Jupiter moon may hold life in ice-locked ocean

The chances of finding alien life in the further reaches of the solar system double today with a persuasive new theory identifying a second hidden ocean among Jupiter's moons.

Scientists have suspected for some time that Callisto, a pitted moon the size of Mercury, could harbour an ocean of salty water, 12 miles deep, beneath an icy sheath.

But until now, there has been no explanation of why all the water did not freeze so far from the heat of the sun.

Now a Spanish scientist, Javier Ruiz of Madrid's Universidad Complutense, has pointed out that in the extreme conditions of the Jovian moons, ice acquires special properties and becomes a heat insulator. Under Callisto's ice blanket, the feeble heat from radioactive minerals is enough to stop the sea freezing.

Another of Jupiter's big moons, Europa, is also thought to have an ocean under its ice covering. It is warmer than Callisto because of enormous tidal energies exerted on it by Jupiter.

Water is essential for life, and many scientists now think that the lunar Jovian oceans, dark and cold as they may be, offer a better prospect for finding alien life forms than Mars.

Ruiz's work is published in the journal Nature. Commenting on his work in the same edition, Kristin Bennett of the Los Alamos national laboratory in New Mexico said that until now Callisto had always been shunned by planetary scientists in favour of other moons, like Europa with its icebergs or Io with its active volcanoes.

"Around 80% of Callisto's surface is cratered and pitted, from which it is inferred that, unlike its companions, Callisto has been volcanically and tectonically dead almost since its origin," said Dr Bennett. "From a geologist's point of view, Callisto was boring.

"But Ruiz shows that an internal water-ice ocean ebbing and flowing under its surface is perfectly plausible."