Water Vapor Discovered on Tiny Moon of Saturn

By Guy Gugliotta
Washington Post Staff Writer
Monday, September 19, 2005

The original idea was to fly within 600 miles of Enceladus, orbiting 148,000 miles from Saturn, but some researchers suspected the icy moon might have an atmosphere. Fly closer, they suggested.

"The rest is history," said planetary scientist Robert H. Brown of the University of Arizona's Lunar and Planetary Laboratory. From a height of 109 miles, the Cassini spacecraft trained instruments on a cloud of water vapor venting from fissures at the moon's south pole.

From that moment, tiny Enceladus, only 310 miles in diameter, joined Mars and Jupiter's moon Europa as the solar system's leading candidates for having liquid water beneath their chilly surfaces -- a likely precondition for harboring life. But why the south pole? And how does something so small have liquid water?

"We haven't a clue," Brown said.

Brown calls the discovery of water vapor at Enceladus "one of the most important things ever to come out of planetary science," but for NASA's Cassini, perhaps the most muscular planetary science mission ever launched, it was just another day's work.

Nearly 15 months after Cassini awed an international viewing audience by flying through the rings of Saturn and settling into orbit around the giant gas planet, the spacecraft is operating impeccably, producing data and discoveries that are occupying the professional lives of as many as 500 scientists and engineers.

"We feel like we're drinking from a fire hose," said Torrence V. Johnson of NASA's Jet Propulsion Laboratory. He is a member of Cassini's imaging science team.

Perhaps the mission's most memorable moment came in January, when the European Space Agency's Huygens probe, released by Cassini 11 days earlier, parachuted to the surface of Saturn's largest moon, Titan, penetrating its smoggy atmosphere to record the first clear images of a forbidding world.

Since then, Cassini has made five more passes, and scientists' perception of Titan has begun to expand. They still regard Titan as a "pre-biotic" world, lifeless but rife with the compounds that existed on Earth before life evolved, but they now find its meteorology just as interesting as its chemistry.

"Titan is probably more Earth-like than anyplace we've looked at anywhere," Johnson said, "and we're seeing it every few weeks." Titan quite likely has a weather system based not on water but on liquid methane, which has eroded Titan's ice cliffs to create a landscape reminiscent of Norwegian fjords.

The methane vents from Titan's interior; and although much of it blows away into space and some of it falls back to the surface as hydrocarbon dust particles, scientists are confident that a great deal of it must also fall as rain. No solid body in the solar system except Earth has such a complex weather system.

And even though Cassini has not yet seen methane rain nor found any evidence of the predicted methane oceans, project scientists say rain is the only explanation that fits the landscape. "We're probably talking about some seasonal differences," said J. Hunter Waite Jr. of the University of Michigan. "There is some evidence from the southern hemisphere of a methane lake, and we'll find more."


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