UVIS results and ring evolution LW Esposito 5 January 2010

Background Accretion is possible in the Roche zone, where rings are found, but limited by tides and collisions (Canup & Esposito, Barbara & Esposito, Karjalainen & Salo) F ring shows clumps and moonlets (Esposito etal, Murray etal); A ring has propellers (Tiscareno, Sremcevic); Embedded moonlets are elongated (Charnoz) Self-gravity wakes (Colwell, Hedman) in A,B rings show temporary aggregations Does the size of aggregates represent an equilibrium between accretion and fragmentation (Barbara & Esposito)?

UVIS occultations UVIS has observed over 100 star occultations by Saturn’s rings Time resolution of 1-2 msec allows diffraction- limited spatial resolution of tens of meters in the ring plane Multiple occultations provide a 3D ‘CAT scan’ of the ring structure Spectral analysis gives characteristics of ring structures and their dimensions

Occultation Analysis Features in F ring (‘kittens’) Ring edge thickness and sharpness Location of F ring core and of B ring edge Sub-km structure seen in wavelet analysis

Features in F ring Esposito etal (2008) identified 13 statistically significant features These were interpreted as temporary clumps and a possible moonlet, ‘Mittens’ Meinke etal (2009) now catalog 39 features from the first 102 stellar occultations For every feature, we have a location, width, maximum optical depth (opacity), nickname

Kittens show dependence on Prometheus-relative longitude Opacity increased in quadrants following Prometheus passage Linear fit to Prometheus-relative longitude has correlation r = 0.49 Synodic period is 68 days

Optical depth: leading vs. trailing features Longitude relative to Prometheus

Optical depth and means by quadrant (all features) Triangles give quadrant means with standard errors

R = 0.49

Ring edge thickness and sharpness Occultation profile at ring edge gives limits on the edge shape and vertical thickness (Lane etal 1982, Colwell etal 2007) Edge of Saturn’s B ring (the last 100m before the Cassini Division) shows major time and longitude variability But, no correlation with Mimas longitude

B ring edge is highly variable Optical depth is correlated with Mimas location and increasing since 2004 Edge location intermittently fit by m=2 pattern, with variable phase lag

B ring edge optical depth Edge profiles exhibit multiple shapes: plains, plateaus, steep hillsides, cliffs, and broad hummocks Highest optical depth at edge when it is inward of Mimas 2:1 ILR location Maxima found at 0 and 180 deg co-rotating longitude from Mimas; this is anti-correlated to sub-km structure maxima at 90 and 270 deg from Mimas (see below) Edge appears to have typical optical depth ~1 present at all times; only recently do certain longitudes show higher optical depths of >2 Optical depth at edge increases since 2004 in both mean and variance

Edge profile variability

Optical depth over time

B ring edge location Clear, but intermittent m=2 pattern (consistent with VIMS, RSS) ~2005: amplitude ~100km; minimum leading Mimas by ~45deg ~2008: amplitude ~150km; minimum trailing Mimas by ~25deg Weak/hidden/absent m=2 pattern ~2007; A simple drift or libration can not account for all the data Mimas 2:1 (ILR) is most dominant (m=2 pattern) starting 2008

Edge has m=2 pattern 2004-05

No m=2 pattern 2006-07

Edge has m=2 pattern 2008-09

Edge location over time

F ring core and kittens Unlike the F ring kittens, the F ring core exhibits no trend in optical depth related to Prometheus F ring core is well-described by freely precessing ellipse to within 50km Variance of F ring core residuals increases in time as Prometheus and F ring near their anti-apse alignment in December 2009

F ring core perturbed by Prometheus [Albers et al. (2009)]

Sub-km structure seen in wavelet analysis varies with time, longitude Wavelet analysis from multiple occultations is added in a probabilistic manner to give a significance estimate For the B ring edge, the significance of features with sizes 200-2000m increases since 2004; and shows maxima at 90 and 270 degrees ahead of Mimas For density waves, significance correlated to resonance torque

Structure increasing near B ring edge

Observational Findings F ring kittens more opaque trailing Prom. B ring edge more opaque leading Mimas Sub-km structure, which is seen by wavelet analysis at strongest density waves and at B ring edge, is correlated with torque (for density waves) and longitude (B ring edge) Variance in B ring location and in the strength of sub-km structure is increasing since 2004 The largest structures could be visible to ISS

Conclusions Cassini occultations of strongly perturbed locations show accretion and then disaggregation: scales of hours to weeks Moons may trigger accretion by streamline crowding (Lewis & Stewart); which enhances collisions, leading to accretion; increasing random velocities; leading to more collisions and more accretion. Disaggregation may follow from disruptive collisions or tidal shedding

Summary Like the global economy, the rings may not have a stable equilibrium for accretion; instead, boom/bust triggered by stochastic events? The year 2009 showed increased structure in both B ring edge and the F ring : this would be a good time to look for embedded objects

Backup Slides

Optical depth of all features by quadrant (outliers excluded) Triangles give quadrant means with standard errors

Optical depth by quadrant (all radial outliers > 100km excluded)

Optical depth versus edge location

Optical depth in relation to Mimas

Edge location with corotating longitude (all seasons)

F ring kittens and core Diamonds: kittens Triangles: core

Mechanisms Collisions may cause stochastic events: compress unconsolidated objects, trigger adhesion or bring small pieces into contact with larger or higher-density seeds The ring system may resemble a non-linear driven oscillator: moon forcing can drive it out of resonance into chaotic response (works for B ring edge, density waves, but not necessarily for F ring..?)

Mechanisms (continued) In the accretion/disruption balance, increased random motions may give the upper hand to disruption… just as ‘irrational exuberance’ can lead to financial panic in the economy; or the overpopulation of hares can lead to boom-and-bust in the population of foxes Non-linear thresholds can thus give rise to episodic cycles in accretion: and thus to the observable ring features that indicate embedded objects increasing since SOI

Density wave sub-km feature structure strength vs. resonance torque