Abstract

The three major classes of chondritic meteorites—enstatite, ordinary, and carbonaceous—exhibit distinct oxidation states and mineral compositions. Traditionally, this diversity has been attributed to formation under different redox conditions or varying gas compositions in the protoplanetary disk. However, such explanations require fine-tuned scenarios that are difficult to reconcile with a nebula of nearly solar composition.

We present results from KineCond, a time-dependent condensation model that simulates the time-dependant condensation of a cooling gas of solar composition under a wide range of pressures and cooling rates. Our simulations reveal that kinetic (i.e., non-equilibrium) condensation naturally leads to three mineralogical types only, with redox states that match those of the enstatite, ordinary, and carbonaceous chondrites when plotted in at Urey-Craig diagram -without requiring any changes in gas composition.

This suggests that the redox and mineralogical diversity of primitive Solar System solids may be the direct result of local thermodynamical conditions during condensation (cooling rates, pressure), rather than local compositional heterogeneity of fO₂. Our results offer a new physical framework for understanding the early chemical architecture and oxidation state of the Solar Nebula.

Context

Chondrites, the most primitive meteorites, preserve a record of the Solar System’s early chemical environment. Chondrites are made of diverse components, namely refractory inclusions (CAIs or AOA), chondrules, metallic inclusions, and matrix. They are traditionally grouped into enstatite (EC), ordinary (OC), and carbonaceous (CC) classes, which differ both in oxidation state and bulk composition. These classes fall into distinct regions on the Urey–Craig diagram, a classical plot of Fe oxidation state. By increasing order of oxidation we have EC, OC and CC. Traditional models explain these differences via the varying C/O ratio or O/H ratio in the protoplanetary disk, but no plausible astrophysical process has been shown to generate the necessary vast range of oxygen fugacities necessary within a solar composition disk. We propose that non-equilibrium condensation in the Solar Nebula a simple, physical explanation for this redox diversity. We show that chondrites mineral precursors formed by kinetic condensation naturally define 3 mineralogical groups, then may represent the 3 main chondrites populations due to their close match in the Urey-Craig diagram but also in bulk mineralogy.

Method

We developed KineCond, a time-dependent condensation model that simulates the cooling of a solar-composition gas from 2000 to 130 K in a closed system. The model tracks 39 direct condensation/evaporation reactions and 38 gas–solid nebular (generalized) reactions, across pressures from 10⁻⁹ to 10⁻¹ bar and cooling timescales from 0.01 to 1000 years. Solids evolve through competition between condensation, evaporation, and solid-gas exchange. Lack of kinetic data on solid-gas reactions are compensated by an exploration of reactions constants on several orders of magnitude. We find that at first order, condensation processes dominate over gas-grain reactions for determining the final mineralogies of precursors.

We define an empirical condensation index X = log₁₀(P/bar) + log₁₀(T_c/yr), which captures the influence of pressure and cooling rate. For each simulation, the resulting mineralogy is projected onto the Urey–Craig diagram and compared to known chondrite groups. X >−5 correspond close-to equilibrium condensation, whereas X<−5 correspond to fast and out-of equilibrium condensation.

Results

Our simulations show that condensates naturally segregate into three distinct mineralogical and redox groups:

• Type A: Metal-rich, reduced, enstatite-rich (X > –2), with sulfides. Recalling EH bulk mineralogy
• Type B: Intermediate oxidation, fayalite with forsterite, Recalling OC bulk mineralogy
• Type C: Oxidized, containing fayalite, magnetite and phyllosilicates (X < –6), Recalling CO-CV bulk mineralogy.

