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Saturn's Deep, Dark Secret

Titan, the only major body in the solar system that we haven't gotten a good look at, is about to be outed.

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MOST OF US PROBABLY FEEL THAT WE SPEND LONG, tedious intervals in the shadows awaiting brief moments of glory.

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But spare a thought for the European Space Agency’s Huygens planetary probe. After a seven-year cruise through space, the 700-pound disk, stuffed with electronic instruments, will spend as little as three minutes on the surface of Titan, the largest moon of Saturn. Then it will either sink beneath a burbling methane ocean or be abandoned in frozen Titanic wastes forevermore. And that’s if everything goes well.

Yet Huygens and its ground controllers in the German suburb of Darmstadt will in those three minutes see what no one has seen before. Titan, which is larger than Mercury and almost half the size of Earth, is surrounded by a dense gaseous atmosphere that has kept its surface hidden—from the Voyager probes, which flew by in 1980 and ’81; from the Hubble Space Telescope, which measured the moon’s surface reflectivity in 1994; and from the cloud-penetrating radar of the Arecibo Observatory in Puerto Rico, which in 2001 and 2002 detected glints from Titan’s radar-reflective areas. To know the surface of Titan, scientists must send an observer to descend beneath the atmosphere. ESA’s Huygenauts don’t even know if their vessel will land on a hard surface or liquid; they figure the chances for either at 50-50.

What they do know is that Titan’s atmosphere, which Huygens will study for a whole two and a half hours before its finale on the surface, is intriguingly like Earth’s probably was four billion years ago, when fledgling organic compounds like methane first began to vary the choking miasma of nitrogen. Life as we know it cannot evolve on Titan, thanks to average temperatures of –289 degrees Fahrenheit. But the great moon is “a unique laboratory for studying the pre-biotic soup,” in the loving phrase of Jean-Pierre Lebreton, the 54-year-old French physicist who has given most of his working life to Huygens and is ESA’s top scientist on the project. “There’s a large list of hydrocarbon molecules, indicating something very strange and complex is going on in the atmosphere and on the surface,” he says. “Basic information about how life was created that is erased on Earth may be frozen for us to study on Titan.”

The Huygens project—named for Christiaan Huygens, the Dutch astronomer who in 1655 discovered Titan—is something of a pioneer back on Earth too. It is Europe’s farthest stab by far into the universe. It is also the first spacecraft of any country to attempt a landing—hairsplitters would say an “impact”—in the outer solar system.

“By the time we get the first pictures, we will have either succeeded spectacularly or failed spectacularly.” So concludes David Salt, one of two British techies minding Huygens’ so-far quiescent bank of computer monitors in a bunker-like control room in the Darmstadt basement. Either way, it will be too late to do anything about it. With Huygens 80 light-minutes from Earth (compared to nine for the Mars landers recently in the news), a round-trip radio signal, even assuming it penetrated the Titanic haze, would take longer than Huygens’ planned 150-minute plunge through the atmosphere. The little craft—all of eight feet in diameter and looking Star Wars-adorable hunkered behind a cap-like heat shield—will be alone in the impenetrable fog of strange and complex hydrocarbons.

At about 200 miles above Titan’s surface, the heat shield will brake the craft from 12,400 mph to 870 mph in less than two minutes. Sensing the reduced speed, accelerometers on board should trigger the release of the first of three parachutes, meant to slow the craft to about 12 mph so its impact with the surface will be survivable. When the first chute deploys and the shell is jettisoned, the science instruments kick in, and the distilled genius of another pint-size explorer—the European Space Agency, which figures its budget at six percent of NASA’s—goes to work.

ESA has had help. Plenty of American know-how is at work on Huygens too. For one thing, the probe is hitching a ride to Saturn on a NASA spacecraft named Cassini (after Renaissance Italian skywatcher Gian Domenico Cassini, who discovered one of the gaps in Saturn’s rings). Cassini will begin to orbit Saturn on July 1 and, sweeping by Titan with its infrared camera and radar, will upstage Huygens with the first glimpses of the moon’s surface. Six and a half months later, Cassini will drop Huygens off to visit Titan, relay (it is hoped) Huygens’ data feeds, then head off on its own four-year tour of the ringed planet.

Huygens carries a transatlantic suite of instruments. An Aerosol Collector and Pyrolyzer, built in Paris, will grab samples of Titan’s atmosphere during descent, vaporize them, and feed the results for analysis to a spectrometer constructed at NASA’s Goddard Space Flight Center in Maryland. The Italian-made Huygens Atmospheric Instrument, which senses lightning and other electrical activity in the moon’s sky, works with a Descent Imager and Spectral Radiometer (that is to say, a camera and radiation sensor), contributed by the University of Arizona.

Cassini-Huygens scientists insist that their real purpose will be fulfilled above Titan’s surface, as the instruments log chemistry, electricity, winds, and the rest of it. “The main mission is going through the atmosphere,” Lebreton says. “The rest is bonus, if we get anything back.”

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