Cassini UV Imaging Spectrograph
Observations Show Active Saturn
Rings
Larry W. Esposito
Joshua E. Colwell
LASP, University of Colorado
16 December 2004
Cassini UV Imaging
Spectrograph
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Spectra and images from 550 -1900A
Hydrogen-Deuterium cell measures D/H
High speed photometer has 20m resolution
Chemistry of Saturn, Titan clouds
Exospheres of moons
Saturn’s magnetosphere neutrals; thermosphere
airglow and aurora
• Ring origin and evolution
Implications of UVIS
observations
• The rise and fall in abundance of OI between Dec
25, 2003 and May 13, 2004 amounts to ~500 Mkg of
mass apparently lost from the system in this
period. Mean inferred loss rate of bulge in mass
during 2 months is ~4 X1027 atoms s-1.
• Total mass of OI + OH in system is ~2200. Mkg.
• The estimated micron sized particles in the E-ring
involved in the Mimas – Tethys region is 600 Mkg.
UVIS RESULTS FOR
MAIN RINGS
• We summed all SOI spectra from same distance to
produce radial profile and color rendition
• UV spectra show water abundance increasing
outward to a peak in outer A ring
• Rings A and B more icy than ring C and Cassini
Division, consistent with VIMS
• More structure than seen in Voyager and HST
images of A ring
A Ring Particle Properties
RING HISTORY
• Large scale variation consistent with
meteoritic pollution of initially pure ice
• Variations over scales of 1000 - 3000 km
too narrow to be explained by ballistic
transport over the age of rings
• One or more 20-km moonlets shattered in
last 10 -100 million years could do it
• Same future result from disrupting Pan
A Ring Particle Properties
YOUTHFUL RINGS: DESTRUCTIVE
PROCESSES ACT QUICKLY
• Grinding and sputtering
• Spreading and momentum transfer to small
moons
• Darkening from meteoroid bombardment
• Ring ages: 107 to 109 years
VOYAGER, GALILEO AND CASSINI
SHOW CLEAR RING - MOON
CONNECTIONS
• Rings and moons are inter-mixed
• Moons sculpt, sweep up, and release ring
material
• Moons are the parent bodies for new rings
COLLISIONAL CASCADE FROM MOONS
TO RINGS
• Big moons are the source for small moons
• Small moons are the source of rings
• Largest fragments shepherd the ring
particles
• Rings and moons spread together, linked by
resonances
• Small moons caught in resonances with
larger moons: this slows linked evolution
COLLISIONAL CASCADE
MARKOV MODEL FOR THE
COLLISIONAL CASCADE
• Improve by considering recycling
• Collective effects: nearby moons can
shepherd and recapture fragments
• Accretion in the Roche zone is possible if
mass ratio large enough (Canup & Esposito
1995)
MODEL PARAMETERS
• n steps in cascade, from moons to dust to
gone…
• With probability p, move to next step
(disruption)
• With probability q, return to start (sweep up
by another moon)
• p + q = 1.
LIFETIMES
• This is an absorbing chain, with transient
states, j= 1, …, n-1
• We have one absorbing state, j=n
• We calculate the ring/moon lifetime as the
mean time to absorption, starting from state
j=1
EXPECTATION VALUES
Lifetimes (steps):
E1=(1-pn)/(pnq)
~n, for nq << 1
(linear)
~n2, for nq ~ 1
(like diffusion)
~2n+1-2, for p=q=1/2
~p-n, as q goes to 1 (indefinitely long)
EXAMPLE: F RING
• After parent body disruption, F ring reaches steady state
where accretion and knockoff balance (Barbara and
Esposito 2002)
• The ring material not re-collected is equivalent to ~6km
moon; about 50 parent bodies coexist…
• Exponential decay would say half would be gone in 300
my.
• Considering re-accretion, loss of parents is linear: as
smaller particles ground down, they are replaced from
parent bodies. The ring lifetime is indefinitely extended
MARKOV MODEL CONCLUSIONS
• Although individual rings and moons are
ephemeral, ring/moon systems persist
• Ring systems go through a long quasi-static
stage where their optical depth and number
of parent bodies slowly declines
• Below some threshold, recycling declines
and the rings are rapidly lost
CAUTIONS
• Some rings are too close for much
recycling: Uranus and Neptune rings may
require flatter strength distribution (Colwell
et al)
• Momentum transfers and moon radial
evolution still a problem: do chaotic
interactions and linkup to larger moons
solve this? (Goldreich and Rappaport)
How big a moonlet, when?
To estima te the time since such a reset even t, we estimate the rate at which diffusion
spreads the material. Using the kin ematic vis cosit y  ~ 280 cm2/sec, mass extinction
coeffi cient  ~ 0.013 cm2/gm (30, 31) and the estim ate of the fraction of regol it h lost in a
colli sion f ~ 0.1 (32) we calculate an effective dif fusion coe ffi cient for interchang ing
rego lit h material in the A ring
DC ~ f *  ~ 30 cm2/sec.
Dim ension al scali ng g ives an e stim ate of the time f or such a renewa l even t to spread
r = 1000 k m as
T = r2 / D C ~ 107 yr, sure ly a lower estim ate since f is unc ertain.
The mass needed to cover 10% o f the surface area of aver age optical depth  = 0.5, ove r
an annu lus r at saturnocen tric dist ance r = 130,000 km is
M = 0.1 * 2r r  /  ~ 3 x 1019 gm.
This mass is equivalent to a moon wit h radius R ~ 20 km.
UVIS data are consistent with
active ring/moon recycling
• Oxygen fluctuations require replacement of
E ring grains from parent bodies
• Radial spectral variations in A ring require
multiple reservoirs and recent release of
purer ice
• 20 km embedded moonlets have right
lifetimes and mass, given estimated
diffusion rates