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Melting curve of superionic ammonia at planetary interior conditions

Nature Physics volume 19, pages 1280–1285 (2023)Cite this article

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An Author Correction to this article was published on 16 June 2023

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Abstract

Under high pressures and temperatures, molecular systems with substantial polarization charges, such as ammonia and water, are predicted to form superionic phases and dense fluid states with dissociating molecules and high electrical conductivity. This behaviour potentially plays a role in explaining the origin of the multipolar magnetic fields of Uranus and Neptune, whose mantles are thought to result from a mixture of H2O, NH3 and CH4 ices. Determining the stability domain, melting curve and electrical conductivity of these superionic phases is therefore crucial for modelling planetary interiors and dynamos. Here we report the melting curve of superionic ammonia up to 300 GPa from laser-driven shock compression of pre-compressed samples and atomistic calculations. We show that ammonia melts at lower temperatures than water above 100 GPa and that fluid ammonia’s electrical conductivity exceeds that of water at conditions predicted by hot, super-adiabatic models for Uranus and Neptune, and enhances the conductivity in their fluid water-rich dynamo layers.

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Fig. 1: Experimental setup with examples of VISAR and SOP data.
Fig. 2: Liquid and phase III pre-compressed Hugoniots and reflectivity.
Fig. 3: Phase diagram and melting temperature of SI NH3.
Fig. 4: Electrical conductivities and melting curves of NH3 and H2O at planetary interior conditions.

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Data availability

Data supporting this study are available in the Supplementary Information, from the online repository66 and from the corresponding author upon request.

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DFT simulations were performed with the proprietary code VASP.

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Acknowledgements

We acknowledge the crucial contribution of the LULI2000 laser and support teams to the success of the experiments. We also thank S. Brygoo and P. Loubeyre for useful discussions. This research was supported by the French National Research Agency (ANR) through the projects POMPEI (grant no. ANR-16-CE31-0008) and SUPER-ICES (grant ANR-15-CE30-008-01), and by the PLAS@PAR Federation. M.F. and R.R. gratefully acknowledge support by the DFG within the Research Unit FOR 2440. M.B. was supported by the European Union within the Marie Skłodowska-Curie actions (xICE grant 894725) and the NOMIS foundation. The DFT-MD calculations were performed at the North-German Supercomputing Alliance facilities.

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Authors and Affiliations

  1. LULI, CNRS, CEA, Sorbonne Université, École Polytechnique – Institut Polytechnique de Paris, Palaiseau, France

    J.-A. Hernandez, A. Benuzzi-Mounaix, M. Guarguaglini, F. Lefevre, T. Vinci & A. Ravasio

  2. Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, Norway

    J.-A. Hernandez

  3. European Synchrotron Radiation Facility, Grenoble, France

    J.-A. Hernandez

  4. École Normale Supérieure de Lyon, Université Lyon 1, Laboratoire de Géologie de Lyon, Lyon, France

    M. Bethkenhagen

  5. Institute of Science and Technology Austria, Klosterneuburg, Austria

    M. Bethkenhagen

  6. Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université, CNRS, MNHN, Paris, France

    S. Ninet & F. Datchi

  7. Institut für Physik, Universität Rostock, Rostock, Germany

    M. French & R. Redmer

  8. CEA, DAM, DIF, Arpajon, France

    F. Occelli

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Contributions

J.-A.H., A.R., S.N. and F.D. designed the project and the experiments. F.D., S.N., F.O., J.-A.H. and A.R. designed the DAC targets, and J.-A.H., S.N., F.D., F.L. and F.O. prepared them. J.-A.H., A.R., A.B.-M., M.G. and T.V. performed the dynamic compression experiments. J.-A.H. analysed the experimental data. M.B. and M.F. designed, performed and analysed the DFT-MD simulations. All authors discussed the data. J.-A.H., M.B. and A.R. wrote the manuscript, and all authors commented on it and improved it.

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Correspondence to J.-A. Hernandez or A. Ravasio.

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Nature Physics thanks David Stevenson, Kenji Ohta and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Sections S1–S7, Figs. S1–S9 and Tables S1–S3.

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Hernandez, JA., Bethkenhagen, M., Ninet, S. et al. Melting curve of superionic ammonia at planetary interior conditions. Nat. Phys. 19, 1280–1285 (2023). https://doi.org/10.1038/s41567-023-02074-8

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