Planetary magnetic fields

As a consequence of the smallness of the electronic fine structure constant, the characteristic time scale for the free diffusive decay of a magnetic field in a planetary core is much less than the age of the Solar System, but the characteristic time scale for thermal diffusion is greater than the age of the Solar System. Consequently, primordial fields and permanent magnetism are small a;d the only means of providing a substantial planetary magnetic field is the dynamo process. This requires a large region which is fluid, electrically conducting and maintained in a non-uniform motion that includes a substantial RMS vertical component. The attributes of fluidity and conductivity are readily provided in the deep interiors of all planets and most satellites, either in the form of an Fe alloy with a low eutectic temperature (e.g. Fe-S-0 in terrestrial bodies and satellites) or by the occupation of conduction states in fluid hydrogen or ‘ice’ (H20-NH3-CH4) in giant planets. It is argued that planetary dynamos are almost certainly maintained by convection (compositional and/or thermal). If alternative mechanisms such as precessional torques work at all, they only work when they are not needed (i.e. when the core is neutrally or unstably stratified because of other larger energy sources). For any plausible convective vigour, it is possible to satisfy the sufficient conditions of dynamo onset (large magnetic Reynolds number, small Rossby number) for every planet and satellite. Estimates of convective vigour are obtained from estimates of likely energy fluxes and a consideration of the form of convective motions in a rotating fluid sphere. The reason that some planets and probably all satellites do not have dynamos is because the fluid regions of their cores are stably stratified and do not convect. Thermal evolution models indicate that any terrestrial body with an entirely fluid (iron alloy) core becomes stably stratified over geologic time and loses heat by conduction only at the present time. The core energy flux is then totally unavailable for dynamo generation. However, terrestrial bodies that nucleate an inner solid core (e.g. Earth) can usually continue to sustain a dynamo because of the resulting gravitational energy release and compositional buoyancy of convective motions. In contrast, giant planets can easily sustain a dynamo by gradual cooling alone. The field amplitudes of planetary dynamos are poorly understood. Existing attempts at ‘scaling laws’ are naive because they deny the diversity of planets, and @ 1983 The Institute of Physics 5 5 5