The radar images, taken by Cassini last October, show dunes 330 feet (100 meters) high that run parallel to each other for hundreds of miles at the moon's equator. One dune field runs more than 930 miles (1,500 kilometers), said Ralph Lorenz of the University of Arizona's Lunar and Planetary Laboratory.

"It's bizarre," Lorenz said. "These images from a moon of Saturn look just like radar images of Namibia or Arabia. Titan's atmosphere is thicker than Earth's, its gravity is lower, its sand is certainly different - everything is different, except for the physical process that forms the dunes and resulting landscape."

A decade ago, scientists thought Titan was too far from the Sun to experience solar-driven surface winds powerful enough to sculpt sand dunes. They also hypothesized that the dark regions at Titan's equator might be an ocean of liquid ethane that would trap sand.

More recently, scientists have learned that Saturn's powerful gravity creates tidal effects in Titan's atmosphere that are roughly 400 times greater than the Moon's tidal pull on Earth.

First seen in circulation models just a few years ago, Lorenz and colleagues reported in the May 5 issue of Science, "Tides apparently dominate the near-surface winds because they're so strong throughout the atmosphere, top to bottom. Solar-driven winds are strong only high up."

The dunes seen by Cassini radar are a particular linear or longitudinal type characteristic of dunes formed by winds blowing from different directions. The tides cause wind to change direction as they drive winds toward the equator, Lorenz explained.

When tidal winds combine with Titan's west-to-east zonal winds, as the radar images show, they create dunes aligned nearly west-east, except near mountains that influence local wind direction.

"When we saw these dunes in radar it started to make sense," Lorenz said. "If you look at the dunes, you see tidal winds might be blowing sand around the moon several times and working it into dunes at the equator. It's possible that tidal winds are carrying dark sediments from higher latitudes to the equator, forming Titan's dark belt."

The team's model of Titan suggests tides can create surface winds that reach about one mile per hour, or half a meter per second).

"Even though this is a very gentle wind, this is enough to blow grains along the ground in Titan's thick atmosphere and low gravity," Lorenz said. Titan's sand is a little coarser but less dense than typical sand on Earth or Mars. "These grains might resemble coffee grounds," he added.

Exactly what the grains are made of - organic solids, water ice or a mixture of both - remains a mystery, although Cassini's Visual and Infrared Mapping Spectrometer could obtain the results in later passes.

Another mystery is how the sand formed. It may have originated when liquid methane rain eroded particles from ice bedrock. Researchers previously thought Titan does not experience enough rain to erode much bedrock, but new observations and models suggest that individual storms could be large and still yield a low average rainfall.

Images taken during the Huygens probe descent to Titan in January 2005 showed gullies, streambeds and canyons in the landscape, and Cassini has confirmed the same features via radar.

These features show when it does rain on Titan, it rains in very energetic events, just as it does in the Arizona desert. Energetic rain that triggers flash floods could be a mechanism for making sand.

Alternatively, the sand may come from organic solids produced by photochemical reactions in Titan's atmosphere.

"It's exciting that the radar, which is mainly to study the surface of Titan, is telling us so much about how winds on Titan work," Lorenz said. "This will be important information for when we return to Titan in the future, perhaps with a balloon."

Related Links
Arizona LPL
Cassini-Huygens at JPL
Cassini Imaging Team