The sands of Titan.

Although the surface of Saturn's moon Titan is cold enough to freeze methane, it has sand dunes like those in the Arabian Desert, radar images taken by the Cassini spacecraft reveal. But instead of being made of ordinary sand, Titan's dunes are probably grains of frozen organic compounds, water ice, or a mixture of both, Ralph Lorenz of the University of Arizona in Tucson and his colleagues report in the May 5 Science.

The parallel, 100-meter-high dunes stretch for hundreds of kilometers at the equator of Titan, whose atmosphere is rich in methane. The team proposes that the grains that make up the dunes might have formed when rare but intense methane rain drove ice particles out of the moon's frozen rocks. Alternatively, the sand could be organic material produced by sunlight-driven reactions involving methane and other compounds.

Once the grains were created, winds driven by Saturn's gravity could have sculpted them into dunes, says lorenz. In that process, the planet's gravity would raise tides in the moon's thick atmosphere. Combined with wind generated by Titan's rotation, the tide-driven breeze would average 0.5 meter per second, the researchers calculate.

Although gentle by terrestrial standards, such a breeze would be strong enough to push grains along Titan's surface and carry dark sediment from the moon's higher latitudes to the equator, Lorenz says.--R.C.