Cassini UVIS results from
Saturn, Titan, icy satellites and
rings
LW Esposito and the UVIS team
23 May 2005
American Geophysical Union
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
Rings Summary
• A Ring has cleanest water ice signature: less contaminants than other
rings, particularly C Ring and Cassini Division.
• Density waves galore:
– Dispersion of waves gives ring surface mass density:
(Cassini Division) << (A Ring).
– New waves seen in Cassini Division and new second order waves
observed.
• Large particle or clump size distributions from occultation statistics:
– Largest particles or clumps (~ 10 m) in A ring, increasing outward to
Encke Gap, then decreasing.
– No significant number of large particles or clumps in C ring, Cassini
Division.
• Correlation between surface mass density and largest particle sizes and
(to a lesser extent) ring ice purity.
• Abrupt density transitions observed (r<50 m): particle “traffic jam” at
perturbed ring edges.
• Unexplained features observed at high resolution.
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.
0
-1
ln(I/I0)
h= 868.4 km
-2
 Sco data rec 41
Model synthesis
N2 = 1.43 x 1019
CH4 = 1.90 x 1017
C2H2= 1.1x1016
-3
C2H4=8.2x1015
C2H6=7.3x1015
-4
HCN=1.90x1016
C4H2=1.3x1015
-5
110
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160
 (nm)
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Species found in Titan Tb occ
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CH4 methane
C2H2 acetylene
C2H4 ethylene
C2H6 ethane
C4H2 diacetylene
HCN hydrogen cyanide
2000
CH4 UVIS
C2H4 UVIS
C2H2 UVIS
C2H6 UVIS
HCN UVIS
C4H2 UVIS
Yelle TA model
Vervack V2 CH4
lsco CH4 model
1800
1600
h (km)
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1200
1000
800
600
400
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13
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Log(n
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UVIS CH4 T
UVIS CH4 H
Lindal et al ingress T
Lindal et al egress T
Yelle et al recommended T
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h (km)
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0
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TCH (K) : HCH (km)
4
4
140
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180
T-3 Titan FUV Observation
February 15, 2005
Start Time: 10:26:52
End Time: 11:34:52
Lyman Alpha Image: 1146 –1271 Å
LBH Image:
1286 – 1427 Å
Solar Image:
1661 – 1913 Å
Titan Results
• Differences with INMS show horizontal
variations at 1200 km
• Evidence for diffusive separation at 900 km
• Mesopause temperature 113K at 615km
• Heavier hydrocarbons peak at 600 -750 km
• Discontinuous aurora and haze layers seen
UVIS H2 band data on Saturn
Saturn in atomic and molecular
hydrogen emission
What is the source of the dark
material on Iapetus?
• Does Phoebe material contaminate the LH
of Iapetus?
– At visible wavelengths, Iapetus’s spectrum is
redder than Phoebe’s
– What do the UV data contribute to this
mystery?
Iapetus
Iapetus compared with Phoebe
Iapetus: Light/Dark Ratio
Looks a lot like H2O or CO2!
Iapetus reflectance: dark and light terrains
Iapetus dark material is dark because of very little frost (H2O and
CO2), NOT because of a lot of dark material
Summary:
Iapetus vs. Phoebe
• What can we say about dark material on Iapetus?
– It does not look like Phoebe
• Phoebe material is too H2O-rich to match Iapetus dark
spectrum, although a good match for bright material!
– We need very little H2O to model Iapetus dark material
• Though if Phoebe material (or any other material) coats
Iapetus, H2O might be lost in the impact process
• So our results do not discriminate between endogenic and
exogenic models for Iapetus dark material
– Loss of volatiles might be associated with either process
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
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
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
Ring Plane Radius (km)
Ring Plane Radius (km)
Ring Plane Radius (km)
Bending Waves in Saturn’s Rings
5:3 BW
5:3 DW
Density Waves
Period (km)
Wavelet Power Spectrum Estimation for Wave Dispersion
Provides local surface mass density.
Ring Plane Radius (km)
Titan
1:01:0
Ringlet
Titan
Inner Inner
Edge Edge
7.2 m res.
29 m resolution
Sharp Edges in the Rings
Encke Gap
Pan Wakes
Density Waves
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
Rings Summary
• A Ring has cleanest water ice signature: less contaminants than other
rings, particularly C Ring and Cassini Division.
• Density waves galore:
– Dispersion of waves gives ring surface mass density:
(Cassini Division) << (A Ring).
• Large particle or clump size distributions from occultation statistics:
– Largest particles or clumps (~ 10 m) in A ring, increasing outward to
Encke Gap, then decreasing.
– No significant number of large particles or clumps in C ring, Cassini
Division.
• Correlation between surface mass density and largest particle sizes and
(to a lesser extent) ring ice purity.
• Abrupt density transitions observed (r<50 m): particle “traffic jam” at
perturbed ring edges.