This means it might be possible to find life without having to drill down to the ocean far beneath the ice. It also means that any terrestrial stowaways on a space probe could easily contaminate Europa.

The cracks would provide an ideal home for life, says Richard Greenberg of the University of Arizona in Tucson. Tides caused by Jupiter's strong gravitational pull make warm water flow up and down the cracks, creating a rich chemistry along the fissures. "That makes Europa extremely habitable," says Greenberg.

There is growing evidence that Europa has an ocean of liquid water beneath its icy crust. But for life to survive there, it would also need energy. Greenberg's analysis of data from the Galileo spacecraft suggests the ice is at most 6 kilometres thick, but even if it's thinner in places it would still block the sunlight.

However, the ice is riddled with cracks caused by tidal forces, he says. Europa circles Jupiter every 3.5 days. These periodic tidal stresses form a network of fissures in the ice, which can open and close by about a metre. When a crack opens, water will rise 90 per cent of the way to the surface, Greenberg says.

At the top of the fissures, ice both freezes and evaporates into the near vacuum. When the walls close again, they squeeze slush to the top, where it freezes. The cyclic flow of water up and down the cracks continually mixes the liquid.

The connection to the surface is vital. Here, solar radiation can split molecules, producing chemicals that could supply energy for life. What's more, enough sunlight might reach the upper few metres to drive photosynthesis.

"You could have organisms a metre down, with roots hanging onto the cracks, unfolding their little leaves," says Greenberg. "If there were life in the cracks, the whole crack would be filled, it's such a hospitable setting." Some organisms might stay in one place, while others could ride up and down with water, he suggests.

Unlike our Moon, which always shows the same face to the Earth, Europa rotates full circle once every 20,000 to 200,000 years relative to Jupiter. This means cracks slowly shift into different positions. As old cracks seal up after thousands of years, organisms would be forced to migrate to new cracks or evolve ways to survive when frozen. "It's a great set-up for advancement," Greenberg says.

This article appeared in the Nov 4 issue of New Scientist New Scientist. Copyright 2000 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.

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