Life on Europa: A possible energy source.

In contemplating likely habitats for life beyond Earth, planetary scientists rank Jupiter's moon Europa as a prime candidate. A boatload of circumstantial evidence suggests that beneath the icy surface of this moon lies a vast ocean. And where there's water, there could be life.

If life on Earth is any guide, however, organisms cannot live by water alone. They also need energy. Scientists have proposed that the same source of heat that may have melted ice and created a Europan ocean may also generate plumes of warm water that could be fueling life. Theoretical calculations, however, don't offer convincing proof that this source--flexing caused by the gravity of two of Europa's sibling moons and Jupiter--was ever strong enough to create subterranean plumes.

A new report suggests another font of life-giving energy. Charged particles striking Europa's surface could generate chemical energy to sustain life beneath the icy shell, suggests Christopher F. Chyba of the SETI Institute in Mountain View, Calif. He outlines the scenario in the Jan. 27 NATURE.

Charged particles, trapped and accelerated by Jupiter's strong magnetic field, are plentiful in Europa's vicinity. When the particles slam into the moon, they probably create molecular oxygen and organic compounds. The molecular oxygen would readily liberate energy as it removes the outermost electrons from the organic compounds. Laboratory experiments in which particles bombard frozen water and carbon dioxide prove the possibility of such reactions. In addition, the Galileo spacecraft has found several organic compounds on Europa's surface (SN: 1/3/98, p. 11).

If the liberated energy remained trapped in Europa's crust, it could not provide the fuel for bacteria or other organisms that may inhabit an ocean buried 100 kilometers beneath the ice. However, several aspects of the moon's appearance suggest ways that such energy may indeed reach the ocean, Chyba argues.

Europa's crust is remarkably free of craters, indicating that the moon's facade gets a facelift about every 10 million years, as new material rises from below and buries older material.

Moreover, some parts of the surface appear cracked and jumbled, an indication of sudden melting. William B. McKinnon of Washington University in St. Louis has proposed that the gravitational flexing could generate heat within the ice. As the ice warms, it becomes easier for heat to flow and melt some of the frozen material between the ocean and the surface.

Microbes could thrive "wherever Europa's surface communicated with its ocean," Chyba suggests.

If the ice delivers 10 percent of the liberated chemical energy to a Europan ocean, it could support one living cell per cubic centimeter, Chyba calculates. That concentration is high enough that organisms that reach the surface could be detected by a lander that would melt ice and filter microbes from the water, Chyba says. NASA expects to launch a Europan lander at the end of the decade.

Charged-particle bombardment is just one of several ways to pump energy into a Europan ocean, Chyba points out.

While scientists continue to debate whether a Europan ocean contains life, most have accepted the presence of the ocean, sight unseen. New measurements of the moon's magnetic field, taken by Galileo, add to the indirect evidence.

Because Europa orbits Jupiter, the planet's magnetic field appears to vary over time. On Jan. 3, as the craft flew within 351 km of Europa, its magnetometer measured a change in the direction of the moon's own magnetic field in sync with the variations in the Jovian field.

The changing direction of Europa's magnetic field can best be explained if the moon contains a shell of electrically conducting material, such as a salty ocean, says Margaret G. Kivelson of the University of California, Los Angeles, leader of the Galileo magnetometer team. Time variations in Jupiter's field would induce electric currents in a salty ocean, which in turn would generate a varying magnetic field at Europa's surface. If Europa possessed a permanent, rather than an induced, magnetic field, it would not change direction, she notes.

"The change is consistent with a [magnetic field] induced in a conductor," Chyba concurs. Kivelson says she has found no other explanation.

Later this year, she hopes to use the magnetometer to search for variations in the magnetic field of Europa's sister moon, Ganymede. Telltale changes there may indicate that Ganymede also contains a hidden ocean.