A MAGNETIC SIGNATURE AT IO - INITIAL REPORT FROM THE GALILEO MAGNETOMETER

During the inbound pass of the Galileo spacecraft, the magnetometer ac quired 1 minute averaged measurements of the magnetic field along the trajectory as the spacecraft flew by lo. A field decrease, of nearly 4 0 percent of the background jovan field at closest approach to lo, was recorded. Plasma sources alone appear incapable of generating perturb ations as large as those observed and an induced source for the observ ed moment implies an amount of free iron in the mantle much greater th an expected. On the other hand, an intrinsic magnetic field of amplitu de consistent with dynamo action at lo would explain the observations. It seems plausible that lo, like Earth and Mercury, is a magnetized s olid planet.