The magnetic signature of double-diffusive convection in planets

The magnetic fields of planets are produced deep in their interiors, in regions composed of electrically conducting fluid. This conducting fluid swirls around due to convection as the planet gradually cools down, generating electric currents that induce the magnetic field via a dynamo process. For some planets in our Solar system, the magnetic field observed at their surface is unusual and difficult to reconcile with the picture produced by numerical models of the convective dynamo region: Mercury’s field is very weak and Saturn’s field is very axisymmetric (i.e. lacking features that depend on longitude). To explain these unusual magnetic features, the prevailing theory evokes the presence of a stable conducting layer (i.e. a region where the mean variation of density with depth is stable to overturning convection) at the top of the dynamo region, which is consistent with the evolution models of these planets. The stable layer is thought to act as a filter for the magnetic field, removing rapidly-fluctuating and non-axisymmetric fields. However, the details of this filtering process are not well understood because current models overlook the presence of hydrodynamic instabilities in the stable layer, due to the dependence of the fluid density on both the temperature and chemical composition. These instabilities drive vertical flows known as double-diffusive convection (DDC), which can distort the magnetic field passing through the stable layer. How DDC interacts with the magnetic field is completely unknown, yet DDC could amplify or reduce the magnetic field, thereby drastically affecting the filtering process in the stable layer. In this project, we will address this problem by studying how DDC in the stable layer shapes the observable planetary magnetic fields using numerical simulations that model the stable layers in Mercury and Saturn. This work is important because magnetic observations are one of very few tools available to probe planetary interiors, so understanding the magnetic signal of stable layers is crucial to exploit this tool accurately. The project is based on our recent work on non-magnetic DDC in rotating, spherical global models. The main objectives of the project are (i) to predict how DDC in the stable layer modifies the primary magnetic field produced in the planet’s deep convective layer; (ii) to determine whether stable layers could generate their own magnetic fields by dynamo action and whether this secondary signal can be identified in magnetic observations.