# Why are rings thin?

 Saturn's rings are 200,000 km wide but a few tens of meters thick -- "as thin as a sheet of tissue paper spread across a football field."  (Cassini views Dione through Saturn's edge-on rings) How do they stay thin?

The particles experience (partially inelastic) collisions, which damp out their vertical motions:

 These collisions keep rings thin, and also moving in circular orbits. The velocity dispersion in Saturn's rings is probably 0.1 cm/s or less (one ten-millionth of their orbital velocity!).

Jupiter's rings are much lower in particle density -- would you expect them to be thinner or thicker than Saturn's rings?

What causes gaps and grooves in rings?

 Orbital resonances with moons. For example, the Cassini division in Saturn's rings corresponds to a 2:1 resonance with Mimas. A ring particle there will be forced into an elongated orbit, where it will collide with particles at other radii, and have its orbit circularized there. Other gaps correspond to resonances with other moons. But they aren't empty gaps -- there are just many fewer ring particles. Why aren't the gaps completely empty? Many more grooves exist than can be accounted for by simple resonances with moons -- the situation is very complex, and not fully understood.

 Saturn's moon Pan,orbiting in the Encke division. What's in store for Pan?

## Fun things to think about

What causes narrow rings?

Notice the very thin outer ring - how did it get that way? Something must be keeping the ring "in line": shepherd moons.

Shepherd moons orbit just inside or just outside of the ring in question.

• A ring particle orbiting just inside a shepherd moon catches up and passes the moon. It is first sped up, then slowed down by the moon, but it spends more time being slowed than being sped up. A net loss of energy occurs and the ring particle moves inwards.
• A particle orbiting just outside a shepherd moon lags the moon and is pulled forward, receiving energy and moving outwards.
The net result is that a pair of shepherd moons can entrain a ring.

The gravitational effects of the shepherd moons can also result in interesting "kinks" and "braids" in the rings.

What causes ring spokes?

Voyager found dark spokes in Saturn's rings. Because the ring particles orbit differentially, spokes should quickly shear out and not exist for very long.  So they can't be masses of darker ring particles -- what are they? (Ring spokes in action)

Spokes move at the planet's rotation rate (much faster than the orbital speed) -- charged dust particles suspended by Saturn's magnetic field! The dust particles cast shadows on the ring surfaces -- the spokes.