The melting curve of iron at extreme pressures: implications for planetary cores

TL;DR

This study uses first-principles simulations to determine the iron melting curve at extreme pressures relevant to super-Earths, revealing that planets over twice Earth's mass likely have solid cores, affecting their magnetic field potential.

Contribution

First-principles molecular dynamics simulations were used to calculate the iron melting curve at super-Earth core conditions, providing new insights into planetary core states.

Findings

Planets over 2 Earth masses likely have solid iron cores.

The iron melting curve has a steeper slope than planetary adiabatic profiles.

Molten metallic cores are less probable in larger terrestrial planets.

Abstract

Exoplanets with masses similar to that of Earth have recently been discovered in extrasolar systems. A first order question for understanding their dynamics is to know whether they possess Earth like liquid metallic cores. However, the iron melting curve is unknown at conditions corresponding to planets of several times the Earth's mass (over 1500 GPa for planets with 10 times the Earth's mass (ME)). In the density-temperature region of the cores of those super-Earths, we calculate the iron melting curve using first principle molecular dynamics simulations based on density functional theory. By comparing this melting curve with the calculated thermal structure of Super Earths, we show that planets heavier than 2ME, have solid cores, thus precluding the existence of an internal metallic-core driven magnetic field. The iron melting curve obtained in this study exhibits a steeper slope than any calculated planetary adiabatic temperature profile rendering the presence of molten metallic cores less likely as sizes of terrestrial planets increase.