Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Implications of the iron oxide phase transition on the interiors of rocky exoplanets

Abstract

The discovery of an extraordinary number of extrasolar planets, characterized by an unexpected variety of sizes, masses and orbits, challenges our understanding of the formation and evolution of the planets in the Solar System and the perception of the Earth as the prototypical habitable world. Many exoplanets appear to be rocky and yet more massive than the Earth, with expected pressures and temperatures of hundreds of gigapascal and thousands of Kelvin in their deep interiors. At these conditions, the properties of bridgmanite and ferropericlase, expected to dominate their mantles, are largely unconstrained, limiting our knowledge of their interior structure. Here we used nano-second X-ray diffraction and dynamic compression to experimentally investigate the atomic structure and density of iron oxide (FeO), one of the end-members of the (Mg,Fe)O ferropericlase solid solution, up to 700 GPa, a pressure exceeding the core–mantle boundary of a 5 Earth masses planet. Our data document the stability of the high-pressure cesium-chloride B2 structure above 300 GPa, well below the pressure required for magnesium oxide (MgO) to adopt the same phase. These observations, complemented by the calculation of the binary MgO–FeO phase diagram, reveal complex stratification and rheology inside large terrestrial exoplanets.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Experimental setup for X-ray diffraction measurements during laser-driven ramp compression.
Fig. 2: d-spacings as a function of pressure for the different FeO structures.
Fig. 3: Pressure–density relation of mantle oxides.
Fig. 4: MgO–FeO phase diagram and interior structure of a 5M rocky exoplanet.

Data availability

Raw data were generated at the Omega large-scale facility. Derived data supporting the findings of this study are available in the Supplementary Information, from the corresponding author upon request and can be downloaded at https://doi.org/10.5061/dryad.pg4f4qrn8.

Code availability

Igor scripts used to analyse the X-ray diffraction and VISAR data described in this study can be obtained from the corresponding author upon reasonable request.

References

  1. 1.

    Unterborn, C. T. & Panero, W. R. The pressure and temperature limits of likely rocky exoplanets. J. Geophys. Res. Planets 124, 1704–1716 (2019).

    Article  Google Scholar 

  2. 2.

    Hinkel, N. R. & Unterborn, C. T. The star–planet connection. I. Using stellar composition to observationally constrain planetary mineralogy for the 10 closest stars. Astrophys. J. 853, 83 (2018).

    Article  Google Scholar 

  3. 3.

    Valencia, D., O’Connell, R. J. & Sasselov, D. Internal structure of massive terrestrial planets. Icarus 181, 545–554 (2006).

    Article  Google Scholar 

  4. 4.

    Seager, S., Kuchner, M., Hier-Majumder, C. A. & Militzer, B. Mass–radius relationships for solid exoplanets. Astrophys. J. 669, 1279–1297 (2007).

    Article  Google Scholar 

  5. 5.

    Unterborn, C. T., Dismukes, E. E. & Panero, W. R. Scaling the Earth: a sensitivity analysis of terrestrial exoplanetary interior models. Astrophys. J. 819, 32 (2016).

    Article  Google Scholar 

  6. 6.

    Wagner, F. W., Tosi, N., Sohl, F., Rauer, H. & Spohn, T. Rocky super-Earth interiors. Structure and internal dynamics of CoRoT-7b and Kepler-10b. Astron. Astrophys. 541, A103 (2012).

    Article  Google Scholar 

  7. 7.

    Boujibar, A., Fei, Y. & Driscoll, P. Super‐Earth internal structures and initial thermal states. J. Geophys. Res. Planets 125, e2019JE006124 (2020).

    Article  Google Scholar 

  8. 8.

    Duffy, T., Madhusudhan, N. & Lee, K. K.M. in Mineralogy of Super-Earth Planets 2nd edn, Vol. 2 (ed. Schubert, G.) Ch. 2.07 (Elsevier, 2015).

  9. 9.

    Umemoto, K. et al. Phase transitions in MgSiO3 post-perovskite in super-Earth mantles. Earth Planet. Sci. Lett. 478, 40–45 (2017).

    Article  Google Scholar 

  10. 10.

    Bitsch, B. & Battistini, C. Influence of sub- and super-solar metallicities on the composition of solid planetary building blocks. Astron. Astrophys. 633, A10 (2020).

    Article  Google Scholar 

  11. 11.

