Solubility of Rock in Steam Atmospheres of Planets

TL;DR

This study investigates how rocky elements dissolve and partition into steam atmospheres of planets, affecting their composition and evolution, with implications for hot exoplanets and Earth's early atmosphere.

Contribution

It provides a comprehensive calculation of chemical equilibrium and vaporization of rocky elements in steam atmospheres, highlighting their potential loss and impact on planetary properties.

Findings

Rocky elements form volatile hydroxide and halide gases in steam atmospheres.

Fractional vaporization can significantly alter planetary bulk composition.

Earth's Si/Mg ratio may result from primordial steam atmosphere loss.

Abstract

Extensive experimental studies show all major rocky elements (Si, Mg, Fe, Ni, Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these results to compute chemical equilibrium abundances of rocky element-bearing gases in steam atmospheres equilibrated with silicate magma oceans. Rocky elements partition into steam atmospheres as volatile hydroxide gases and via reaction with HF or HCl as volatile halide gases in much larger amounts than expected from their vapor pressures over volatile-free solid or molten rock at the same temperature. We compute the extent of fractional vaporization by defining gas to magma partition coefficients and show Earth's sub-solar Si to Mg bulk elemental ratio may be due to loss of a primordial steam atmosphere. We conclude hot rocky exoplanets that are undergoing or have undergone escape of steam atmospheres may experience fractional vaporization and loss of Si, Mg, Fe, Ni, Ca, Al, Na, and K. This loss may modify their bulk composition, density, heat balance, and internal structure.