Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 39: Planetary Rings

Key Ideas:

All Jovian planets have rings: Jupiter: faint, dusty rings; Saturn: bright, spectacular rings; Uranus: dark, thin rings; Neptune: dark, thin rings and ring arcs
Ring Properties: Composed of dusty particles and ice balls; Often confined by Shepherd Moons via orbital resonances
Roche (tidal) radius

Jovian Ring Systems

Jupiter has faint, dusty rings: Discovered in 1979 by Voyager 1 and 2; Can be seen from Earth using infrared images
Saturn's rings are bright and broad: Discovered in 1659 by Christiaan Huygens
Uranus and Neptune have thin, dark rings: Discovered in 1977 and 1985 by watching stars disappear behind them (stellar "occultation")

Jupiter's Dusty Rings

Jupiter has faint, dusty rings:: Made of micron-sized dark dust particles; Total mass is < 10^-12 Earth masses
Ring material may have been knocked off the moons of Jupiter or captured from comets

Saturn's Rings

Most spectacular in the Solar System:: Broad bands of bright, icy material; Broad gaps (Cassini Division and Encke Gap); Extend from 73,000 km to 140,000 km from the center of Saturn (1.2 to 2.3 times Saturn's radius)
The rings are extremely thin:: Thickness is < 100 meters; Equivalent to a sheet of paper 1 mm thick and 10 km wide!

Saturn's Ring Composition

Rings are not solid:: Billions of ice balls in independent orbits; Range in size from centimeters to 5 meters; Total mass about 10^-6 the mass of the Earth
As the iceballs collide: Stick into larger ice balls; Chip off fragments or break into smaller chunks
Collisions keep the iceballs bright and shiny

Thin Rings of Uranus

Dark Rings of Neptune

Faint, very dark rings: Only a few km wide; Best seen backlit by the Sun; Ring "arcs" seen in the outermost ring

Shepherd Moons

Cassini Division

Caused by a 2:1 resonance with Mimas: Mimas orbits every 22 hours; Iceballs in the Cassini Division orbit every 11 hours
Every 2 orbits:: Iceballs in the Casssini Division get tugged towards Mimas; This eventually cleared a persistent gap

Origin of the Rings

Rings are not very massive: Jupiter's, Uranus', and Neptune's rings are the mass of an icy moon ~10 km across; Saturn's rings have the mass of a mid-sized icy moon ~200 km across
Possible origins of the ring material: An icy moon that got too close to the planet and was disrupted by tidal forces; A moon that could not form initially because of tidal forces

Roche Radius

Distance where tidal forces on a moon equal the gravitational force holding it together: If a moon gets closer than the Roche radius of its parent planet, it will be pulled apart by tides; Icy particles orbiting inside the Roche radius of a planet cannot aggregate into a moonlet
All rings are located inside the Roche Radii of their parent planets

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.