May 7, 2006
 
SCIENCE: Dunes Gird Titan, Saturn’s Most Mysterious Moon
 
By David Perlman
San Francisco Chronicle
 
Titan, the planet Saturn's most mysterious moon, now poses a fresh puzzle: mile upon mile of rippling dunes similar to the desert sand dunes of the Sahara or Arabia have been discovered on its tarry surface -- but what they're made of nobody knows.
 
Scientists have puzzled over the startling features they've found on Titan ever since the European Space Agency's Huygens probe parachuted to a safe landing on the moon's dark orange surface last year.
 
The Huygens probe's brief life on Titan's surface -- and the powerful cameras and radar eyes aboard the Cassini spacecraft far above -- have revealed persistent rainfalls of liquid methane; flash floods of organic compounds filling rivers, creeks and drainage channels; eroding rocks of water ice; a porous, crusty surface; and hills of unknown composition rising above the shorelines of what may be vast basins of methane.
 
But now the Cassini-Huygens science team is baffled by the latest discovery on Titan by radar astronomers: the rippling dunes that at the very least provide unambiguous evidence that gentle winds are blowing steadily across the satellite's surface while tides generated by the gravity of its parent planet Saturn are also molding the surface.
 
A team of 40 scientists and engineers, headed by Ralph D. Lorenz of the University of Arizona's Lunar and Planetary Laboratory in Tucson, is reporting on the unexpected dune discovery today in the journal Science. Laurence Soderblom of the U.S. Geological Survey is the radar team leader at NASA's Jet Propulsion Laboratory in Pasadena.
 
"How those dunes are formed we have no idea," Lorenz said in an interview Wednesday. "Our best candidate is that they pile in rows of fine material raining down from Titan's atmosphere, although they may also come from dirty ices eroded out of the rocks that the Huygens probe detected when it landed there last year."
 
The radar images gathered during Cassini's recent flights past the planet's enigmatic moon reveal wind-blown dune fields hundreds of yards wide and more than 1,000 miles long -- most of them near Titan's equator.
 
The images show clear evidence that the parallel rows upon rows of dunes bend around small mountains and rock formations as they drift across the moon's equatorial surface from northeast to southwest -- in the same way sand dunes on Earth's deserts bend around obstacles in their path.
 
The ridges of the dunes, according to the radar data, may be up to 150 yards high, and have a striking resemblance to the parallel dunes of sand that cover vast desert areas of Namibia, the Sahara and the fabled Rub al-Khali, or Empty Quarter, of Saudi Arabia.
 
Although the Huygens probe encountered winds up to 250 mph as it descended for 700 miles in free fall from its berth aboard Cassini, as it neared Titan's surface its parachute deployed and the probe drifted gently under a breeze no greater than 1 mph as it neared the surface.
 
That same gentle wind speed is what appears to be forming the dunes of Titan, according to Lorenz and his colleagues, and from their radar data they infer that the dunes have actually circumnavigated the moon's surface many times during the billions of years since Saturn and its dozens of moons first formed.
 
"The morphology of these beautiful features, familiar to us from terrestrial arid regions, is a comforting sign that even though the environment and working materials on Titan are exotic, the physical processes that shape Titan's surface can be understood and studied here on Earth," Lorenz and his colleagues wrote in their Science report.
 
Other scientists on the Cassini-Huygens mission were equally impressed. "It shows again and again that Titan is not at all what we'd thought it would be," said Christopher McKay, an astrophysicist at NASA's Ames Research Center in Mountain View who recently returned from studying a vast dune field in North Africa's Sahara Desert in order to compare them to dunes known to exist on Mars.
 
"When Huygens was built," McKay said Thursday, "it was built like a boat to float on a Titan sea, but now we have a desert, and what we'd thought would be a surface of gooey organic stuff now turns out to be dry and wind-blown. It's a real surprise."
 
To Carolyn Porco, leader of the Cassini imaging team and senior research scientist at the Space Science Institute in Boulder, Colo., the discovery of Titan's dunes "is one of the most spectacular things that's been discovered in the Saturn system."
 
"It's telling us that the surface winds are blowing consistently, that tides are a significant influence, and that the dune material has to be really dry -- although we'll still be struggling to learn just what it's made of. "We still have a major puzzle about its composition, although it all looks so magical, and so much like the deserts here on Earth."
 
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