    Scora, J., Valencia, D., Morbidelli, A. & Jacobson, S. Chemical diversity of super-Earths as a consequence of formation. Mon. Not. R. Astron. Soc. 493, 4910–4924 (2020).

    Article  Google Scholar 

  12. 12.

    Jang, B. G., Kim, D. Y. & Shim, J. H. Metal–insulator transition and the role of electron correlation in FeO2. Phys. Rev. B 95, 075144 (2017).

    Article  Google Scholar 

  13. 13.

    Ozawa, H., Takahashi, F., Hirose, K., Ohishi, Y. & Hirao, N. Phase transition of FeO and stratification in Earth’s outer core. Science 334, 792–794 (2011).

    Article  Google Scholar 

  14. 14.

    Coppari, F. et al. Experimental evidence for a phase transition in magnesium oxide at exoplanet pressures. Nat. Geosci. 6, 926–929 (2013).

    Article  Google Scholar 

  15. 15.

    Duffy, T. S., Hemley, R. J. & Mao, H.-k Equation of state and shear strength at multimegabar pressures: magnesium oxide to 227 GPa. Phys. Rev. Lett. 74, 1371–1374 (1995).

    Article  Google Scholar 

  16. 16.

    Dorfman, S. M., Prakapenka, V. B., Meng, Y. & Duffy, T. S. Intercomparison of pressure standards (Au, Pt, Mo, MgO, NaCl and Ne) to 2.5 Mbar. J. Geophys. Res. 117, B08210 (2012).

    Google Scholar 

  17. 17.

    McWilliams, R. S. et al. Phase transformations and metallization of magnesium oxide at high pressure and temperature. Science 338, 1330–1333 (2012).

    Article  Google Scholar 

  18. 18.

    Bouchet, J. et al. Ab initio calculations of the B1–B2 phase transition in MgO. Phys. Rev. B 99, 094113 (2019).

    Article  Google Scholar 

  19. 19.

    Musella, R., Mazevet, S. & Guyot, F. Physical properties of MgO at deep planetary conditions. Phys. Rev. B 99, 064110 (2019).

    Article  Google Scholar 

  20. 20.

    Soubiran, F. & Militzer, B. Anharmonicity and phase diagram of magnesium oxide in the megabar regime. Phys. Rev. Lett. 125, 175701 (2020).

    Article  Google Scholar 

  21. 21.

    Lazicki, A. et al. Metastability of diamond ramp-compressed to 2 terapascals. Nature 589, 532–535 (2021).

    Article  Google Scholar 

  22. 22.

    Mao, H.-k, Shu, J., Fei, Y., Hu, J. & Hemley, R. J. The wüstite enigma. Phys. Earth Planet. Inter. 96, 135–145 (1996).

    Article  Google Scholar 

  23. 23.

    Fei, Y. in Mineral Spectrosopy: A Tribute to Roger G. Burns (eds Dyar, M. D et al.) 243–254 (The Geochemical Society, 1996).

  24. 24.

    Sata, N. et al. Compression of FeSi, Fe3C, Fe0.95O, and FeS under the core pressures and implication for light element in the Earth’s core. J. Geophys. Res. 115, B09204 (2010).

    Google Scholar 

  25. 25.

    Ohta, K. et al. Experimental and theoretical evidence for pressure-induced metallization in FeO with rocksalt-type structure. Phys. Rev. Lett. 108, 026403 (2012).

    Article  Google Scholar 

  26. 26.

    Fei, Y. & Mao, H. K. In situ determination of the NiAs phase of FeO at high pressure and temperature. Science 266, 1678–1680 (1994).

    Article  Google Scholar 

  27. 27.

    Murakami, M. et al. High pressure and high temperature phase transitions of FeO. Phys. Earth Planet. Inter. 146, 273–282 (2004).

    Article  Google Scholar 

  28. 28.

    Campbell, A. J. et al. High pressure effects on the iron–iron oxide and nickel–nickel oxide oxygen fugacity buffers. Earth Planet. Sci. Lett. 286, 556–564 (2009).

    Article  Google Scholar 

  29. 29.

    Ozawa, H., Hirose, K., Tateno, S., Sata, N. & Ohishi, Y. Phase transition boundary between B1 and B8 structures of FeO up to 210 GPa. Phys. Earth Planet. Inter. 179, 157–163 (2010).

    Article  Google Scholar 

  30. 30.

