Hydrocarbon mixtures are extremely abundant in the Universe, and diamond formation from them can play a crucial role in shaping the interior structure and evolution of planets. With first-principles accuracy, we first estimate the melting line of diamond, and then reveal the nature of chemical bonding in hydrocarbons at extreme conditions. We finally establish the pressure-temperature phase boundary where it is thermodynamically possible for diamond to form from hydrocarbon mixtures with different atomic fractions of carbon. Notably, here we show a depletion zone at pressures above 200 GPa and temperatures below 3000 K-3500 K where diamond formation is thermodynamically favorable regardless of the carbon atomic fraction, due to a phase separation mechanism. The cooler condition of the interior of Neptune compared to Uranus means that the former is much more likely to contain the depletion zone. Our findings can help explain the dichotomy of the two ice giants manifested by the low luminosity of Uranus, and lead to a better understanding of (exo-)planetary formation and evolution.
BC thanks Daan Frenkel for stimulating discussions. We thank Aleks Reinhardt, Daan Frenkel, Marius Millot, Federica Coppari, Rhys Bunting, and Chris J. Pickard for critically reading the manuscript and providing useful suggestions. BC acknowledges resources provided by the Cambridge Tier-2 system operated by the University of Cambridge Research Computing Service funded by EPSRC Tier-2 capital grant EP/P020259/1. SH acknowledges support from LDRD 19-ERD-031 and computing support from the Lawrence Livermore National Laboratory (LLNL) Institutional Computing Grand Challenge program. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. MB acknowledges support by the European Horizon 2020 program within the Marie Skłodowska-Curie actions (xICE grant number 894725), funding from the NOMIS foundation and computational resources at the North-German Supercomputing Alliance (HLRN) facilities.
Cheng B, Hamel S, Bethkenhagen M. Thermodynamics of diamond formation from hydrocarbon mixtures in planets. Nature Communications. 2023;14. doi:10.1038/s41467-023-36841-1
Cheng, B., Hamel, S., & Bethkenhagen, M. (2023). Thermodynamics of diamond formation from hydrocarbon mixtures in planets. Nature Communications. Springer Nature. https://doi.org/10.1038/s41467-023-36841-1
Cheng, Bingqing, Sebastien Hamel, and Mandy Bethkenhagen. “Thermodynamics of Diamond Formation from Hydrocarbon Mixtures in Planets.” Nature Communications. Springer Nature, 2023. https://doi.org/10.1038/s41467-023-36841-1.
B. Cheng, S. Hamel, and M. Bethkenhagen, “Thermodynamics of diamond formation from hydrocarbon mixtures in planets,” Nature Communications, vol. 14. Springer Nature, 2023.
Cheng B, Hamel S, Bethkenhagen M. 2023. Thermodynamics of diamond formation from hydrocarbon mixtures in planets. Nature Communications. 14, 1104.
Cheng, Bingqing, et al. “Thermodynamics of Diamond Formation from Hydrocarbon Mixtures in Planets.” Nature Communications, vol. 14, 1104, Springer Nature, 2023, doi:10.1038/s41467-023-36841-1.
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