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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 VIMSconfirmed 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 Ring Age Tracebility Ring Featu re Inf erred/observed age Notes N arrow ringlets in gaps F ring clumps months F ring moonlets tens to millions of years 100 ,000 years V ariable during C as s in i mis sion Sizes not a c ollisional dis tribution C reate fans and jets C as sin i Division dens ity waves E mbedded moonlets "P ropeller" objects Ring pollution (from color) A B C olor/s pectrum varies in A Shepherd moons Self-gravity wakes months millions of years millions of years 107 - 1 08 years 108 - 1 09 years 106 - 1 07 years Breakup: 1 07 years M omentum: 107 years days OLD YOUNG RENEWED OK OK OK OK OK Low mass quickly ground to dust Low bulk density s hows ac cretion Steep size distribution from recent disruption NO OK NO OK E xpected more polluted than B M eteoroid flux not s o high? M ore massive? Ring c omposition not homogenized NO NO OK OK NO OK if massive OK NO OK OK OK Breakup/momentum: N o c ontradiction in ages ! Partic les c ontinually c ollide; s elf-gravity and adhesion enhance aggregation OK OK OK OK OK Table LWE 2. Inferred ages of various ring features and consistency with 3 models for ring formation. OK: Can be accommodated; NO: Serious contradiction; Blank : Unclear, or deserves more study. Adopted from Esposito 2006, presented at Montana Rings Workshop.