps and moonlets Propellers in A ring Under-dense ringmoons Self-gravity wakes and auto-covariance show heterogeneous rings Low mass density in Cassini Division gives gross erosion time of 30,000 years

F Ring Search Method Search tuned for 1 VIMS-confirmed event Optimal data-bin size min VIMS UVIS Pywacket -15 km 0 15 km

Key Model Results Ring dynamics: Temporary aggregations Competition between fragmentation and accretion produces bi-modal distribution Meteor impacts can explain the color and morphology if rings are about 108 years old Aggregates mean that if ring mass was under-estimated, pollution would be less: Recycling of ring material can extend the ring lifetime “Nice” model of solar system evolution can produce the rings by shattering a moon during LHB

Robbins & Stewart simulation grows clumps!

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.

Physical approach Meteorites strike surface element If the impact penetrates the regolith, it breaks and excavates new material For any impactor size distribution, only impactors larger than a(h) will penetrate a regolith of present depth h(t) The ejecta are emplaced on the surface uniformly: every surface element is as likely to recapture ejecta

Mathematical approach Take h(t), regolith depth, as a stochastic variable This is a Markov chain: discrete values of h are the states of the chain; transitions occur when a meteorite strikes; transition probabilities can be calculated from the mass flux and size distribution We do not need to know the exact strike location, just that the strikes are uniformly distributed D drops out, since the probability of a strike and the area its ejecta cover both scale as D2

Realistic case for Saturn Use Cuzzi and Estrada (1998) impactor size distribution, extended to 100m Allow for disruption of ring bodies by largest impactors: redistribute ejecta among surviving bodies Model the size distribution with a broken power law to improve numerical performance

Larger ring particles grow deeper regoliths

10 meter ring particles reach 1% pollution in 2x109 years

More massive rings show insignificant spectral changes

Conclusions Saturn’s rings appear young… but may confuse ‘age’ with most recent renewal! Cassini shows ring heterogeneity and more massive rings, consistent with little observed pollution in ring B Detailed regolith models predict insignificant UV spectral differences for 10m particles (this is 10x current mass estimate from Esposito etal 1983)

Backup Slides

Self-Gravity “Wake” Model

Reinterpretation of P-11 results: Colwell’s ‘Granola Bar’ Model Cooper etal 1983 assumed a uniform ring to calculate secondary fluxes from GCR flux But, secondary fluxes are double-valued! Self-gravity wakes say B ring density is also… Instead, assume surface density ring C = ring A = 60g/cm2. For ring B: 80% with 500g/cm2, 20% with 60g/cm2, consistent with Colwell etal 2007. This yields predicted fluxes (Cooper Fig 5): Protons 40 (measured 50 +/- 11) Gammas 118 (measured 180 +/- 45) in ( m2-sec-ster)-1