Saturn is best known for its fabulous ring system, but it also has an amazing moon system. Here are ten facts to introduce this marvel to you.
1. Cassini and Huygens observed Saturn in the 17th century and again in the 21st century.
Giovanni Domenico Cassini (1625-1712) was the director of the Paris Observatory. His many achievements included the discovery of four of Saturn's moons. His contemporary Christiaan Huygens (1629-1695) discovered Saturn's moon Titan.
NASA and ESA (European Space Agency) launched the Cassini-Huygens mission to Saturn in 1997. Cassini is still studying the Saturnian system. Huygens was a probe that landed on Saturn's moon Titan on January 14, 2005. You can see images taken by Huygens during its descent to the surface by clicking here
2. Saturn has at least 62 moons.
When Cassini-Huygens left Earth in 1997, we knew of only eighteen Saturnian moons, but there were 62 at the end of 2010. So you can see that the past decade and a half have been quite productive. Interestingly, only some of the moons were discovered by the Cassini probe. Ground-based telescopes discovered most of them.
3. Titan is the second largest moon in the Solar System and vastly bigger than all of the rest of Saturn's moons put together.
The mass of Titan forms 96% of the combined mass of Saturn's entire moon system. The six comparatively large moons - Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus - account for most of the remaining mass. The other 53 moons are about 0.04% of it.
4. Mimas is the smallest known round body in the Solar System.
Any body that has enough mass will collapse under its own gravity into a sphere. Asteroids generally have irregular shapes because they are too small to do this. The mass needed for icy bodies to collapse is lower than that for rocky bodies, but astronomers aren't sure exactly where the cut-off is. The smallest spherical body that we know of is Saturn's moon Mimas (pronounced MY-mass). It's about 400 km (250 miles) in diameter.
5. Iapetus is a two-tone moon, with one hemisphere ten times brighter than the other.
Click here to see a picture of Iapetus (pronounced eye-AP-i-tus). It looks as if the moon is in shadow on the right hand side, but that is actually the dividing line between a bright hemisphere and one which is as dark as coal. The dark terrain is called Cassini Regio after Giovanni Cassini.
The dark side is fairly smooth, showing that new material is being deposited which covers old cratering. Astronomers are still studying the Cassini data in order to work out what is happening. You can see the two hemispheres compared here.
6. Some of Saturn's rings have "shepherd" moons.
Shepherds herd sheep and shepherd moons herd rings! Some small moons have orbits to one side of a ring and their gravitational influence helps define the ring shape and maintain the gaps between them. To see Prometheus and Pandora shepherding Saturn's F rings, click here.
7. Titan is the most Earthlike body in the Solar System.
Titan has a thick nitrogen atmosphere rich in organic chemicals. Scientists think that it may be a cold version of ancient Earth. Even today our atmosphere is about 80% nitrogen and on the ancient Earth, it would have been almost all nitrogen. Oxygen only entered our atmosphere in quantity after photosynthesizing organisms evolved.
8. Hyperion is not tidally locked to Saturn.
On Earth we always see the same side of the Moon, because it's tidally locked to Earth. Therefore it takes just as long to rotate on its axis as it does to orbit the Earth. This occurs because of the gravitational interaction of a moon and its planet, and itís common throughout the Solar System (and beyond). Saturn's moon Hyperion (hi-PEER-ee-on) is the notable exception, because the influence of the nearby large moon Titan causes its motion to be chaotic.
9. Enceladus is geologically active
Enceladus (en-SELL-uh-dus) is one of four Solar System bodies which have been observed erupting. Enceladus is heated by tidal effects. Most people know that the Moon raises tides on the Earth, but many don't realize that the Earth also affects the Moon. There aren't any oceans there, but Earth's gravity pulls on the Moon's crust. The moons of giant planets can experience extreme tidal effects that cause internal heating and even geological activity.
10. None of Saturn's moons was given a name until the nineteenth century.
To Huygens Titan was "Saturn's moon" and Cassini referred to his four moons as "Louis's stars" in honor of his patron King Louis XVI. Astronomers called the moons by numbers (in order from Saturn), but that got quite confusing as more moons were discovered.
Finally, in 1847 John Herschel suggested names from the mythology of Saturn (Cronos in Greek). Cronos was a Titan, one of the children of Uranus and Gaia. The largest moon was named Titan and the others were given names of Titans. Since there were only a limited number of Titans, other mythologies (Inuit, Norse, Celtic) have been used for the newer discoveries.
(1) Abby Cessna, "Saturn's Moons" http://www.universetoday.com/56037/saturns-moons/ (February 15, 2010)
(2) Cassini Solstice Mission "About Saturn & Its Moons" http://saturn.jpl.nasa.gov/science/moons/