Probe spots Xanadu on Saturnian moon

NASA's Cassini spacecraft has sent back these two views of Titan, Saturn's most mysterious moon. The bright feature known as Xanadu can be made out in the sunlit portion of the disk at right.
NASA's Cassini spacecraft has sent back these two views of Titan, Saturn's most mysterious moon. The bright feature known as Xanadu can be made out in the sunlit portion of the disk at right.NASA / JPL / SSI
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The Cassini spacecraft has returned its first images of the smog-shrouded moon Titan that reveal surface features, including a bright area dubbed Xanadu that so far has eluded explanation.

"We have at last glimpsed the surface of the fabled world, Titan, Saturn's largest moon and the greatest single expanse of unexplored territory remaining in the solar system today," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute.

Titan's frigid surface may contain seas of methane and other hydrocarbons that stay liquid at extreme temperatures.

"Imagine Lake Michigan brimming with paint thinner," Porco suggests.

Twilight at noon
Because the methane fuels a thick, global layer of smog, astronomers have never gotten a good look at the Mercury-sized moon. Porco imagines "the Titan nights starless and the days eerie dark, where high noon is as dim as deep Earth twilight."

Titan is thought to contain organic chemistry similar to early Earth, prior to the development of life. Scientists are eager to learn more about the moon and its composition, because they think it might provide clues to how life developed out of an organic soup here.

The new images were taken in mid-April and released Thursday. They are similar to the best ground-based efforts to photograph Titan, and Porco said the successful imaging from a difficult angle and long distance portends better views to come as the spacecraft speeds closer to the Saturnian system.

Better imaging can be done with the sun being directly behind the spacecraft, she told, rather than the moon being sunlit from the side and reflected light having to slice through more of the atmosphere, as in the latest pictures.

The two new views of Titan show an area from its south pole up to about 50 degrees north latitude. The second was taken after the moon had rotated 90 degrees from the position in the first image.

A large, bright region in one of the pictures is the area previously named Xanadu. It could be a mountain range, a giant basin, a smooth plain or some combination of these, scientists say. It is certainly not a blissful locale from a human perspective. The average temperature on Titan is minus-289 degrees Fahrenheit (-178 Celsius).

Almost there
Cassini will enter orbit around Saturn on July 1 and study the planet, its rings and the many moons. On July 2, it will pass 217,500 miles (350,000 kilometers) from Titan. That will be the first chance to date to see small-scale features on the moon, down to 1.2 miles (2 kilometers).

Porco doesn't know how well the surface will be resolved, however. To peer through the methane-rich atmosphere, Cassini's camera uses a filter designed to block out all but a narrow band of near-infrared light that passes through methane.

Unfortunately, she explained, light in this narrow band is somewhat scattered by aerosols in Titan's atmosphere.

The aerosol haze, or smog, is not well-understood. Scientists think it starts about 62 miles (100 kilometers) above the surface and extends another 120 miles (200 kilometers) upward. But that's based on computer modeling, not observations. "Models are often wrong," Porco pointed out.

The actual thickness of the haze will determine the extent to which the narrow band of near-infrared sunlight is scattered as it goes through the atmosphere, reflects off Titan's surface, and heads back into space.

Even closer
Scientists have another chance to get a better view of Titan.

Early next year, the Huygens probe, which is piggybacked on Cassini, will plunge through the moon's atmosphere and land on the surface, or possibly splash down. It will take many measurements of the atmosphere, as well as pictures. And if all goes well it will send back images from the surface.

Porco speculated on what might be found:

"Over eons, smog particles have drifted downwards, growing as they fell, to coat the surface in a blanket of organic matter. On high, steep slopes, methane rains have washed away this sludge, revealing the bright bedrock of ice," she says. "Occasional bolts of lightning momentarily brighten the gloomy landscape, and wind-blown waves lap the shores of hydrocarbon lakes and seas dotting the scene."