A GLOBAL ocean almost certainly lurks beneath the surface of Europa, Jupiter鈥檚 fourth-largest moon. But the big question is whether that ocean, and the life it may support, is locked away under tens of kilometres of ice or laps tantalisingly close to the surface. Two teams of planetary scientists have come up with radically different answers.
James Head and Robert Pappalardo of Brown University in Providence, Rhode Island, believe that the ocean is capped by a layer of ice between 10 and 15 kilometres thick. They say the pits, domes, ridges and dark spots seen on Europa can only be explained by a thin layer of brittle ice riding on a thicker ice layer that is warm enough to deform and flow, as glaciers do on Earth. Heated by the ocean, ice blobs shoulder their way towards the surface, producing the features revealed by the Galileo spacecraft. 鈥淲arm ice is playing the principal role in shaping the surface,鈥� says Pappalardo.
But a team led by Richard Greenberg at the University of Arizona鈥檚 Lunar and Planetary Laboratory in Tucson argues for a much thinner crust. Greenberg鈥檚 colleagues Randy Tufts and Greg Hoppa have shown that Europa鈥檚 tides have created chains of crescent-shaped cracks, each 100 kilometres long, which stretch across the moon鈥檚 surface. 鈥淔or these cracks to occur, you have to have a substantial ocean. And the crust has to be thin relative to the ocean,鈥� Greenberg says.
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Both teams would like to see Europa鈥檚 surface probed for signs of life. A thin crust would allow chemicals into the ocean and signs of life to reach the surface. But Head thinks life could also have evolved in an ocean covered by a thick, slowly cycling ice layer.
