Saturn’s Moon System Most extensive, complex moon system in the solar system. Over 40 known moons.

Most moons are covered with snow and ice. Some are probably made almost entirely of H 2 O ice.

Three natural groups exist: over 11 “small” moons, 6 “medium-sized” moons, 1 “large” moon

The one large moon is Titan km in diameter Titan is the second largest moon in the solar system, only Ganymede is larger.

Titan has a reddish color caused by an atmosphere!! Voyager I passed very close to Titan. (This cost Voyager I any more exploration).

A hazy smog layer obscures the surface.

Titan was explored by a lander as part of the Cassini mission.

The lander found lakes and rivers of hydrocarbons and also hydrocarbon rain.

Titan’s atmosphere is thicker and denser than Earth’s. The composition is N %, Ar-10%, CH 4 -1 to 2%.

Sunlight causes chemical reactions in the atmosphere, which might produce some of the colors seen.

Titan’s atmosphere contains ten times as much gas as Earth’s. It extends ten times as far out into space as Earth’s (due to lower gravity). The lower surface temperature (94K) favors the retention of the atmosphere.

The six medium- sized moons are all spherical, and are between 400 and 1500 km in diameter.

The inner five are all composed of largely rock and water ice. These five have circular, synchronous orbits inside Titan’s orbit. We discuss these in order from innermost to outermost.

Mimas - gravitational resonance affects the ring particles causing gaps. Mimas has an enormous crater on its leading face. This crater is named “Herschel” and has a diameter 1/3 of the moon’s diameter.

Enceladus - Similar to Mimas. There is evidence of large-scale volcanic activity. Many lava- covered impact craters (the lava is water). Volcanism on Enceladus replaces the particles in the E-ring of Saturn.

Tiger stripes

Tethys - heavily- cratered reflective surfaces. Tethys has a huge impact crater called Odysseus (2/5 the diameter of the moon).

Dione - The trailing face of Dione has prominent bright streaks.

Rhea - Saturn’s second largest moon. Heavily cratered with an albedo of 0.6. The water ice on its surface is very hard, so its craters look like the Moon’s.

Rhea has the wispy terrain. Light colored streaks on the trailing side of the moon.

Dione and Tethys

Iapetus - orbits far beyond Titan on an inclined, elliptic path. It has a huge black spot called the Cassini Regio.

Iapetus has a dark leading face with an albedo of 0.03 and a light trailing face with an albedo of 0.5.

Smaller Moons

Smaller moons: Some of the moons are “sweepers” that sweep out gaps in the rings.

Atlas (top) and Pan (bottom)

Prometheus and Pandora are the shepherd moons for the F-ring.

Prometheus

Pandora

PrometheusPrometheus

Prometheus

Beyond the F-ring lie the Co-orbital Satellites, Janus and Epimethius. These two moons share an orbit.

The inner moon takes approximately 4 years to “lap” the outer moon, and they exchange orbits when they meet.

Janus

Epimethius

The small moons, Telesto and Calypso have orbits that are synchronized with the moon Tethys. One orbits 60° ahead and the other orbits 60° behind Tethys. These locations are called Lagrange Points.

The moon Helene is similarly synchronized with Dione.

The two outermost moons are Hyperion and Phoebe. Phoebe is the only moon of Saturn with a retrograde orbit. It is possible that it is a captured moon.

Hyperion’s rotation is not synchronous with its orbit. This is because Hyperion’s orbit is not circular due to the gravitational pull of Titan in addition to Saturn.

The combination of these two gravitational pulls causes Hyperion to change its rotation speed and its rotation axis. This is called chaotic rotation.

Mimas Enceladus Tethys Dione Rhea Titan Hyperion Iapetus Phoebe