First Space Lakes Found on Saturn Moon

Richard A. Lovett
for National Geographic News
July 27, 2006
A flyby of Saturn's largest moon, Titan, has finally revealed what scientists were searching for: lakes. Lots and lots of lakes.

This makes Titan the only body in our solar system, other than Earth, with lakes on its surface.

The discovery was made on July 22, when NASA's Cassini spacecraft, now orbiting Saturn, did its 16th close flyby of Titan.

The craft zoomed some 600 miles (965 kilometers) above a strip approximately 3,000 miles (5,000 kilometers) long—not to be confused with Titan's Earthlike "continent," announced last week.

Much of the terrain around the lakes looks like Earth's northern regions, says Ralph Lorenz of the University of Arizona's Lunar and Planetary Laboratory in Tucson.

Lorenz compares the area to Finland and Sweden or parts of Canada and Minnesota.

"It is a real potpourri of what look like lakes," added Jonathan Lunine, another planetary scientist from the Lunar and Planetary Laboratory.

On radar the lakes show as dark, smooth blotches surrounded by hills.

"The contrast with the surrounding areas is very obvious," Lunine said. "It's not at all subtle."

"We don't know for sure that they're liquid," he added. "But that's the simplest interpretation."

No Water

On Titan bitterly cold temperatures would turn any watery lake to ice.

So what are the lakes made of?

Titan's lakes, the scientists say, are most likely composed of hydrocarbons, most likely liquid methane, possibly mixed with ethane.

They range from slightly more than 0.6 mile to 50 miles (1 kilometer to 80 kilometers) wide.

Channels connect some of the lakes. And while some lakes have shallow basins—one appears to be shallow enough for the radar to have revealed features on its bottom—others lie in steep-sided valleys.

Scientists had long suspected that Titan has liquid on its surface. But prior to the latest flyby, the only other possible lake had been found in Titan's southern hemisphere, now in its summer season.

Lakes are harder to find in summer, because they are likely to be evaporating.

The newly discovered lakes are in the north, just now emerging from Titan's winter, when surface liquids are expected to be more abundant.

Even so, the lakes were only found north of about 70 degrees latitude. South of that, Lunine says, is a belt of what appear to be dry lake beds.

Methane Rain

The lake discovery is only one of two major Titan findings announced this week.

The other appears in two studies in today's issue of the journal Nature.

One of the articles, by a European and U.S. team, shows how methane could fall from Titan's atmosphere in a gentle drizzle, even as far south as the equator.

The other study, by a team of Spanish researchers, describes how the planet could generate methane thunderstorms.

Like the lakes, such storms had been expected. That's because the Huygens probe (a European Space Agency craft that parachuted from Cassini to Titan's surface on January 14, 2005) sent back photos of what appeared to be river channels strewn with rounded rocks that appeared to have tumbled downstream in flash floods.

All of the week's Titan discoveries are related.

Without the lakes, there could be no source of methane to evaporate into rain clouds—and therefore no drizzles or storms to trigger stone-smoothing flash floods.

Future flybys may allow additional opportunities to study the lakes—and how they change over the course of a Titan year.

"Toward the end of the mission," Lunine said, "we may see [the lakes] get smaller as spring approaches and they start to lose methane."

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