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Last Updated: Thursday, 4 January 2007, 10:25 GMT
'Proof' of methane lakes on Titan
The lakes should grow and shrink with Titan's seasons

The Cassini probe has spotted what scientists say is unequivocal evidence of lakes of liquid methane on Titan, Saturn's largest moon.

Radar images reveal dark, smooth patches that range in size from three to 70km across (two to 44 miles).

The team says the features, which were spied in the moon's far north, look like crater or caldera lakes on Earth.

The researchers tell the journal Nature that everything about the patches points to them being pools of liquid.

"They look very similar to lakes on Earth," explained Dr Ellen Stofan, a Cassini radar team member from Proxemy Research in Washington DC, US.

"They have channels feeding into them just like you have rivers feeding into lakes on the Earth. Their shapes, their shore-lines, all of those geologic aspects are actually very familiar."

Northern strip

The atmospheric chemistry on Titan is dominated by nitrogen and carbon-based compounds.

And with temperatures on the Saturnian satellite rarely venturing above -179C (-290F), it has long been hypothesised that abundant volumes of methane should pool on the surface into lakes, and even large seas.

Cassini has been investigating Saturn and its moons since 2004
But evidence for current bodies of liquid material on the surface has until now been sparse and equivocal.

Cassini must use radar to pierce the photochemical haze that obscures Titan's surface from its optical camera system.

The latest data was obtained last July, when the probe made its most northern radar pass of Titan to date.

The spacecraft imaged a narrow strip about 250km wide and over 1,000km long. It was found to contain more than 75 lakes.

Everything scientists know about the atmospheric chemistry on Titan suggests the liquid in the lakes should be predominantly methane, with some ethane also mixed in.

Some of the liquid would be expected to rain out of the sky, some could have welled up from below the surface.

Methane cycle

"The methane-ethane would become transparent, the way water is on Earth; it would be behaving like water, the lakes could have small waves on the surfaces," speculated Dr Stofan.

"So if it was possible for you to stand on Titan and look at the lakes, you wouldn't really know it's this weird chemistry."

Scientists have long predicted the existence of lakes

On Earth, the cycling of water between the atmosphere, the land and oceans is known as the hydrological cycle. Titan would appear to be the only other place in the Solar System to have a similar, active fluid cycle. Scientists have already dubbed it the "methane-ologic cycle".

Last month, it was announced that the radar instrument on Cassini had found an enormous mountain range on Titan.

The range lies south of the equator and is about 150km long (93 miles), 30km (19 miles) wide and about 1.5km (nearly a mile) high.

Scientists told the American Geophysical Union (AGU) Fall Meeting that the range was probably as hard as rock, but made of icy materials.

The mountains appeared in the radar images to be coated with layers of material that researchers thought could be methane "snow".

The Cassini-Huygens mission is a cooperative project of the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).

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