Saturn's Moon Titan Has Seas of Sand, International Team Finds
08 May 2006
Dunes 100 meter high cover hundreds of miles at Titan's equator
Washington – Until a couple of years ago, scientists thought the dark equatorial regions of Titan might be liquid oceans.
New radar evidence shows they are seas – but seas of sand dunes like those in the Arabian or Namibian deserts, a University of Arizona (UA) member of the Cassini radar team and colleagues from the United States, France and Italy report in the May 5 edition of the journal Science.
Radar images taken when the Cassini spacecraft flew by Titan last October show dunes 100 meters high that run parallel to each other for hundreds of miles at Titan's equator, according to a May 4 UA press release.
One dune field is more than 1,500 kilometers long, said Ralph Lorenz of UA's Lunar and Planetary Laboratory.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and Agenzia Spaziale Italiana, the Italian space agency.
"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 years ago, scientists believed that Saturn's moon Titan was too far from the sun to have solar-driven surface winds powerful enough to sculpt sand dunes. They also theorized the dark regions at Titan's equator might be liquid methane oceans that would trap sand.
But researchers have since learned that Saturn's powerful gravity creates significant tides in Titan's atmosphere. Saturn's tidal effect on Titan is roughly 400 times greater than the moon's tidal pull on Earth.
As first seen in circulation models a couple of years ago, Lorenz said, "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 that is 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 said.
And when the tidal wind combines with Titan's west-to-east zonal wind, as radar images show, it creates 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," he 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 researchers' model of Titan suggests tides can create surface winds that reach about a half-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 likely is a little coarser but less dense than typical sand on Earth or Mars.
"These grains might resemble coffee grounds,” he added. But whether the grains are made of organic solids, water ice, or a mixture of both is a mystery.
The variable tidal wind combines with Titan's west-to-east zonal wind to create surface winds that average about a half a meter per second. Average wind speed is a bit deceptive because sand dunes would not form on Earth or Mars at their average wind speeds.
Additional information about Saturn and the Cassini-Huygens mission is available at the NASA Web site. The full text of the press release is available at the UA Web site.
(Distributed by the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)
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