The effect of a small amount of hydrogen in the atmosphere of ultrahot magma-ocean planets: Atmospheric composition and escape

¹ Université de Paris Cité, Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, 75005 Paris, France
e-mail: charnoz@ipgp.fr
² Laboratoire AIM, CEA, CNRS, Univ. Paris-Sud, UVSQ, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
³ Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, 91191 Gif-sur-Yvette, France
⁴ Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland
⁵ CEED, PHAB, University of Oslo, Oslo, Norway

Received: 22 December 2022
Accepted: 21 April 2023

Abstract

Context. Ultrahot (>1500 K) rocky exoplanets may be covered by a magma ocean, resulting in the formation of a vapor rich in rocky components (e.g., Mg, Si, Fe) with a low total pressure and high molecular mass. However, exoplanets may have also captured a significant amount of hydrogen from the nebular gas during their formation. Ultrahot rocky exoplanets around the Fulton gap (~1.8 R_⊕) are sufficiently large to have retained some fraction of their primordial hydrogen atmosphere.

Aims. Here, we investigate how small amounts of hydrogen (much smaller than the mass of the planet) above a magma ocean may modify the atmospheric chemistry and its tendency to thermally escape.

Methods. We use a chemical model of a magma ocean coupled to a gas equilibrium code (that includes hydrogen) to compute the atmospheric composition at thermodynamical equilibrium for various H contents and temperatures. An energy-limited model is used to compute atmospheric escape and is scaled to consider H-rich and H-poor atmospheres.

Results. The composition of the vapor above a magma ocean is drastically modified by hydrogen, even for very modest amounts of H (≪10⁻⁶ planetary mass). Hydrogen consumes much of the O₂(g), which, in turn, promotes the evaporation of metals and metal oxides (SiO, Mg, Na, K, Fe) from the magma ocean. Vast amounts of H₂O are produced by the same process. At high hydrogen pressures, new hydrogenated species such as SiH₄ form in the atmosphere. In all cases, H, H₂, and H₂O are the dominant nonmetal-bearing volatile species. Sodium is the dominant atmospheric metal-bearing species at T < 2000 K and low H content, whereas Fe is dominant at high H content and low temperature, while SiO predominates at T > 3000 K. We find that the atmospheric Mg/Fe, Mg/Si, and Na/Si ratios deviate from those in the underlying planet and from the stellar composition. As such, their determination may constrain the planet’s mantle composition and H content. As the presence of hydrogen promotes the evaporation of silicate mantles, it is conceivable that some high-density, irradiated exoplanets may have started life as hydrogen-bearing planets and that part of their silicate mantle evaporated (up to a few 10% of Si, O, and Fe) and was subsequently lost owing to the reducing role of H.

Conclusions. Even very small amounts of H can alter the atmospheric composition and promote the evaporation to space of heavy species derived from the molten silicate mantle of rocky planets. Through transit spectroscopy, the measurement of certain elemental ratios, along with the detection of atmospheric water or hydrogen, may help to determine the nature of a surface magma ocean.