Published online 8 June 2005 | Nature | doi:10.1038/news050606-10

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Titan disappoints ocean hunters

Saturnian moon has no seas, but may well be volcanically active.

This false colour picture of Titan was taken during Cassini's 26 October flyby. The inset appears to reveal an icy volcano.This false colour picture of Titan was taken during Cassini's 26 October flyby. The inset appears to reveal an icy volcano.© NASA

Although pictures of Titan seem to show rivers, deltas and oceans, Saturn's giant moon is as dry as a bone, scientists say.

Researchers know that the moon has methane in its atmosphere, and they had speculated that this came from fumes evaporating from giant lakes of hydrocarbons. They envisaged rainstorms of liquid methane and other wild weather events on the alien surface.

So they eagerly anticipated pictures from the Cassini probe, a joint venture between NASA and the European Space Agency, which flew past Titan on 26 October 2004 and peered through the moon's thick smog using its Visual and Infrared Mapping Spectrometer. Surprisingly, a detailed analysis of these images, published this week in Nature1, has shown no significant bodies of liquid anywhere on the moon.

Instead there are signs of a volcano. And this could be a source for the 'river channels' that Cassini's piggybacking partner, Huygens, saw when it landed on the moon.

"It was a big surprise," says Christophe Sotin, a planetary scientist from the University of Nantes, France, who led the study. "It would have been so neat to find a body with liquids on the surface, just like Earth," he says.

Mark Leese, part of the Huygens team from the Open University, Milton Keynes, UK, says the data do not "absolutely rule out" the possibility of liquid on the surface. But they do certainly exclude the possibility of giant lakes or oceans, he says.

Flow charts

The survey team found a 30-kilometre-wide dome, which could be a volcano. It has a depression at the top that looks like a caldera, the collapsed top of a volcanic vent. And flow lines radiate from the dome, as if particles of ice have been scattered by an eruption.

This might explain the river channels, says Sotin. He thinks that Titan's surface is a crust of dirty ice, tens of kilometres thick, which is slowly circulating and drawing up material from deep inside the moon.

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Rising chunks of ice could bring trapped methane with them, which would be released as a gas when it reached the surface, he says. The methane could then gather into hydrocarbon storm clouds that pour rain on to the surface, causing flash floods that carve the deep channels, he explains.

"The features at the Huygens landing site do look like river channels, and I've no doubt they were made by liquid methane," says Leese. "So maybe we're just in a dryish period at the moment."

The key question to answer is whether the channels were carved by an annual flood, or by a freak event millions of years ago, he adds. The absence of significant craters on Titan suggests that the surface has been swept clean relatively recently, and observations during Cassini's remaining three years around Saturn should provide interesting data.

"The methane story isn't over yet," promises Leese. 

  • References

    1. Sotin C., et al. Nature, 435. 786 - 789 (2005). | Article |