1. Planetary Rings

2. Rings are swarms of orbiting particles Orbits have to be very circular Elliptical orbits will result in collisions, destroying the ring

3. Rings Have to be Very Flat Inclined orbits will result in collisions and destruction of the ring

4. Rings have to be Equatorial Tilted orbits precess; the plane of the orbit rotates due to the planet’s equatorial bulge, other satellites, and the Sun’s gravity

5. A tilted ring won’t stay flat very long Particle orbits will precess at different rates Collisions will soon destroy the ring

6. Rings, Gaps, and Resonances When two objects return to the same relative positions regularly, they are said to be in resonance Some resonances are stable. Mercury’s 3:2 resonance between its rotation and its orbit is an example. Pluto’s period is 3/2 that of Neptune’s. This resonance keeps the two planets from ever colliding even though their orbits cross.

7. Rings, Gaps, and Resonances Some resonances create gaps. There are no asteroids with one-half Jupiter’s period because repeated pulls by Jupiter eventually would change the asteroid’s orbit. Gaps in Saturn’s rings are due to resonances with Saturn’s satellites.

8. Shepherd Moons Shepherd Moons help maintain the sharp edges of rings

9. A Shepherd Moon Outside a Ring Tends to Drag Stray Particles Back, Causing Them to Fall Back Into the Ring

10. Outer Shepherd Moon

16. A Shepherd Moon Inside a Ring Tends to Speed Stray Particles Up, Causing Them to Rise Back Into the Ring

17. Inner Shepherd Moon

23. Rings Are Probably Short-Lived Gravity of the Sun and planet’s satellites constantly disturbs orbits of particles Collisions probably happen frequently Particles should drift out of the ring Rings around outer planets have probably come and gone repeatedly during history of Solar System