One scientist who'll be watching closely is Dr. Vikki Meadows, an astrobiologist at NASA's Jet Propulsion Laboratory. Meadow's team is creating computer simulations of all the known terrestrial planets with atmospheres - Earth, Mars, and Venus - and also the mysterious moon Titan.

Their computer models will provide the scientific foundation for Terrestrial Planet Finder, a future NASA mission to detect and characterize Earthlike planets elsewhere in the galaxy.

"Our angle is that Titan is basically another terrestrial planet, but it's often not recognized as one because it happens to be orbiting Saturn," Meadows said.

"Despite its low temperatures and icy composition, it has a solid surface, a dense atmosphere, and a number of processes that we often attribute to terrestrial planets. It's an alien Earth, right here in our solar system."

Titan is bigger than the planet Mercury and only slightly smaller than Mars. Its dense 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 rains gasoline-like liquids that accumulate into lakes or oceans on the planet's surface. However, little is known about the surface of Titan because its hazy atmosphere has been impenetrable at visible wavelengths.

In 1980, NASA's Voyager 1 spacecraft tried to take close up images of the natural features of Titan's landscape but was unable to peer through the thick clouds.

The intrigue deepened in 1994, when NASA's Hubble Space Telescope recorded near-infrared pictures of Titan. The images showed light reflecting from what might be a "continent" the size of Australia on one hemisphere.

More recent observations by ground-based telescopes have revealed bright regions that could be icy landmasses and dark regions that could be seas of liquid hydrocarbons. These images suggest Titan has the distinction of being the only other body in the solar system, besides Earth, with open seas.

Not exactly desirable real estate

Even so, Titan isn't really that much like home.

"The analogies are not that close, because Titan is such a cold place," said Dr. James Kasting of Pennsylvania State University. Because of its great distance from the Sun, Titan's surface temperature is about minus 178 Celsius (minus 289 Fahrenheit), too cold for liquid water, and too cold for life. Furthermore, the atmosphere is laced with poisonous substances and devoid of oxygen.

Kasting, whose research focuses on the early evolution of Earth's atmosphere, believes Titan's atmosphere may resemble the primordial cloud that cloaked our home planet 2.3 billion years ago, before life emerged. Titan's chemically rich sheath contains many of the same molecules as early Earth. A closer examination may unravel some of the secrets of the beginning of life.

That long-awaited examination is at hand with the arrival of Cassini mission at Saturn on June 30. During dozens of flybys, the Cassini orbiter will map Titan with cloud-penetrating radar and collect atmospheric data.

In December, the Huygens probe will be released for a three-hour plunge through Titan's atmosphere. Tethered to a parachute, the robotic laboratory will scrutinize the clouds, atmosphere, and surface of the moon, and perhaps even land on the surface.

"Just what Huygen's will see beneath Titan's shroud is pretty much anybody's guess," said Cassini's deputy project scientist, Dr. Linda Spilker. "Possible surface features include lakes or oceans of liquid hydrocarbons, mountains, and plains. Who knows, maybe it will look like my home state of Minnesota - lots of lakes!"

A world without life (probably)

The ultimate goal of planet-finding missions like the Space Interferometry Mission and Terrestrial Planet Finder is to discover habitable worlds - those that could support life. And frigid Titan doesn't qualify.

"Since there's no water, you would have to come up with a different type of biochemistry. Most of us would be surprised to find that," Kasting said.

But Terrestrial Planet Finder may discover planets similar to the moon Titan: cold, terrestrial bodies with dense atmospheres that are essentially uninhabitable. And being able to identify uninhabitable or pre-habitable worlds is just as important as recognizing our exact twin, Meadows said.

"The more we understand about Titan, the better equipped we are to go out searching for Earthlike planets around other stars," she said. "No matter what we discover about Titan, it's going to be fantastic. It's very relevant to understanding how life arose in our solar system."

Regardless of its potential for life, Titan remains one of the most fascinating places in the solar system. If nothing else, it may give us a sneak preview of the panoply of exotic, Earthlike worlds waiting to be detected by Terrestrial Planet Finder.

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