Recent work by Christopher Chyba (SETI Institute) and Kevin Hand (Stanford University) suggests that there may be ways to nourish biology on watery environments where the Sun's rays don't penetrate. The two researchers have published their work in the June 15 issue of Science.

"Most surface life on Earth - on land or in the seas - depends on photosynthesis," notes Chyba. "The first link in the food chain is chlorophyll's conversion of sunlight into chemically stored energy. But imagine an ocean on Europa, a huge, bottled-up body of water capped with miles of ice. Photosynthesis isn't going to work there. Nonetheless, there are other ways to make a metabolic living in those dark seas."

Recent results from NASA's Galileo spacecraft have strongly suggested the presence of subsurface oceans not only on Europa, but also its sister moons, Callisto and Ganymede. Since liquid water is usually considered a prerequisite for the development of life, these nearby worlds are intriguing locales to search for extraterrestrial biology.

However, more than water is required. An energy source is necessary to support life. Chyba and Hand point out that this is usually obtained by oxidation-reduction reactions in which two substances (for example, carbon and oxygen) bond to share an electron, releasing energy during the reaction. An important oxidizing agent in Earth's oceans is molecular oxygen (O2), the product of photosynthesis. But one would expect this to be in short supply in the inky abysses of the Jovian moons.

However, Chyba and Hand note that Europa's icy exterior is routinely bombarded with high-speed particles accelerated in Jupiter's magnetosphere. When they slam into the Europan ice, they form oxidants such as H2O2 and O2. If, as could be the case, this surface food supply eventually gets churned into the ocean below, it could provide sustenance to a substantial biomass.

"We can't be certain at this point whether the oxidants would actually make it into the water, even over geological time scales," says Chyba. "But if not, there are other mechanisms that might be a source for molecular oxygen in the oceans."

One of these is the radioactive decay of a potassium isotope 40K, which would be present in both the ice crust and the liquid water. The decay splits water molecules and produces O2. Although the quantity of oxidant produced in this way is less than could be supplied by the surface effects of charged particles, it would still be enough to support a biosphere.

"Obviously, we don't know if life exists on these moons," Chyba emphasizes, "but at least we can say that if the oceans are there, the compounds that could supply energy for life seem likely to be present."

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