Saturday, January 06, 2007

Saturn Moon Has Lakes, "Water" Cycle Like Earth's, Scientists Say - National Geographic News

Richard A. Lovett
for National Geographic News
January 5, 2007

Saturn's giant moon has lakes and a "water" cycle remarkably similar to Earth's, new evidence suggests.

But Titan's lakes aren't made of water. Instead, they probably consist of liquid methane, which plays the role of water in Titan's superchilled climate, the researchers say.

The lakes were discovered by radar mapping when the Cassini spacecraft, now orbiting Saturn, did a close flyby of northern Titan last July.

The flyby revealed dozens of large, dark patches resembling lakes, up to 40 miles (70 kilometers) in diameter. (See more Saturn photos from Cassini.)

When the lakes were first discovered, the scientists noticed riverlike drainage channels that probably conducted moisture from the surrounding highlands.

This indicates that the lakes were fed by methane rains falling at higher elevations, said Ellen Stofan, lead author of a study in yesterday's issue of the journal Nature.

But some lakes had no such inlet streams. That means that there must be methane aquifers—"methanofers," Stofan called them—not far below the surface.

"Just like on Earth, if you dig deep enough, the depression fills up with water," said Stofan, who shares her time between the Proxemy Research corporation in Virginia and University College London in England. "There's a subsurface methane table."

Some of Titan's lakes also appear to lie in calderas formed by "cryovolcanism," but this doesn't mean that the methane in these lakes came from the volcanoes.

"Just like at Crater Lake in Oregon, once you have a depression, it will fill up with liquid," Stofan says.

Just Like Earth

Like other Titan researchers, Stofan is amazed by how many similarities Titan has to Earth.

"It's amazing that so far away, with such exotic materials, there's this hydrological cycle that's occurring that we dont see anywhere else in the solar system but on Earth," she said.

"If you were standing on the shores of these lakes, in some ways they would look familiar," she added.

"The methane would be transparent, so you could see pebbles on the bottom. And the largest lake is 70 kilometers [43 miles] across, so there's probably enough [distance] for waves to form."

In fact, she said, some of the radar images appear to show that the lake surfaces are choppier near the shores, a possible indicator of wave action.

The next step, Stofan said, is to watch how the lakes change as Titan's 29-year-long seasonal cycle switches from winter to spring.

Learning more about Titan's methane cycle will help us understand our own planet better, added Christophe Sotin of the University of Nantes in France.

"Titan is the only other place in the solar system where liquid is present at the surface," Sotin said by email.

Liquids are believed to be necessary for life.

"On Titan we don't expect life at the surface, because the temperature is too small—minus 179 degrees Celsius [minus 290 degrees Fahrenheit]," Sotin said.

But, he added, the study of methane helps us understand the processes that allow the existence of any liquid, including water. For liquids to persist for billions of years, for example, there must be a long-term equilibrium between such forces as evaporation and rainfall.

"By studying Titan, we may better understand the evolution of any planet, including Earth," he said.

Cassini is slated to study Saturn and its moons until at least June 2008.


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