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Titan conditions cry 'Surf's up!' So why can't scientists catch a wave?

The dense atmosphere on Saturn's moon Titan can generate winds that have raised 300-foot-tall dunes near the equator. The lack of observed waves on its hydrocarbon seas and lakes has been a puzzle.

By Staff writer / July 22, 2013

This undated true color image by the Cassini spacecraft released by NASA shows Saturn's largest moon, Titan, passing in front of the planet and its rings.

NASA/AP

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When it comes to building waves, Saturn's moon Titan has it all – liquid hydrocarbon seas and lakes, plus a dense atmosphere capable of generating winds that can raise 300-foot-tall dunes near the equator.

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Waves on seas and lakes, however, are another story. In the nine years since the joint NASA-European Space Agency Cassini-Huygens mission to Saturn and its moons arrived, nary a ripple has been spotted.

Now, an international team of scientists has offered an explanation for Titan's unexpectedly calm "waters," offering a theory that describes what the small waves should look like for a certain wind speed and a certain range of recipes for the liquid hydrocarbon.

If these small features show up, they could yield important clues about the composition of the liquid hydrocarbons filling the seas and lakes, the researchers say.

"We're laying the groundwork for understanding wave generation on Titan, which is something we didn't have until now," says Alexander Hayes, a planetary scientist at Cornell University and the lead author of the formal paper that lays out the explanation.

As for no detection so far? The mission to date was observing the lakes and seas on Titan during the wrong season there, the researchers suggest. Nearly all of Titan's existing lakes and seas appear in the moon's northern polar region, which has been slowly emerging from winter into spring and summer. The analysis, based on what the team says are the most comprehensive calculations and modeling to date related to wave action on Titan, suggests that the winter chill in the north brought winds there to a virtual standstill.

Now the northern hemisphere is warming up. The models suggest that winds will pick up sufficiently to begin the wave-building process – if the liquid hydrocarbons aren't too viscous for the winds to tussle lake or sea surfaces.

The waves Cassini can detect would be on the order of only a few centimeters tall, based on the characteristics of the radar the craft uses to explore the surfaces of objects in the Saturn system. Depending on how thick or thin the liquid hydrocarbons are, wavelettes could start to appear at windspeeds on the order of about 1 mile an hour.

Some 1 to 2 percent of Titan's surface is covered with liquid hydrocarbons, with 87 to 88 percent of the liquid held in three large seas, or mare. The largest, Kraken Mare in the northern hemisphere, is about the size of the Caspian Sea on Earth. No small part of Titan's fascination for planetary scientists and astrobiologists is its status as the only other body in the solar system with liquids stable on its surface. Like Earth, Titan has an active hydrological cycle – even if at far lower temperatures than any seen at Earth's surface. It's average surface temperature is a flash-freezing 323 degrees below zero Fahrenheit – cold enough to liquify methane and ethane, gases at Earth's balmy temperatures.

Most of the liquid bodies currently appear in the northern hemisphere. Based on observations, researchers estimate that the "wettest" hemisphere changes from north to south and back over 10,000-year cycles – Titan's version of Earth's transition into and out of ice ages.

Hunting for waves on Titan's lakes and seas has yielded measurements of just how remarkably calm the moon's winds can get.

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