Saturn's moon awash in methane

Rainstorms of liquid methane wash the surface of Saturn's moon Titan, spawning rivers that tumble onto dark, gooey plains of hydrocarbons in an other-worldly version of Earth's water cycle, scientists announced Friday.

A new analysis of data from the European Huygens space probe shows a world with its own complex climatological cycle played out on a frigid landscape chilled to minus 290 degrees Fahrenheit.

Scientists at a news conference in Darmstadt, Germany, said rain falls so regularly that it may have soaked the moon's surface within days of the space probe's landing last week.

"There are truly remarkable processes at work on the surface of Titan," said Jean-Pierre Lebreton, Huygens mission manager for the European Space Agency. "There is liquid flowing on Titan. It has been raining not long ago."

When Huygens landed on Titan Jan. 14, mission scientists looking over hundreds of images returned from spacecraft immediately recognized evidence of erosion on the moon's surface. After a week of digging into the data, they now believe that liquid methane is a primary force in the moon's climate and topography.

"We now have the key to understanding what shapes Titan's landscape," said Martin Tomasko, principal investigator of the probe's descent camera. "Geological evidence for precipitation, erosion, mechanical abrasion and other (river-based) activity says that the physical processes shaping Titan are much the same as those shaping Earth."

On Earth, methane is a colorless, odorless gas that is a prime constituent of natural gas. On frozen Titan, methane can be a liquid under the higher pressure at the moon's surface. Water, on the other hand, is frozen and hard as rock.

Unlike Earth, Titan is far too cold for plant life to take hold and breathe oxygen into the atmosphere. That is a good thing. With all the inflammable chemicals in the atmosphere, the addition of oxygen would create an explosive world.

Scientists have known about Titan's methane-rich atmosphere since Voyager 1 flew by the moon 25 years ago and remotely measured its smog-like haze.

But what fueled the decision to spend $3.3 billion on a mission to Saturn and Titan was the moon's usefulness as a laboratory to study what Earth was like before life formed billions of years ago. Titan, the only moon in the solar system with an atmosphere, is believed to resemble early Earth, before life formed.

"We've got a very primitive environment here," Toby Owen, of the Institute for Astronomy in Hawaii, said at the news conference.

The Cassini-Huygens mission, jointly undertaken by NASA, ESA and the Italian Space Agency, was launched on Oct. 15, 1997.

The spacecraft traveled seven years with the hubcap-shaped Huygens probe as a hitchhiker before reaching Saturn. Last month, Huygens was cast off from Cassini and traveled another three weeks in space before parachuting to Titan's surface, making the most distant spacecraft landing yet.

As the craft descended, it took several hundred photographs of the moon. The images showed strong evidence of erosion and the presence of streams and riverbeds, proving there had, at one time, been liquid coursing over the surface.

The probe transmitted information from cameras, surface instruments and other devices for 72 minutes after landing. The craft is no longer active, its battery power having run out.

One piece of evidence that convinced scientists that methane rain continues to fall was the bright and clean ice hills in the descent images. They indicated that something had washed off the accumulation of atmospheric dust.

The other indication was a puff of methane gas, released from the soil when the hot spacecraft landed. It showed that methane must be present in large quantities near the surface.

Confirming this, a mass spectrometer aboard Huygens also found that the amount of methane in the atmosphere jumped by 30 percent near the surface.

Immediately after the probe's landing on Jan. 14, some scientists speculated that the darkly hued plain nearby might be a lake. The science team now believes the plain is dry, the liquid methane having soaked into the surface, awaiting the next rainfall.

Some researchers compared the landing site to Arizona, where rain quickly is absorbed by the desert.

That doesn't mean Titan is a parched place. Using pictures flashed on a large screen, University of Arizona planetary scientist Martin Tomasko pointed out "a river system that flows into a delta . . . draining into a broad plateau. . . . It must rain fairly frequently."

Over the past few days, scientists also have conducted a series of experiments to find out what the surface is made of. Early on, some scientists speculated that organic particles accumulated on the surface could be sticky like tar or brittle like plastic.

Measurements from Huygens indicated the probe landed on the moon at about 10 mph, and then settled into the surface up to six inches. In their experiments, scientists dropped weights on different substances to try to recreate the landing force.

They were finally able to reproduce the results on Titan using finely ground glass particles to simulate icy sand, said John Zarnecki, the principal investigator for the mission's surface science experiments.

Whether that material would suck at an astronaut's boots, or crack under it like a piece of glass remains unclear.

Scientists are still puzzling over the origin of Titan's methane. Researchers believed that if it was just in the atmosphere, it would have dissipated millions of years ago.

On Earth, methane is a greenhouse gas, and is produced by decaying organic matter.

Scientists said Friday that the level of Argon measured in the atmosphere shows that volcanic activity may also be playing a role in shaping Titan's chemical makeup. Only instead of lava, Titanian volcanoes spew ice and ammonia.

"There's lots of evidence of familiar, Earth-like processes, but with very exotic materials," Tomasko said.

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