A New Global Chemical Equilibrium Code: Refractory Element Signatures in Super-Earths and Sub-Neptunes
Abstract
The atmospheres of super-Earths and sub-Neptunes can be strongly modified by chemical exchange with their molten interiors during long-lived magma ocean phases. Interpreting atmospheric observations requires fast models that self-consistently couple atmospheric chemistry to the composition of the planetary interior. We present an updated, CPU-based implementation of the global chemical equilibrium (GCE) framework from H. E. Schlichting & E. D. Young (2022), which computes the equilibrium composition of a coupled metal-silicate-gas system. The numerical solver has been improved using a gradient-based optimizer, reducing the computational cost of solving the chemical network by more than two orders of magnitude and enabling large parameter studies. We apply the framework to a large synthetic population of planets and explore the imprint of bulk refractory composition of Mg, Si, and Fe on atmospheric properties in the end member case that hydrogen dissolution increases unbound with pressure. We consider planets with different masses, thermal states, and volatile inventories. We find that the atmospheric mass fraction and atmospheric metal mass fraction are primarily controlled by the temperature at the atmosphere-magma ocean interface and the planetary water budget, while the accreted hydrogen mass fraction plays a minor role because most hydrogen dissolves into the interior. For planets that accreted water, the refractory ratios Mg/Si and Fe/Si strongly influence carbon partitioning between the gas, silicate, and metal phases, producing large variations in atmospheric metal mass fraction and C/O ratios. These results demonstrate that atmospheric compositions of sub-Neptunes depend sensitively on both the volatile inventory and the bulk composition of rocky material, providing new constraints for interpreting atmospheric observations. The new GCE code is open-source.
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BibTeX
@misc{grimm2026global,
title = {{A New Global Chemical Equilibrium Code: Refractory Element Signatures in Super-Earths and Sub-Neptunes}},
author = {Simon Grimm and Marie-Luise Steinmeyer and Aaron Werlen and Caroline Dorn and Hilke Schlichting and Edward D. Young},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2605.07833},
eprint = {2605.07833},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.07833},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A New Global Chemical Equilibrium Code: Refractory Element Signatures in Super-Earths and Sub-Neptunes
AU - Simon Grimm
AU - Marie-Luise Steinmeyer
AU - Aaron Werlen
AU - Caroline Dorn
AU - Hilke Schlichting
AU - Edward D. Young
PY - 2026
DA - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2605.07833
UR - https://arxiv.org/abs/2605.07833
AB - The atmospheres of super-Earths and sub-Neptunes can be strongly modified by chemical exchange with their molten interiors during long-lived magma ocean phases. Interpreting atmospheric observations requires fast models that self-consistently couple atmospheric chemistry to the composition of the planetary interior. We present an updated, CPU-based implementation of the global chemical equilibrium (GCE) framework from H. E. Schlichting & E. D. Young (2022), which computes the equilibrium composition of a coupled metal-silicate-gas system. The numerical solver has been improved using a gradient-based optimizer, reducing the computational cost of solving the chemical network by more than two orders of magnitude and enabling large parameter studies. We apply the framework to a large synthetic population of planets and explore the imprint of bulk refractory composition of Mg, Si, and Fe on atmospheric properties in the end member case that hydrogen dissolution increases unbound with pressure. We consider planets with different masses, thermal states, and volatile inventories. We find that the atmospheric mass fraction and atmospheric metal mass fraction are primarily controlled by the temperature at the atmosphere-magma ocean interface and the planetary water budget, while the accreted hydrogen mass fraction plays a minor role because most hydrogen dissolves into the interior. For planets that accreted water, the refractory ratios Mg/Si and Fe/Si strongly influence carbon partitioning between the gas, silicate, and metal phases, producing large variations in atmospheric metal mass fraction and C/O ratios. These results demonstrate that atmospheric compositions of sub-Neptunes depend sensitively on both the volatile inventory and the bulk composition of rocky material, providing new constraints for interpreting atmospheric observations. The new GCE code is open-source.
ER -