Cassini UVIS Observations of
Saturn’s Rings
Josh Colwell, UVIS Co-Investigator
Larry Esposito (UVIS P.I.),
the UVIS Team, and
Glen Stewart, Heather Tollerud, Jeff
Cuzzi (Rings and Dust IDS)
Saturn’s Rings:
Age and origin unknown
Cassini ISS image:
SSI (Boulder),
NASA/JPL.
Cassini Division
Approach picture from
Cassini:
F
May 10, 2004
Dist: 27 million km.
Pixel: 161 km.
C
B
A
Moon: Prometheus
Cassini ISS image:
Space Science
Institute (Boulder),
NASA/JPL.
Encke Gap
W~350 km
3 Types of Ring Observation with UVIS
•Spectra and images from 550 -1900A.
•Stellar occultations with spatial resolution of 10 m.
•Meteoroid impact detection.
“Spokes” Observed by Voyager
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
But not by Cassini.
SSI, NASA, JPL.
It may not be spoke season yet due to the photoelectron layer over the rings.
QuickTime™ and a
DV/DVCPRO - NTSC decompressor
are needed to see this picture.
Detecting (spoke-forming) meteoroid impacts on the rings:
28 readings of 6
2 readings of 7
3.18 million samples
Mean = 0.454
SQRT(mean) = 0.674
Std. Dev. = 0.676
Mean = .526
Std. Dev. = .721
Root Mean = .725
2 readings of 8
10 readings of 7
4.98 million readings
Poisson Statistics

e 
P(x) 
x!
UVIS_00CRI_IMPACT001:
Mean = 0.454
SQRT(mean) = 0.674
Std. Dev. = 0.676

UVIS_00CRI_IMPACT002:
Mean = 0.526
SQRT(mean) = 0.725
Std. Dev. = 0.721
x
Expectation values:
m(6) = P(6)xN = 24.6
m(7) = P(7)xN = 1.6
m(8) = 0.09
Observed values:
28 readings of 6
2 readings of 7
0 readings of 8
Expectation values:
m(7) = P(7)xN = 6.5
m(8) = 0.43
Observed values:
10 readings of 7
2 readings of 8
So far, no smoking gun (or ring particle)
•Distance to Rings:
6.3 million km.
•Radial Coverage:
–Inner C ring
–Outer B ring (mostly
opaque to this star)
–Cassini Division
–A ring (to 135,500
km).
•Sampling Interval:
–0.88 to 1.0 km/s.
–8 msec sampling.
–7-8 m samples.
–~30 m Fresnel zone
radius ( d ).

Stellar Occultation Raw Data
Density Waves in Saturn’s Rings
– Separation of azimuthal (Ω), radial (), and vertical
() orbital frequencies around an oblate planet.
(RL )  m(RL )  mM  nM  pM
m, n, p are integers, and M refers to the moon, and RL is the resonance
location.
Strongest horizontal forcing when n=p=0:
(RL )
m

M
m 1

- Packing density of ring particles varies with the gravity of the ring
propagating the wave.
- Propagation of wave gives ring surface mass density.

m = 2 streamlines
m = 2 streamlines
affected by perturber
and self-gravity
More on resonances
(RL )  m(RL )  mM  nM  pM
Suppose m=1, n=p=0. Then:
M  (RL )  (RL )
In other words, the azimuthal motion of the moon is equal to the apsidal
precession rate of ring particles at RL.
Suppose m=2, n=p=0. Then:

2M  2(RL )  (RL )  (RL )
Which is therefore called a 2:1 resonance.
Now suppose m=3, n=0, p=1, then:

3M  M  3(RL )  (RL )  2(RL )
Which is therefore called a 4:2 resonance. 4:2 resonance ≠ 2:1 resonance.
Cassini ISS image:
SSI (Boulder),
NASA/JPL.
Ring Plane Radius (km)
Ring Plane Radius (km)
Ring Plane Radius (km)
Bending Waves in Saturn’s Rings
(RV )  m(RV )  mM  nM  pM
Strongest vertical forcing when n=1, p=0:
m(RV )  (RV )  mM  M
- Vertical corrugation or warping of the ring.


5:3 BW
5:3 DW
Cassini ISS image:
SSI (Boulder),
NASA/JPL.
Density Waves
Wave dispersion relation:
(  m)2   2  2G k  0
Dispersion of wave spreads power over many frequencies
Period (km)
Wavelet Power Spectrum Estimation for Wave Dispersion
Provides local surface mass density.
Ring Plane Radius (km)
New Density Waves
Atlas 5:4 in Cassini Division shows  << (A Ring)
Atlas 5:4
Measuring Particle Size or Clumps from Occultation Statistics
Particles << sample size.
Particles ~ sample size.
Observed  = Poisson 
Observed  > Poisson 
Region of ring observed in one sample.
Titan
1:01:0
Ringlet
Titan
Inner Inner
Edge Edge
7.2 m res.
29 m resolution
Same Edge at Higher Resolution
Sharp Edges in the Rings
What Does It All Mean?
• No clear signal of meteoroid impacts yet:
– Detection may not work as expected;
– May be fewer meteoroids than expected.
• Ring edges unresolved at 30m resolution:
– Particle traffic jams;
– Direct observations of large clumps or
particles?
• Composition, particle size, surface mass
density  recent resurfacing or creation of
some ring regions.
• And certainly much more…
Future
• 100’s of hours of impact observations ahead.
• 100’s of hours of ring UV images ahead.
• 6 more stellar occs between May and
September.
• >60 stellar occs by the end of the nominal
mission on June 30, 2008.
• Two-year extended mission anticipated.
• Best views of rings still to come.
• Stay tuned…
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.