A map of the dynamo lifetime of rocky planets

Planetary-scale magnetic fields provide a unique window into a planet’s deep interior. From the magnetic field of the Earth to that of Jupiter, we have tied the existence of such fields to the presence of an electrically conductive convecting fluid (dynamo source region) in the interior. Thus, detections of planetary-scale magnetic field signals offer constraints on the planets’ thermal state, interior structure, and dynamics. The discovery of exoplanets and the ubiquitousness of magnetospheric emissions within the solar system motivate theoretical work on exoplanets to predict which ones may host the necessary condition for a dynamo source, i.e. having an electrically conductive convecting fluid with a magnetic Reynolds number greater than ~50. Previous studies of rocky planets have focused on Earth-like ones with an iron-dominated core, silicate mantle and a core mass fraction (CMF) ~ 0.33 in the habitable zone. Here, we calculate the thermal evolution of rocky planets with a greater parameter space, with planetary masses from 0.5 to 10 Earth-masses, and CMFs from 0.0 to 1.0 and equilibrium temperatures from 255K to 2000K. Our aim is to map out the lifetime of the possible dynamo source regions in both the liquid core and magma ocean of these planets. To achieve this, we couple a 1D thermal evolution model with a Henyey solver to calculate their thermal evolution. Our model solves the energy balance equation in both the core and the mantle. We use the modified mixing length theory appropriate for fluids with high and low Reynolds number to model the convective heat flow. In addition, by including the Henyey solver, we self-consistently account for adjustments in the interior structure and heating (cooling) due to planet contraction (expansion). For each combination of planetary mass and CMF, we explore 4 scenarios, with and without compositional convection in the liquid outer core due to inner core solidification, as well as mobile and stagnant lid tectonic regimes. The result will be the first-step to aid future surveys searching for magnetic field signals from rocky exoplanets.