showed that volatile species exhibit contrasting partitioning be-
haviour during magma ocean evolution. Owing to its low solu-
bility in silicate melts, carbon-bearing species preferentially re-
side in the atmosphere, being efficiently outgassed from the ear-
liest stages of crystallisation (Bower et al. 2022). In contrast,
solved in the melt, favouring retention within the interior until
the final stages of solidification (Suer et al. 2023; Sossi et al.
2023; Bower et al. 2022, 2025). Water-rich atmospheres enhance
thermal blanketing and suppress radiative cooling, extending the
molten phase. Therefore, the initial water inventory plays a key
role in setting magma ocean lifetimes. In contrast, atmospheres
ally more transparent to outgoing longwave radiation and there-
fore less effective at retaining heat (Kiehl & Trenberth 1997; Sal-
vador et al. 2023). Stellar evolution can produce a similar effect,
as high stellar irradiation may suppress efficient cooling through
atmospheric buffering of infrared emission, potentially sustain-
ing long-lived magma oceans. However, recent studies indicate
that this behaviour is not governed by a universal critical flux
threshold but depends more on atmospheric and interior proper-

⋆Corresponding author: m.c.villamil.sastre@rug.nl
Article number, page 1 of 19 ties (Hamano et al.

2013, 2015; Salvador et al. 2017; Boer et al. 2025).

Exoplanet surveys provide an opportunity to test models of coupled interior–atmosphere evolution during magma ocean phases (Kreidberg & Stevenson 2025; Lichtenberg et al. 2025).

Recent JWST observations have transformed our understanding of rocky ultra-short-period exoplanets, revealing that some retain volatile secondary atmospheres rather than being entirely desiccated.

Using JWST/NIRCam and MIRI observations, Hu et al. (2024) detected a persistent volatile envelope around 55 Cnc e, inconsistent with a bare-rock or silicate-vapour atmosphere and likely dominated by CO or CO2.

Similarly, Teske et al. (2025) reported the dayside emission spectrum of TOI-561 b, whose brightness temperatures require a thick atmosphere that redistribute heat away from the planet’s dayside.

These results provide the first evidence that volatile-rich envelopes can survive on highly irradiated rocky planets.

Because these highly irradiated planets exist at thermodynamic conditions that may favour persistent dayside or global magma ocean regimes, these detections open a new avenue for linking observable secondary atmospheres to their geochemical histories.

Therefore, interpreting these observations requires a detailed understanding of the coupled evolution of planetary interiors and atmospheres, and of the processes that regulate volatile release and retention.

Although atmospheric processes have received substantial attention, the geochemical and geophysical evolution of the interior is equally important in shaping a planet’s long-term climate and thermodynamic state.

A key factor in this evolution is the oxidation state of the silicate mantle, which influences melting behaviour, volatile speciation, and interior–atmosphere exchange (Hirschmann 2012; Lichtenberg et al. 2021; Gaillard et al. 2022; Wordsworth & Kreidberg 2022; Salvador et al. 2023; Nicholls et al. 2024).

The mantle redox state reflects the equilibrium distribution of electrons among multivalent elements, particularly the balance between Fe2+ and Fe3+ in silicate phases (Cottrell et al. 2025).

Controlled by the oxygen chemical potential, this thermodynamic state is commonly quantified by the oxygen fugacity, fO2, which represents the thermodynamic availability of oxygen for redox reactions, even in the absence of free molecular oxygen (Wade & Wood 2005; Kasting 2014; Salvador et al. 2023).

High oxygen fugacity corresponds to more oxidizing mantle conditions, whereas low oxygen fugacity favours reduced chemical species.

As the dominant multivalent species in the mantle, iron simultaneously records and controls redox conditions through the balance of its oxidation states (Salvador et al. 2023).

The oxygen fugacity of a system is set by equilibrium among coexisting redox-sensitive phases, which buffer the chemical potential of oxygen at specific conditions.

These equilibrium reactions define reference oxygen fugacity states that are commonly referred to as redox buffers (Frost & McCammon 2008; Hirschmann 2012; Cottrell et al. 2025).

One of the most commonly used redox buffer is the iron–wüstite (IW) buffer Eq. 1., which represents the equilibrium between metallic iron (Fe) and ferrous iron incorporated in wüstite (FeO, iron(II) oxide).