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).

The oxygen fugacity relative to this buffer is expressed in log units as ∆log fO2(IW) = log fO2 −log fOIW 2 , where fOIW
is the oxygen fugacity at the IW equilibrium at the same pressure and temperature.
2 O2 ⇌FeOw¨ustite. (1)
Femetal + 1
This buffer is characteristic of moderately reducing conditions in planetary interiors.

With higher relative fO2 correspondArticle number, page 2 of 19
ing to greater stabilization of oxidized iron species and lower values favouring metallic iron and more reducing conditions (Salvador et al. 2023).

Previous studies have shown that oxygen fugacity and crystallisation regimes jointly regulate the thermal and chemical evolution of planetary magma oceans (O’Neill & Eggins 2002; Armstrong et al. 2015; Sossi et al. 2020; Lichtenberg et al. 2021; Bower et al. 2022; Gaillard et al. 2022; Shorttle et al. 2024; Nicholls et al. 2024).

Under reducing conditions relative to the IW buffer, coupled interior–atmosphere models predict C- and H-rich atmospheres dominated by H2 and CO, whereas oxidizing conditions favour H2O- and CO2-dominated compositions.

CO remains stable across a wide redox range.

Complementary to these redox-driven trends, melt–solid separation modulates evolution (Boukaré et al. 2025).

For instance, Bower et al. (2022) showed that fractional crystallisation (crystal segregation) and equilibrium crystallisation (no crystal segregation) lead to different melt redistribution, affecting the timing of cooling and outgassing processes.

In addition, redox-dependent changes in atmospheric speciation influence radiative properties and thermal evolution.

For example, by treating fO2 as a free parameter, Nikolaou et al. (2019); Katyal et al. (2020) showed that changes in the oxidation state modify the equilibrium between interior and atmosphere, alter volatile partitioning, and modulate the outgoing radiative flux, thereby shaping planetary cooling.

At the same time, reducing conditions can favour the formation of thick H2-rich atmospheres that, at sufficiently high pressures, also enhance greenhouse blanketing and prolong magma-ocean cooling times (Pierrehumbert & Gaidos 2011; Lichtenberg et al. 2021; Nicholls et al. 2024).

When volatile species such as sulphur are included, the redox state further controls the emergence of H-C versus CN-S-dominated atmospheric compositions (Gaillard et al. 2022).

However, most existing models consider volatile inventories dominated by H2O and CO2 (Lebrun et al. 2013; Salvador et al. 2017; Hier-Majumder & Hirschmann 2017; Wordsworth et al. 2018; Nikolaou et al. 2019; Barth et al. 2021) H2O (Hamano et al. 2015; Schaefer et al. 2016), or H2O, CO2, and O2 (Krissansen-Totton et al. 2021).

In parallel, coupled frameworks linking radiative–convective atmospheres with vertically resolved mantle evolution have also been developed (Lichtenberg et al. 2021).

Their model included seven atmospheric species (H2, H2O, CO2, CH4, CO, O2, and N2) and simulated the coupled evolution of the magma ocean solidification, and the resulting atmospheric spectral signatures.

From these simulations, Lichtenberg et al. (2021) showed that atmospheric composition strongly controls cooling timescales, with H2-dominated atmospheres producing the longest-lived magma oceans.

More recently, fully coupled models have emerged that include the regulation of the atmospheric inventory through feedbacks between the interior and atmosphere, and compute volatile outgassing and atmospheric composition during magma-ocean evolution.