Understanding dense hydrogen at planetary conditions

Abstract

Materials at high pressures and temperatures are of great interest for planetary science and astrophysics, warm dense-matter physics and inertial confinement fusion research. Planetary structure models rely on an understanding of the behaviour of elements and their mixtures under conditions that do not exist on Earth; at the same time, planets serve as natural laboratories for studying materials at extreme conditions. The topic of dense hydrogen is timely given the recent accurate measurements of the gravitational fields of Jupiter and Saturn, the current and upcoming progress in shock experiments, and the advances in numerical simulations of materials at high pressure. In this Review we discuss the connection between modelling planetary interiors and the high-pressure physics of hydrogen and helium. We summarize key experiments and theoretical approaches for determining the equation of state and phase diagram of hydrogen and helium. We relate this to current knowledge of the internal structures of Jupiter and Saturn, and discuss the importance of high-pressure physics to their characterization.

Key points

  • Modelling planetary interiors relies on a profound knowledge of the behaviour of materials at high pressures and temperatures. For the gas giant planets, these materials are hydrogen and helium.

  • Progress in high-pressure experiments using diamond anvil cells and shock waves is critical for understanding hydrogen under extreme conditions and for calibrating theoretical models

  • Simulations of hydrogen at high pressure are essential to understand fundamental physical problems such as its rich phase diagram; assist the experimental realization and interpretation of new materials; and predict its behaviour for parameters at which experiments cannot be performed.

  • Jupiter and Saturn are expected to have complex interiors in which hydrogen metallizes and helium separates from hydrogen. The full understanding of these processes is still a major challenge in high-pressure physics.

  • Although the structure and evolution of gas giant planets are dominated by hydrogen and helium, the planets contain other, heavier elements and can have complex interiors that include composition gradients and inhomogeneous regions.

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Fig. 1: Phase diagram of H and D.
Fig. 2: Principal Hugoniot data of H.
Fig. 3: Numerically predicted phase diagram for a H–He mixture of proto-solar composition and typical isentropes of Jupiter and Saturn.
Fig. 4: The mass–radius relation of H–He-dominated planets.
Fig. 5: Sketches of the internal structures of Jupiter and Saturn.

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Acknowledgements

The authors thank the anonymous referees for comments that helped to improve the manuscript. The authors also acknowledge support from W. Nellis, F. Soubrian, S. Sorella, D. Stevenson, N. Nettelmann, J. J. Fortney, Y. Miguel, S. Müller, C. Valletta and A. Cumming. R.H. acknowledges support from the Swiss National Science Foundation (SNSF grant 200020_188460) and thanks the members of the Juno science team for discussions. R.R. acknowledges support by the Deutsche Forschungsgemeinschaft via the projects FOR 2440 and SPP 1992.

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Helled, R., Mazzola, G. & Redmer, R. Understanding dense hydrogen at planetary conditions. Nat Rev Phys 2, 562–574 (2020). https://doi.org/10.1038/s42254-020-0223-3

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