Planetary Ring Systems

Rings: A B C

4/4 Giant Worlds Have Rings

• Jupiter: broad, dark, fine particles• Saturn: broad, bright, complex, icy particles• Uranus – narrow, dark, fine particles• Neptune: uneven, fine particles• All consist of independently orbiting small

chunks of material within very thin layers. Saturn’s rings span 100,000 miles, are only a few yards thick in places.

Why rings?

Tidal forces destroy a large solid moon insides a planet’s Roche limit. Ring systems are always found inside the Roche limit (about 1.44 planet diameters above the surface).

Collisions make rings the final configuration for swarms ofIndividual particles in orbit; they sap energy but not momentum.

Comet Shoemaker-Levy 9 following a 90-minute excursionwithin Jupiter’s Roche limit.

This is not a large gravitationally-bound moon

How do they stay there?

Random motions should make some particles leave therings and limit their lifetime. External effects can helpherd stragglers back. Main example: shepherd moons.

Pan and Prometheus shepherd Saturn’s outer thin F ring

Internal structures

Rings can be very thin. Radial structures can be producedby gravitational influences (such as perturbations and tidesfrom nearby moons). Example: Cassini division in Saturn’srings. Weaker disturbances and wave patterns can dividea ring into myriads of ringlets.

Wave patternsin Saturn’s rings(unprocessedCassini data,July 2004)

Arcs of enhancedparticle density inrings of Neptune(Liberte, Egalite,Fraternite; they werefound in 1989)

OriginsSome ring systems are intimately tied to small satellitesas sources of particles. But where did all that ice aroundSaturn come from?

Puzzles

• Radial spokes in Saturn’s rings• How long have rings been there? Are they

part of a perpetual juggling act?

Voyager 1 time-lapse movie (contrast enhanced) of ring spokes