Magazine article Science News

Galileo Probes Structure of Jovian Moons

Magazine article Science News

Galileo Probes Structure of Jovian Moons

Article excerpt

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. …

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