P - ρ - T data for H2O up to 260 GPa under laser-driven shock loading

Abstract:

H₂O is believed to be one of the most abundant compounds in ice giants including Neptune and Uranus¹. Therefore, equation of state (EOS) for H₂O is critical for understanding the formation and evolution of these planets. Various EOS models have been suggested for modeling the interior structure of the ice giants^2-4. The recent shock experiments reported that their P - ρ data of H₂O are in agreement with those of the QMD based EOS model⁵, indicating that this model is most suitable for modeling H₂O in the ice giants.

Whether H₂O is in the solid or liquid state in the planetary interior has a great importance to understand their internal structures⁶. While the QMD model predicted that the solid H₂O is present in deep interior of their planets above ~100 GPa⁴, the recent measurements revealed that H₂O remains in the liquid state even at the deep interior conditions⁷. This discrepancy between experimental and theoretical studies suggests that the QMD based EOS model is disputable for modeling the planetary interior. Indeed, the comparison between data obtained from the shock experiments and the QMD based EOS did not cover the temperature⁵.

We have obtained P - ρ - T data for H₂O up to 260 GPa by using laser-driven shock compression technique. The diamond cell applied for the laser shock experiments was used as the sample container in order to achieve temperature conditions lower than the principal Hugoniot states. This shock technique combined with the cell can be used for an assessment the EOS models because it is possible to compare the states under the conditions that the contrast between the models clearly appears. Our data covering P – ρ - T on both the principal and the off Hugoniot curves agree with those of the QMD model, indicating this model to be adopted as the standard for modeling the interior structures of Neptune, Uranus, and exoplanets.

References

¹W. B. Hubbard et al., The interior of Neptune: Neptune and Triton(Univ. Arizona Press, Tucson, 1995) p.109-138.

²S. P. Lyon and J. D. Johnson, Los Alamos Technical Report No. LA-UR-92-3407, 1992.

³F. H. Ree, Lawrence Livemore Laboratory Technical Report No. UCRL-52190, 1976.

⁴M. French et al., Phys. Rev. B 79, 054107 (2009).

⁵M. D. Knudson et al., Phys. Rev. Lett. 108, 091102 (2012).

⁶ R. Redmer et al., Icarus 211, 798 (2011).

⁷T. Kimura et al., J. Chem. Phys. 140, 074501 (2014).