By Robert Roy Britt
Senior Science Writer
SPACE.com

In one of the most distant weather reports ever received, clouds and even rain showers seem to have been spotted on Titan, Saturn's largest moon. Along with vast seas and modest mountains, a picture is emerging of a place more like Earth than anywhere else in the solar system.

Scientists have already labeled Titan a hot spot in the search for extraterrestrial life, and the new work adds to that enthusiasm.

Titan is a cold, dark, smog-shrouded world larger than the planet Mercury and nearly half as big as Earth. Known for decades as the only moon in the solar system with an atmosphere, Titan has many of the raw materials for life, including nitrogen, carbon and water.

But at 1.4 billion kilometers (886 million miles) away from the Sun, most if not all of Titan's water is locked in ice. So the distant moon's rain is of a different sort, thought to be methane -- the primary component of the common heating fuel, natural gas. Methane is also an important raw material for other organic compounds.

In a new study, researchers looked at non-visible light emitted by Titan, spotting small clouds that developed and disappeared daily, most likely after causing a methane rain shower.

The results of the study, which confirm earlier suspicions about Earth-like weather on Titan, appear in the October 20 issue of the journal Science.

"Titan is a planet-sized laboratory hosting perhaps the kinds of organic chemical reactions that preceded and initiated life on Earth 4 billion years ago," said Jonathan Lunine, a professor at the Lunar and Planetary Laboratory who was not involved in the study. "To see methane in action as a cloud-forming and rain-forming condensate lends further credence to the view that Titan is a very attractive astrobiology target."

The driving force

On Earth, weather is largely driven by heat from the Sun, which creates temperature differences that move large masses of wind and foster huge oceanic currents. But Titan receives about 100 times less solar energy; temperatures hover around -178 degrees Celsius (-288 degrees Fahrenheit).

So a different force is thought to drive weather on Titan, weather that is mild by terrestrial standards.

"We propose latent heat [released when a gas condenses] plays a large role in driving Titanian weather," said Caitlin Griffith, lead researcher. "These bizarre conditions conspire to bring about strange clouds. On Titan, clouds are rare, usually covering less than 1 percent of the globe, compared to the Earth's 50-percent coverage."

Titan's gravity is only about one-seventh that of Earth. The intense chill, however, means a low-energy atmosphere that hangs around, instead of escaping this relatively weak force of gravity. So Titan's atmosphere is denser than Earth's and extends much higher into the sky.

Titan's clouds hover about 25 kilometers (15 miles) up, said Griffith, a researcher at Northern Arizona University in Flagstaff. Earth's clouds, by contrast, range from the ground to occasionally as high as 17-18 kilometers (11 miles) in the middle of the worst thunderstorms and hurricanes.

While weather is relatively docile on Titan, and rainfall is probably sparse, it may come down in buckets now and then.

"Most rivers on Titan may run dry, but river valleys may nevertheless be abundant and deep," writes the Lunar and Planetary Laboratory's Ralph Lorenz in an analysis of the study.

Search for life

Increasingly, Titan is being seen as a good place to look for extraterrestrial life. Like Jupiter's moon Europa, Titan is expected to be hiding liquid water in its belly, says Lorenz, who models geophysical processes on Titan's surface but did not work on Griffith's study.

Lorenz said Titan's surface is too cold for liquid water, the requirement for life as we know it. But deep beneath the surface, it may be warm enough to melt the ice.

And already, researchers have evidence of simple organic molecules -- similar to gasoline -- on the smoggy moon. These molecules are very far from life, but are a first step, says Lorenz.

"The one thing Titan does have on its side is all these organics, which Europa doesn't have," Lorenz told SPACE.com. "That gives it better stuff to work with to create life -- a more 'hearty' primordial soup, if you like."

Let's go see

There is no evidence that either Titan or Europa do harbor life, and scientists agree that we won't know until more research is done, most likely with a spacecraft to perform direct measurements. Answers may come as soon as 2004. The Cassini spacecraft, en route to study Saturn, its rings and moons, is scheduled to drop the Huygens probe into Titan's atmosphere.

The probe will take readings in the atmosphere during a 2.5-hour descent. On the surface, it is expected to survive and collect data for at least three minutes, and perhaps longer.

But a recently discovered hardware problem aboard Cassini may leave it unable to receive much of the science data the probe will collect. The European Space Agency hopes to have a plan in place to deal with the glitch by next summer.

Meanwhile, firm proof that clouds exist on Titan may depend on the Huygens probe. One researcher who studies Titan and is familiar with Griffith's study isn't entirely convinced of the findings.

"I don't think we can say for sure one way or the other at this point" whether the clouds exist, said Athena Coustenis of the Paris-Meudon Observatory. Coustenis said the results need to be confirmed by other researchers.