Saturn Moon Has Lake Effect Clouds?

Richard A. Lovett

Saturn's moon Titan has odd clouds forming downwind of its lakes—apparently much like clouds seen near North America's Great Lakes, scientists said Monday.

The clouds appear to form from liquid evaporating from the lakes, which then recondense over land, said Mike Brown, a planetary astronomer at the California Institute of Technology.

Near the Great Lakes, similar ribbons of clouds called lake effect clouds form downwind on cold winter days, according to data from NASA's Cassini orbiter.

"Titan is like Buffalo, [New York], without the Bills," Brown quipped.

(Related: "Huge Space Lake Confirmed on Saturn's Moon Titan" [July 31, 2008].)

Scientists have long been interested in a cloud cap above Titan's north pole.

The cloud cap's main layer sits about 40 to 50 kilometers (about 25 to 30 miles) above the moon's surface.

But recent measurements have found another cloud layer—apparently made of the chemical compound methane—about about 6 to 12 miles (10 to 20 kilometers) below that.

"If you look closely at [these] clouds, you see knots and streaks in them," Brown said at an American Geophysical Union meeting in San Francisco.

"They change very rapidly."

Plausible Explanation

In addition to observing lake effect cloud shapes on Titan, Brown noticed that the supposed lake effect clouds only occur downwind of a high-latitude region known to have large lakes.

The clouds tend to cluster particularly densely above a hilly region near the lakes, in keeping with the behavior of Earth's lake effect clouds.

"You never see them to be on the other side [of the moon] or [directly] over the lakes," he said.


He noted, though, that the Cassini spacecraft's flybys have tended to pass more frequently over the downwind region, so it is possible that clouds elsewhere might have escaped observation. And Brown admitted he hasn't proven that the clouds are undoubtedly created by lakes.

But, he said, "I'm sure this is a plausible explanation." Proving it may take a few more years of observation, he added.

The find is another example of the many Earthlike processes that have been found to be operating on Titan, said Ralph Lorenz, a planetary scientist from Johns Hopkins University who was not involved in the study.

"The physical conditions and working materials are different, but the processes themselves and the phenomena that result—whether sand dunes, river channels, or rain clouds—are remarkably similar," he said in an email.

Unknown Liquids

Ultimately, the clouds may also help scientists determine whether liquid methane or liquid ethane makes up Titan's lakes.

The apparent methane lake effect clouds detected under the cloud cap could be formed by either type of lake, experts say.

In the case of methane lakes, warm methane would evaporate into colder air passing over the lakes—exactly what happens on Earth.

But ethane lakes could also form methane lake effect clouds by warming the atmosphere as air blows across the lakes, causing air to rise until clouds condense.

Viewed over a short time frame, these two processes produce very similar looking clouds.

But as Titan's seasons progress through their slow, 16-year cycle, methane and ethane lakes would cool differently, producing different cloud patters as the seasons change.

"Over time, we'll be able to watch these things," Brown said