The Many Dimensions of Dynamos and Non-dynamos in Small Planets and Satellites

All giant planets have dynamos because they convect and they have deep fluid interiors that are adequately good electrical conductors (not good metals). The "solid" planets (including large icy satellites) are expected to have liquid iron-alloy cores; perhaps even including Pluto. Callisto may be an exception because it failed to separate most of the rocky component from the ice. Partially liquid cores are hard to avoid except in oxidized conditions (no metallic Fe alloy possible) because of the sulfur antifreeze. In icy bodies, accretional heating will not drive prompt core formation but percolative downward aggregation of iron alloy can arise from radiogenic heating, aided by the higher potassium-40 relative to Earth. The remarkable range of magnetic behaviors of these otherwise seemingly similar bodies suggests there are other dimensions to the existence or absence of a dynamo than the issue of liquid core existence or overall size. The currently operating dynamo in Ganymede is striking, not because it is hard to do (one can certainly generate models that permit this behavior) but because one must then explain why Titan fails. There are three other dimensions to this puzzle (at least), none of which is well understood: The role of chemistry (e.g., the "rocky" component of Titan is different from that of Ganymede or Europa), the role of different histories (rock and ice separated during rapid accretion of Ganymede whereas Titan accreted slower and had delayed differentiation), and the role of other heating (primarily tidal) which may prevent the core from cooling efficiently. The one case that resists this kind of analysis is our Moon where the field amplitude requirement is high and any dynamo explanation is likely to require mechanical rather than thermal/compositional convective driving.