Studies suggest Jupiter's moon looks like a home from home

Icebergs, oxygen and liquid water - Jupiter's moon Europa looks like a chilly home away from home according to four new studies published today.

The Galileo space probe, which arrived at Jupiter on December 7th, 1995, opened up a whole new vista on this giant and its moons. There have already been numerous reports on Europa, Io and Titian but most of the attention centres on Europa.

Earlier studies highlighted what looked like an arctic-like terrain, ice fields covered with broken ice floes. The four new studies, published in the current issue of Nature, go further however, speculating on the availability of liquid water just below the surface ice and the possibility of upwelling water through breaks between the floes.

Galileo carried out a "Europa Campaign" during its initial orbits of the jovian system. The high resolution images returned by the spacecraft have provided the new clues about Europa's surface reported by groups in California, Rhode Island, Arizona and New York.

Powerful tidal forces from Jupiter's enormous gravitational pull may heat Europa's interior enough to produce a relatively warm subterranean ocean, experts believe. The new data is consistent with liquid water or slush existing at depths of no more than three to 10 km, according to a team led by Dr James Head, of Brown University, Providence, Rhode Island.

They analysed pits, domes and spots on the surface, suggesting that they were caused by diapirs - warm localised ice masses rising up and pushing through the upper ice layers. They could be produced by convection currents within an ice shell overlying a liquid water layer.

The team said the features, together with the low crater density which implies a young surface age, "increases the likelihood that liquid water may persist to the present day beneath the surface of Europa".

Dr Michael Carr and colleagues from the US Geological Survey submitted the "iceberg" theory to explain 200 metre high blocks of ice, up to 20 kilometres across, that appeared to have moved sideways as much as several kilometres, rotated and in some cases tilted.

Dark bands, seen in previous images but now observed at greater resolution, were studied by Dr Robert Sullivan and colleagues from Cornell University, Ithaca, New York.

They were suggestive of ice or water welling up and filling cracks splitting the crust. This team also pointed to the low crater density on Europa, which suggests a young surface age that could be explained by fresh ice formation.

A paper on Europa's unusual spinning behaviour from Dr Paul Geissler and researchers at the University of Arizona, Tucson, discussed the moon's resistance to being locked by gravity towards Jupiter. Normally it would be expected to have synchronous rates of rotation and revolution.

Dr Geissler's group showed that Europa seemed to be spinning faster than the synchronous rate. This indicated "decoupling" of the surface from the interior, with the core detached from the ice crust by a liquid or viscous layer. "Such decoupling could be produced by a subsurface ocean or possibly a layer of ductile ice," said the scientists.