This behavior is remarkably robust: varying elemental ratios (e.g., Mg/Si, Al/Si) affects mineral proportions but not the redox state. Cooling rate and pressure, however, are decisive. Highpressure, slow-cooling conditions reproduce equilibrium results, and close to EC mineralogy, but under realistic nebular conditions (e.g., P ∼ 10⁻⁴ bar at 1 AU), kinetic effects dominate. Fast condensation leads to oxidation because under rapid cooling, atoms and molecules in the gas phase do not have enough time to fully equilibrate with condensates. Oxygen, being the most abundant

Figure 1: Urey-Craig diagram. X axis: Number oxidized Fe atoms/Si and normalized to solar Fe/Si, Y axis : fraction of metal (Fe+FeS) and normalized to Solar . Colored shapes shows measurements  in chondrites populations. Solids lines show results of Kinecond condensation simulations. In blue : slow condensation (type A condensates), in blue : moderately fast condensation (type B condensates), in red : very fast and out of equilibrium condensation (type C condensates). Oxidation state of CM and CO are never matched (parent body water circulation could explain it).

condensable specie, is rapidly incorporated into solid phases as they supersaturate and condense upon rapid cooling conditions. In contrast, reduced minerals are typically produced in equilibrium condensation, that require slower reaction times or higher temperatures to establish. As a result, fast condensation ”freezes in” more oxidized mineral phases than would form under equilibrium or slower kinetics. We find that minerals typical of CAIs, AOAs and chondrules, metal nuggets, and also phillosilicates are formed under specific condensation conditions, depending on X.

Transient heating followed by recondensation (e.g., via bow shocks) leads to similar groupings.

Discussion

Kinetic condensation offers a natural explanation for the redox and mineralogical diversity of chondrite precursors. Unlike equilibrium models, it does not require ad hoc parameters (like the ”isolation factors”) or large-scale chemical heterogeneities to reproduce at the same time reduced and oxidized mineralogies. The key factor is the pressure and the condensation time. Fast condensation at low pressure favors oxidized forms, whereas slow condensation at high pressure favors reduced minerals. The emergence of three stable mineralogical regimes from solar gas alone reproduces the key structure of the Urey–Craig diagram (Fig. 1).

This suggests that the redox properties of chondrites may reflect early condensation kinetics rather than later alteration.  Our results suggest a paradigm in which the redox properties of planetary building blocks are inherited from non-equilibrium condensation.

Acknowledgments

This work was supported by the DISKBUILD project (ANR-20- CE49-0006), the LabEx UnivEarthS initiative (ANR-10-LABX-0023 and ANR-18-IDEX-0001) and by the French space agency CNES (Centre National d’Etudes Spatiales), the Swiss National Science Foundation (SNSF) , Eccellenza Professorship (203668) , SERI contract No. MB22.00033, a SERI-funded ERC Starting grant “2ATMO”

How to cite: Charnoz, S., Jérôme, A., Chaussidon, M., Sossi, P. A., Marrocchi, Y., and Franco, P.: Forming the first solids precursors in the Solar System through kinetic condensation, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-162, https://doi.org/10.5194/epsc-dps2025-162, 2025.

The quantity of radioactive isotopes in a planet’s mantle and the evolution of its heating due to the isotopes’ radioactive decay determines the capability of that planet to develop geological features associated with a habitable environment, such as surface crust and plate tectonics. When our solar system was formed, large quantities of Potassium (K), a major element available in the interstellar medium at the time, got subsequently deposited inside our planet’s mantle and crust. Potassium’s long-lived radioactive isotope ⁴⁰K is still present in large quantities inside the planet. The beta particles that it emits while decaying have been heating up earth’s mantle for the last several billions of years and largely contribute to the habitable nature of Earth. Predicting the amount of ⁴⁰K enrichment in the solar system of an exoplanet would be key for a reliable calculation of the planet’s heating evolution and would allow us to make estimates on the likely existence or not of a habitable environment. Potassium, however, has a complex production and (destruction) mechanism in the cosmos. From a nucleosynthesis point of view, the uncertainty in the abundance of ⁴⁰K is associated with the reactions that create and destroy ⁴⁰K in stellar nucleosynthesis processes and the corresponding reaction rates. In my talk, I will discuss the importance of potassium in the context of exoplanet-related research, the origin of potassium in stars, the nuclear physics aspects that affect the existence of ⁴⁰K, and current experimental efforts to constrain relevant reaction rates.

How to cite: Perdikakis, G.: The radiogenic heating of planets and the thermonuclear rate for the destruction of 40K in stars , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1771, https://doi.org/10.5194/epsc-dps2025-1771, 2025.

