Under the icy shell covering Titan, Saturn's largest moon, lies a "dead sea" as salty as any ocean on Earth, making it an unlikely place to look for any alien life, scientists say.

Measurements of the moon's significant gravity by the NASA/European Space Agency Cassini spacecraft suggests the ocean assumed to exist under the moon's frozen surface would have to be of relatively high density, they say.

A global body of water with high levels of dissolved salts of sodium, potassium and sulphur would be dense enough to fulfill the requirement, they say.

That would be as salty as Earth's Dead Sea, which can reach salt concentrations of around percent as compared to a global Earth average of 3.5 percent.

"This [Titan ocean] is an extremely salty ocean by Earth standards," study lead author Giuseppe Mitri of the University of Nantes in France says. "Knowing this may change the way we view this ocean as a possible abode for present-day life, but conditions might have been very different there in the past."

The ice shell that covers the surface of Titan is not consistent in its thickness, suggesting the salty ocean beneath may be slowly crystallizing in some areas and itself freezing into ice.

Such ongoing freezing could limit the possibility of Titan's ocean being habitable for any kind of life, the researchers said, since it would create a barrier limiting the exchange of materials between the moon's atmosphere and the freezing ocean underneath the surface.

Astronomers have detected significant amounts of methane and other hydrocarbon compounds in Titan's atmosphere, a mix some say is similar to the "prebiotic" chemistry existing on the early Earth.

Sunlight normally breaks down such compounds in just millions of years, suggesting the methane in Titan's atmosphere has somehow been constantly replenished during the moon's 4.5 billion year history.

Outgassing of methane from the salty underground ocean into the atmosphere is likely occurring at widely scattered "hot spots," like the volcanic hot spot forming the chain of the Hawaiian Islands over eons here on earth.

Given the rigidity and overall stability of Titan's surface ice cover, processes like plate tectonics or convection are unlikely sources for such outgassing, scientists say.

More research will be needed to conclude that for sure, they say.

"Our work suggests looking for signs of methane outgassing will be difficult with Cassini, and may require a future mission that can find localized methane sources," study co-author Jonathan Lunine, a Cassini mission scientist with Cornell University says. "As on Mars, this is a challenging task."

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