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Some Peculiar Features In Saturn’s Rings
Quentin Donnellan
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“In the universe there are spheres and there are discs; given the correct mechanisms one can become the other”
Carolyn Porco
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Basics…
Cassini radio image> 5 cm
< 5 cmlarge
99.9% pure water ice3x1019 kg5-30 m thick
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Particle size
Roche limit
Particles over a certain size will be torn apart by tidal forces
Accretion discs inside the Roche limit should not coalesce beyond a certain size
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Dominant features
*Titan
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Dominant features
*Mimas
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Dominant Features
Pan Daphnis
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Moons carve out gaps/features
Rings are Keplerian…
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Some disturbances are quite elegant
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Prometheus
Pandora
F-Ring
Teamwork
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Some are not big enough…
Moonlets embedded in the rings create keplerian wakes
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Propellers
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Upon further inspection, it ain’t that simple…
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Density WavesIn the 1960’s astronomers theorized the origins of spiral arms in galaxies as artifacts of “density waves”
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Density Waves
“Traffic Jams” of material increase the local density of the galactic disc and increase the local rate of star formation
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Well, the rings around Saturn are essentially just a “disc”…
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Density Waves at Saturn
Analogous to spiral arms of galaxies, just much more tightly wound, caused by resonance excitation by moons
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Also bending…
2004, A ringDensity wave
bending wave
(Vertical undulations)
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This is advanced dynamics
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So we can explain all of the peculiarities in the rings by the tugging of moons…
… right?
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Viscosity
B ring: very dense
We would expect such a high viscosity to dampen any waves excited into this ring
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Viscosity
But the viscosity of the ring is such that the random motions of the particles in the ring actually amplify waves
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The rings, much like guitar strings, have natural modes which can become self-excited and amplified due to “viscous overstability”
http://www.youtube.com/watch?v=tA2eqTZijwI
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The equinox reveals more clues
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I know you’re there Janus
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Back to Daphnis
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Vertical Structures
1-3 km in height
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Back to the B-Ring
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Back to the propellers
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Complicated system…
So the rings are very complicatedWaves induced by moonsPaths cut out by moonsMaterial dragged up and down by moonsWaves induced by the rings themselves
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Extensions
Can we treat all disc structures similarly?