Devolatilization — depletion of volatile elements (e.g., C, O, N, S, Na and K) in rocky planets relative to their host stars — is a common feature that has been observed in both the Solar System and exoplanet systems. Competing mechanisms have been proposed to explain this common feature, ranging from incomplete condensation of dust materials from an ultra-hot nebula with a host stellar composition, partial evaporation of planetesimals by short-lived radiogenic heating and/or collisional kinetic energy, as well as thermal processing of primordial pebbles in evolving protoplanetary disks and through accretion process. The compositional outcome of prevalent theories will be simulated and then tested against a wide range of empirical/observational data with the solar system rocky bodies and exoplanetary rocky materials as inferred from both ‘polluted’ white dwarfs and co-natal pairs of planet-hosting stars. It is anticipated to bridge theories and observations to understand in depth this potentially universal phenomenon of devolatilization in rocky planet formation – a pillar underpinning the chemical foundations of rocky worlds.

How to cite: Wang, H., Bonsor, A., Liu, F., and Johansen, A.: Devolatilization during rocky planet formation: Bridging theories and observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-697, https://doi.org/10.5194/epsc-dps2025-697, 2025.

Abstract

Rocky planets in the Solar System exhibit element depletions relative to the Sun that correlate with volatility during disk condensation. To investigate whether this trend extends to exoplanets, we analyze how element volatility varies with proto-planetary disk composition using a statistical sample of stellar abundances. We find that volatility behavior depends strongly on the disk’s C/O ratio, revealing a regime (C/O = 0.75–0.95) where Mg and Si become more volatile while Ca, Al, and Fe retain near-constant volatility. This leads to the formation of planets with high core mass fractions and Ca/Al-rich mantles—potentially distinguishable from both graphite-crusted and Earth-like planets based on bulk density.

Introduction

Rocky planets inherit their material from proto-planetary disks, which reflect  the composition of their host stars. To constrain bulk rocky exoplanet compositions, which governs various aspects of their evolution, we can use observed stellar abundances. This requires an additional step of volatility-based element depletion, as observed for the Earth, Mars, and Vesta. Element volatility can vary significantly with bulk gas composition, as proven for disks with varying molar C/O ratios (Larimer, 1979). Here, we aim to extend our understanding of how element volatility varies with bulk proto-planetary disk composition, by simulating condensation sequences for a large statistical sample of disk compositions based on stellar abundance observations.

Methods

We use the GGchem code (Woitke+, 2018) to simulate the composition of condensed solids in chemical equilibrium with the proto-planetary disk gas, as it cools from 2500 K to 400 K, at 1e-4 bar (based on the Solar nebula). We characterize element volatility by the 50% condensation temperatures (Tc), since the Earth shows a direct correlation between Tc values and the depletion of elements in Earth compared to the Sun (e.g., Wang+ 2019). We run these models for a sample of 1,000 stellar abundances taken from the GALAH catalogue (DR3.2, Buder+ 2021), and create parametrizations for element condensation temperatures as a function of bulk disk composition, for major rock-forming and metal-forming elements. We finally apply element depletion, correlated to condensation temperatures updated with these parametrizations, to all stellar abundances in the GALAH catalogue to simulate bulk rocky exoplanet compositions.

Results

We find that the Tc of major rock-forming cations (Ca, Al, Mg, Si, and Na) increases as bulk disk C/O increases, similar to previous research. However, we find that Mg and Si experience a drop in Tc at lower C/O than Ca and Al (0.75-0.85 vs. 0.94-0.95; Fig. 1). This leads to refractory-rich planetary material in this C/O range. On the contrary, Fe has Tc only dependent on its own abundance, and is independent of the disk C/O.

Applying the Earth-Sun devolatilization trend, accounting for disk chemistry-dependent element volatility, reveals a wider range of bulk planet compositions than considered before. Especially, planets with C/O between 0.75 and 0.94 display greater Mg and Si depletions, leading to planets with higher core mass fractions and higher concentrations of Al and Ca in their mantles (Fig. 2). These planets could be observationally distinguishable from lower C/O planets with smaller cores, or higher C/O planets with extensive light graphite crusts, based on their mass-radius relation. Further, if these planets would form with a steeper devolatilization pattern (e.g., a Vesta-like pattern), they could reach Mercury-like core mass fractions, potentially explaining super-Mercury observations.

