

Dione and Saturn



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 -180oC (-292oF). 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 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



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



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 -201oC (-330oF). 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



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.