Water Enrichment Of Forming Sub-Neptune Envelopes Limited By Oxygen Exhaustion

The interaction between a magma ocean and a primordial atmosphere is increasingly recognized as a key process in shaping planetary envelope compositions.

This coupling should strongly influence gas accretion, yet its role during the disk-embedded stage remains poorly constrained. We develop a time-dependent model that couples solid accretion, nebular-gas accretion, and water enrichment and partitioning through magma-atmosphere interactions, along with post-disk thermal evolution and escape.

We find that, for super-Earth-mass planets, water production is generally limited by the magma oxygen budget and typically ceases before disk dispersal. Subsequent nebular-gas accretion dilutes the envelope toward hydrogen-dominated compositions, largely independent of the initial magma redox state.

This establishes an upper bound on the envelope water fraction — the oxygen exhaustion limit — primarily set by the reactive-oxygen inventory and the planet mass. After disk dispersal, degassing increases the water fraction only in Earth-mass planets undergoing strong escape, while super-Earths exhibit little change because surface pressures are hardly affected by escape.

Magma-atmosphere coupling alone therefore cannot maintain water-rich envelopes in sub-Neptunes and produces a strong mass-composition relation imposed by the oxygen exhaustion limit. Highly enriched sub-Neptunes would therefore imply additional mechanisms such as late volatile delivery or post-disk giant impacts.

The relation between planetary radius and envelope composition offers a means to infer magma properties, providing a pathway to connect present-day observables with early formation histories.

Tadahiro Kimura, Tim Lichtenberg

Comments: 18 pages, 9 figures, accepted for ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2603.02423 [astro-ph.EP] (or arXiv:2603.02423v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.02423
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Submission history
From: Tadahiro Kimura
[v1] Mon, 2 Mar 2026 22:08:03 UTC (1,185 KB)
https://arxiv.org/abs/2603.02423
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