To assess the geophysical implications of these compositions, we modeled mantle mineralogy as a function of bulk planet composition. We use a Gibbs energy minimization algorithm (Perple_X, Connolly 2005) and thermodynamic databases valid for the whole mantle (Stixrude+Lithgow-Bertelloni, 2024). We confirm the existence of planets with the weak mineral ferropericlase in their upper mantles (Fig. 3). Further, we find that Si-rich planets with quartz present in their bulk mantle, which would have a low-density crust and could be locked out of Earth-like plate tectonics behaviour, are exceedingly rare.

Conclusion

Element volatility in proto-planetary disks varies with bulk composition, particularly the molar C/O ratio. In disks with intermediate C/O values (0.75–0.95), Mg and Si become more volatile than other refractory elements, resulting in rocky planets with elevated core mass fractions and Ca- and Al-enriched mantles. These distinct compositions may produce observable differences in mass-radius relationships and offer a potential explanation for high-density “super-Mercury” exoplanets. Our results highlight a new class of rocky planets shaped by disk chemistry that expands the known diversity of planetary interiors.

Fig. 1. 50% condensation temperatures (Tc) of key rock-forming cations Mg, Si, Ca, and Al, as a function of bulk proto-planetary disk molar C/O ratios. Tc values are based on condensation sequences of various disk compositions, calculated with GGchem, at 1e-4 bar.

Fig. 2. Bulk planet compositions in terms of mass fraction of the metallic iron core (CMF) and minor element fraction (Ca+Al) of the mantle, as a function of host stellar molar C/O ratios. Planet compositions are calculated by applying element volatility-dependent depletion factors to stellar abundances.

Fig. 3.  Predicted distribution of rocky exoplanet bulk mantle Mg/Si ratios (top), and mantle mineralogy (P = 3 GPa) of exoplanets as a function of bulk mantle Mg/Si (bottom). Mineralogy is calculated with Gibbs energy minimization algorithm Perple_X.

Bibliography

Larimer, J. W., & Bartholomay, M. (1979). The role of carbon and oxygen in cosmic gases: some applications to the chemistry and mineralogy of enstatite chondrites. Geochimica et Cosmochimica Acta, 43(9), 1455-1466.

Woitke, P., Helling, C., Hunter, G. H., Millard, J. D., Turner, G. E., Worters, M., ... & Stock, J. W. (2018). Equilibrium chemistry down to 100 K-Impact of silicates and phyllosilicates on the carbon to oxygen ratio. A&A, 614, A1.

Wang, H. S., Lineweaver, C. H., & Ireland, T. R. (2019). The volatility trend of protosolar and terrestrial elemental abundances. Icarus, 328, 287-305.

Buder, S., Sharma, S., Kos, J., Amarsi, A. M., Nordlander, T., Lind, K., ... & Galah Collaboration. (2021). The GALAH+ survey: Third data release. MNRAS 506(1), 150-201.

Connolly, J. A. (2005). Computation of phase equilibria by linear programming: a tool for geodynamic modeling and its application to subduction zone decarbonation. EPSL, 236(1-2), 524-541.

Stixrude, L., & Lithgow-Bertelloni, C. (2024). Thermodynamics of mantle minerals–III: the role of iron. GJI, 237(3), 1699-1733.

How to cite: Spaargaren, R., Herbort, O., Wang, H., Mojzsis, S., and Sossi, P.: Compositional effects on element volatility in extra-Solar proto-planetary disks, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1612, https://doi.org/10.5194/epsc-dps2025-1612, 2025.

Exoplanet super-Earths and sub-Neptunes are likely more prevalent than stars, yet their nature remains uncertain. Notably, sub-Neptunes orbit in close proximity to their host stars, often enveloped in substantial and hot hydrogen-rich atmospheres. These conditions, characterized by intense irradiation and the presence of greenhouse gases, accelerate the melting of the underlying silicate surface and facilitate the exchange of volatiles with the atmosphere. In this study, we conduct extensive large-scale ab initio simulations to investigate the reaction between hydrogen gas and molten multi-component silicates. Our findings reveal that magma oceans possess the capability to dissolve up to 2 wt% hydrogen at typical conditions pertinent to the interface between the atmosphere and the condensed surface of such planets. The influx of hydrogen into the molten silicate results in a reduction in magma density, leading to the formation of a substantial buffer layer between the condensed interior and the outer atmosphere. This layer effectively suppresses convection and restricts chemical exchanges within these planets.

