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, self-gravity 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

We identify this structure as temporary clumps

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

Buerle etal 2010: Bright spots cast shadows.

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 100m 1km 5km

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)

Clump observed in UVIS line of sight 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 observation We would observe a clump if it were within distance a of the observation path, so the width of the observed area is L=2a The length of that area is the width of the ring, deltaR. Extrapolate to the total number in the F ring by accounting for the total area observed in N_occs.

Coagulation equation describes competing accretion and disruption Pre-collision bodies Post-collision bodies M-m1 M m1 Fragmentation Accretion m m m m m m Collision of bodies 1 and 2 result in redistribution of fragments M m m2 m1 m m m m m m m m m m m m m

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 enhanced growth Body within area of enhanced growth Body after encounter with area of enhanced growth λ0 Vorb ΩF ΔR Δλ Vorb ΩF λ0+Δλ Ωparticles~ΩF HDR ΩHDR=ΩProm ΔΩ = ΩF-ΩHDR Angular speed at which the body moves through the HDR HDR HDR fixed to Prometheus

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

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 What is the lifecycle of moonlets in the F ring?

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

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] [gravitational stirring] - A0 cos(ωt) [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.