On Saturn's moon Titan, bring an umbrella

CHICAGO (Reuters) - The daily weather forecast on Saturn's largest moon Titan appears to be a steady drizzle of liquid methane, at least around the bright, exotically named region known as Xanadu, U.S. researchers said on Thursday.

But this is hardly the paradise romanticized by the Samuel Taylor Coleridge poem "Kubla Kahn."

New images from Hawaii's W.M. Keck Observatory and Chile's Very Large Telescope show nearly global cloud cover at high elevations and a dreary morning drizzle that seems to dissipate around midmorning local time -- which is about three Earth days after sunrise.

Scientists had expected rain in the atmosphere of this planet-sized moon, but these near-infrared images for the first time have revealed a persistent drizzle of methane off the western foothills of Xanadu.

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"We expected that perhaps it was raining. It was reasonable that it could be raining. We just didn't know if it was raining right now," said Mate Adamkovics, a University of California, Berkeley researcher whose paper appears in the journal Science.

Titan is larger than the planet Mercury, but much, much colder, with surface temperatures of minus 297 degrees Fahrenheit (minus 183 degrees Celsius) -- cold enough to turn an explosive gas like methane into a liquid form.

It is well known for its hydrocarbon lakes and methane cloud cover. Radar images of Xanadu taken in 2006 showed deep channels that cut through plains and wind around hills.

And now it appears these moisture-laden clouds rain down on Xanadu.

"The question is, is it liquid methane that is sitting in a cloud, or is it falling through the sky," Adamkovics said in a telephone interview.

His hunch is that it is falling, given the massive size of these raindrops, which Adamkovics believes are about 1,000 times bigger than rain on Earth.

"Because there is a bit less gravity and the atmosphere is thicker on Titan, the rain drops and the cloud drops are really big," he said.

Whereas raindrops on Earth are micrometer sized, he said on Titan they appear to be a millimeter or bigger in size.

"The droplet gets so big it can't hold itself together anymore," Adamkovics said.

He and colleagues are now speculating about just what is causing the rain, and whether it follows weather patterns similar to those on Earth.

"The first thing you need is either air being pushed upward or you need the temperature to drop. What we didn't expect is there to be a big temperature drop daytime to nighttime on Titan," he said.

"Since we have seen the rain, we thought maybe there is enough of a temperature drop to cause the rain to start," he said, but right now that is just speculation.

"How it happens is unclear. We just know that it does," he said.

Xanadu, a region about the size of Australia, was first discovered in 1994 by NASA's Hubble Space Telescope.