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Cassini UVIS Observations: Structure and History of Saturn’s Rings Larry Esposito 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 Butterball Fluffy Broad and narrow features in gamma Arae 7 Feature 9 8 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. 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 This implies young rings? The fractional pollution of the regolith, fp, is given by fp = FG mÝt / h(t) For ring particles, fp is 0.01 in 108 years, a rough upper limit from ring observations at microwave Estimating ring age from the volume pollution rate For a ring system with surface mass density, , we have fp(vol) = 8 2 10 g /cm / year t So, fp(vol) = 0.01 and = 100g/cm2 also gives t=108 years, consistent with CE98 Ring age • Ring particles should develop reflectance spectra showing 1% pollution in 108 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 Numerical simulations show collisions and self-gravity effects will create transient elongated trailing structures. Pollution Conclusions • Our stochastic regolith model shows the surfaces of ring particles could be 1% meteoritic after only 108 years • If rings recycle ejecta, the volume pollution rate is the same: 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 Numerical simulations show a spider-web structure. The ring opacity underestimates mass of the B ring! UVIS occs inverted 2D autocorrelation => FFT => image 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. These are reasonable values, no free parameters varied! • This yields predicted fluxes (Cooper Fig 5): – Protons 40 (measured 50 +/- 11) – Gammas 118 (measured 180 +/- 45) in ( m2-sec-ster)-1 UVIS spectrum as a function of age and grain size Comparison of numerical models with different time steps Regolith depth simulation data CE98 Distribution / not extended Attribute Diameter amax Squarings 10x deltaT years Value 91.98269 2.3585305 54 23.64365713 1.35E+10 Units cm cm # sec years Non Zero Probabilities (first row) Location Probability 0 1 1 4.67E-13 10 4.11E-14 61 4.77E-15 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 Backup Slides 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 = H max Ý/ FGYm UVIS HSP 2D autocorrelation