Solubility of Iron in Metallic Hydrogen and Stability of Dense Cores in Giant Planets

TL;DR

This study uses ab initio calculations to show that iron dissolves readily in metallic hydrogen at high temperatures, suggesting giant planet cores are likely eroded and redistributed over time, affecting their evolution.

Contribution

It provides the first detailed ab initio analysis of iron solubility in metallic hydrogen across a wide pressure and temperature range, highlighting core erosion processes.

Findings

Iron dissolves strongly above 2000 K across 0.4-4 TPa

Giant planet cores are in thermodynamic disequilibrium with surrounding layers

Differences in solubility influence planetary interior evolution

Abstract

The formation of the giant planets in our solar system, and likely a majority of giant exoplanets, is commonly explained by the accretion of nebular hydrogen and helium onto a large core of terrestrial-like composition. The fate of this core has important consequences for the evolution of the interior structure of the planet. It has recently been shown that H2O, MgO and SiO2 dissolve in liquid metallic hydrogen at high temperature and pressure. In this study, we perform ab initio calculations to study the solubility of an innermost metallic core. We find dissolution of iron to be strongly favored above 2000 K over the entire pressure range (0.4-4 TPa) considered. We compare with and summarize the results for solubilities on other probable core constituents. The calculations imply that giant planet cores are in thermodynamic disequilibrium with surrounding layers, promoting erosion and redistribution of heavy elements. Differences in solubility behavior between iron and rock may influence evolution of interiors, particularly for Saturn-mass planets. Understanding the distribution of iron and other heavy elements in gas giants may be relevant in understanding mass-radius relationships, as well as deviations in transport properties from pure hydrogen-helium mixtures.