Saturn's largest moon, Titan, is second in size only to Jupiter's Ganymede among the moons of our solar system. It's so big that if it were orbiting the sun directly, we'd be calling it a planet. Titan was discovered by the astronomer Christian Huygens in 1655.
Titan is the only moon in the solar system with a significant atmosphere, a hazy orange one that is 90% nitrogen and 10% methane and other carbon compounds. It's thick, blocking just about all human attempts to peer through it.
But Peter Smith of the University of Arizona found a way. Using Hubble Space Telescope data, Smith mathematically removed the specific colors of the haze in order to "see" past it. His work revealed continent-sized masses of varying colors.
Today's Titan might resemble the early Earth. It might have liquid lakes of hydrocarbons on its surface—many of those compounds are the basic organic building blocks of life on Earth. But a frigid surface temperature of minus 182 degrees Celsius (minus 296 Fahrenheit) likely rules out the possibility of life there.
Starting in 2004, the spacecraft Cassini is to make repeated flybys of Titan and map this intriguing moon using radar. A small probe dispatched from the Cassini mission, called Huygens, is to send atmosphere data back during a parachute descent toward a landing on Titan's surface. But that surface could be liquid—an ocean with giant waves in Titan's low gravity is quite possible—or slush, or dangerous icy crags. The probe will find out.
Titan is the biggest of them, but Saturn has more than 60 moons, second in number only to Jupiter.
Six medium-sized moons—between 400 and 1,500 kilometers (250 miles and 940 miles) in diameter—orbit in Saturn's equatorial plane. From largest to smallest, they are: Rhea, Iapetus, Dione, Tethys, Enceladus, and Mimas. All six were discovered before 1800, exhibit synchronous rotation, and are made mostly of ice.
In addition, Saturn has at least eleven smaller inner moons. The gravitational influence of the innermost moons (Atlas, Prometheus, Pandora, and Pan) affects Saturn's rings, compressing some rings and creating gaps between them. These are the shepherd satellites, watching over the rings.
Beyond the "big seven," Saturn has at least 14 very distant satellites. This gang is about 20 million kilometers (32 million miles) away from Saturn. They have very irregular orbits, some going around Saturn in opposite—retrograde—directions from the rest, which suggests they might be captured asteroids. Thirteen of these satellites seem to belong in three orbital clusters—possibly because they are pieces of larger bodies that broke apart after impacts with other objects.