Planetary science

Sebastien Hamel, Miguel Morales, Kyle Caspersen

The predictive capability of QMD simulations allows us to determine the properties of materials subject to planetary interiors conditions. The equation-of-state (EOS) of the planetary materials, specifically the pressure as a function of density, temperature, and composition, are required in order to close the set of hydrostatic equations used in planetary models [1,2,3].

A large part of the uncertainty in models of Saturn and Jupiter lies in the existence and location an inhomogeneous region where helium separates from hydrogen to form helium rich droplets that fall deeper into the planet due to their larger density. We use first-principles simulations to predict the temperature, as a function of pressure, at which helium becomes insoluble in dense metallic hydrogen [4] and study the impact of helium on the equation of state of hydrogen and the properties of that mixture [5]. The presence of other elements will also modify the properties of hydrogen and their impact at high pressure and temperature is also of interest. First-principles simulations are also an important tool in the design and analysis of laboratory astrophysics experiments [6,7,8,9,10] performed at various facilities such as the Omega laser at the Laboratory for Laser energetics in Rochester, NY, the Z Pulse power facility at Sandia National Laboratory, Albuquerque, NM or the National Ignition Facility here at LLNL.
In ice giants such as Uranus and Neptune, the uncertainty in the planetary models comes mainly from the properties of mixtures of water, ammonia and methane (referred to as “ices”) at high pressures and temperatures [11,12,13]. Many observables properties of these planets, such as gravitational moments, atmospheric composition and magnetic fields, are thought to be determined by the physical and chemical properties of matter within this ice layer. Of particular interest is the impact of the complex organic chemistry on the fluid properties at these extreme conditions [7]. Recent experiments on water have lead to the discovery of a super-ionic phase [9] with a very high melting point and the determination of the its crystal structure (face centered cubic) [10]. The existence of this 18th phase of water has important implications for the magnetic fields of Neptune and Uranus.

[1] D. C. Swift, J. H. Eggert, D. G. Hicks, S. Hamel, K. Caspersen, E. Schwegler, G. W. Collins, N. Nettelmann  and G. J. Ackland Mass-Radius Relationships for Exoplanets. Astro. Phys. J. 744, 59 (2012)

[2] R. G. Kraus, S. T. Stewart, D. C. Swift, C. A. Bolme, R. F. Smith, S. Hamel, B. D. Hammel, D. K. Spaulding, D. G. Hicks, J. H. Eggert, G. W. Collins Shock vaporization of silica and the thermodynamics of planetary impact events. J. Geophys. Research – Planets 117, E09009 (2012)

[3] N. Nettelmann, K. Wang, J.J. Fortney, S. Hamel, S. Yellamilli, M. Bethkenhagen, R. Redmer, Uranus evolution models with simple thermal boundary layers. Icarus Vol. 275, pages 107-116 (2016)

[4] Morales, MA; Schwegler, E; Ceperley, D; Pierleoni, C; Hamel, S; Caspersen, K. Phase separation in hydrogen-helium mixtures at Mbar pressures. P. Natl. Acad. Sci. USA, 106, 1324-1329, (2009)

[5] S. Hamel, Miguel Morales and Eric Schwegler, Signature of helium segregation in hydrogen-helium mixtures. Phys. Rev. B 84, 165110 (2011)

[6] Stephanie Brygoo, Marius Millot, Paul Loubeyre, Amy E Lazicki, Sebastien Hamel, Tingting Qi, Peter M Celliers, Federica Coppari, Jon H Eggert, Dayne E Fratanduono, Damien G Hicks, J Ryan Rygg, Raymond F Smith, Damian C Swift, Gilbert W Collins, Raymond Jeanloz, Analysis of laser shock experiments on precompressed samples using a quartz reference and application to warm dense hydrogen and helium. Journal of Applied Physics 118 (19), 195901 (2015)

[7] R. Chau, S. Hamel, and W. J. Nellis Chemical Processes in the Deep Interior of Uranus. Nature Communications, 2, (2011)

[8] Ping Y, Coppari F, Hicks DG, Yaakobi B, Fratanduono DE, Hamel S, Eggert JH, Rygg JR, Smith RF, Swift DC, Braun DG, Boehly TR, Collins GW, Solid iron compressed up to 560 GPa. Phys Rev Lett. 111 065501 (2013)

[9] Marius Millot, Sebastien Hamel, J. Ryan Rygg, Peter M. Celliers, Gilbert W. Collins, Federica Coppari, Dayne E. Fratanduono, Raymond Jeanloz, Damian C. Swift & Jon H. Eggert, Experimental evidence for superionic water ice using shock compression. Nature Physics 14, pages 297–302 (2018)

[10] Marius Millot, Federica Coppari, J. Ryan Rygg, J, Antonio Correa Barrios, Sebastien Hamel, Damian C. Swift & Jon H. Eggert, Nanosecond X-ray diffraction of shock-compressed superionic water ice. Nature 569, pages 251-255 (2019)

[11] M. French; S. Hamel and R. Redmer Dynamical Screening and Ionic Conductivity in Water from Ab Initio Simulations. Phys. Rev. Lett., 107, 185901 (2011)

[12] M. Bethkenhagen, E. R. Meyer, S. Hamel, N. Nettelmann, M. French, L. Scheibe, C. Ticknor, L. A. Collins, J. D. Kress, J. J. Fortney, and R. Redmer, Planetary Ices and the Linear Mixing Approximation. The Astrophysical Journal Vol. 848, page 67 (2017)

[13] Mandy Bethkenhagen, Daniel Cebulla, Ronald Redmer, and Sebastien Hamel,
Superionic Phases of the 1:1 Water–Ammonia Mixture
 The Journal of Physical Chemistry A 119 (42), 10582-10588 (2015)