In the search for microbial life on Jupiter's icy moon Europa, a new idea has emerged, suggesting that heavy doses of lethal radiation surrounding the massive planet might spur chemical reactions on its tiny satellite, providing fuel for life in the suspected liquid ocean below.

In a paper in the January 27 issue of the journal Nature, Christopher Chyba of the SETI Institute lays out a novel method by which biology might arise.

As Chyba explained to space.com, fostering life as we know it is something like operating your Chevy. You need fuel and a little oxidant.

Europa has in recent years been nominated by numerous exobiologists as the likeliest candidate in our solar system for extraterrestrial life, however teensy a form it might take. Most everyone agrees there are no fish swimming around on Europa. We're talking instead of small creatures. Wee little things that might stretch current definitions of "life as we know it."

But finding one of these teensy critters would be no small fish tale, implying, as it would, that life can arise amid conditions very unlike those on Earth and, hence, that life could be ubiquitous in the universe.

Roughly the size of our moon, Europa is thought to contain an ocean of water nearly twice as voluminous as all of Earth's oceans. The overlaying ice is expected to be at least half a mile (a kilometer) thick, and perhaps much more. Sunlight cannot possibly penetrate the ice, and therefore photosynthesis cannot be the driving force for any possible sub-surface life. At the surface, radiation is likely too intense for life to survive.

Seafloor hydrothermal vents have been suggested as a possible energy source; after all, they support life on Earth. But oxidants are still needed, and in Earth's oceans they migrate down from the surface. No one has figured out where oxidants might originate on Europa.

Hoping to sidestep these theoretical shortcomings, Chyba sought to come up with a "speculative ecology," a previously unimagined way to power a biosphere which, he is careful to point out, may or may not exist. His starting point -- that radiation powers chemistry in ice -- is not a new idea.

What is new is how Chyba, the Carl Sagan Chair for the Study of Life at SETI (Search for Extraterrestial Intelligence), applies the idea to create his hypothetical biosphere.

Laboratory experiments show that when ice is bombarded with radiation, a host of possible fuels are produced -- including formaldehyde, off of which common soil bacteria live. Radiation, interacting with water, also produces a suite of oxidants.

On Jupiter, charged particles in the planet's magnetosphere (similar to Earth's, but much stronger) bombard Europa.

"When the particles hit the ice, they tunnel through and create a high-temperature cylinder," Chyba explains. "For a brief time, chemistry takes place along that path."

The resulting formaldehyde, carbon dioxide, fragments of water and other substances become trapped as the tunnel re-freezes.

How the life-giving chemicals might make their way to the suspected liquid ocean below is not clear. But Chyba says the young surface of Europa, as seen in photographs, indicates some ongoing process of mixing with the sub-surface occurs.

"Spots on the surface show evidence of flooding, where water has filled in low-lying areas," Chyba said. "One way to get [the chemicals to the sub-surface] is to slowly bury them by flooding. There might be more dramatic ways."

Photographs of tilted chunks of ice hint at one possibility, where some unknown heat source triggers a massive melting. Chunks of ice might break off from large ice sheets, tilt and founder in a large pool of water that would mix surface substances with the ocean below, before re-freezing.

Other researchers say Chyba's idea is highly speculative and, like many theories about un-visited places, could turn out to be inaccurate. Then again, as Kenneth Nealson puts it, "We need ideas like this."

Nealson, a bacteriologist, is director of the astrobiology group at NASA's Jet Propulsion Laboratory. Along with colleagues, he wrote an article for the journal Science last June questioning whether Europa had the stuff of life as we know it. The researchers were skeptical about how the necessary oxidants might form.

If Chyba's radiation-chemistry idea works, Nealson said in an interview, then it should be measurable.

'It all comes down to the great unknown of how much mixing occurs with the surface and the deep ocean waters," Nealson said.

But until hard facts are gathered by a mission to Europa (the first is scheduled to depart in 2003) most researchers, including Chyba, agree that we simply won't know if conditions are ripe for life on the remote moon.