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No. 128: MAR-APR 2000

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The Ganymede Magnetic Paradox

In December 1995, the Galileo space-craft was injected into orbit around Jupiter, thereby becoming the first known artificial satellite of this giant planet. In the five years that have transpired, Galileo has radioed back voluminous data about Jupiter itself and its four large Calilean satellites. These natural satellites have turned out to be a disparate bunch. Three have iron cores, but Callisto breaks the mold with an unusual core of mixed ice and rock. Europa probably possesses an ocean, and Callisto might also. Only one of Jupiter's large satellites, Ganymede, boasts a magnetic field. In fact, Ganymede is apparently the only satellite in the solar system to display an intrinsic, dipole magnetic field like the earth's.

Although Ganymere's magnetic field is like that produced by a permanent bar magnet, its core is much too hot for permanent magnetism. Again like the earth, Ganymede's field is theorized to be generated by the convection of electrically conducting liquid in its core -- a dynamo of sorts. All well and good, but Ganymede is so small that it should have cooled off billions of years ago thereby freezing its metallic core. So then, whence its magnetic field?

One way out of this box it to suppose that about a billion years ago Ganymede was circling Jupiter in an orbit that took it much closer to this ponderous planet. Then, Jupiter's powerful gravitational field would have gently kneaded Ganymede's structure creating what is called "tidal heating," which kept the core liquid and able to generate a magnetic field.

(Johnson, Torrence V.; "The Galileo Mission to Jupiter and Its Moons," Scientific American, 282:40, February 2000.)

Comment. Sounds good, but there is a puzzle piece missing: What catastrophic event catapulted Ganymede into its pre-sent orbit? It's as big as Mercury!

From Science Frontiers #128, MAR-APR 2000. � 1997 William R. Corliss

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