Inward solidification of planetary cores as a mechanism for dynamo generation

Although magnetic fields appear to have been generated in a diverse range of planetary bodies in the past, only three rocky bodies in our solar system (Earth, Mercury and Ganymede) are known to produce active fields today. Current dynamo theory cannot explain the divergent magnetic field evolution of these otherwise similar rocky bodies, and the implications for their long-term thermochemical evolutions. The current magnetic field of the Earth is powered by light elements being expelled from the growing inner core. However, there is still major debate surrounding both the age of the inner core, and how the geodynamo was driven prior to solidification.

Magnetic fields in rocky bodies are powered by a dynamo: convection in the molten part of their electrically conductive metallic cores. The requirement for an at least partially molten core means dynamos are linked to a planet's thermal history. Cores of rocky planets consist primarily of iron and nickel, with some additional lighter elements, such as sulfur, oxygen and silicon. For example, seismic studies of the Earth's core suggest it contains 5 - 10% of these lighter elements. This convection can be powered by a combination of three sources: temperature induced density differences (thermal convection), crystallisation induced density differences (compositional convection), and mechanical forcing (e.g. tidal stirring). Thermal convection requires a high heat flux across the core-mantle boundary (CMB) and the core to have a low thermal conductivity, whilst compositional convection is affected by the core temperature and composition gradients. This means that magnetic field generation is intrinsically
linked to core composition and how planetary bodies cool with time. We cannot observe planetary interiors directly, but the absence/presence of magnetic fields is a powerful and unique window into the properties of the inaccessible interiors of Earth, other planets and moons and their thermal histories.

When thermal convection alone is not strong enough to generate a dynamo, compositional convection can provide an additional power source with the two mechanisms combining in thermo-chemical convection. Compositional convection arises when the core crystallises and light elements are expelled from the solidifying iron, resulting in density differences which can drive convection. Whether crystallisation of the core starts at the CMB or the centre of the core depends on how the core temperature (adiabat) and crystallisation temperature (liquidus) vary with depth. When the core crystallises from the centre (bottom-up), light elements which are rejected from the solid are less dense than the surrounding core so rise buoyantly to the CMB. This is the process which generates the Earth's dynamo today. Dynamos generated when the core crystallises from the CMB inwards (top-down) are poorly understood, but are significant for dynamos in the numerous smaller bodies in the Solar System, where the CMB is at lower pressure. This includes asteroids, moons and potentially Mercury, which has a large core and thin mantle. There are several possible crystallisation mechanisms, including sinking of km size iron chunks or iron snow. In particular, iron snow is an important mechanism for convection because it produces sustained dynamo activity. Iron snow occurs when solid iron particles ( 10-2mm) crystallise at the CMB. The crystals fall through the crystallised region (the snow zone) until they reach the base where they remelt. This forms a more dense pure iron layer on top of the less dense iron alloy in the liquid part of the core, which is gravitationally unstable so sinks and drives convection.