Diversity of low-mass planet atmospheres in the C-H-O-N-S-Cl system with interior dissolution, nonideality, and condensation: Application to TRAPPIST-1e and sub-Neptunes

TL;DR

The paper addresses how interior compositions influence low-mass exoplanet atmospheres, focusing on magma-ocean exchanges that control early atmospheric chemistry. It presents Atmodeller, an open-source Python/JAX toolkit that unifies volatile solubility, nonideality, and condensation within a mass-balance and chemical-equilibrium framework. The method employs the extended law of mass-action ($xLMA$) to solve a closed system of nonlinear equations connecting atmosphere, condensates, and mantle, with flexible constraints such as $f_{O2}$ or $IW$. Applications to TRAPPIST-1e-like planets and sub-Neptunes reveal how nonideal gas behavior and solubility shape atmospheric composition across pressure regimes, informing interpretation of observations and habitability prospects.

Abstract

A quantitative understanding of the nature and composition of low-mass rocky (exo)planet atmospheres during their evolution is needed to interpret observations. The magma ocean stage of terrestrial- and sub-Neptune planets permits mass exchange between their interiors and atmospheres, during which the mass and speciation of the atmosphere is dictated by the planet's volatile budget, chemical equilibria, and gas/fluid solubility in molten rock. As the atmosphere cools, it is modified by gas-phase reactions and condensation. We combine these processes into an open-source Python package built using JAX called Atmodeller, and perform calculations for planet sizes and conditions analogous to TRAPPIST-1e and K2-18b. For TRAPPIST-1e-like planets, our simulations indicate that CO-dominated atmospheres are prevalent during the magma ocean stage, which, upon isochemical cooling, predominantly evolve into CO2-rich atmospheres of a few hundred bar at 280 K. Around 40% of our simulations predict the coexistence of liquid water, graphite, alpha-sulfur, and ammonium chloride, which are key ingredients for surface habitability. For sub-Neptune gas dwarfs, pressures are sufficiently high (a few GPa) that gas fugacities deviate from ideality, thereby drastically enhancing solubilities. This buffers the total atmospheric pressure to lower values than for the ideal case. These effects conspire to produce CH4-rich sub-Neptune atmospheres for total pressures exceeding around 3.5 GPa, provided H/C is approximately 100x solar and fO2 moderately reducing (3 log10 units below the iron-wustite buffer). Otherwise, molecular hydrogen remains the predominant species at lower total pressures and/or higher H/C. For all planets at high temperature, solubility enriches C/H in the atmosphere relative to the initial composition.