    Fischer, R. A. et al. Equation of state and phase diagram of FeO. Earth Planet. Sci. Lett. 304, 496–502 (2011).

    Article  Google Scholar 

  31. 31.

    Weerasinghe, G. L., Pickard, C. J. & Needs, R. J. Computational searches for iron oxides at high pressures. J. Phys. Condens. Matter 27, 455501 (2015).

    Article  Google Scholar 

  32. 32.

    Lavina, B. et al. Discovery of the recoverable high-pressure iron oxide Fe4O5. Proc. Natl Acad. Sci. USA 108, 17281–17285 (2011).

    Article  Google Scholar 

  33. 33.

    Hu, Q. et al. FeO2 and FeOOH under deep lower-mantle conditions and Earth’s oxygen–hydrogen cycles. Nature 534, 241–244 (2016).

    Article  Google Scholar 

  34. 34.

    Vassiliou, M. S. & Ahrens, T. J. The equation of state of Mg0.6Fe0.40 to 200 GPa. Geophys. Res. Lett. 9, 127–130 (1982).

    Article  Google Scholar 

  35. 35.

    Zhang, N. B. et al. Spin transition of ferropericlase under shock compression. AIP Adv. 8, 075028 (2018).

    Article  Google Scholar 

  36. 36.

    Deng, J. & Lee, K. K. M. Viscosity jump in the lower mantle inferred from melting curves of ferropericlase. Nat. Commun. 8, 1997 (2017).

    Article  Google Scholar 

  37. 37.

    Kondo, T., Ohtani, E., Hirao, N., Yagi, T. & Kikegawa, T. Phase transitions of (Mg,Fe)O at megabar pressures. Phys. Earth Planet. Inter. 143, 201–213 (2004).

    Article  Google Scholar 

  38. 38.

    Wicks, J. K. et al. Thermal equation of state and stability of (Mg0.06Fe0.94)O. Phys. Earth Planet. Inter. 249, 28–42 (2015).

    Article  Google Scholar 

  39. 39.

    McCammon, C. A., Ringwood, A. E. & Jackson, I. Thermodynamics of the system Fe–FeO–MgO at high pressure and temperature and a model for formation of the Earth’s core. Geophys. J. R. Astron. Soc. 72, 577–595 (1983).

    Article  Google Scholar 

  40. 40.

    Dubrovinsky, L. S. et al. Stability of ferropericlase in the lower mantle. Science 289, 430–433 (2000).

    Article  Google Scholar 

  41. 41.

    Murakami, M., Hirose, K., Kawamura, K., Sata, N. & Ohishi, Y. Post-perovskite phase transition in MgSiO3. Science 304, 855 (2004).

    Article  Google Scholar 

  42. 42.

    Ritterbex, S., Harada, T. & Tsuchiya, T. Vacancies in MgO at ultrahigh pressure: about mantle rheology of super-Earths. Icarus 305, 350–357 (2018).

    Article  Google Scholar 

  43. 43.

    Karato, S.-I. Rheological structure of the mantle of a super-Earth: some insights from mineral physics. Icarus 212, 14–23 (2011).

    Article  Google Scholar 

  44. 44.

    Reali, R. et al. Modeling viscosity of (Mg,Fe)O at lowermost mantle conditions. Phys. Earth Planet. Inter. 287, 65–75 (2019).

    Article  Google Scholar 

  45. 45.

    Thielmann, M., Golabek, G. J. & Marquardt, H. Ferropericlase control of lower mantle rheology: impact of phase morphology. Geochem. Geophys. Geosyst. https://doi.org/10.1029/2019GC008688 (2020).

  46. 46.

    Shahnas, M. H., Pysklywec, R. N. & Yuen, D. A. Penetrative convection in super-Earth planets: consequences of MgSiO3 postperovskite dissociation transition and implications for super-Earth GJ 876 d. J. Geophys. Res. Planets 123, 2162–2177 (2018).

    Article  Google Scholar 

  47. 47.

    Yamazaki, D., Yoshino, T., Matsuzaki, T., Katsura, T. & Yoneda, A. Texture of (Mg,Fe)SiO3 perovskite and ferro-periclase aggregate: implications for rheology of the lower mantle. Phys. Earth Planet. Inter. 174, 138–144 (2009).

    Article  Google Scholar 

  48. 48.

    Stamenković, V., Breuer, D. & Spohn, T. Thermal and transport properties of mantle rock at high pressure: applications to super-Earths. Icarus 216, 572–596 (2011).

