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Titan's Sahara

Earthlike dunes on Saturn's moon Titan give scientists atmospheric clues.
Cat scratches
"Cat scratches," believed to be windblown dunes, appear in this radar image returned by the Cassini spacecraft of Saturn's moon Titan. Individual dunes measure up to .6 mile (1 kilometer) long and are oriented east-west.
NASA / JPL
June 13, 2006
Radar images returned by the Cassini spacecraft now circling Saturn contained a surprise about the gas giant's planet-size moon Titan: Its frozen surface is covered with dunes that rival Earth's deserts. "We've been bewildered for some time by the forms we've seen in the Cassini data," says Ralph Lorenz, Cassini team member at the Lunar and Planetary Laboratory in Tucson, Arizona. "The exciting thing is that we can now start to make sense of things on Titan," he says.

Lorenz and others believe the dunes — one of the few earthlike formations seen on the bizarre moon — may consist of pulverized water ice. Titan's surface comprises primarily water ice frozen rock-hard in Titan's frigid temperature (–290º Fahrenheit, –179º Celsius). Titan's ice plays the same role that stone does on Earth. Relentless winds may break down the ice into particles similar to terrestrial sand. The roughly parallel dunes spread across regions up to 930 miles (1,500 kilometers) long and are similar in form to the majority of dunes seen on Earth in sites such as Africa's Namib Desert.

But even more alien processes may be at work, Lorenz reports in the May 5 issue of Science. A constant rain of organic material drizzles from Titan's ruddy sky. This sooty sludge may pile up into material that is blown into the dune-like formations visible in Cassini images.
Seasonal methane rainstorms
Seasonal methane rainstorms may pool in Titan's low-lying areas, leaving vast dune fields behind. Left: Cassini radar image. Center: The same landscape filled with liquid methane lakes. Right: The landscape as it may appear today.
NASA / JPL; Michael Carroll
Most significant, however, is what the dunes tell astronomers about Titan's environment. Lorenz feels the dunes are evidence of an atmospheric phenomenon not visible anywhere else in the solar system. Researchers didn't expect to see high winds — the kind of winds that generate dune fields — on Titan due to the low solar heating from the distant Sun. But Saturn's gravity pulls on Titan's dense atmosphere, creating a tide of pressure that moves across the moon's face. This tide sets up alternating east-west directional winds.

The dunes' shapes and locations are consistent with this unique tidal wind, which may dominate the near-surface environment. "This is probably one of the neatest implications of our study," says Lorenz. The circulation of Titan's atmosphere may echo that of Earth's Hadley cells, air columns that bring dry air to desert regions while circulating moist air into higher latitudes.

On Titan, any moisture is in the form of liquid methane. Titan's chilly temperature holds methane to a consistency of thin gasoline. If a Hadley cell-like dynamic is present on Titan, researchers may see more dune-covered areas across Titan's equatorial region. Nearer the poles, liquid methane may pool into ponds, flowing rivers, or even vast lakes. Future Cassini passes will image higher latitudes. With Cassini's continued return of rich data, it may be a simple matter of time before scientists decode this mystery.

Michael Carroll is a science writer and astronomical artist, and author of nearly 20 children's science books. He lives with his family in Colorado.
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