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 of Individual particles in orbit; they sap energy but not momentum.

Comet Shoemaker-Levy 9 following a 90-minute excursion within Jupiter’s Roche limit. This is not a large gravitationally- bound moon

How do they stay there? Random motions should make some particles leave the rings and limit their lifetime. External effects can help herd 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 produced by gravitational influences (such as perturbations and tides from nearby moons). Example: Cassini division in Saturn’s rings. Weaker disturbances and wave patterns can divide a ring into myriads of ringlets.

Wave patterns in Saturn’s rings (unprocessed Cassini data, July 2004)

Arcs of enhanced particle density in rings of Neptune (Liberte, Egalite, Fraternite; they were found in 1989)

Origins Some ring systems are intimately tied to small satellites as sources of particles. But where did all that ice around Saturn 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