Thermal Evolution and magnetic history of rocky planets

TL;DR

This study models the thermal and magnetic evolution of rocky planets, revealing how factors like temperature, mass, and composition influence the potential for magnetic dynamo activity in planetary cores and magma oceans.

Contribution

It introduces a coupled thermal evolution model with a Henyey solver to self-consistently analyze dynamo viability in rocky planets based on interior structure and thermal dynamics.

Findings

Magnetic dynamo lifetime depends on planet mass and core properties.

Magma ocean solidification is governed by equilibrium temperature.

Core convection and dynamo activity are limited by thermal and structural factors.

Abstract

We present a thermal evolution model coupled with a Henyey solver to study the circumstances under which a rocky planet could potentially host a dynamo in its liquid iron core and/or magma ocean. We calculate the evolution of planet thermal profiles by solving the energy balance equations for both the mantle and the core. We use a modified mixing length theory to model the convective heat flow in both the magma ocean and solid mantle. 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). We evaluate whether a dynamo can operate using the critical magnetic Reynolds number. We run simulations to explore how planet mass ($M_{pl}$), core mass fraction (CMF) and equilibrium temperature ($T_{eq}$) affect the evolution and lifetime of possible dynamo sources. We find that the $T_{eq}$ determines the solidification regime of the magma ocean, and only layers with melt fraction greater than a critical value of 0.4 may contribute to the dynamo source region in the magma ocean. We find that the mantle mass, determined by $M_{pl}$ and CMF, controls the thermal isolating effect on the iron core. In addition, we show that the liquid core last longer with increasing planet mass. For a core thermal conductivity of 40$\ \mathrm{Wm^{-1}K^{-1}}$, the lifetime of the dynamo in the iron core is limited by the lifetime of the liquid core for 1$M_{\oplus}$ planets, and by the lack of thermal convection for 3$M_{\oplus}$ planets.