Evidence for ultra-water-rich ammonia hydrates stabilized in icy exoplanetary mantles
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arXiv
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| Main Authors: | , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866912539767472128 |
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| author | Mondal, Anshuman Mohrbach, Katharina Fedotenko, Timofey Bethkenhagen, Mandy Liermann, Hanns-Peter Sanchez-Valle, Carmen |
| author_facet | Mondal, Anshuman Mohrbach, Katharina Fedotenko, Timofey Bethkenhagen, Mandy Liermann, Hanns-Peter Sanchez-Valle, Carmen |
| contents | 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 MEarth 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2508_11924 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Evidence for ultra-water-rich ammonia hydrates stabilized in icy exoplanetary mantles Mondal, Anshuman Mohrbach, Katharina Fedotenko, Timofey Bethkenhagen, Mandy Liermann, Hanns-Peter Sanchez-Valle, Carmen Earth and Planetary Astrophysics Materials Science Geophysics 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 MEarth 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. |
| title | Evidence for ultra-water-rich ammonia hydrates stabilized in icy exoplanetary mantles |
| topic | Earth and Planetary Astrophysics Materials Science Geophysics |
| url | https://arxiv.org/abs/2508.11924 |