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 said yesterday.

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

Scientists at a news conference in Darmstadt, Germany, said the rain 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 (ESA). "There is liquid flowing on Titan. It has been raining not long ago."

Mission scientists poring over hundreds of images sent by Huygens after it landed Jan. 14 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.

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

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 three more weeks in space before parachuting to Titan's surface, making the most distant spacecraft landing yet.

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.

Immediately after the probe landed, 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. A sensor about the size of a police officer's nightstick on the front of Huygens probed beneath the moon's crust and found a material with the consistency of loose sand.

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."

The riverbeds are darkened by what seem to be particles of smog that fall out of Titan's atmosphere, coating the terrain. The dirt apparently gets washed off the ridges by the methane rain to collect in the river channels.

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 produced by decaying organic matter.

Scientists said yesterday 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. But instead of lava, Titanian volcanoes spew ice and ammonia.

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

Material from The Associated Press is included in this report.

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European Space Agency:

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