Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 36: Ringed Saturn

Key Ideas:

Saturn is the second largest planet: Gas giant planet much like Jupiter; 47+ moons and brighter rings
Titan: Largest of Saturn's Moons
Rings of Saturn: Orbiting chunks of ice and rock

Spacecraft to Saturn

Fly-bys:: Pioneer 11 (Sept 1979); Voyager 1 (Nov 1980); Voyager 2 (Aug 1981)
Cassini Orbiter:: Launched Oct 1997; Arrived July 2004; Huygens Titan probe, landed January 2005

Saturn vs. Jupiter

Saturn differs from Jupiter in many ways: Saturn is less dense (0.7 g/cc, less than water); More flattened at the poles (~10%) by rotation due to its lower density; Less metallic hydrogen due to lower overall interior pressure (lower mass); Less Helium (~1/4) than Jupiter, probably due to "rainout" in the deep interior

Atmosphere of Saturn

The atmosphere is divided into dark bands and bright zones like Jupiter's, but ...: Saturn is farther from the Sun, and colder; Makes the bands less chemically complex, and so more subtle and less colorful than Jupiter's
West-to-East winds are very strong: ~1800 km/hr, faster than on any other planet; Fewer and shorter-lived cyclonic storms; Occasional very powerful storms are seen

Moons of Saturn

Saturn has 47 moons: 1 giant moon (Titan) and 46 much smaller
Small moons:: Sizes: 20-1500 km; Those > 300 km are spherical; Those < 300 km are irregular; Density: 0.3 - 1.5 g/cc; Rock and ice, or mostly icy; All have heavily cratered, ancient surfaces
Irregular Moons include:: Epimetheus, Pandora, Prometheus
Large Moons include:: Titan (giant), Hyperion, Dione, Phoebe Rhea, Enceladus
Enceladus: Most reflective moon in the Solar System; Has a thin atmosphere fed by water vaopr heated by tides and welling up through surface cracks

Titan

Saturn's giant moon: Radius: 2575 km; Density: ~1.9 g/cc; Icy mantle over a rocky core
Only moon with a dense atmosphere: Cold enough to retain a heavy atmosphere

Rings of Saturn

Billions of tiny chunks of ice and rock: Few centimeters to many meters; Follow independent orbits in the same plane; Collisions keep the surfaces shiny so the rings are bright
Most elaborate of the Jovian ring systems:: Broad bands of bright, icy material; Broad gaps (Cassini Division, Encke Gap); Extend from 73,000 km to 140,000 km from the center of Saturn (1.2 - 2.3 R_Saturn)
The rings are very thin:: Thickness is <100 meters; Comparable to a sheet of paper 1mm thick and 10 km wide!

Cassini Division

2:1 resonance with Mimas: Mimas orbits every 22 hours; Iceballs in the Cassini Division orbit every 11 hours
Every two orbits:: Iceballs in the cassini Division get tugged toward Mimas; Eventually clears a persistent gap

Orbiting Iceballs

Rings are not solid: Billions of ice balls in independent orbits; Range in size from centimeters to 5 meters
Total mass ~10^-6 M_Earth: About the mass of a 100 km icy/rocky moon
As the iceballs collide:: Stick into larger ice balls; Chip off fragments or break into smaller chunks
Collisions keep the iceballs bright and shiny

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