I just read a recent report on May 13, 1996 that the Galileo spacecraft may have discovered that Io has an iron core that takes up half of the volume of Io's interior, and that the Galileo instruments now have indications that a magnetic field is present around this satellite; the first 'moon' in the solar system to have a known magnetic field. The strength of this field is about 1/40th of Earth's field, but that a satellite has such a field at all is remarkable. Here is a short excerpt from a paper which discusses the findings:
A Magnetic Signature at Io: Initial Report from the Galileo
Magnetometer
M. G. Kivelson, K. K. Khurana, R. J. Walker, C. T. Russell,
J. A. Linker, D. J. Southwood, C. Polanskey
During the inbound pass of the Galileo spacecraft, the magnetometer acquired 1 minute averaged measurements of the magnetic field along the trajectory as the spacecraft flew by Io. A field decrease, of nearly 40 percent of the background jovian field at closest approach to Io, was recorded.(See above plot on the right) Plasma sources alone appear incapable of generating perturbations as large as those observed and an induced source for the observed moment implies an amount of free iron in the mantle much greater than expected. On the other hand, an intrinsic magnetic field of amplitude consistent with dynamo action at Io would explain the observations.
It seems plausible that Io, like Earth and Mercury, is a magnetized solid planet. Jupiter's moon Io has repeatedly surprised planetary scientists. First, Io's orbital position was unexpectedly found to control decametric radio emission from Jupiter's ionosphere (1). Early explanations suggested that the emissions were generated by magnetic field-aligned currents linking Io and Jupiter (2). These ideas were refined and linked to Alfvénic disturbances generated by the interaction of the flowing plasma of Jupiter's magnetosphere with an electrically conducting Io (3,4). After the discoveries of a large cloud of neutral sodium surrounding Io (5) and of a torus of ionized sulfur encircling Jupiter at the distance of Io's orbit (6), Voyager 1 found volcanic plumes distributed on the surface of the moon (7). The Voyager 1 magnetometer detected magnetic perturbations of ~5% of the ambient jovian magnetic field (~1900 nT) as it crossed Io's magnetic flux tube about 11 RIo (radius of Io 1821 km) below Io (8), thereby confirming the presence of a field-aligned current flowing several thousand kilometers away from the spacecraft and carrying more than 106 A into the jovian ionosphere.
Copyright 1997 Dr. Sten Odenwald
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