Around 4:30 A.M. eastern standard time oil the morning of January 14, 2005, a flyingsaucer-shaped object named Huygens will encounter an atmosphere for the first time since it left Earth, in 1997. In that atmosphere's thin, cold gas, the object, roughly nine feet in diameter and hurtling through space at 13,500 miles an hour, will make its first palpable contact with Titan, the largest moon of the planet Saturn. Ever so slightly, the friction with Titan's atmosphere will slow down the spacecraft, triggering a complex sequence of events that, in the ensuing few hours, should unravel some of the secrets of what could be the most exotic environment in our solar system. By all signs to date, that environment could be dominated by complex organic molecules and seas made of liquefied hydrocarbons. A day at a Titanian beach would be spent freezing to death under hazy skies made of methane and nitrogen, a scenario similar to what would have taken place oil the early Earth (though our young planet probably wasn't freezing).

The high-speed entrance of a flying saucer into Titan's atmosphere is not so much a beginning as a culmination of a much longer process, which started more than twenty, years ago. Not long after the two Voyager spacecraft visited Saturn, in 1980 and 1981, three planetary scientists, each studying different aspects of the outer solar system, hatched a plot to convince two space agencies, NASA and the European Space Agency (ESA), to mount a major mission specifically to study the Saturnian system.
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Voyager 1 and Voyaager 2 had flown past Saturn and its retinue of satellites. Observations by those spacecraft had revealed the staggering beauty and complexity of the planet's ring system as never before, not to mention the diverse nature of its satellites and the enormous extent of the planet's magnetosphere, the magnetic "bubble" that surrounds Saturn. Those data, though, were mere snapshots, taken by two interplanetary tourists as they raced on epic journeys through and beyond our solar system. The three planetary scientists--Daniel Gautier of the University of Paris, Tobias C. Owen of the University of Hawai'i in Honolulu, and Wing-Huen Ip, then at the Max Planck Institute for Aeronomy in Katlenburg-Lindau, West Germany--felt that what the Voyagers had shown deserved a second look. A dedicated mission of long duration could refine and expand on the many tantalizing clues about the unique Saturnian system that investigators first glimpsed in the early 1980s.

Such a project would be neither cheap nor easy to mount, and numerous other scientific and technical projects would be competing for the resources of the space agencies. Persuading those agencies to undertake a new mission to Saturn would be an uphill battle. Furthermore, the time schedules announced by the two agencies for selecting and approving space missions rarely seemed to coincide. Yet, whether by good fortune, the power of their arguments, or the support of the relevant scientific communities in the United States and Europe, the three investigators succeeded. In the late 1980s both NASA and ESA approved a mission for which NASA would provide a craft to orbit Saturn and ESA would provide a dedicated probe. Once in the vicinity of Saturn, the orbiter would release the probe, which would descend through Titan's atmosphere to land on its surface.

Gautier, Owen, and Ip are among the latest in a long line of investigators that stretches back to the era just after the invention of the telescope. One of the first observers off Titan was the Dutch physicist and astronomer Christiaan Huygens, who deduced in 1655 that Titan was in orbit around Saturn. At the time, little information could be gleaned, aside from the fact that Titan was fairly large. The state of knowledge about the moon remained ranch as Huygens had left it until 1944, when Gerard Kuiper, a Dutch astronomer working in the U.S., detected the unmistakable signature of methane gas in the spectrum of Titan, revealing the existence of an atmosphere.

The Voyager flybys were a great leap forward in the human knowledge of Titan. Precise measurements pegged the moon's diameter at 3,200 miles, larger than the diameter of Mercury. But it is Titan's atmosphere, not its size, that makes the moon of such enormous interest. The Voyager spacecraft revealed that atmosphere to be thicker than Earth's--its column mass, the total mass of gas in a column extending from the surface to the top of the atmosphere, is some ten times that of our own planet's atmosphere. (Titan's relatively small surface gravity, compared with the Earth's, makes its pressure at "sea" level, as measured by Voyager 2, about 1.5 times that of the Earth's surface atmospheric pressure.) Furthermore, the composition of Titan's atmosphere is complex: it is made up primarily of nitrogen, but a variety of hydrocarbons, such as methane, ethane, ethyne, and propane, are also present.

Titan is the only satellite in the solar system to possess an atmosphere, though trace amounts of gases are present on a few other satellites. But Titan's atmosphere is unique in the solar system, primarily because of its mixture of hydrocarbons. Theoretical models predict that methane gas exposed to ultraviolet light (even the small amount that reaches Titan from the distant Sun) undergoes a series of chemical reactions. Those reactions could give rise to the approximate concentrations of the various gases other than methane that the Voyager flybys observed. Furthermore, the theoretical models predict that, with time, the ultraviolet radiation would supply the energy needed to form increasing quantities of complex, long-chain hydrocarbon molecules. The most intriguing conclusion of this line of reasoning is that among the longchain molecules could be precursors of molecules needed for life.