By BBC News Online Science Editor Dr David Whitehouse

A radiation-driven ecosystem could exist in the ocean thought to lie beneath the surface of Jupiter's moon Europa, a scientist has suggested.

Ever since the Voyager spacecraft flew past the Jupiter system in the 1970's, astronomers have been fascinated by Europa and its bizarre striped surface and the prospects for primitive forms of life on the satellite.

But life needs energy. It has been suggested that on the floor of the suspected subterranean ocean there may be hydrothermal vents like those found on Earth.

These vents, which gush hot water and minerals, could provide both the energy and the food sources for primitive Europans.

Mighty Jupiter

However, some analyses of energy sources on Europa have been pessimistic saying that there may not be enough free-energy available to support life, or at least life as we know it.

If Europa really does have an ocean, it lies beneath an ice layer that is too thick to permit photosynthesis to be a source of energy for life-forms.

Hydrothermal vents may or may not exist, but such ecology as exists on Earth depends largely on oxidants that filter down from the surface of the ocean and this would not happen on Europa.

In a new analysis, published in the journal Nature, Chris Chyba of the Seti (Search for Extra-Terrestrial Intelligence) Institute in California suggests that charged sub-atomic particles that are swirled around Jupiter by its mighty magnetic field should interact with Europa's icy surface providing energy that primitive life-forms could tap.

Unusual colours

"Microbial life could exist in concentrations detectable by surface landers able to filter meltwater from Europa's ice," says Dr Chyba.

He suggests that Europa could have a radiation-driven ecosystem and has calculated the minimum amount of biomass in this ecosystem by comparing it to ecosystems on Earth. His estimate is 20 billion grams.

Microbial blooms could also occur wherever the ocean beneath the ice broke through. Dr Chyba suggests that any unusual colours detected on the surface of the moon may be an indicator of this.

So far, this is all speculation. These calculations provide a plausible argument for one of many possible types of charged-particle or radiation-driven ecosystem. But "only direct exploration," Dr Chyba concludes, "will reveal whether life on Europa actually exists."