Additional Heat Source for Planetary Dynamos at the Inner Core Boundary of Rocky Worlds Due to Tides

Short-period, rocky exoplanets that reside in the traditional habitable zone around M dwarf stars are subjected to much higher FUV flux and flaring events than are experienced on Earth. The retention and chemistry of any atmosphere will depend, in part, on the presence, magnitude, and longevity of a protective magnetic field. The geodynamo within Earth’s core drives its magnetic field, connecting atmospheric and surface conditions to the core’s thermal state. This link may also be present on rocky exoplanets. Unlike Earth, the close proximity of short-period worlds to their stellar hosts leads to high tidal susceptibility and overlap of the system’s habitable and tidal zones. The additional heat created by tidal dissipation can act to delay or disrupt planetary dynamos by altering the thermal gradient across the liquid outer core. This has been explored by modeling the thermal state of a planet’s mantle and core-mantle-boundary (CMB). In this work, we instead look at a new mechanism that can deposit some of this tidal energy into a thin, low-viscosity zone at the inner core boundary (ICB) of rocky exoplanets. This exploration is guided by observations of a mushy layer at Earth’s ICB which exhibits mechanical properties that are conducive to frictional heating from tidal stresses. Heat deposited at the base of the liquid core can drive outer core convection that may otherwise be stymied by an elevated CMB temperature.

The structure and composition of Earth’s core are still not well understood, however, the importance of any present magnetic field to an exoplanet’s habitability motivates us to explore a large phase space of possible interior states to determine what conditions result in significant heating at the ICB. Using a tidal stress model that accounts for compressibility at the extreme pressures found near the ICB, we find that nonisotropic heating in the gigawatt-to-terawatt range is possible. Follow-on work will examine how this level of heating influences the dynamo and production of magnetic fields.