Some Peculiar Features In Saturn’s Rings

Quentin Donnellan

“In the universe there are spheres and there are discs; given the correct mechanisms one can become the other”

Carolyn Porco

Basics…

Cassini radio image> 5 cm

< 5 cmlarge

99.9% pure water ice3x1019 kg5-30 m thick

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

Dominant features

*Titan

Dominant features

*Mimas

Dominant Features

Pan Daphnis

Moons carve out gaps/features

Rings are Keplerian…

Some disturbances are quite elegant

Prometheus

Pandora

F-Ring

Teamwork

Some are not big enough…

Moonlets embedded in the rings create keplerian wakes

Propellers

Upon further inspection, it ain’t that simple…

Density WavesIn the 1960’s astronomers theorized the origins of spiral arms in galaxies as artifacts of “density waves”

Density Waves

“Traffic Jams” of material increase the local density of the galactic disc and increase the local rate of star formation

Well, the rings around Saturn are essentially just a “disc”…

Density Waves at Saturn

Analogous to spiral arms of galaxies, just much more tightly wound, caused by resonance excitation by moons

Also bending…

2004, A ringDensity wave

bending wave

(Vertical undulations)

This is advanced dynamics

So we can explain all of the peculiarities in the rings by the tugging of moons…

… right?

Viscosity

B ring: very dense

We would expect such a high viscosity to dampen any waves excited into this ring

Viscosity

But the viscosity of the ring is such that the random motions of the particles in the ring actually amplify waves

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

The equinox reveals more clues

I know you’re there Janus

Back to Daphnis

Vertical Structures

1-3 km in height

Back to the B-Ring

Back to the propellers

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

Extensions

Can we treat all disc structures similarly?