erate lower-pressure but higher mean molecular weight (MMW) envelopes.

The radiative consequences are notable: high pressure H2+CO atmospheres efficiently trap heat within the planet, through collision-induced absorption (Borysow 2002), potentially prolonging magma ocean lifetimes (Nicholls et al. 2024; Krissansen-Totton et al. 2024).

Oxidized atmospheres, though IR-active, are less massive and offer a weaker blanketing effect at late times.

This redox–volatility coupling offers a possible explanation for the atmospheric diversity now being revealed in close-in rocky planets around M dwarfs.

Observations of 55 Cnc e (Hu et al. 2024) and TOI-561 b (Teske et al. 2025), for example, indicate volatile-rich envelopes dominated by CO and CO2, broadly consistent with the oxidized to moderately reduced regimes identified here.

However, other ultra-short period exoplanets do not exhibit signs of an atmosphere (Lichtenberg & Miguel 2025).

The high surface pressures and mixed C–O chemistry of the scenarios inferred for these systems could reflect planets that partially solidified from initially reducing magma oceans and later oxidized through volatile loss or mantle crystallisation.

Water storage in oxidized mantles has been predicted to affect the longterm bulk volatile composition of observed super-Earths (Dorn & Lichtenberg 2021).

Conversely, more reduced cases in our grid predict thick H2+CO atmospheres.

Therefore, our simulations illustrate that redox trends inherited from accretion can affect the distribution of melting in the deep interior, and, by extension, the storage capacity of the mantle over Gyr timescales.
4.2.

Benchmarking case GJ 1132 b provides a particularly valuable benchmark for testing coupled interior–atmosphere evolution models, given the availability of high-precision observational constraints.

Early groundbased transmission spectroscopy observations (Diamond-Lowe et al. 2018) already indicated a largely featureless spectrum, disfavouring H2-rich atmospheres and suggesting either a high mean molecular weight atmosphere or absence of an atmosphere.

Similarly, recent JWST/NIRSpec transmission spectroscopy observations shows that the planet spectrum is best described by a flat continuum (Bennett et al. 2025), consistent with either a very thin steam atmosphere or an atmosphere-free surface.

Nevertheless, when combined with MIRI/LRS thermal emission measurements, these data favour the presence of a very thin atmosphere explained by a nearly bare rocky planet (Xue et al. 2024).

In this way, forward modelling of the dayside emission spectrum rules out Earth-like atmospheres (P ∼1 bar) containing even modest amounts of H2O, excludes atmospheres of any thickness (10−4-102 bar) with ≳1% CO2, and disfavours thick, Venus-like envelopes (P ≳100 bar) with trace amounts of CO2 or H2O, indicating that GJ 1132 b most likely lacks a significant volatile atmosphere (Xue et al. 2024).

Within the context of our simulations, the apparent lack of a substantial atmosphere on GJ 1132 b is most consistent with volatile-poor initial conditions and an oxidized deep-mantle state fOmelt
that limits sustained melt fractions and long-term volatile release.

Reduced and volatile-rich evolutionary pathways in our models tend to produce thick H2- and CO-rich atmospheres during early magma-ocean phases, which, although potentially short-lived due to escape, would be expected to leave behind more substantial secondary atmospheres.

This interpretation is consistent with the evolutionary models of Schaefer et al. (2016), who investigated the coupled magma ocean–atmosphere evolution of GJ 1132 b.

They find that retaining a substantial water-rich atmosphere requires extreme initial volatile invento-
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ries ≳5 wt% (percent by weight) of H2O, while most evolutionary pathways lead to tenuous atmospheres.

In their models, photodissociation of water and preferential hydrogen escape result in the build-up of abiotic O2, with the magma ocean acting as only a limited oxygen sink.

As a consequence, the most common outcome is a thin, O2-dominated atmosphere, unless a long-lived magma ocean or unusually high initial water content is maintained.

The observational evidence for GJ 1132 b therefore favours an evolutionary history in which efficient early atmospheric loss and limited late-stage outgassing suppress the persistence of a detectable present-day atmosphere.

Future detections of even weak CO2- or H2O-dominated envelopes would provide valuable constraints on the planet’s initial volatile inventory and redox state, highlighting the potential of GJ 1132 b as a probe of coupled interior–atmosphere evolution (Nicholls et al. 2025b).