Furthermore, the incorporation of hydrogen into the molten silicate alters the redox state of the magma ocean, causing it to become reduced. Consequently, a substantial outflux of oxygen is generated, which combines with atmospheric hydrogen to produce substantial amounts of water vapor. Consequently, the release of oxygen will profoundly alter the atmospheric chemistry and manifest spectral signatures detectable from space telescopes. These atmospheric composition modifications constitute a testable signature of concealed magma oceans on exoplanets. The uptake of volatiles by the magma implies that sub-Neptunes exhibit a higher volatility compared to previously assumed. Additionally, our atomistic simulations suggest that the dissolution and evaporation process is chemically intricate, resulting in the formation of hundreds of distinct molecular species within the atmosphere. These magma-buffered atmospheric recipes will necessitate consideration in future studies focused on the origins of exoplanet atmospheres.

How to cite: Caracas, R., Kite, E., and Chen, H.: Large-scale hydrogen reactions with magma oceans change the atmosphere of exoplanets, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-635, https://doi.org/10.5194/epsc-dps2025-635, 2025.

Highly irradiated rocky exoplanets are exposed to extreme temperatures that cause the presence of  magma oceans on their surfaces. The direct exchange of material between these magma oceans and their overlying atmospheres offers a window into the planets’ interiors. To understand how the properties of the magma shape atmospheric composition and observability, detailed chemical‑equilibrium modelling is essential. Until now, however, most models have considered only refractory (non‑volatile) components when simulating lava evaporation. 
In this presentation, we examine how adding volatile species containing carbon, hydrogen, nitrogen, sulphur and phosphorus changes equilibrium vaporisation outcomes in vaporization codes and, in turn, the atmospheric makeup of hot rocky exoplanets.
In order to accomplish this, we modify our open-source code LavAtmos to be able to solve for the oxygen partial pressure that satisfies both mass‑action and mass‑conservation laws in a melt‑plus‑volatiles system. Coupling our code to the FastChem solver expanded the gas‑phase network to 523 species. We used this scheme to compute “pure” atmospheres containing only one volatile element (C, H, N, S or P) and mixed atmospheres with all five, and we applied it to two literature scenarios for 55 Cnc e.
We show how introducing volatiles consistently raises the equilibrium partial pressures of rock‑derived gases (SiO, SiO₂, Na and K) relative to volatile‑free calculations. It also boosts the atmospheric oxygen inventory, altering key species such as CO₂ and H₂O. For 55 Cnc e, the model indicates that a low carbon‑to‑oxygen ratio could signal an exposed magma ocean beneath a volatile‑rich sky on this ultra‑short‑period planet.

How to cite: Miguel, Y., van Buchem, C., Zilinskas, M., and van Westrenen, W.: Volatile‑Rich Magma–Atmosphere Equilibria on Hot Rocky Exoplanets, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1367, https://doi.org/10.5194/epsc-dps2025-1367, 2025.

To aid the search for atmospheres on rocky exoplanets, we should know what to look for. An unofficial paradigm is to anticipate CO2 present in these atmospheres, through analogy to the solar system and through theoretical modelling. This CO2 would be outgassed from molten silicate rock produced in the planet’s mostly-solid interior—an ongoing self-cooling mechanism that should proceed, in general, so long as the planet has sufficient internal heat to lose.

Outgassing of CO2 requires relatively oxidising conditions. Previous work has noted the importance of how oxidising the planet interior is (the oxygen fugacity), which depends strongly on its rock composition. Current models presume that redox reactions between iron species control oxygen fugacity. However, iron alone need not be the sole dictator of how oxidising a planet is. Indeed, carbon itself is a powerful redox element, with great potential to feed back upon the mantle redox state as it melts. Whilst Earth is carbon-poor, even a slightly-higher volatile endowment could trigger carbon-powered geochemistry.

