Comparing F ring to B ring
outer edge: implications for
accretion
Larry W. Esposito
LASP, University of Colorado
10 June 2009
Are Saturn’s Rings Young or Old?
• Voyager found active processes and short
inferred lifetimes: we concluded the rings
were created recently
• It is highly unlikely a comet or moon as big as
Mimas was shattered recently to produce
Saturn’s rings; Are we very fortunate?
• Cassini observations show a range of ages,
some even shorter… and even more massive
rings!
Key Cassini Observations
•
•
•
•
Changes since Voyager and even since SOI
F ring clumps and moonlets
Propellers in A ring
Under-dense ringmoons with equatorial
bulges
• Self-gravity wakes and auto-covariance show
heterogeneous rings
• Low mass density in Cassini Division gives
gross erosion time of 30,000 years
F Ring Search Method
• Search was
tuned for 1
VIMSconfirmed
event:
– Optimal
data-bin
size
–  threshold
VIMS
UVIS
Pywacket
-15 km
0
15 km
Shadows at B ring edge show clumps like in F ring? The
F ring is relevant to similar processes in main rings!
Key Model Results
• Ring dynamics: Temporary aggregations form
• Competition between fragmentation and
accretion produces bi-modal distribution
• Meteor impacts can explain the color and
morphology if rings are about 108 years old
• Aggregates mean that if ring mass was
under-estimated, pollution would be less:
Recycling of ring material can extend the ring
lifetime
• “Nice” model of solar system evolution can
produce the rings by shattering a moon
during LHB
Robbins & Stewart simulation grows clumps!
F ring: similarities to B ring
outer edge
• Observations by Cassini’s UVIS
instrument show highly perturbed,
transient structures in the F ring and at
the outer edge of the B ring.
• These features are much more extreme
than predicted by models that assume
the rings behave like a fluid.
More similarities
• Like the F ring structures, the B ring
outer edge wanders unpredictably
• Multiple structures are evident: jagged
peaks, plateaus, gradual gradients
• These structures are transient, apparent
at some longitudes and absent at others
Power spectral similarities
• Spectral analysis indicates structures
having widths 200-2000 meters, even
more erratic than in the F ring
• This spectral behavior is only seen at
the F ring, the B ring outer edge and at
the strongest density waves in the A
ring
Possible explanations
• The chaotic behavior arises from the
resonant forcing on ring particles that
causes them to jam together, like ‘traffic
jams’ seen in the computer simulations
(Lewis and Stewart 2005)
• Attraction among the ring particles
creates temporary clumps that further
disrupt the rings around them
Implications
• The exotic F ring structures have
parallels in the main rings
• They may also result from temporary
aggregations like those inferred from
images by Murray etal and from UVIS
• Similar processes of tidally limited
accretion (Barbara and Esposito 2002)
may be responsible
Backup Slides
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.
Physical approach
• Meteorites strike surface element
• If the impact penetrates the regolith, it
breaks and excavates new material
• For any impactor size distribution, only
impactors larger than a(h) will penetrate
a regolith of present depth h(t)
• The ejecta are emplaced on the surface
uniformly: every surface element is
equally likely to recapture ejecta
Mathematical approach
• Take h(t), regolith depth, as a stochastic
variable
• This is a Markov chain: discrete values of h
are the states of the chain; transitions occur
when a meteorite strikes; transition
probabilities can be calculated from the mass
flux and size distribution
• We do not need to know the exact strike
location, just that the strikes are uniformly
distributed
• D drops out, since the probability of a strike
and the area its ejecta cover both scale as D2
Realistic case for Saturn
• Use Cuzzi and Estrada (1998) impactor
size distribution, extended to 100m
• Allow for disruption of ring bodies by
largest impactors: redistribute ejecta
among surviving bodies
• Model the size distribution with a broken
power law to improve numerical
performance
Larger ring particles grow deeper regoliths
10 meter ring particles reach 1% pollution in 2x109 years
Spectral time variation for 1m moonlets
More massive rings show insignificant spectral changes
Conclusions
• Saturn’s rings appear young… but may
confuse ‘age’ with most recent renewal!
• Cassini shows ring heterogeneity and
more massive rings, consistent with little
observed pollution in ring B
• Detailed regolith models predict
insignificant UV spectral differences for
10m particles (this is 10x current mass
estimate from Esposito etal 1983)
Self-Gravity “Wake” Model
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, consistent with Colwell etal 2007.
• This yields predicted fluxes (Cooper Fig 5):
– Protons 40 (measured 50 +/- 11)
– Gammas 118 (measured 180 +/- 45) in ( m2-sec-ster)-1
Ring Age Tracebility
Ring Featu re
Inf erred/observed
age
Notes
N arrow ringlets
in gaps
F ring clumps
months
F ring moonlets
tens to millions of
years
100 ,000 years
V ariable during C as s in
i
mis sion
Sizes not a c ollisional
dis tribution
C reate fans and jets
C as sin
i Division
dens ity waves
E mbedded
moonlets
"P ropeller"
objects
Ring pollution
(from color)
A
B
C olor/s pectrum
varies in A
Shepherd moons
Self-gravity
wakes
months
millions of years
millions of years
107 - 1 08 years
108 - 1 09 years
106 - 1 07 years
Breakup: 1 07 years
M omentum: 107
years
days
OLD
YOUNG
RENEWED
OK
OK
OK
OK
OK
Low mass quickly ground to
dust
Low bulk density s hows
ac cretion
Steep size distribution from
recent disruption
NO
OK
NO
OK
E xpected more polluted than B
M eteoroid flux not s o high?
M ore massive?
Ring c omposition not
homogenized
NO
NO
OK
OK
NO
OK if
massive
OK
NO
OK
OK
OK
Breakup/momentum: N o
c ontradiction in ages !
Partic les c ontinually c ollide;
s elf-gravity and adhesion
enhance aggregation
OK
OK
OK
OK
OK
Table LWE 2. Inferred ages of various ring features and consistency with 3
models for ring formation. OK: Can be accommodated; NO: Serious
contradiction; Blank : Unclear, or deserves more study. Adopted from Esposito
2006, presented at Montana Rings Workshop.