A SELF-SUSTAINED MAGNETIC-FIELD ON IO

The thermal-orbital evolution of Io is discussed with particular empha sis on the evolution of its fluid Fe-FeS core and the energy balance f or a possible magnetohydrodynamic dynamo. It is found that magnetohydr odynamic dynamo action is unlikely if the surface heat flow and the ti dal heating rate are in long-term approximate equilibrium with fluctua tions in surface heat flow by no more than a few tens of percent and w ith characteristic periods less than some 10(6) years. In this case, t he tidal heating rate will dominate the energy balance of the mantle a nd little heat will be removed from the core. The core is then unlikel y to be convecting, in which case dynamo action is impossible. Because the mantle is likely to be partially molten and because mantle solidu s temperatures are larger than core liquidus temperatures, bouyancy re leased upon growth of an inner core is also not available to drive a d ynamo. If the thermal and orbital variables are varying strongly with time, for instance because of an instability of the thermal-orbital st ate, with disequilibrium between tidal heating and surface heat flow, then periods of dynamo action alternating with periods of no magnetic field generation are likely. The dominant period associated with oscil lations of the thermal-orbital variables is about 10(8) years. The pea k dipole moment could be large enough to cause an open magnetosphere a nd to sustain an Ionian ionosphere. Magnetic field measurements by the upcoming Galileo mission may thus provide important constraints for t he discussion of Io's evolution and interior dynamics.