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 diffractionlimited 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
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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
trailing
leading
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

[Albers et al. (2009)]
F ring core perturbed by Prometheus
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 boomand-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