Transcript Document

Wavelike Structure in
Saturn’s Rings
Benjamin Pollard
2/09/06
Overview
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Ring System
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History
Description
Satellites
Types of Structure
Voyager Occultation Experiments
Data and Analysis Method
Observed Ring Structure
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Density waves
Wakes
Irregular Structure
History
• Galileo – 1610
• Christian Huygens - 1655
• Jean Chapelain – 1660
• Jean-Dominique Cassini – 1660’s
• James Clerk Maxwell - 1857
Ring Particle Models
Saturn’s Ring System
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Approximately 340,000 km from edge to edge
100 meters thick
7 major rings
Less than 10 million years old?
Saturn’s Ring System
Cassini Division
C
B
Encke Gap
A
Cassini ISS
Satellites
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39 known satellites
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Diameter of 7 km to
5150 km
Responsible for some
ring structure
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Produce waves in the
rings
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Spiral density waves
Bending waves
Wakes
Shepherd F ring
Gaps and Ringlets
Wavelike Structure
Density Waves
 Bending Waves
 Wakes
 Irregular Structure
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Cassini ISS
Cassini ISS
Cassini UVIS
Voyager Occultation Experiments
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Voyager 1
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November 13, 1980
Radio Science Experiment (RSS)
Spacecraft occulted from earth
Maximum resolution of 0.2 km
(RSS)
Voyager 2
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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)
Methods
20 km
Spiral Density Waves
F (cy/km) Optical Depth
Optical Depth
0
0
1
1
2
2
F (cy/km)
Optical Depth
0
0
0
1
1
2
2
F (cy/km)
Log S
(σ cm4/s2)
Density Waves in the A ring
PPS
0
0
0
0
0
0
0
0
0
0
1
1
2
2
0
0
0
0
PPS
1
RSS X
1
RSS S
Calculated Surface Mass Densities From Spiral Density Waves
10
Surface Mass Density (g/cm2)
8
6
4
2
120
RANGE (km)
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Searched C ring and
Cassini Division for
possible wave
formations
Two promising
features appeared in
the Cassini Division
Frequency (cycles/km) Frequency (cycles/km)
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Optical Depth
Feature Search
Possible Density Wave
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1.
Frequency (cycles/km)
1.
.
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-.
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Range (km)
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
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Very high value for
Cassini Division
Prometheus 5:4 Surface Mass
Density Results
.
Frequency (cycles/km)
.
.
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.
12
RSS X Band
Mimas 4:1
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Located in the C ring
at 74892 km
16 km long wavetrain
Previously calculated
surface mass density
1.0 +- 0.1 g/cm2 and
1.1 +- 0.1 g/cm2
Our calculated
surface mass density
~5 g/cm2
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Optical Depth
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7
The Gap
1.
Frequency (cycles/km)
1.
.
-.
Range (km)
Wakes
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Caused by moonlets
embedded in rings
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Similar to boat traveling
through water
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Wake features can be
modeled based upon
position of moon
Pan
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Located in the Encke Gap
in Saturn’s A ring
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Approximately 10 km in
diameter
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Discovered in 1990 using
Voyager occultation data
Possible Wakes near 119950 km
Satellite located somewhere near 119950 km
Gap is about half the size of Encke Gap
Possible wakes visible in both RSS and PPS data
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2.
Optical Depth
.
1.
.
-.
11
Possible Wake?
Wake Fits
RSS inner fit θ = 295º
PPS inner fit θ = 247º
RSS outer fit θ = 65º
PPS outer fit θ = 113º
Irregular Structure
Sources of Irregular Structure
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Ballistic Transport from meteoritic collisions
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100 km wavelengths
C Ring and Inner B Ring
Durisen, R.H. An Instability in Planetary Rings Due to Ballistic Transport. Icarus 115, 66-85
(1995).
Ring Particle Assemblies
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Up to 50 - 100 km wavelengths
B Ring
Tremaine, S. On the Origin of Irregular Structure in Saturn’s Rings. The Astronomical
Journal 125, 894 – 901 (2003).
Inner
B
Ring
–
Large
Structure
PPS
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Irregular
structure visible
from 92500 –
104500 km
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Wavelengths
between 100
and 500 km
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Appears very
similar to
structure studies
previously using
an image
derived ring
profile
100 - 500 km wavelength
350 km window size
UVS
100 - 500 km wavelength
350 km window size
Image derived ring profile
350 km window size
Horn and Cuzzi
Inner B Ring Finer Irregular Structure
PPS
~50-100 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
C Ring 1000 km waves
RSS - Peaks removed
RSS -Original
~1000
~1000 km
km wavelength
wavelength
1500
1500 km
km window
window
~800-1200 km
wavelength
2500 km window
Ringlets in the C Ring
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6 Ringlets searched
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1 found to contain similar wavelike structure in both pps and rss datasets
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89190 – 89300 km
Ringlet between 89190 – 83000 km
RSS
~5-10 km wavelength
18km window
~5-7 km wavelength
12 km window
PPS
~3-5 km wavelength
21 km window
~3-5 km wavelength
27 km window
Observations
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A Ring
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B Ring
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Measured the surface mass density over 30
density waves
Large and fine irregular structure
C Ring
Large structure
 3-7 km wavelength in one ringlet
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Cassini Division
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Two unexplained features
Acknowledgements
Linda Spilker
 Stuart Pilorz
 Idaho Space Grant Consortium
 David Atkinson
 Mark Showalter
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Cassini ISS