Earthlike "Continent" Found on Saturn Moon

Richard A. Lovett
for National Geographic News

July 21, 2006

Radar images of Titan, Saturn's largest moon, reveal a continent-size region of river valleys, hills, plains, and mountains. The area looks remarkably similar to terrain here on Earth, scientists say.

Mapping the region, called Xanadu, was one of the primary goals of the Cassini probe, now orbiting Saturn (Saturn wallpaper). The images were taken on an April 30 flyby that used radar to map a 2,800-mile-long (4,500-kilometer-long) strip of the area.

Xanadu has enticed scientists since 1994, when infrared images from the Hubble Space Telescope found a large, Australia-size bright spot on one side of Titan.

The bright spot, Xanadu, is so big it covers a tenth of the moon's surface.

"It is the only part of Titan that was named before Cassini got there, because it was such a strikingly large feature," said Jonathan Lunine, a planetary scientist from the Lunar and Planetary Laboratory at the University of Arizona in Tucson.

The name Xanadu comes from Samuel Taylor Coleridge's famous poem "Kubla Khan," about a mysterious realm where a river runs to a sunless sea.

Dense clouds shroud Titan's surface, making it mysterious and sunless as well. And the name turns out to be prescient, because Xanadu is chock full of river channels.

Earthlike Landscape

Xanadu also looks remarkably familiar.

"The images look much like radar images of Earth," Lunine said in a telephone interview. "I'm staring at an image of Arizona on my wall, and it doesn't look a lot different."

The pictures reveal a convoluted land of hills, river channels, and mountains that might be several thousand feet (one kilometer) tall—roughly the height of the U.S. Appalachian Mountains.

But the area more closely resembles the U.S. Southwest. That's because the river networks look like those found in deserts, where tributary systems are simpler than in wetter areas.

"But the liquid isn't water, and the mountains aren't rock," Lunine said. "Almost certainly, the liquid is methane and the mountains are water ice."

Intermixed with the peaks are dark, flat areas that might be lakes or playas (dry lake beds). "The radar doesn't tell us if it's liquid, but between the hills are some flat plains," Lunine said.

Some of the mountains form ridges, but again the similarity is to desert, because the ridges are relatively short and scattered.

"It's not like the Rocky Mountains, where you've got this huge chain that stretches for a very long distance," Lunine said.

It's still rugged terrain, though.

"I wouldn't want to drive a rover over that stuff," said fellow Titan researcher Ralph Lorenz via email.

"We expected from early low-resolution radar data that Xanadu had to be rough," added Lorenz, also of the Lunar and Planetary Laboratory.

"So for once—Titan has given us so many surprises—we got something right."

Sand Dunes

Xanadu appears to rise above a dry surrounding lowland. On one side Xanadu is bounded by vast, spreading sand dunes, similar to those seen on other parts of Titan (read "Saturn Moon Has Seas of Sand, Images Reveal" [May 4, 2006]).

But the dunes don't resume on Xanadu's far side.

"It seems that Xanadu is interrupting the wind flow," Lunine said

The next radar-mapping flyby is scheduled for tomorrow, when Cassini will pass over Titan's far north, where scientists hope to find lakes of liquid methane.

All told, Cassini is scheduled to do radar flybys of about 20 to 25 percent of Titan's surface—a number that might be stretched to 30 percent if the orbiter doesn't suffer a major breakdown.

But don't expect Cassini to reveal all of Titan's secrets. "Every pass looks different," Lunine said. "If you covered only 30 percent of the Earth, think of all the things you would have missed."

"It really [shows] how varied Titan's landscape is," Lorenz said.

"You have these dark plains with sand dunes around much of the equator, and then on the leading face of Titan, you hit this 'continent' of rugged mountains. It really argues for further exploration by a balloon or other aircraft that can visit all these terrains."

Steve Wall of NASA added in a statement, "We have a newly discovered continent to explore, just like the early explorers of America."

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