A magnetic signature at Io: Initial report from the Galileo magnetometer

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 lo. A field decrease, of nearly 40 percent of the background jovan field at closest approach to lo, was recorded. 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 lo would explain the observations. It seems plausible that lo, like Earth and Mercury, is a magnetized solid planet.