Ganymede May Have Vast Hidden Ocean.

Move over, Europa. Make way, Callisto. You're not the only moons of Jupiter that might hold seas of liquid water. Big brother Ganymede may also harbor an ocean beneath its icy surface, three new studies suggest. And where there's water, there could be life.

A layer of saltwater, at least several kilometers deep and buried some 150 kilometers beneath Ganymede's surface, is the best explanation for magnetic measurements that the Galileo spacecraft recorded, says Margaret G. Kivelson of the University of California, Los Angeles.

Magnetic readings of Ganymede, taken during flybys over several years, reveal that Jupiter's largest moon has both a fixed magnetic field of its own and a secondary field induced by Jupiter. It's the induced field that suggests an unseen, salty ocean, Kivelson reported Dec. 16 at a meeting of the American Geophysical Union in San Francisco.

As Ganymede orbits Jupiter, it encounters a magnetic field that varies. Galileo measured an ever-so-slight change of direction that this Jovian field produces in Ganymede's field.

To create an induced magnetic field on Ganymede, Jupiter must set up electric currents within the icy moon. That's possible only if Ganymede contains an electrically conducting medium. Ice is a poor conductor, but a layer of salty water within the ice would allow current to flow, says Kivelson.

Her team has used similar arguments to suggest that both Europa, which lies closer to Jupiter, and Callisto, which lies much farther away, have hidden oceans (SN: 1/29/00, p. 70). These moons have no magnetic fields of their own, making it easier to discern induced fields.

Detecting Ganymede's induced field "is trickier," Kivelson says, because the moon's permanent field can confound results. In the unlikely event that the moon's field is sufficiently complex, it could mimic an induced field. In that case, the moon need not have an ocean.

Two other findings support the ocean option, however. At last week's meeting, Thomas B. McCord of the University of Hawaii in Honolulu presented Galileo spectra showing several mineral salts on Ganymede. The salts "suggest that brine from [an] ocean reached the surface," he says.

Additional evidence comes from close-up images of Ganymede's Arbela Sulcus region. They reveal a long band, similar to those seen on Europa's surface of brittle ice. The band indicates that Arbela Sulcus consists of sections that slid past each other, as if they had glided over a layer of pliable, warm ice. The band appears to have been pulled apart, with the warm ice rising to fill the gaps, Robert T. Pappalardo and James W. Head of Brown University in Providence, R.I., and their colleagues reported at the meeting.

The findings suggest that Ganymede has enough internal heat to maintain a layer of liquid beneath that ice. Natural radioactivity from rocks within Ganymede could generate the required heat, notes David J. Stevenson of the California Institute of Technology in Pasadena.

In contrast, Europa's heat stems from gravity. Jupiter's tug on Europa generates the equivalent of ocean tides on the moon's solid surface. As Europa moves around Jupiter, the tides vary in strength, causing the moon to flex and heat up. This extensive heating may explain why Europa's putative ocean lies just a few tens of kilometers from its surface.

Sandwiched between layers of ice, Ganymede's ocean may once have been larger and as close to the moon's surface as Europa's is today, Pappalardo speculates. Although Ganymede now receives little tidal heating, the moon may have been subject to much more in the past if it held a particular position relative to the other moons, he suggests. The flexing would have dumped additional energy into Ganymede, enlarging the ocean and thinning the moon's icy surface. The bands seen on Arbela Sulcus may have arisen during this ancient epoch, he says.

Europa may offer a more promising locale for life because its suspected ocean would lie above rock rather than ice, Stevenson says. Volcanic and hydrothermal activity at the ocean-rock boundary could provide a source of energy for organisms. On the other hand, notes Head, "I look upon all these places as laboratories for the study of conditions that might have led to life."