Saturn: Moons
Saturn, the sixth planet from the Sun, is home to a vast array of intriguing and unique worlds. From the cloud-shrouded surface of Titan to crater-riddled Phoebe, each of Saturn's moons tells another piece of the story surrounding the Saturn system.
Christiaan Huygens discovered the first known moon of Saturn. The year was 1655 and the moon is Titan. Jean-Dominique Cassini made the next four discoveries: Iapetus (1671), Rhea (1672), Dione (1684), and Tethys (1684). Mimas and Enceladus were both discovered by William Herschel in 1789. The next two discoveries came at intervals of 50 or more years - Hyperion (1848) and Phoebe (1898).
As telescopic resolving power increased through the 19th century, Saturn's family of known moons grew. In 1966 Epimetheus and Janus were discovered. By the time Cassini-Huygens was launched in 1997, Saturn's moon count had reached 18. The number of known moons soon increased with high-resolution imaging techniques used on ground-based telescopes. Cassini discovered four more moons after its arrival at Saturn and may find even more during its mission.
We've discovered a total of 52 natural satellites orbiting Saturn. Each of Saturn's moons bears a unique story. Two of the moons orbit within gaps in the main rings. Some, such as Prometheus and Pandora, interact with ring material, shepherding the ring in its orbit. Some small moons are trapped in the same orbits as Tethys or Dione. Janus and Epimetheus occasionally pass close to each other, causing them to periodically exchange orbits. Here's a sampling of some of the unique aspects of the moons:
- Titan Titan is so large that it affects the orbits of other near-by moons. At 5,150 kilometers (3,200 miles) across, it is the second largest moon in the solar system. Titan hides its surface with a thick nitrogen-rich atmosphere. Titan's atmosphere is similar to Earth's atmosphere of long ago, before biology took hold on our home planet. Titan's atmosphere is approximately 95% nitrogen with traces of methane. While Earth's atmosphere extends about 60 kilometers (37 miles) into space, Titan's extends nearly 600 kilometers (ten times that of Earth's atmosphere) into space.
- Iapetus has one side as bright as snow and one side as dark as black velvet, with a huge ridge running around most of its dark-side equator.
- Phoebe orbits the planet in a direction opposite that of Saturn's larger moons, as do several of the recently discovered moons.
- Mimas has an enormous crater on one side, the result of an impact that nearly split the moon apart.
- Enceladus displays evidence of active ice volcanism: Cassini observed warm fractures where evaporating ice evidently escapes and forms a huge cloud of water vapor over the south pole.
- Hyperion has an odd flattened shape and rotates chaotically, probably due to a recent collision.
- Pan orbits within the main rings and helps sweep materials out of a narrow space known as the Encke Gap.
- Tethys has a huge rift zone called Ithaca Chasma that runs nearly three-quarters of the way around the moon.
- Four moons orbit in stable places around Saturn called Lagrangian points. These places lie 60 degrees ahead of or behind a larger moon and in the same orbit. Telesto and Calypso occupy the two Lagrangian points of Tethys in its orbit; Helene and Polydeuces occupy the corresponding Lagrangian points of Dione.
- Sixteen of Saturn's moons keep the same face toward the planet as they orbit. Called 'tidal locking,' this is the same phenomenon that keeps our Moon always facing toward Earth. The Cassini spacecraft will fly past Titan 45 times during its four-year primary mission. In addition, Cassini will gather data about many of the other satellites in an effort to fully understand the nature, formation, and dynamics of Saturn's moons.
References:
USGS Astrogeology: Gazetteer of Planetary Nomenclature Planetary Body Names and Discoverers
Solar System Dynamics - Planetary Satellite Discovery Circumstances
