A Titan of a Mission: Parachuting through Smog to Saturn's Moon

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On Jan. 14, a flying saucer will parachute through the thick orange haze of a distant moon's atmosphere. Descending through the hydrocarbon smog, the probe could crash into an icy mountain, plop in a pool of organic goo, or dive into a methane ocean. Welcome to Saturn's largest moon, Titan, a place where organic chemistry appears to be a carbon copy of the infant Earth's just before life got a foothold. The saucer-shaped Huygens probe, named for the 17th-century Dutch astronomer who discovered Titan, has been riding piggyback on the Cassini spacecraft since it left Earth in October 1997. The craft arrived at Saturn on June 30 and has now embarked on a 4-year tour of the planet and its moons.

Radar data from Cassini, taken during its first close flyby of Titan on Oct. 26, reveal dark patches that might be lakes of methane. Streaks imaged by visible-light cameras during that flyby could be caused by the flow of a hydrocarbon fluid or by wind eroding solid material (SN: 11/6/04, p. 291)

Titan has fascinated researchers for 6 decades, ever since astronomer Gerard Kuiper analyzed sunlight reflecting off the moon and discovered methane in its atmosphere. But interest escalated in 1980, when the Voyager 1 spacecraft revealed that methane is a small but key component of a nitrogen-rich atmosphere too thick to see through. The craft also confirmed the presence of ethane, acetylene, propane, and other hydrocarbons. Bombarded by energetic charged particles from Saturn as well as by ultraviolet light from the sun, methane breaks down in Titan's upper atmosphere to form this complex array of organic compounds.

The chemicals may even rain out of the atmosphere to form hydrocarbon ponds or vast lakes on the moon's surface. Molecules that evaporate from these liquid reservoirs would end up back in the atmosphere, replenishing the supply, just as water in Earth's oceans resupplies our planet's atmosphere.

That would make Titan, the second-biggest moon in the solar system (after Jupiter's moon Ganymede), the only one with liquid at its surface. Radar beamed from Earth suggests that radio waves are indeed reflecting off a Titan lake or ocean, but results from visible-light studies are less clear-cut.

Yet even if Huygens doesn't plunge into a methane bath, its findings are likely to make quite a splash.

It isn't just Titan's mix of organic compounds that intrigues planetary scientists. The moon also has reserves of frozen water that occasionally melt when struck by comets. The overall chemical cocktail appears to offer researchers the only available glimpse of conditions like those on Earth just before life got started. On our planet, traces of this long-ago era have been erased by the actions of life itself. But Titan, residing in the chilly outer solar system and protected by a thick atmosphere, may have preserved for billions of years the conditions that were necessary for life to begin.

Life is unlikely to have sprung up on Titan, which has an average temperature of--180 [degrees]C. In exploring the moon, "we're trying to understand about the origin of life in the solar system, which is very different from searching for life," says Larry Soderblom of the U.S. Geological Survey in Flagstaff, Ariz. Titan could reveal how the raw materials for life--organic compounds--collected into pockets of varying concentrations, where biological action could begin, he adds.

"Titan is more like the prebiotic Earth than any other site in the solar system;' asserts planetary scientist Jonathan I. Lunine of the University of Arizona in Tucson.

PREPARE TO DIVE Before Huygens can take the big plunge, it will have to execute the big escape--separating from its mother craft, Cassini. On Christmas Day, engineers will radio a final set of commands for the parting. Explosive bolts will fire, springs will give a gentle push to the probe, and Huygens will coast into space. A device on Cassini will twirl Huygens as it detaches, giving the probe a spin of seven revolutions per minute that will prevent it from tumbling end over end. …