On Titan, soft breeze = big sand dunes

Dark patches once thought to be seas on Saturn's largest moon may instead be vast swaths of desert.

If Hollywood ever remakes "Lawrence of Arabia" with a sci-fi twist, it could try Saturn's exotic moon, Titan, for the setting.

Planetary scientists have discovered on Titan vast swaths of "sand seas" covered with row after row of dunes from 300 to 500 feet high. Radar images of these seas, which stretch for hundreds of miles, bear a stunning likeness to ranks of dunes in Namibia and Saudi Arabia.

This wasn't the kind of sea scientists initially expected to find. These desert formations appear in dark patches of the moon's surface once suspected to be seas of liquid methane and ethane. Their "sand" is unique, perhaps composed of tiny grains of ice or hydrocarbons. And their presence is giving researchers new insights into the geological and atmospheric conditions shaping the moon.

Indeed, these features may dominate the dark regions at the moon's equator, the researchers say.

The discovery "is very exciting from a Titan-system perspective," says Ralph Lorenz, a planetary scientist at the University of Arizona in Tucson. "There are reasonably good ideas about how dunes form and move on Mars and Earth," he says. "Titan gives us a whole new laboratory to explore these kinds of processes because it has such different circumstances."

Dr. Lorenz led the team reporting the results in the current issue of the journal Science and has written a book on this moon. Images underpinning the team's report come from radar on the Cassini orbiter, part of a US-European mission to study the Saturn system.

On Earth or Mars, he explains, heat from sunlight drives the winds that form sand dunes. On Titan, sunlight is too weak to do the job. Instead, Saturn's gravity acts as bellows - pushing and pulling on Titan's dense atmosphere as the moon traces an elliptical orbit around planet. These tidal forces are 400 times stronger on Titan than on Earth, where sunlight trumps any effect of lunar gravity on wind.

By Earth standards, wind speeds on Titan - typically about 1 mile an hour - would barely qualify as a puff of breeze. But the moon's atmosphere is so dense and its gravity is so weak, researchers reason, that the the feeble wind is enough to sweep small particles for long distances across the surface.

The typical size of sand grains is remarkably constant, whether one looks at Titan, Mars, or Earth, adds Ronald Greeley, a planetary geologist at Arizona State University in Tempe. The grains are roughly the size of granulated sugar. With colleague James Iverson, Dr. Greeley ran lab experiments in the 1980s that suggested Titan's winds might be strong enough to move material around the surface. The new images confirm that work.

"It's great fun to think about one fundamental process operating in such drastically different environments," he says.

The source of Titan's sand remains a puzzle. The team so far has failed to find any obvious ones. One possibility: The supply of sand-like grains have built up over the moon's 4.5 billion-year-history through sunlight-driven chemical reactions in Titan's stratosphere. The resulting particles could have rained to the surface over time. Or, liquid methane flowing on the surface could erode exposed formations of ice "bedrock." Where that would occur is unclear, given the absence so far of images of fluids. But images from the European Huygens lander, as well as from Cassini, show features in boulders and the landscape shaped by fluids.

Lorenz points to one possible explanation: a picture of the planet's climate offered earlier this year by two French researchers who published the results of modeling studies in Science. In essence, the planet's atmospheric circulation patterns dry out regions around the moon's equator but allow for torrential storms near the poles. These cloudbursts, though relatively rare, could provide fluids that flow with erosive force sufficient to grind loose solids on the surface into grains. The team's radar observations would seem to confirm the equator-is-dry portion of the model's results.

Indeed, researchers say, the model calculations and radar results raise the possibility that the dark patches of surface at higher latitudes may also be reservoirs of sand. So far, these patches show no dune-like patterns. If they are sand, Lorenz and his team suggest, the areas may be kept smooth because winds are weaker there or the areas are moist.

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