Titan Covered With Sand Dunes

"It shows the full range of processes that modify the Earth's surface seem to be active on Titan, too," says team leader Ralph Lorenz at the University of Arizona in Tucson, US. "Some process makes the sand, and something moves it around."

That was a surprise, he told New Scientist. Until recently, it was not clear that enough erosion took place on Titan to form sand in the first place. And though the moon has a dense atmosphere, it is so far from the Sun that the sunlight it does receive was thought to be too weak to drive winds that could blow the sand into dunes.

But Cassini and its passenger probe, Huygens, which dropped to Titan's surface in January 2005, revealed erosion from rivers of methane that once coursed across the moon. And recent computer models suggest Saturn's gravity could pull on Titan's atmosphere enough to drive winds. The dunes, which run east-west, may also help map out these wind patterns, says Lorenz. "That will be very helpful for understanding Titan's weather, and for planning future missions," he says, adding that these might include balloon-borne experiments.

But it remains unclear what the sand is made of or how it formed. Lorenz and his colleagues offer two possibilities. In one scenario, water ice "rocks" are flushed down liquid methane streams during periodic storms on the moon. The rocks would break up into sand-sized particles as they tumbled down the streams, and when the streams dried up, the particles could be blown by the wind to form dunes. In the second scenario, organic compounds in the atmosphere could somehow clump together into sand-sized particles when they fall to the surface. But Lorenz says it is not yet clear what process would make them stick together. "We can't really tell between [the two possibilities] yet," he admits. But he says data from an imaging spectrometer on Cassini, called VIMS, will reveal spectral data on the dunes that could "indicate how icy versus organic-rich" they are.

The radar observations, made during a Cassini flyby in October 2005, also suggest large areas on Titan may be dry, bolstering previous observations by the craft's infrared and visible light cameras. Seas of liquid methane had previously been thought to be the source of the short-lived methane gas in the moon's atmosphere. The dunes cover a substantial portion of the region around Titan's equator, where the recent radar observations were made. But atmospheric models "suggest higher latitudes are damp", says Lorenz. "So I'm hoping perhaps to see lakes when Cassini's radar sees Titan's northern polar region" later this year.