A self sustained magnetic field on Io?

The thermal-orbital evolution of Io is discussed with particular emphasis on the evolution of its fluid FeFeS core and the energy balance for a possible magnetohydrodynamic dynamo. It is found that magnetohydrodynamic dymamo action is unlikely if the surface heat flow and the tidal heating rate are in longterm approximate equilibrium with fluctuations in surface heat flow by no more than a few tens of percent and with characteristic periods less than some 106 years. In this case, the tidal heating rate will dominate the energy balance of the mantle and little heat will be removed from the core. The core is then unlikely to be convecting, in which case dynamo action is impossible. Because the mantle is likely to be partially molten and because mantle solidus temperatures are larger than core liquidus temperatures, bouyancy released upon growth of an inner core is also not available to drive a dynamo. If the thermal and orbital variables are varying strongly with time, for instance because of an instability of the thermal-orbital state, 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 periods associated with oscillations of the thermal-orbital variables is about 108 years. The peak dipole moment could be large enough to cause an open magnetosphere and to sustain an Ionian ionosphere. Magnetic field measurements by the upcoming Galileo mission may thus provide important constraints for the discussion of Ioʹs evolution and interior dynamics.