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Last Updated: Friday, 9 September 2005, 10:39 GMT 11:39 UK
Titan moon occupies 'sweet spot'
By Paul Rincon
BBC News science reporter, in Cambridge

Titan, Nasa
Wind, rain and volcanism play a big role on Titan
Earth and Saturn's moon Titan show striking similarities because both occupy "sweet spots" in our Solar System, researchers have said.

Many processes that occur on Earth also take place on this moon, say scientists participating in the US-European Cassini-Huygens mission.

Wind, rain, volcanism and tectonic activity all seem to play a role in shaping Titan's surface.

One scientist even sees a way that life could survive on the freezing world.

"Titan is perhaps the most Earth-like place in the Solar System other than Earth, in terms of the balance of processes," says Jonathan Lunine, of the University of Arizona, US, who is an interdisciplinary scientist for Cassini-Huygens.

Titan is perhaps the most Earth-like place in the Solar System other than Earth
Jonathan Lunine, University of Arizona
"Wind-driven processes, river channels, evidence of rain, possible lakes and geological features that may have to do with volcanism and tectonism."

Different chemistry

But the chemistry that drives these processes is radically different between the two worlds. For example, methane seems to perform many of the same roles on Titan that water plays on Earth.

Dr Lunine believes that Earth and Titan both have similar processes occurring because they occupy "sweet spots" in the Solar System. Being in one these spots requires striking a balance between size, or mass, and distance from the Sun.

To demonstrate the idea, Dr Lunine considered three planets in the inner Solar System: Venus, Earth and Mars.

The mass of a body corresponds to an ability to sustain heat flow from its interior, while distance from the Sun is correlated with the ability to retain liquid water, a driver of geological activity on Earth.

Venus is about the same size as Earth. But it is so close to the Sun that any water it had must have boiled off. As such, there is no hydrological cycle and no tectonic activity, says Lunine.

Mars is distant enough from the Sun to retain water. But its small size caused it to cool quickly, turning water to ice and ending large-scale geological activity. Earth occupies an intermediate position - the "sweet spot".

The researcher then turned to three bodies in the outer Solar System: Ganymede, Titan and Triton. The chemistry is different, but similar principles apply.

Jupiter's moon Ganymede, the closest of the three to the Sun, is similar in size to Titan, but lacks the methane and nitrogen that drive liquid processes on the Saturnian moon: "It's a kind of baked out version of Titan," said Lunine.

Neptune's moon Triton, much further from the Sun than both Ganymede and Titan, possesses methane and nitrogen. But its small size caused them to freeze, ending any prospect of geological activity.

Scientists have been revealing new details about Titan at the meeting in Cambridge. Ralph Lorenz of the University of Arizona, US, said that the river channels and flows on Titan were fashioned by "monsoon" events.

'Catastrophic rains'

It takes a relatively long time for methane to build up to a point where it can rain down on Titan's surface. Scientists, therefore, think rains are only occasional, but catastrophic, when they occur.

Evidence also suggests Titan is constantly being resurfaced by a fluid mixture of water and ammonia spewed out by volcanoes and hot springs, explaining why Titan is not littered with impact craters like its neighbours

Earth from space, Nasa
Many processes that occur on Earth also take place on Titan
A surface feature called Ganesa Macula may show just such a flow emanating from a volcanic crater.

The moon's icy surface is also covered with a film, or patina, of organic compounds, Cassini-Huygens data shows.

One researcher has even proposed a way for life to survive on the giant Saturnian satellite. It is too cold for organisms to survive on the surface of Titan, where temperatures are about -178C (-289F).

But David Grinspoon, of the Southwest Research Institute, US, says organisms could occupy specific niches, such as hot springs. They could use acetylene, in reaction with hydrogen gas, to release enough energy to power metabolism, and possibly to heat their environments.

The Cassini spacecraft entered orbit around Saturn on 1 July, 2004, on a mission to explore the ringed planet and its satellites.

In December, it released the piggybacked Huygens probe on a collision course with Titan. Two weeks later, Huygens tumbled through the moon's atmosphere and made a successful touchdown on the surface.

New results from the mission were presented at the American Astronomical Society Division of Planetary Sciences meeting in Cambridge, UK.

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