"One could expect life in such oceans," said Krishan Khurana, a geophysicist at the University of California at Los Angeles and lead author of the research published Thursday in the journal Nature.
Heat and water in liquid form, as opposed to ice, are considered fundamental requirements for life. The latest findings were put together by a team from UCLA, the California Institute of Technology, and NASA's Jet Propulsion Laboratory.
Ice-encrusted moons like Europa and Callisto, and other frozen moons even farther from the sun, were long thought to be too cold for life to form.
"If we find out 4 and a half billion years after the formation of the solar system that there's still enough heat that ice will melt on the interior of these bodies, we have to do a little bit of rethinking," said one of the researchers, UCLA physicist Margaret G. Kivelson.
Earth is 93 million miles from the sun; Jupiter, 483 million.
The scientists hypothesized the existence of underground seas from data collected by the Galileo spacecraft. Galileo measured strong disturbances to Jupiter's magnetic field as the spacecraft zipped past Europa and Callisto.
However, the moons lack strong magnetic fields of their own that could exert such a force. The scientists concluded that Jupiter's magnetism must be giving rise to secondary fields within some powerful electrical conductor on each moon - a phenomenon known as electromagnetic induction, which is at work on Earth in generators and electromagnets.
The researchers could imagine only one sufficiently strong conductor that is plausible on those moons: huge bodies of salt water. They would be within about 60 miles of the frozen surface, kept from icing over by heat within the interior of the moons. Assuming they are as salty as Earth's oceans, they would be about six miles deep.
"It's making all of us go back to our models and think about our understanding of these icy bodies," said Ronald Greeley, an Arizona State University geologist who studies moons.
Europa was already viewed as a leading candidate in the search for life, because its jigsaw crust resembles broken icebergs and thus points to the possibility of oceans at least in the past. However, Callisto's pockmarked surface contains little visible trace of liquid movement below.
Written by Jeff Donn