Magma ocean interactions can explain JWST observations of the sub-Neptune TOI-270 d

TL;DR

The study investigates whether magma-ocean interactions can reproduce the JWST-observed atmosphere of the sub-Neptune TOI-270 d. It develops a coupled framework that links magma-ocean equilibrium chemistry at the atmosphere–mantle boundary (Schlichting2022) with upper-atmosphere radiative-convective structure (HELIOS), rainout condensation (FastChem Cond), photochemistry (Photochem), and forward spectra (Aura-3D); by exploring a grid of boundary temperatures and iron mass fractions, the authors identify interior configurations that match the retrieved metallicity and C/O and reproduce detected species such as H$_2$O, CH$_4$, and CO$_2$ within observational uncertainties, notably for $T_{ m m-a}=3000$ K, $T_{ m c-m}=4000$ K, and $x_{ m Fe}=33$–$50$ ext% with $K_{zz}=10^{7}$ cm$^2$ s$^{-1}$. They show that CO$_2$ can be elevated by vertical mixing and that ammonia remains difficult to detect, though nitrogen enhancements can shift the upper-atmosphere chemistry. The results demonstrate that magma-ocean processes offer a viable interior pathway to metal-rich sub-Neptune atmospheres without requiring extensive icy accretion, while also acknowledging alternatives and the need for further evolution and chemistry inclusions to robustly distinguish formation scenarios. This work advances the interpretation of JWST spectra by linking interior thermodynamics to observable atmospheric composition, motivating broader incorporation of interior–atmosphere coupling in exoplanet atmosphere studies.

Abstract

Sub-Neptunes with substantial atmospheres may possess magma oceans in contact with the overlying gas, with chemical interactions between the atmosphere and magma playing an important role in shaping atmospheric composition. Early JWST observations have found high abundances of carbon- and oxygen-bearing molecules in a number of sub-Neptune atmospheres, which may result from processes including accretion of icy material at formation or magma-atmosphere interactions. Previous work examining the effects of magma-atmosphere interactions on sub-Neptunes has mostly been limited to studying conditions at the atmosphere-mantle boundary, without considering implications for the upper atmosphere which is probed by spectroscopic observations. In this work, we present a modeling architecture to determine observable signatures of magma-atmosphere interactions. We combine an equilibrium chemistry code which models reactions between the core, mantle and atmosphere with a radiative-convective model that determines the composition and structure of the observable upper atmosphere. We examine how different conditions at the atmosphere-mantle boundary and different core and mantle compositions impact the upper atmospheric composition. We compare our models to JWST NIRISS+NIRSpec observations of the sub-Neptune TOI-270 d, finding that our models can provide a good fit to the observed transmission spectrum with little fine-tuning. This suggests that magma-atmosphere interactions may be sufficient to explain high abundances of molecules such as H$_2$O, CH$_4$ and CO$_2$ in sub-Neptune atmospheres, without additional accretion of icy material from the protoplanetary disk. Although other processes could lead to similar compositions, our work highlights the need to consider magma-atmosphere interactions when interpreting the observed atmospheric composition of a sub-Neptune.

Figures (8)

• Figure 1: Schematic of the modeling framework used in this study. Chemical equilibrium between the core, mantle and atmosphere is calculated using the model from Schlichting2022. Upper atmospheric temperature profiles are calculated using HELIOSMalik2017. Fastchem condStock2022 is used to find equilibrium chemical abundances throughout the upper atmosphere, accounting for rainout condensation. After iterating between HELIOS and Fastchem cond until convergence is reached, we account for photochemical processes and vertical mixing using PhotochemWogan2023Wogan2025.
• Figure 2: Temperature profiles for TOI-270 d from HELIOS. Each panel shows a different combination of core-mantle and mantle-atmosphere boundary temperatures. The different colors and linestyles for each profile represent different iron mass fractions, shown as a percentage of the mass of the nucleus of the planet (i.e. the iron and silicate components).
• Figure 3: Metallicity ([M/H], left) and C/O ratio (right) of the upper atmosphere of TOI-270 d ($P=1\,$mbar) for different magma ocean model scenarios. [M/H] and C/O are plotted as a function of the iron mass fraction, shown as a percentage of the total mass of the nucleus (core + mantle). The metallicity [M/H] is calculated as [(C+O)/H] relative to solar abundances Asplund2009, since carbon and oxygen-bearing species are the main drivers of the metallicity constraints presented in Benneke2024. Different line styles represent different combinations of mantle-atmosphere and core-mantle boundary temperatures ($T_{\rm m-a}$ and $T_{\rm c-m}$ respectively). The green shaded regions show the constraints on [M/H] and C/O reported by Benneke2024, inferred from the JWST transmission spectrum of the planet. The black dotted line on the right-hand panel shows the solar C/O value from Asplund2009.
• Figure 4: Volume mixing ratio of H$_2$O (left) and CH$_4$ (right) in the upper atmosphere of TOI-270 d ($P=1\,$mbar) for different magma ocean model scenarios. We show models with $K_{zz}=10^7$cm$^2$s$^{-1}$, although the difference in H$_2$O and CH$_4$ volume mixing ratio is negligible for different values of $K_{zz}$. Volume mixing ratios are plotted as a function of the iron mass fraction, shown as a percentage of the total mass of the nucleus (core + mantle). Different line styles represent different combinations of mantle-atmosphere and core-mantle boundary temperatures ($T_{\rm m-a}$ and $T_{\rm c-m}$ respectively). The green shaded regions show the constraints on the volume mixing ratios of H$_2$O and CH$_4$ reported by Benneke2024, inferred from the JWST transmission spectrum of the planet.
• Figure 5: Volume mixing ratios of CO (green, solid) and CO$_2$ (red, dashed) in the upper atmosphere of TOI-270 d ($P=1\,$mbar) as a function of $K_{zz}$. $T_{\rm{m-a}}=3000\,$K, $T_{\rm{c-m}}=4000\,$K and $x_{\rm Fe}=33\%$ for all models shown here. The red shaded region shows the constraints on the volume mixing ratio of CO$_2$ reported by Benneke2024, and the green dotted line shows the reported 2$\sigma$ upper limit for CO.