Saturn’s Rings:
A Post-Equinox View
Larry W. Esposito
LASP
14 March 2012
Launched on October 15,
1997 from KSC
Cassini Mission to Saturn
7 Year cruise with VenusVenus-Earth-Jupiter Gravity
Assist trajectory
30 June / 1 July 2004
Saturn’s Rings
•
•
•
•
•
Billions and billions of small satellites
Each on its own orbit
Continually colliding
Beautiful
Resemble planet forming disks
Saturn’s rings were a 17-th century puzzle!
They are still today.
Are Saturn’s Rings Young or Old?
• Voyager found active processes and inferred
short 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?
• New Cassini observations show a range of
ages, some even shorter… and even more
massive rings!
Saturn Equinox 2009: when
the Sun set on the rings
• 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,
for example, F ring (French etal)
Sub-km structure seen that 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 Kittens
• UVIS occultations initially found 13
statistically significant features
• Interpreted as temporary clumps and a
possible moonlet, ‘Mittens’
• Meinke etal (2012) now catalog 27
features from the first 102 stellar
occultations
• For every feature, we have a location,
width, maximum optical depth (opacity),
nickname
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
34
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!
After the Equinox
• Saturn’s rings are made of nearly pure
ice and constantly changing
• Clumping and disaggregation occur in
perturbed regions
• Moonlets are indicated in all rings
• Clumps may form moons that renew the
rings: Saturn’s rings are both old and
young!