Louise Prockter, a researcher at the Applied Physics Laboratory at Johns Hopkins University, also doubts there is any place on Europa where the crust is thin enough to drill through. Though some studies have suggested a thin crust is possible, at least in patches, "I think the evidence is mounting against this view," she said in an e-mail interview.

In a separate newly released study that Prockter led, gray bands of material on Europa, which were thought to have been created by water, were found in fact to have been generated by ice that long ago rose to the surface.

"One of the bands we looked at stands about 100 meters [330 feet] higher than the surrounding terrain," Prockter explained. "If the band was formed of water we would expect it to overflow onto areas around it."

The overflow would have left signature features, which do not exist. Further, Prockter said the interiors of several bands contain mounds and ridges that are hard to form with liquid water.

The research will be reported in an upcoming issue of Journal of Geophysical Research-Planets.

Prockter said the overall body of research on Europa suggests the ice shell is at least 9 miles (15 kilometers) thick.

Long before any of the present day surface features of Europa were created, every planet and moon in the solar system was bombarded by a rain of early debris. Many researches suspect these asteroids and comets, known to have pummeled Earth up to about 3.8 billion years ago, brought life's raw materials to our own planet.

Chyba and Pierazzo figure the same scenario could have paved the way for life on Europa. They analyzed impacts on our Moon -- which maintains a remarkable record of the events -- and other bodies in the solar system, then combined the data with known compositions of comets.

Billions of tons of carbon would have been delivered to Europa, they say. Earlier studies have suggested that substantial amounts of another important class of prebiotic chemicals carried by comets, called amino acids, would have survived the fiery and explosive impacts.

Much of the cometary chemicals would have escaped back into space, but enough should have remained, the new study showed.

"If these chemicals find their way into the ocean," Pierazzo said, "and if there exists a mechanism that could take them through the formation of increasingly complex organic molecules, those elements could ultimately evolve into living cells."

Experts agree, however, that making life out of some ingredients involves unknown natural instructions, and nobody can say that it has ever happened anywhere but on Earth.

"Given what we know about Europa now, the energy sources are there that could sustain a biosphere," Chyba said at the STScI conference. "The possibility that life could exist there is not the same thing as the [possibility that the] origin of life could have occurred there. The only way to answer this question is to go."