Magnesium silicate condensation in sub-Neptune envelopes: the fundamental link between chemistry, structure, and observables

[2608.24873] Magnesium silicate condensation in sub-Neptune envelopes: the fundamental link between chemistry, structure, and observables Skip to main content Search Submit Donate Log in Astrophysics > Earth and Planetary Astrophysics arXiv:2608.24873 (astro-ph) [Submitted on 25 Aug 2026 ( v1 ), last revised 5 Nov 2026 (this version, v3)] Title: Magnesium silicate condensation in sub-Neptune envelopes: the fundamental link between chemistry, structure, and observables Authors: William Misener , Peter Gao , Nicole L. Wallack , Maggie A. Thompson , Anat Shahar View PDF HTML (experimental) Abstract: Chemical interactions between the hydrogen-dominated envelopes and silicate-rich interiors of sub-Neptunes likely play a key role in shaping their atmospheric structure, mass-radius relations, and upper atmosphere composition. While atmospheric abundances and structure deeply influence each other, many existing models have either considered the effects of chemical interactions without the structural implications or have modeled the envelope structure using oversimplified chemical networks. In this work, we introduce Rocky Raccoon, a coupled chemical equilibrium-atmospheric structure model. This model incorporates Mg, Si, O, C, and H species and produces self-consistent atmospheric chemical and thermal profiles for sub-Neptune envelopes, treating multi-species condensation for the first time. We find that the condensation sequence of magnesium silicates above a magma ocean is determined by the basal magma composition. We show that these condensation sequences drive the upper atmospheric composition to two endmembers: high oxygen abundances in the underlying melt produce compositions rich in oxygen-bearing volatiles (sub-solar C/O) and higher mean molecular weight atmospheres with $\mu \sim 4$ amu, while oxygen-poor melts produce lower mean molecular weight atmospheres dominated by methane and silane (super-solar C/O). The transition between the two regimes is abrupt and depends on melt properties like Mg/Si ratios and oxygen abundances. The different condensation sequences also lead to different thermal profiles, as deep convection is inhibited over different regions due to varying molecular weight gradients. Further experiments and simulations are key to resolving critical uncertainties in the condensation sequences and the corresponding significant impacts on sub-Neptune composition and thermal structure. Comments: 26 pages, 10 figures, accepted in ApJ Subjects: Earth and Planetary Astrophysics (astro-ph.EP) Cite as: arXiv:2608.24873 [astro-ph.EP] (or arXiv:2608.24873v3 [astro-ph.EP] for this version) https://doi.org/10.48550/arXiv.2608.24873 Focus to learn more arXiv-issued DOI via DataCite Submission history From: William Misener [ view email ] [v1] Tue, 25 Aug 2026 17:54:18 UTC (667 KB) [v2] Thu, 27 Aug 2026 19:26:07 UTC (480 KB) [v3] Thu, 5 Nov 2026 13:13:25 UTC (471 KB) Full-text links: Access Paper: View PDF HTML (experimental) TeX Source view license Current browse context: astro-ph.EP < prev | next > new | recent | 2026-08 Change to browse by: astro-ph References & Citations SciX Google Scholar Semantic Scholar export BibTeX citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer ( What is the Explorer? ) Connected Papers Toggle Connected Papers ( What is Connected Papers? ) Litmaps Toggle Litmaps ( What is Litmaps? ) scite.ai Toggle scite Smart Citations ( What are Smart Citations? ) Code, Data, Media Code, Data and Media Associated with this Article alphaXiv Toggle alphaXiv ( What is alphaXiv? ) Links to Code Toggle CatalyzeX Code Finder for Papers ( What is CatalyzeX? ) DagsHub Toggle DagsHub ( What is DagsHub? ) GotitPub Toggle Gotit.pub ( What is GotitPub? ) Huggingface Toggle Hugging Face ( What is Huggingface? ) ScienceCast Toggle ScienceCast ( What is ScienceCast? ) Demos Demos Replicate Toggle Replicate ( What is Replicate? ) Spaces Toggle Hugging Face Spaces ( What is Spaces? ) Spaces Toggle TXYZ.AI ( What is TXYZ.AI? ) Related Papers Recommenders and Search Tools Link to Influence Flower Influence Flower ( What are Influence Flowers? ) Core recommender toggle CORE Recommender ( What is CORE? ) IArxiv recommender toggle IArxiv Recommender ( What is IArxiv? ) Author Venue Institution Topic About arXivLabs arXivLabs: experimental projects with community collaborators arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website. 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