Jovian Planet Moons and Rings Each planet has multiple moons and seem like “mini-solar systems” Moons are mostly made of ice, so quite soft and can be more active than similar sized rocky planets/moons Rings are made of individual small particles, orbiting like “micromoons”

RINGS: Saturn’s First Rings as Seen from Earth: Huygens: Galileo's bumps (1610), or ears (1616) were rings (1655). Main gap, about 2/3rd out, seen by Giovanni Cassini in 1675: Cassini’s division

Ring Names A ring is outer bright one (with Encke gap in it): between 122,000 and 137,000 km B ring (inside Cassini division) is brightest C ring, inside B, is much fainter (down to 75,000 km) Later, D, inside C; then F, G, E (all outside A) were discovered

What are they? The rings can't be solid bodies, nor could they be liquid or gas -- solids would be ripped apart, liquids or gases would dissipate Maxwell suggested many small bodies, each in Keplerian orbit (1857); different Doppler shifts confirmed hypothesis in 1895. Rings are very thin and let light through; collisions damp out motions perpendicular to plane so they are very narrow (<100 m thick!). High reflectivity implies ice balls, or ice on rocks.

The Roche Limit Most ring particles around 10 cm in diameter; lots of tinier ones and a small number of ones up to 10 m or so. ROCHE LIMIT is the radius at which tidal stresses rip a body apart --- no large moons can survive within about 2.8 RS So rings probably shredded moons or mashed captured asteroids, likely after collisions. Estimated lifetime under 107 yr “temporary”.

Rings as Seen from Voyager THOUSANDS OF RINGLETS in known rings Due to spiral density waves, produced by small moons in and near the rings Encke division due to ``SWEEPING UP'' by Pan, a 20 km diameter Moon

Cassini and many other divisions are due to RESONANCES with Moons (and Moonlets) These rhythmic gravitational tugs make it very difficult to stay in orbits whose periods are fractions of a Moon's period. Cassini division is 2:1 resonance with Mimas Even Cassini division not empty --- just many fewer particles in that region

Weird Rings Sharp outer edge of A ring is due to little moon Atlas, stuck in a 3:2 resonance with Mimas D ring, very thin, inside of C and down to cloud-tops  F ring, very narrow and braided, is held by SHEPARDING SATELLITES E ring, very far out, probably volcanic debris from Enceladus  Radial ``Spokes'' on B ring: tiny charged dust above ring plane, orbiting w/ Saturn's magnetic field

Last Word on Saturn’s Rings Moon repels ring particles as it gives and takes orbital energy from them; sheparding moons force narrow ring(lets)

JUPITER HAS RINGS TOO Roughly 50,000 km above cloud layers A few thousand km wide, most only tens of km thick Particles smaller and darker than those of Saturn So must be replenished, via collisions of asteroids with each other or J's moons.

ALL OUTER PLANETS HAVE RINGS URANUS has 11 sets of rings between 1.45 and 2.00 planetary radii Five densest discovered by stellar occultation in 1977 4 more seen from Earth, 2 more only by Voyager in 1986 Much narrower, darker, with wider gaps than Saturn's Kept in line by sheparding satellites NEPTUNE has 5 sets of rings between 1.65 and 2.54 planetary radii discovered by Voyager 2 in 1989 They are incomplete -- more arcs than rings Since ring systems are believed to be transitory, they must be crated fairly easily and often as we have found them around all 4 Jovian planets!

Stellar Occultation Technique

Rings of Uranus & Neptune Uranus; Occulted Neptune and Shepards (Cordelia, Opehlia) for Uranus’ epsilon ring

All Rings Lie Within Roche Limits