We offer a new framework for how carbon is transported from solid planetary interior to atmosphere. The model incorporates realistic carbon geochemistry constrained by recent experiments on CO2 solubility in molten silicate, as well as redox couplings between carbon and iron that have never before been applied to exoplanets. We also incorporate a coupled 1D energy- and mass-balance model to provide first-order predictions of the rate of volcanism.

We show that carbon-iron redox coupling maintains interior oxygen fugacity in a narrow range: more reducing than Earth magma, but not reducing enough to destabilise CO2 gas. We predict that most secondary atmospheres, if present, should contain CO2, although the total pressure could be low. An atmospheric non-detection may indicate a planet either born astonishingly dry, or having shut off its internal heat engine.

How to cite: Guimond, C. M., Shorttle, O., and Pierrehumbert, R. T.: A geochemical view on the ubiquity of CO2 on rocky exoplanets with atmospheres, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1714, https://doi.org/10.5194/epsc-dps2025-1714, 2025.

Constraining the heat flow out of a planetary core over geological time remains a significant challenge, as
the extreme pressures and temperatures involved push the limits of experimental capabilities. However,
this heat flow is crucial for understanding planetary processes that are often linked to habitability in
planetary sciences. A geodynamo driven by core cooling, mantle convection, and plate tectonics is
strongly influenced by this value.
Recent studies coupling higher-dimension visco-plastic mantle convection with core evolution models
have demonstrated correlations for Earth, highlighting the need for more comprehensive models to
explore these interactions in other rocky planets. One such potential correlation is between a planet’s
surface magnetic field strength and its surface cooling regime, such as plate tectonics.
Here, we present a new 2D mantle convection model coupled with a core evolution model, incorporating
state-of-the-art equations of state for core and mantle minerals, to study the well-known exoplanet
class: Super-Earths. Our results reveal a previously overlooked mechanism—an inner-core–mantle
thermal feedback loop—emerging from our coupling approach. The Earth reference cases examined
here further support the necessity of an additional geodynamo-driving mechanism in early Earth to
resolve the "new core paradox". Additionally, we find that surface magnetic field intensities for super-
Earths range from 25 to 360 μT. Notably, we observe that "hot" super-Earths (<3M⊕) exhibit geodynamo
evolutions independent of surface regime, while those with a mobile lid (>3 M⊕) experience enhanced
geodynamo lifetimes and stronger surface fields. This suggests a small but significant link for future
observational detections.

How to cite: Müller, L., Kislyakova, K., and Noack, L.: Geodynamos of Super-Earths, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1764, https://doi.org/10.5194/epsc-dps2025-1764, 2025.

This talk will cover the state of the art in whole-planet subNeptune modelling, and needs for the future.  Inferences about the composition of the deep envelope can be made on the basis of the way chemical transformations in the deep envelope may be evidenced in the observable atmosphere, such as has been attempted, for example, regarding the presence or absence of NH3 in the observable atmospheres of subNeptunes.  Such inferences require an understanding not only of deep envelope chemistry, but also of vertical mixing processes. The mixing process engages a number of poorly understood phenomena, such as mixing rates through stably stratified (nonconvective) internal radiative layers.  The occurrence of such radiative layers can be induced by compositional suppression of convection (e.g. due to high molecular weight H2O in an H2-rich atmosphere). We will review our modelling studies regarding this phenomenon.  Typically, the envelope-silicate interface is hot enough that the interface takes the form of a magma ocean, so compositional interchange with the magma ocean becomes crucial. This exchange includes rock vapours as well as lower molecular weight volatiles.  Our work on magma ocean exchanges will be reviewed. We highlight the importance of mineral physics experiments and molecular dynamics to provide crucially needed (and largely absent) thermodynamic parameters, particularly at high pressure.  At sufficiently high temperatures, silicate itself can become supercritical so that the distinction between silicate melt and silicate vapour disappears and the silicate substance becomes completely miscible with the lower molecular weight envelope.  Modeling and experiment regarding this novel and largely unexplored regime is particularly needed.

How to cite: Pierrehumbert, R.: What are subNeptunes made of?, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1855, https://doi.org/10.5194/epsc-dps2025-1855, 2025.