Icy Worlds

Evidence For Ultra-water-rich Ammonia Hydrates Stabilized In Icy Exoplanetary Mantles

By Keith Cowing

Status Report

astro-ph.EP

August 19, 2025

Filed under ammonia dihydrate, ammonia hemihydrate, astrochemistry, astrogeology, exoplanet, ice, icy world, Water

Evidence For Ultra-water-rich Ammonia Hydrates Stabilized In Icy Exoplanetary Mantles — Buoyancy-driven layering in the icy mantle of water-rich exoplanets. The assembly ADH-DMA + NH3.6H2O + H2O ice VII will be stabilized below 1800 km depth in the interior of TRAPPIST-1f like icy bodies as long as temperatures above 750-800 K are reached. The buoyancy contrast between the water-rich ammonia hydrates, ADH-DMA and NH3.6H2O, and H2O ice (Fig. 3) will promote the formation of ammonia-bearing upwellings within the icy mantle layer, leading to phase separation and compositional layering. Note that ammonia concentration gradients may be plausible in turn within the segregated layer due to the density contrast between the ADH and NH3.6H2O phases (Fig. 3). Indicated pressures correspond to the reaction pressure at 750 K between AHH-D(I)MA and H2O ice VII (Fig. 3) and to the pressure at ice/rock interface of water-rich exoplanets such as TRAPPIST-1f (56) and GJ 1214b (14), where 40-140 GPa indicates the range of pressures estimated depending on the bulk H2O content assumed in the interior model). Note: Figure is not drawn to scale. — astro-ph.EP

Understanding the behavior of the water-ammonia system at high pressure-high temperature conditions is important for modeling the internal dynamics of exoplanet icy mantles.

Conventionally, mixtures of ammonia hemihydrate AHH (2:1 ammonia-water molar ratio) and H₂O ice VII have been regarded as the ultimate solid phase assembly in the system. Here we report evidence for chemical reactions between AHH and ice VII above 750 K and 16 GPa that stabilize water-rich ammonia hydrates, including a novel ultra-water rich hydrate NH₃.6H₂O (1:6 ratio) coexisting with ammonia dihydrate ADH (1:2 ratio) and excess ice VII. This assembly is stable up to at least 30 GPa and 1600 K and can be quenched to room temperature.

Our results demonstrate that water-rich ammonia hydrates are favored in the icy mantle of 1-2 M_Earth exoplanets regardless of the ammonia content of the hydrate crystallized during accretion and/or evolution as long as excess H₂O ice is available. The buoyancy contrast between water-rich hydrates and ice VII may lead to chemical stratification in exoplanet icy mantles, hence affecting their thermal evolution.

Anshuman Mondal, Katharina Mohrbach, Timofey Fedotenko, Mandy Bethkenhagen, Hanns-Peter Liermann, Carmen Sanchez-Valle

Comments: Main text (22 pages and 5 figures) and Supplementary information (Text S1, Figures S1 to S13, Tablers S1 and S2)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Materials Science (cond-mat.mtrl-sci); Geophysics (physics.geo-ph)
Cite as: arXiv:2508.11924 [astro-ph.EP] (or arXiv:2508.11924v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.11924
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Submission history
From: Anshuman Mondal
[v1] Sat, 16 Aug 2025 06:08:31 UTC (2,602 KB)
https://arxiv.org/abs/2508.11924

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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