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Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions

Nature Astronomy volume 1, pages 606–611 (2017)Cite this article

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Abstract

The effects of hydrocarbon reactions and diamond precipitation on the internal structure and evolution of icy giant planets such as Neptune and Uranus have been discussed for more than three decades1. Inside these celestial bodies, simple hydrocarbons such as methane, which are highly abundant in the atmospheres2, are believed to undergo structural transitions3,4 that release hydrogen from deeper layers and may lead to compact stratified cores5,6,7. Indeed, from the surface towards the core, the isentropes of Uranus and Neptune intersect a temperature–pressure regime in which methane first transforms into a mixture of hydrocarbon polymers8, whereas, in deeper layers, a phase separation into diamond and hydrogen may be possible. Here we show experimental evidence for this phase separation process obtained by in situ X-ray diffraction from polystyrene (C8H8) n samples dynamically compressed to conditions around 150 GPa and 5,000 K; these conditions resemble the environment around 10,000 km below the surfaces of Neptune and Uranus9. Our findings demonstrate the necessity of high pressures for initiating carbon–hydrogen separation3 and imply that diamond precipitation may require pressures about ten times as high as previously indicated by static compression experiments4,8,10. Our results will inform mass–radius relationships of carbon-bearing exoplanets11, provide constraints for their internal layer structure and improve evolutionary models of Uranus and Neptune, in which carbon–hydrogen separation could influence the convective heat transport7.

Being composed of highly abundant elements, hydrocarbons are one of the most common chemical species throughout the Universe. A considerable amount exists inside giant planets, especially icy giants such as Neptune and Uranus, which are also being found in steadily increasing numbers in extrasolar planetary systems. Chemical processes involving hydrocarbons can participate in shaping the interior of these planets where gravity compresses mixtures of light elements to densities of several grams per cubic centimetre while the temperature reaches thousands of kelvins, resulting in thermal energies of the order of chemical bond energies and above2. Attempts have been made to investigate the structural transitions of such systems in the laboratory by studying methane inside laser-heated diamond anvil cells and in shock experiments with gas guns4,10,12,13. In diamond anvil cells, evidence for dissociation and polymerization has been found at pressures of 10–50 GPa and below the carbon melting temperature4,10, resulting in a heavy hydrocarbon fluid8. Similar structural transitions are expected to happen in various hydrocarbons under shock compression6,12,13, and this led to the postulation of possible diamond precipitation inside the icy giants of our Solar System1. However, owing to the lack of in situ measurements in previous shock experiments, details of whether, when and how diamonds can be created are not fully understood.

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Fig. 1: Schematic of the experimental set-up at the Matter at Extreme Conditions end-station of the LCLS.
Fig. 2: Hydrodynamic simulations of the two-stage shock compression.
Fig. 3: Diffraction line-outs.
Fig. 4: Summary of diamond formation.

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Acknowledgements

We thank L. Divol, L. R. Benedetti, S. Hamel and L. X. Benedict for discussions. This work was performed at the Matter at Extreme Conditions (MEC) instrument of LCLS, supported by the US Department of Energy (DOE) Office of Science, Fusion Energy Science, under contract no. SF00515. D.K., A.M.S. and R.W.F acknowledge support by the DOE Office of Science, Fusion Energy Sciences and by the National Nuclear Security Administration under awards DE-FG52-10NA29649 and DE-NA0001859. D.K., N.J.H. and A.K.S. were supported by the Helmholtz Association under VH-NG-1141. SLAC HED is supported by DOE Office of Science, Fusion Energy Science under FWP 100182. S.F. and M.R. were supported by German Bundesministerium für Bildung und Forschung project no. 05P15RDFA1. E.E.M. was supported by funding from Volkswagen Stiftung. The work of A.P., S.F. and T.D. was performed under the auspices of the US DOE by Lawrence Livermore National Laboratory under contract no. DE-AC52-07NA27344.

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Authors and Affiliations

  1. Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328, Dresden, Germany

    D. Kraus, J. Vorberger, N. J. Hartley & A. K. Schuster

  2. Department of Physics, University of California, Berkeley, CA, 94720, USA

    D. Kraus, A. M. Saunders & R. W. Falcone

  3. Institute of Solid State and Materials Physics, Technische Universität Dresden, 01069, Dresden, Germany

    D. Kraus

  4. Lawrence Livermore National Laboratory, Livermore, CA, 94550, USA

    A. Pak, S. Frydrych & T. Döppner

  5. Open and Transdisciplinary Research Institute, Osaka University, Suita, Osaka, 565-0871, Japan

    N. J. Hartley

  6. SLAC National Accelerator Laboratory, Menlo Park, CA, 94309, USA

    L. B. Fletcher, E. Galtier, E. J. Gamboa, S. H. Glenzer, E. Granados, M. J. MacDonald, A. J. MacKinnon, E. E. McBride, I. Nam, P. Sun & T. van Driel

  7. Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstrasse 9, 64289, Darmstadt, Germany

    S. Frydrych & M. Roth

  8. Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

    D. O. Gericke

  9. University of Michigan, Ann Arbor, MI, 48109, USA

    M. J. MacDonald

  10. European XFEL GmbH, Holzkoppel 4, 22869, Schenefeld, Germany

    E. E. McBride

  11. GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstrasse 1, 64291, Darmstadt, Germany

    P. Neumayer

  12. Department of Physics, Stanford University, Stanford, CA, 94305, USA

    P. Sun

  13. Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA

    R. W. Falcone

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Contributions

D.K., R.W.F., J.V., T.D., A.P., S.H.G., D.O.G., M.R., E.Ga., E.Gr., P.N. and A.J.M. were involved in the project planning. D.K., E.Ga., N.J.H., A.P., T.D., P.N., E.E.M., A.M.S., S.F., L.B.F., P.S., M.J.M., E.J.G., E.Gr., I.N. and T.v.D. carried out the experiment. Experimental data were analysed and discussed by D.K., J.V., N.J.H., A.K.S., S.H.G., D.O.G., A.P., E.E.M. and T.D. The manuscript was written by D.K., J.V., A.P., N.J.H., M.J.M., D.O.G. and T.D.

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Correspondence to D. Kraus.

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Kraus, D., Vorberger, J., Pak, A. et al. Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions. Nat Astron 1, 606–611 (2017). https://doi.org/10.1038/s41550-017-0219-9

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