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Monday, July 05, 2004

Cassini probes Titan's mysteries news serviceThe latest images from Cassini are completely reversing scientists' ideas about Saturn's giant moon Titan.

The space probe flew within 340,000 kilometres of Titan on Friday. It has revealed methane clouds and a strangely smeary surface that may include tectonic features and huge impact craters.

Before the flyby, planetary scientists had some low resolution images of Titan. The bright areas shown by the images were thought to be water ice, and the dark areas probably hydrocarbon gunge. But the new images suggest the opposite.

Cassini's VIMS instrument has analysed infrared light from Titan's surface which shows that the bright patches are rich in hydrocarbons and the dark areas are relatively pure ice. It may be that the huge bright region called Xanadu is not a mountain range of ice, as had been suspected, but instead a sticky plain - perhaps even a methane sea.

"The VIMS data have turned the theory on its head," says Elizabeth Turtle of the University of Arizona, a member of the Cassini imaging team.

Cumulus clouds

Meanwhile, the main cameras cut through Titan's orange haze using two filters, one polarising and one to select a particular wavelength of light that can sneak through the smog.

In the images, a field of methane clouds floats fifteen kilometres up, near Titan's south pole. "They are similar to cumulus clouds on Earth," says Kevin Baines of NASA's Jet Propulsion Laboratory in Pasadena.

It is summer in the south of Titan at the moment, which may explain why the clouds are only found there. The faint warmth of the distant Sun may be able to evaporate methane from the surface only where it shines constantly, near the south pole.

Much of the rest of Titan looks fuzzy, even though the resolution of the images is quite sharp. "It may be an atmospheric effect," says Turtle. Or it may be that many surface features simply do not have sharp edges - that Titan is a blurry world.

Giant waves

The pictures are sharp enough to show strong linear features, such as a giant letter H, which probably have a tectonic origin. That means they are created by some process internal to Titan. "But we can't say what it is yet," says Turtle.

There are also round patches, more likely to be impact craters. If so, there is at least one enormous crater more than a thousand kilometres across.

But these images are just a taster. On 26 October, Cassini will make the first of 45 close passes of Titan, flying to within about 1200 kilometres. It will get much finer images, and use its radar to map the surface in 3D. Among other things, scientists are hoping to see lakes or seas of methane, which should be whipped into giant waves by Titan's windy atmosphere.





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