    Article  Google Scholar 

  49. 49.

    Tackley, P. J., Ammann, M., Brodholt, J. P., Dobson, D. P. & Valencia, D. Mantle dynamics in super-Earths: post-perovskite rheology and self-regulation of viscosity. Icarus 225, 50–61 (2013).

    Article  Google Scholar 

  50. 50.

    Vilim, R., Stanley, S. & Elkins-Tanton, L. The effect of lower mantle metallization on magnetic field generation in rocky exoplanets. Astrophys. J. Lett. 768, L30 (2013).

    Article  Google Scholar 

  51. 51.

    McCammon, C. A. & Liu, L.-g The effects of pressure and temperature on nonstoichiometric wüstite, FexO: the iron-rich phase boundary. Phys. Chem. Miner. 10, 106–113 (1984).

    Article  Google Scholar 

  52. 52.

    Boehly, T. R. et al. Initial performance results of the OMEGA laser system. Opt. Commun. 133, 495–506 (1997).

    Article  Google Scholar 

  53. 53.

    Coppari, F. et al. Optimized X-ray sources for X-ray diffraction measurements at the Omega Laser Facility. Rev. Sci. Instrum. 90, 125113 (2019).

    Article  Google Scholar 

  54. 54.

    Celliers, P. M. et al. Line-imaging velocimeter for shock diagnostics at the OMEGA laser facility. Rev. Sci. Instrum. 75, 4916–4929 (2004).

    Article  Google Scholar 

  55. 55.

    Rygg, J. R. et al. Powder diffraction from solids in the terapascal regime. Rev. Sci. Instrum. 83, 113904 (2012).

    Article  Google Scholar 

  56. 56.

    Rygg, J. R. et al. X-ray diffraction at the National Ignition Facility. Rev. Sci. Instrum. 91, 043902 (2020).

    Article  Google Scholar 

  57. 57.

    Maw, J. R. A characteristics code for analysis of isentropic compression experiments. AIP Conf. Proc. 706, 1217–1220 (2004).

    Article  Google Scholar 

  58. 58.

    Rothman, S. D. & Maw, J. Characteristics analysis of isentropic compression experiments (ICE). J. Phys. IV France 134, 745–750 (2006).

    Article  Google Scholar 

  59. 59.

    Bradley, D. K. et al. Diamond at 800 GPa. Phys. Rev. Lett. 102, 75503 (2009).

    Article  Google Scholar 

  60. 60.

    Wicks, J. K. et al. Crystal structure and equation of state of Fe–Si alloys at super-Earth core conditions. Sci. Adv. 4, eaao5864 (2018).

    Article  Google Scholar 

Download references

Acknowledgements

We thank A. Correa Barrios, C. Davis, T. Uphaus and R. Wallace for assistance in target preparation, the operation staff at the OMEGA Laser Facility for supporting the experiments and S. Ritterbex for careful reading of the manuscript and constructive comments. F.C. is grateful to L. Stixrude and R. Wentzcovitch for insightful discussions and to R. Fischer, H. Ozawa and A. Boujibar for sharing some of their previous data. This work was performed under the auspices of US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported, in part, by the US DOE, Office of Science, Office of Fusion Energy Sciences. The research was supported by NNSA/DOE through the National Laser Users’ Facility Program under contract numbers DE-NA0002720 and DE-NA0003611.

Author information

Affiliations

Authors

Contributions

F.C. designed and performed the experiments, analysed, interpreted and modelled the data and wrote the manuscript. R.F.S. contributed to the experiments’ execution and data analysis. M.M. and T.S.D. contributed to data interpretation and writing of the manuscript. J.W. helped with the experiments’ execution and, together with D.K., prepared the samples. S.H. helped to model the solid solution behaviour. J.H.E. and J.R.R. developed data analysis codes and contributed to data interpretation. T.S.D. was the principal investigator on the National Laser Users’ Facility (NLUF) experimental proposal. All authors discussed the results and reviewed the manuscript.

Corresponding author

Correspondence to F. Coppari.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Geoscience thanks Yingwei Fei and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figs. 1–14, Tables 1–5 and discussion.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Coppari, F., Smith, R.F., Wang, J. et al. Implications of the iron oxide phase transition on the interiors of rocky exoplanets. Nat. Geosci. 14, 121–126 (2021). https://doi.org/10.1038/s41561-020-00684-y

Download citation

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing