for National Geographic News
When scientists first began using powerful instruments to peer through the dense atmosphere of Titan, Saturn's largest moon, they found sprawling dark areasregions that looked remarkably like oceans.
Now the latest images from a close flyby last October by NASA's Cassini probe reveal that these dark areas are seas of sand, complete with vast complexes of startlingly Earthlike dunes.
The dunes are up to 150 meters (500 feet) tall and hundreds of kilometers long, dominating large areas of Titan's surface near the equator.
They are long, linear dunes similar to a type commonly seen in Namibia (see map), the Sahara, parts of Australia, and the Arabian Peninsula.
The discovery was surprising, says Ralph Lorenz, a researcher with the Lunar and Planetary Lab based at the University of Arizona in Tucson.
Titan was not expected to have winds strong enough to produce dunes, even in gravity that is only one-seventh that of Earth's.
That's because wind is generally driven by solar heat, and Titan receives only a thousandth as much solar energy as Earth does.
What the predictions overlooked was Titan's proximity to a massive planet, says Lorenz, who is lead author of a study appearing in tomorrow's issue of the journal Science.
Saturn's gravity (wallpaper: Saturn) stirs Titan's atmosphere in the same way our moon's gravity stirs tidal currents in Earth's oceans.
Tidal winds, in fact, exist on Earth but are so weak they are barely detectable, even on the most sensitive instruments.
Titan's winds turn out to be strong enough to blow around sand-grain-size particles.
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