Dione and Saturn

The Moons
On March 24, 1655, only Earth and Jupiter were known to have moons. The next day, Christiaan Huygens added Saturn to the list when he discovered its largest moon, Titan.

Since then, scientists have discovered 34 natural satellites orbiting Saturn. Some, like Pan, Atlas, Prometheus, and Pandora, are "shepherd moons" that herd Saturn's orbiting particles into distinct rings. Others produce twisting and wave patterns in the rings.

But it is mysterious Titan that intrigues scientists most.

Titan
Titan is the second-largest moon in the entire solar system (Jupiter's Ganymede is slightly larger). It's bigger than two planets, Mercury and Pluto. Orbiting Saturn far from the Sun, its surface temperature is only -180^oC (-292^oF). And it's the only moon with a dense atmosphere -- so dense, in fact, that Titan's near-surface atmospheric pressure is about 60 percent greater than Earth's. That's about what a scuba diver feels under 20 feet of water.

Titan's thick atmosphere

Titan orbits Saturn at a distance of about 1.2 million kilometers (745,000 miles), taking about 16 days to complete a full orbit -- 15.94 days to be exact.

Titan is of great interest to scientists because it is the only moon in the solar system known to have clouds and a mysterious, thick, planet-like atmosphere. Scientists have tried for decades to penetrate that thick haze with a variety of telescopes, but got only vague hints at the shape of the surface below.

That changed in 2004 when the Cassini spacecraft's powerful instruments were aimed at the mysterious moon from close range. Cassini's radar instruments are revealing a complex geological surface with very few craters - a sign the surface may be relatively young.

Even with the improved view, Titan remains mysterious. It is still unclear how much of the surface is liquid and how much is solid. There may be lakes, ridges and channels.

Close-up of Titan's surface

"Unveiling Titan is like reading a mystery novel," said Dr. Charles Elachi, director of NASA's Jet Propulsion Laboratory and team leader for the radar instrument on Cassini. "Each time you flip the page you learn something new, but you don't know the whole story until you've read the whole book. The story of Titan is unfolding right before our eyes, and what we are seeing is intriguing."

During dozens of flybys, the Cassini orbiter will continue to map Titan with cloud-penetrating radar and to collect atmospheric data. In January 2005, the European Space Agency's Huygens Probe will dive through Titan's dense atmosphere with instruments capable of analyzing its components.

Combined with the big picture information that that the orbiter Cassini will collect during Titan flybys, data from the Huygens Probe will provide scientists with critical information that may shed light on ancient questions, such as how the planets formed and the evolution of our solar system.

Because of the extremely cold temperatures typical of celestial bodies that are that far away from the sun, the structure of Titan's chemical atmosphere is in a state of deep freeze.

It is this chemical composition that interests scientists a great deal because Titan's atmosphere might consist of compounds similar to those present in the primordial days of the Earth's atmosphere. Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but may contain much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere might indicate that this fascinating moon could harbor some form of life.

Saturn with Tethys (above) and Dione

More Moons
Most of Saturn's moons get their names from Greek mythology, such as Pandora, Prometheus, Enceladus and Hyperion. Some of the more recently discovered moons are identified by numbers that indicate the year and order of their discovery, such as S/2004 S1 and S/2004 S2.

Saturn's moons vary considerably in shape and size. Some appear to be porous, icy bodies with craters, ridges and valleys -- others show corrugated, irregular terrain. Some appear to have formed billions of years ago, while others appear to be part of a bigger, fragmented body. Some moons appear to have rocky surfaces, and might be covered by organic material similar to the complex substances found in the most primitive meteorites.

One moon, Enceladus, is one of the shiniest objects in the solar system. It's about as wide as Arizona and covered with water ice that reflects sunlight like freshly fallen snow. That makes it extremely cold, only about -201^oC (-330^oF). It may be that volcanoes on this moon erupted the icy particles that form Saturn's E-ring, and that they continuously snow back down onto its surface.

Epimetheus and Janus trade orbits with each other every few years, taking turns being closer to their planet.

Herschel Crater on Mimas

Mimas, only 392 km (244 miles) in diameter, has a giant crater one-third as wide as the moon itself. And in its center is a peak about two-thirds the height of Mt. Everest, the hightest point on Earth.

For the most part, very little is known about Saturn's moons, except through data gathered that measures their brightness. These moons' estimated sizes are based on assumptions of their reflectivity.

During its four-year mission in this immense region, the Cassini spacecraft will extensively photograph these moons, and collect data that will increase our understanding of their composition.