Lakes of liquid methane have been discovered on the surface of Titan, Saturn’s largest moon, by a spacecraft that is exploring the ringed planet and its satellites.

Radar observations of Titan’s northern hemisphere have revaled several large standing bodies of liquid, up to 70 kilometres (43 miles) in diameter, suggesting that the moon is much more like Earth than had been thought.

Before the Cassini mission and its Huygens probe, which landed on Titan in January 2005, scientists had believed it highly likely that the moon had lakes, seas and even oceans formed of the hydrocarbons methane and ethane. Although these are gases on Earth, in the low temperatures of Titan they could in theory exist at the surface in liquid form. Telescopes had shown large, smooth patches on Titan’s surface that appeared to be liquid, though these were impossible to resolve clearly through the moon’s thick, smoggy atmosphere.

Cassini and Huygens failed initially to find any liquid lakes and oceans on the moon’s surface, though they did discover evidence that liquid methane falls as “rain” and flows intermittently in streams and rivers.

A Cassini fly-past on July 22, however, confirmed that methane lakes do exist, as originally predicted. Near the moon’s north pole the probe found more than 75 patches that showed up dark and smooth under radar analysis. Some were circular and some irregular.

They occur in depressions, and their similarity to lake-like features on Earth has convinced the research team that they are filled with liquid methane. Details of the research are published today in the journal Nature.

Ellen Stofan, of University College London, who led the team, said: “The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions.

“The surface of Saturn’s haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface.

“Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surfaces. Here, we provide definitive evidence for the presence of lakes on the surface of Titan.”

The findings indicate that Titan is the only body in the solar system, apart from the Earth, with a hydrological cycle — the process by which liquids evaporate from the surface and fall as precipitation, in the form of rain and snow.

This makes Titan even more intriguing to scientists as a model for what the Earth might have been like in its very early life. Our planet also once had an atmosphere composed primarily of methane and nitrogen, like Titan’s, about four billion years ago, shortly before the emergence of life.

Christophe Sotin, of the University of Nantes, western France, who wrote about the discovery for Nature, said: “As far as we know, there is only one planetary body that displays more dynamism than Titan. Its name is Earth.”

World apart

Source: Nasa/Times archive

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