Space / Solar System

For centuries, stargazers have dreamed of finding Earthlike worlds elsewhere in the universe. The Italian monk Giordano Bruno was burned at the stake in 1600 for pushing the idea; Galileo was convinced that alien beings lived on Jupiter; and in the late 1700s, Sir William Herschel, who discovered Uranus, wrote of his certainty that the moon must be inhabited. At the end of the 19th century, canals on Mars were the rage, and well into the 20th, serious scientists imagined swamps and rain forests on Venus.

None of these lush idylls exist, but the dream of life on other worlds lives on. That helps explain why Titan, Saturn’s largest and most exotic moon, has planetary scientists so excited. Over the past two years, a pair of intrepid probes—Cassini, which has orbited Saturn since July 2004, and that satellite’s envoy, Huygens, which plummeted onto Titan’s surface in January 2005—have discovered features that are uncannily reminiscent of Earth. Titan, a world bigger than Mercury, has mountains, volcanoes, dunes, riverbeds, and rainstorms. To cap it all, scientists announced in January that Cassini’s powerful imaging radar had uncovered a system of more than 75 lakes, some up to 40 miles across, dotting the north polar region.

There’s one big difference. At almost one billion miles from the Sun, Titan has a surface temperature that hovers around minus 290 degrees Fahrenheit. As a result, all those familiar-looking features are constructed from decidedly unfamiliar materials. The mountains are made primarily of rock-hard water ice; the dunes are most likely ice granules coated with hydrocarbons; volcanoes probably belch methane and ammonia, and methane fills the lakes, evaporates to form clouds, and rains back down to carve out river channels.

Yet deeper similarities also exist. Titan’s chemistry, replete with organic molecules, may be a facsimile of the early Earth’s, but stuck in a deep freeze. Life as we know it couldn’t exist on Titan—but if most of life’s needs exist there, they might exist on balmier worlds around other stars. Each new Titan discovery seems to bolster the old faith that the universe is full of worlds that mirror our own. “It’s extremely exciting, extremely surprising,” says geologist and Cassini researcher Ellen Stofan, of University College London. “It’s been so much fun, like Christmas morning.”