As sand dunes march across the Sahara, vast dunes cross the surface of Saturn's largest moon, Titan. New research from a refurbished NASA wind tunnel reveals the physics of how particles move in Titan's methane-laden winds and could help to explain why Titan's dunes form in the way they do. The work is published online Dec. 8 in the journal Nature.

"Conditions on Earth seem natural to us, but models from Earth won't work elsewhere," said Bruce White, professor of mechanical and aerospace engineering at the University of California, Davis, and a co-author on the study. "This paper gives us the thresholds to work out what models for Titan would look like."

Earth's dunes are made of silica sand, while Titan's dunes, revealed by the Cassini space probe, are made of coated grains of crystalline water. Titan's atmosphere is 95 percent nitrogen, 5 percent methane and about half again as thick as that of Earth.

When a fluid flows over a layer of particles, there is a threshold speed at which the particles start to move. On Earth, air blowing over sand will start to kick up grains when it reaches a wind speed of about 4 meters per second. But flowing water, which is closer in density to silica, will move sand at much lower speeds.

White and colleagues used a wind tunnel in the Planetary Aeolian Laboratory at NASA's Ames Research Center to establish threshold wind speeds at which grains would start to move on Titan. They found that the threshold was higher than predicted from models based on terrestrial systems.

They were able to reconcile their experiments with the models by allowing for the low ratio of density between particles and atmosphere on Titan.

The new results should help in understanding atmospheric forces on other icy moons and planets with very thin or thick atmospheres, such as Neptune's moon Triton, Pluto or on comets. They can also help us better understand movement of particles in fluids in general. Particle flows are important in a wide range of situations, including coal-mine or grain-elevator dust explosions, environmental pollution and lubricants.

For White, it's a return to work he did almost 40 years ago, before joining the faculty at UC Davis. After completing his doctoral research at Iowa State University on the physics of Martian dust storms, White helped the late Ron Greeley at NASA Ames build a wind tunnel for research on Mars and Venus, which became known as the Planetary Aeolian Laboratory. The facility was mothballed in the mid-1980s, but recently refurbished by a group led by Devon Burr at the University of Tennessee-Knoxville, who is first author on the Nature paper.

"It's very pleasing to be able to hand this on to a new generation of researchers," White said.

Other co-authors on the paper are: Nathan Bridges, Johns Hopkins University; John Marshall, SETI Institute, Mountain View; James Smith, Arizona State University; and Joshua Emery, University of Tennessee-Knoxville. The work was supported by NASA.