But last summer a group of planetary scientists from the California Institute of Technology and Jet Propulsion Laboratory threw water on the theory -- so to speak -- when they took a novel approach and concluded that advanced life forms were not likely.

"Water is a good place to look for life, but is only one ingredient for life," says Kenneth Nealson, an astrobiologist who holds joint appointments at Caltech and JPL, and who was a coauthor of the 1999 paper on Europa.

"You also need energy and, probably, organic carbon."

Nealson and his colleagues Eric Gaidos and Joseph Kirschvink (both of Caltech) wrote in the controversial 1999 Science paper that life on Earth is not necessarily the best analogy for life on another world. In other words, astrobiologists should be prepared to use chemistry and physics to analyze the possibilities for extraterrestrial life, rather than merely assuming life will exist wherever there is water.

Specifically, the authors showed that nearly all forms of energy used by life on Earth would be unavailable to the organisms that might live beneath Europa's surface ice layer. This did not preclude primitive unicellular organisms, but boded poorly for anyone hoping to someday see Europan creatures with gills and backbones.

"There is a trap in the thinking, because on Earth, virtually everywhere you find water you also find life," Nealson says. "And conversely, on Earth, about the only thing you can associate with lifelessness is the lack of water.

"But on another planet, just because you find water doesn't mean you're necessarily going to find life there."

Nealson says that a very likely place to look for life forms is any place where there is an energy gradient of some sort. Some potential energy gradients that might be available on Europa might arise from the gravitational and magnetic fields of Jupiter, which would almost certainly grind things around inside the moon and result in a heat source.

But when Nealson and his colleagues last year analyzed the closed system beneath Europa, they concluded that this source of energy alone was probably insufficient for multicellular life to survive. Also, they concluded that the redox energy (or available chemical energy) of the moon would also be inadequate for complex life of the kind we are familiar with on Earth.

"Still, I think Europa is a great place to look for very simple organisms," Nealson says today.

Another salubrious way to look for life is to look carefully at any place there is a water cycle, however small. If any of the other Jovian moons, such as Ganymede or Callisto, have a hydrological cycle in which moisture precipitates and runs underground, is heated by an internal source, and ultimately is returned to the surface, then the planet or moon would have the potential for energy gradients, energy flow, and geochemical cycling. All of these may be key to the existence of global life.

And the water cycle could be entirely subterranean and could even be a very limited, closed loop, Nealson says. For example, Mars may still have frozen subterranean waters that are occasionally melted by the planet's internal heat, but never result in water vapor actually surfacing. In such a case, there could be bacterial life that has lived in a closed loop beneath the Martian surface for billions of years.

"There's certainly no present-day atmospheric water cycle on Mars -- no rain, no aquifers to collect the rainfall, no recycling," he says. "So if there's life on Mars, it has a hard time existing, and we'd have a hard time finding it without drilling."

While a drilling excavation to Mars is still a few decades in the future, Nealson hopes that one of the orbiters to Mars will soon include a deep- sounding radar instrument. Such an instrument can detect either liquid or frozen water beneath the surface.

The Mars orbiter scheduled for launch in 2003 by the European Space Agency (in conjunction with scientists from JPL) is scheduled to have deep-sounding radar for the detection of subsurface liquid water. A similar device will eventually be sent to Europa.

Perhaps later, the search could be extended to other Jovian moons, as well as the moons of Saturn and even Uranus.

"The moons of Jupiter have changed the way I feel about life in the solar system," Nealson says. "Each of the four large moons has different properties, different energy flows, different likelihoods of water.

"It's important to keep an open mind," he says.