Galileo probes structure of Jovian moons.

Halfway through its 2-year Jovian tour, the Galileo spacecraft has taken a peek at Jupiter's four largest moons. Gravitational and magnetic field maps, along with standard images, are allowing scientists to probe Ganymede, Io, Europa, and Callisto from the inside out. Galileo scientists recently released new information on three of these moons.

The craft confirmed that Ganymede has a magnetic field, making it the only moon known to have one (SN: 7/20/96, p. 37). The evidence includes a surge in electromagnetic emission that researchers heard as soaring whistles and hissing static when the craft passed through ionized gas around Ganymede. The signals indicate that the moon sports a magnetic field large enough to deflect that of Jupiter.

A gravitational map, deduced from the motion of the craft as it orbited Ganymede, reveals that the moon has a dense core. Taken together, the new findings indicate that Ganymede has a three-layered structure. Its core consists of molten iron, whose internal circulation generates the magnetic field. A rocky mantle surrounds the core, and a thick shell of ice forms Ganymede's exterior.

The presence of a molten core reflects an episode of heating well after Jupiter and its moons were born, argues Gerald Schubert of the University of California, Los Angeles. He suggests that Ganymede initially consisted of a uniform mixture of material. Later, the gravity of Jupiter and the other large moons distorted and flexed Ganymede, producing heat that melted and separated out the iron-rich constituents. More recently, the moon's orbit changed and the distortions lessened. The melting could not have happened just as Jupiter formed, notes Schubert, because the core would have had time since then to cool and resolidify, eliminating the magnetic field.

John D. Anderson of NASA's Jet Propulsion Laboratory in Pasadena, Calif., notes that Ganymede is structurally akin to volcanically active Io, which lies closer to Jupiter. "If you stripped away the icy shell from Ganymede, it would look very much like Io," he notes.

The Galileo team describes their studies in the Dec. 12 Nature. At a NASA press briefing, Kelly Bender of Arizona State University in Tempe unveiled the first images of Callisto, the coldest, outermost moon. They show bright regions of material, as if older and darker ice had slumped downhill and exposed more pristine ice underneath.

At last month's American Geophysical Union meeting in San Francisco, Galileo researchers also reported that Callisto appears to have no magnetic field and a uniform composition. Callisto may reside too far from Jupiter for heating to have altered its structure. Some theorists suggest that this icy moon represents what the other large Jovian moons looked like soon after their formation.

Finally, a new image of Europa shows evidence of volcanic activity. Any heat source that could generate volcanoes would also melt ice. Therefore, the volcanism makes scientists more confident that Europa has an ocean beneath its icy exterior.