Chilly moon's icy secrets

Is there an ocean beneath Callisto's icy surface?

By BBC News Online science editor Dr David Whitehouse

Jupiter moon Callisto may not be a boring lump of rock and ice after all.

A new study, by Javier Ruiz at the Universidad Complutense, Madrid, Spain, suggests that the cratered and pitted surface of Callisto may conceal a deep ocean.

Ruiz employs a more sophisticated analysis of ice than previously used, showing that ice in the moon's outer layers would not freeze solid.

His calculations suggest that a 20-kilometre- (12-mile-) deep ocean of water could exist some 150 km (93 miles) below Callisto's surface. Two other Jupiter moons, Ganymede and Europa, are also thought to have subsurface oceans.

Ruiz's conclusion is based on a reappraisal of data from a flyby by the Galileo spacecraft, which surprised astronomers by detecting a magnetic field around the moon.

Saltwater ocean

Because Callisto does not have a metallic core - the usual source of a magnetic field - excited researchers believed that this field was caused by an ocean of salt water whose currents were conducting an electric current.

But these hopes seemed dashed when calculations, based on the rate at which Callisto's surface convects heat, seemed to show that any subsurface ocean would have frozen solid long ago.

But, Ruiz, writing in the journal Nature, now shows those calculations to be flawed.

"His analysis brings into question our understanding of ice and water in the outer Solar System, and will force a re-evaluation of the thermal and structural models of the largest 14 or so moons of Jupiter," comments Kristin Bennett of Los Alamos National Laboratory in New Mexico, US.