Lava / Volcanic Worlds

Absence Of A Runaway Greenhouse Limit On Lava Planets

By Keith Cowing

Press Release

May 20, 2025

Filed under astrogeology, atmosphere, exoplanet, Habitable Zone, lava world, Runaway Greenhouse, Spectroscopy

Absence Of A Runaway Greenhouse Limit On Lava Planets — Illustration of the two different degassing scenarios used in this work, which bracket the potential pathways of mantle melting and crystallization of planetary evolution. The range of orange colours corresponds to temperatures as defined on the bottom abscissa. The figures contain T-P profiles that are shown in black, with a surface temperature as defined by their lowermost endpoint; the corresponding black-body radiation curves are presented by striped lines in shades of orange and their radiance intensity and wavelength range are defined on the upper abscissa and right ordinate (in orange text). The black arrows denote the interaction between the atmosphere and the planetary mantle when determining the equilibrium composition of the atmosphere, as described in the Methods section. (A) The quenched scenario is representative of a case where the primordial magma ocean equilibrates last with the atmosphere above the liquidus, i.e., when the entire mantle is molten. In this case, the chemical systems of the mantle and atmosphere are assumed to be ’closed’ and no further in- or outgassing is taking place at lower surface temperatures. (B) The open scenario represents the opposite end-member assumption: (partially) molten mantle and atmosphere are allowed to equilibrate at all surface temperatures, i.e., in- and outgassing is taking place over the whole temperature range, and the mantle is assumed to be an open system towards the atmosphere. — astro-ph.EP

Climate transitions on exoplanets offer valuable insights into the atmospheric processes governing planetary habitability.

Previous pure-steam atmospheric models show a thermal limit in outgoing long-wave radiation, which has been used to define the inner edge of the classical habitable zone and guide exoplanet surveys aiming to identify and characterize potentially habitable worlds.

We expand upon previous modelling by treating (i) the dissolution of volatiles into a magma ocean underneath the atmosphere, (ii) a broader volatile range of the atmospheric composition including H₂O, CO₂, CO, H₂, CH₄ and N₂, and (iii) a surface temperature- and mantle redox-dependent equilibrium chemistry.

We find that multi-component atmospheres of outgassed composition located above partially or fully-molten mantles do not exhibit the characteristic thermal radiation limit that arises from pure-steam models, thereby undermining the canonical concept of a runaway greenhouse limit, and hence challenging the conventional approach of using it to define an irradiation-based habitable zone.

Our results show that atmospheric heat loss to space is strongly dependent on the oxidation and melting state of the underlying planetary mantle, through their significant influence on the atmosphere’s equilibrium composition. This suggests an evolutionary hysteresis in climate scenarios: initially molten and cooling planets do not converge to the same climate regime as solidified planets that heat up by external irradiation.

Steady-state models cannot recover evolutionary climate transitions, which instead require self-consistent models of the temporal evolution of the coupled feedback processes between interior and atmosphere over geologic time.

Iris D. Boer, Harrison Nicholls, Tim Lichtenberg

Comments: Accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2505.11149 [astro-ph.EP] (or arXiv:2505.11149v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.11149
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Related DOI:
https://doi.org/10.3847/1538-4357/add69f
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Submission history
From: Iris Boer
[v1] Fri, 16 May 2025 11:50:05 UTC (2,084 KB)
https://arxiv.org/abs/2505.11149
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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