1. Wavelike Structure in Saturn’s Rings
Benjamin Pollard
2/09/06

2. Overview Ring System Voyager Occultation Experiments
History
Description
Satellites
Types of Structure
Voyager Occultation Experiments
Data and Analysis Method
Observed Ring Structure
Density waves
Wakes
Irregular Structure

3. History Galileo – 1610 Christian Huygens - 1655 Jean Chapelain – 1660
Jean-Dominique Cassini – 1660’s
James Clerk Maxwell
Galileo first glimpsed in Saturn in July 1610
- Thought it was a triple planet, but largely uninterested
- Looked again in 1612 – Now a ball
- Looked once again 1616 – Now it had handles
Christian Huygens used a far higher quality telescope and discovered Titan. He theorized the ‘handles’ were actually a disk or ring
Jean Chapelain suggested the rings were tiny satellites.
Cassini noticed the gap between the B and A rings. Suggested there were two solid rings instead of one.
James Clerk Maxwell correctly theorized the rings were composed of many particles and explained why it was possible.
Various others were involved in proving Maxwell’s theory correct and advancing our knowledge of Saturn

4. Ring Particle Models

5. Saturn’s Ring System Approximately 340,000 km from edge to edge
100 meters thick
7 major rings
Less than 10 million years old?
The distance between the earth and the moon is 384, 000 km
km to the start of the E ring
km width of E ring
-The particles are similar in composition to the icy moons around the planet
-Nearly pure ice
-Most range between the size of micrometer (millionth) to a meter
-The different colors depend upon their contaminates
Picture is a computer model of a 3 meter square
-The method of sorting of ring material is unknown
Rings in order:
D – C – B – A – F – G - E

6. Saturn’s Ring System C A B
Encke Gap
Cassini Division
Cassini ISS

7. Satellites 39 known satellites Responsible for some ring structure
Diameter of 7 km to 5150 km
Responsible for some ring structure
Produce waves in the rings
Spiral density waves
Bending waves
Wakes
Shepherd F ring
Gaps and Ringlets
Pan 10 km
Titan 5150 km
We see ring structure at every resolution
Prometheus and Pandora shepherd the F ring
Southernmost - Rhea
Southeast – Dione

8. Wavelike Structure Density Waves Bending Waves Wakes
Irregular Structure
Cassini ISS
3 wave types are associated with periodic gravitation forcing.
We have studied density waves and bending waves in the past.
Around 99% of the structure in the rings is irregular structure.
Could be mulitiple causes
UVIS false color image
Left Density Wave - Janus
Right density wave – Pandora
New Moon in Keeler Gap – 7km wide
Cassini ISS
Cassini UVIS

9. Voyager Occultation Experiments
November 13, 1980
Radio Science Experiment (RSS)
Spacecraft occulted from earth
Maximum resolution of 0.2 km (RSS)
Voyager 2
August 25, 1981
Photopolarimeter Experiment (PPS)
Ultraviolet Spectrometer Experiment (UVS)
The star δ Scorpii occulted
Maximum resolution of 0.1 km (PPS) and 3.5 km (UVS)
How do we know all of this
Radio Science 3.6 cm and 13 cm wavelength
PPS
-every 10 msec sample recorded
-6 inch Schmidt-Cassegrain Telescope
-2650 Angstrom
Visible light at ~4000 to 8000 angstroms
hundred-millionth of a cm
Each set has data number and optical depth in relation to the radius of Saturn
Voyager 2 video clip
Voyager 1 and 2 were not the first shuttles to pass by Saturn, Pioneer 11 flew by in 1979

10. Methods
20 km

11. Spiral Density Waves

12. Density Waves in the A ring
(σ cm4/s2)
Log S
PPS
PPS
Optical Depth 1 1 2 2
F (cy/km) 1
RSS X
Optical Depth 1 1 2 2
F (cy/km) 1
RSS S
Optical Depth 1 1 2 2
F (cy/km) 1

13. Calculated Surface Mass Densities From Spiral Density Waves10 8 6
Surface Mass Density (g/cm2) 4
It looks quite scattered at first glance, but look at the squares and triangles. They are the x band .2 and .5 km surface mass density values.
Rosen did not calculate the first two
Most of our calculated RSS values fall very near Rosen’s
Two values really stick out 2
120
RANGE (km)

