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A Season in Saturn’s Rings: Cycling, Recycling and Ring History Larry W. Esposito, Bonnie K. Meinke, Nicole Albers and Miodrag Sremcevic LASP 19 March 2012 Key Cassini Observations • High resolution SOI images of straw, propellers, embedded moons, F ring objects. Occ’s confirm structure, selfgravity wakes, overstability • Multiple UVIS, RSS and VIMS occ’s • Equinox images of embedded objects Saturn Equinox 2009 • Oblique lighting exposed vertical ring structure and embedded objects • Rings were the coldest ever • Images inspired new occultation and spectral analysis • Steady progress and new discoveries continue: More complex, time variable Sub-km structure seen in wavelet analysis varies with longitude • Wavelet analysis from multiple UVIS occultations is co-added to give a significance estimate • For the B ring edge, the significance of features with sizes 200-2000m shows maxima at 90 and 270 degrees ahead of Mimas • For density waves, significance correlated to resonance torque from the perturbing moon Edges also show structure • Some explained by multiple modes • Other sharp features appear stochastic, likely caused by local aggregates From Albers etal 2012 F ring Observations • 27 significant features in F ring: ‘Kittens’ from 22m to 3.7km, likely they are elongated and transient • Icicles have weak correlation to Prometheus, may evolve into moonlets New Features I Gatti di Roma: temporary features in an ancient structure We identify our ‘kittens’ as transient clumps Prometheus excites F ring structures Meinke Dissertation Results: Size Evolution Models • Wavy, quasi-periodic behavior in the size distribution is due to sharp thresholds and their echoes. Multiple modes are not just artifacts! • Porosity evolution makes larger objects more compact and persistent • Matching the observed kitten shallow size distribution requires enhanced accretion for larger objects • This may result from passage through high density regions, triggered by Prometheus streamline crowding Parameters for this model are: qswarm=qej rkitten= 640 m ΣHDR= 40 g cm-2 μcrit = 100 BEST FIT MODEL Upper limit on object like S/2004 S 6 10m 100 m 1km 5km 21 Visibility of Propellers • Moonlet perturbation larger than random motions: Rmoonlet > H; Lewis Mmoonlet > 30 Mmax • Moonlet perturbation larger than caused by SGW accelerations: Rmoonlet > λcrit (Michikoshi) • Propeller width ~ 2-4 * Rmoonlet • Propeller length ~ 50 * Rmoonlet; longer in occultations? • Evidence for moonlets in Rings A, B, C, CD Predator-Prey model of Moon-triggered Accretion? Phase plane trajectory V2 M Observations • Small bodies in the F ring and outer B ring cast shadows • Vertical excursions evident at ring edges and in other perturbed locations • Multimodal ringlet and edge structure: free and forced modes, or just stochastic? • Temporary F ring aggregates • Propellers and gaps in A, B, C rings Rare accretion can renew rings • Solid aggregates are persistent , like the absorbing states in a Markov chain • Even low transition probabilities can populate the states: e.g., 10-9 per collision to an absorbing state • These aggregates – shield their interiors from meteoritic dust pollution – release pristine material when disrupted by an external impact Analogy: Coast Redwoods 1 in 104 seeds grows to a tree! Like Beijing, rings contain both new and ancient structures! Backup Slides F ring Kittens • UVIS occultations initially found 13 statistically significant features • Interpreted as temporary clumps and a possible moonlet, ‘Mittens’ • Meinke etal (2012) now catalog 25 features from the first 102 stellar occultations • For every feature, we have a location, width, maximum optical depth (opacity), nickname Model consistent with observations must include enhanced growth of larger bodies • The largest bodies in the system are the only ones that have increased accretion in the HDRs because gravitational instabilities form around them • The numbers of the smallest bodies decrease as the larger bodies sweep them up • This “flattens” the distribution by preferentially removing small bodies • Thus, the “kittens” that UVIS sees may be themselves swept up by even larger moonlets (S/2004 S 6) 37 Clump observed in UVIS line of sight r ΔR a λ We detect clump if center of clump is within a semi-major axis length of the occultation track. This defines the region of 41 observation Coagulation equation describes competing accretion and disruption Fragmentation Accretion Pre-collision bodies Post-collision bodies m 1 Mm1 M m m 1 m2 m m m m m m m M m m m m m m m m m m m m Collision of bodies 1 and 2 result in redistribut ion of 44 fragments Body experiences enhanced accretion when it enters a High Density Region (HDR) Body approximated at a line mass (line along the azimuth where clumps are likely elongated, triaxial ellipsoids) Body approaching area of λ0 Vorb enhanced ΩF growth Body within area of enhanced Δ Vorb growth λ ΩF Ωparticles~ ΩF HD R Body after encounter with λ0+Δλ area of enhanced Ω = growth Δ R HDR ΩProm ΔΩ = ΩFΩHDR Angular speed46 at which the Observations and Model tell a story of how moonlets are made • Observations show us: – Compaction occurs, but is rare – Clumps are correlated to Prometheus • Model shows us: – Binary accretion is not sufficient to match observations – Bodies must have enhanced growth, and Prometheus provides that opportunity • Together: – Complicated moonlet-construction occurs in the F ring – Moonlets are rare but possible – Accretion is winning in the F ring long-term 47 The F ring is a natural lab for studies of accretion • 30+ years of observations • Models to date: – Beurle, et al. (2010) show that Prometheus makes it possible for “distended, yet long‐lived, gravitationally coherent clumps” to form – Barbara and Esposito (2002) show bimodal distribution of F ring material, which predicts a belt of ~1 km-sized moonlets 48 F rin What is the lifecycle of moonlets in the The F ring may be the easiest place to observe aggregation/disaggregation Increasing accretion 1000x gives consistent slope, but predicts larger clumps that would have seen by UVIS ~few km: UVIS should have seen more than 3 51 Alternate explanation: clumps grow where streamlines crowd F ring model profiles show streamline crowding (Lewis & Stewart) Predator-Prey Equations M= ∫ n(m) m2 dm / <M>; Vrel2= ∫ n(m) Vrel2 dm / N dM/dt= M/Tacc – Vrel2/vth2 M/Tcoll [accretion] [fragmentation/erosion] dVrel2/dt= -(1-ε2)Vrel2/Tcoll + (M/M0)2 Vesc2/Tstir [dissipation] - A0 cos(ωt) [gravitational stirring] [forcing by streamline crowding] Amplitude proportional to forcing B ring phase plane trajectories Wavelet power seen is proportional to resonance torques F ring phase lag Post-Equinox View • Cassini Equinox observations show Saturn’s rings as a complex geophysical system, incompletely modeled as a single-phase fluid: clumps evident; particles segregate by size; viscosity depends on shear; shear reverses in perturbed regions; rings are far from equilibrium in perturbed regions • Self-gravity causes wakes, viscosity, overstabilty and local aggregate growth • Larger fragments: seeds for growth Implications • Self-gravity is key: Rings may suffer viscous and 2-stream instability • Resonances and Kepler shear provide the forcing for a multitude of dynamics • Structure forms at scales from meters to kilometers • Accretion may continue today to renew the ring material.