14. Feature Search
Searched C ring and Cassini Division for possible wave formations
Two promising features appeared in the Cassini Division
Optical Depth
Frequency (cycles/km)
Looked closely around gaps for wakes and near resonance points for density waves
Most objects appeared only in one data set and then searched for in the other data sets.
Frequency (cycles/km)

15. Possible Density Wave
Prometheus 5:4 located in the Cassini Division at 120,302 km
Surface mass density measured in all three data sets
Surface mass density of ~30 g/cm2
Very high value for Cassini Division 1. 1.
Frequency (cycles/km) .
Name all three data sets
Most prominent in the RSS X band
Surface mass density is especially high
Comparable to A ring if it was really the Prometheus 5:4 density wave -.
Range (km)

16. Prometheus 5:4 Surface Mass Density Results.
RSS X Band . .
Frequency (cycles/km) . . 12

17. Mimas 4:1 Located in the C ring at 74892 km 16 km long wavetrain
Previously calculated surface mass density g/cm2 and g/cm2
Our calculated surface mass density
~5 g/cm2 . . .
Optical Depth . .
Why did we measure?
Optically similar to the Cassini Division 7

18. The Gap 1. 1.
Frequency (cycles/km) . -.
Range (km)

19. Wakes Caused by moonlets embedded in rings
Similar to boat traveling through water
Wake features can be modeled based upon position of moon
Keeler Gap
250 kilometers wide
Outside of the A Ring
Wake can be modeled based upon the position of the moon
Position of moon -> the radial location of the moon and the how long ago the moon passed by (it’s longitude)
Moonlet clears out gap in the ring

20. Pan Located in the Encke Gap in Saturn’s A ring
Approximately 10 km in diameter
Discovered in 1990 using Voyager occultation data
Discovered by Mark Showalter
Created a computer program to search through Voyager images

21. Possible Wakes near km
Satellite located somewhere near km
Gap is about half the size of Encke Gap
Possible wakes visible in both RSS and PPS data 2. . 1.
Optical Depth .
- . 11

22. Possible Wake?

23. Wake Fits RSS inner fit θ = 295º RSS outer fit θ = 65º
PPS inner fit θ = 247º
PPS outer fit θ = 113º

24. Irregular Structure

25. Sources of Irregular Structure
Ballistic Transport from meteoritic collisions
100 km wavelengths
C Ring and Inner B Ring
Durisen, R.H. An Instability in Planetary Rings Due to Ballistic Transport. Icarus 115, (1995).
Ring Particle Assemblies
Up to km wavelengths
B Ring
Tremaine, S. On the Origin of Irregular Structure in Saturn’s Rings. The Astronomical Journal 125, 894 – 901 (2003).

26. Inner B Ring – Large Structure
PPS
Irregular structure visible from – km
Wavelengths between 100 and 500 km
Appears very similar to structure studies previously using an image derived ring profile
km wavelength
350 km window size
UVS
km wavelength
350 km window size
UVS values ->10^3 minimum to 10^4 minimum
PPS -> 10^4 to 10^5 minlevel
Image derived ring profile
350 km window size
Horn and Cuzzi

27. Inner B Ring Finer Irregular Structure
PPS
~ km wavelength
105 km window
~100 km wavelength
210 km window
UVS
~50 – 100 km wavelength
105 km window
~100 km wavelength
210 km window

28. C Ring 1000 km waves RSS - Peaks removed RSS -Original
~ km wavelength
2500 km window
RSS - Peaks removed
RSS -Original
~1000 km wavelength
1500 km window
~1000 km wavelength
1500 km window

29. Ringlets in the C Ring 6 Ringlets searched
1 found to contain similar wavelike structure in both pps and rss datasets
89190 – km
Growth distance of some less than 50 meters

30. Ringlet between 89190 – 83000 km RSS PPS ~5-10 km wavelength
18km window
~5-7 km wavelength
12 km window
PPS
~3-5 km wavelength
27 km window
21 km window
Ringlet between – km .5 .2

31. Observations A Ring B Ring C Ring Cassini Division
Measured the surface mass density over 30 density waves
B Ring
Large and fine irregular structure
C Ring
Large structure
3-7 km wavelength in one ringlet
Cassini Division
Two unexplained features

32. Acknowledgements Linda Spilker Stuart Pilorz
Idaho Space Grant Consortium
David Atkinson
Mark Showalter
Cassini ISS