Extreme temperature and pressure conditions within the planets make very hard, even impossible, the experimental characterization of these environments and their related physical phenomena. Numerical simulation is therefore essential to go beyond experiment and study the composition and the properties of the planet cores. Dr Razvan Caracas’s team from the Laboratoire de Geologie de l’Ecole Normale Supérieure de Lyon develops an innovative approach which combines multi-scale modeling of planetary interiors with ab initio calculations based on fundamental physical laws.

One of the major advancement made by the team was the computation of the electrical conductivity of iron in the Earth’s core. If experiments are limited at 150 GPa pressure range (~ 1.5 million atmospheres), it’s still far from the pressure range within the Earth’s core (350 GPa).

The calculations determined the electrical conductivity of iron for pressures up to 400 GPa, with an excellent agreement in the range covered by experiments. The computed electrical conductivity turns out to be two times higher than those in use in previous core studies. Researchers doing geodynamics were able to use these results to model the crystallization of the solid inner core, predict the kinetics of crystallisation and determine its age which appears to be much younger than in the earlier estimates.

In general, the composition of planetary cores, including the Earth’s core, are still not well known because of the lack of direct information. It’s generally accepted that the Earth’s core is predominantly composed by iron (80 %) and nickel (~10 %), but one or more light elements, still unknown, are also part of its composition. Calculations on the isotope splitting between the silicates of the mantle and the iron alloys of the core, showed for the first time that carbon should not be part of the core composition, unlike oxygen and silicon.

Since 2007, the research group also contributes to the WURM[1] project which aims at building an open-access database providing, for materials of mineralogical interest, computed physical properties such the crystal structure, the Raman spectra and the dielectric properties. These physical properties are exclusively computed within the framework of ab initio methods using the ABINIT[2] code, and made available for the community. This project also aims to support the NASA’ Mars 2020[3] exploration mission which will carry two Raman spectrometers onto the surface of Mars (instrument Supercam of CNES which will be mounted on the rover of the NASA).

Molecular dynamics simulations have been also performed on water ice at extreme conditions, similar to those in the interior of ice giant planets like Neptune or Uranus. At high temperatures (beyond 1300 K) and pressures (beyond 10 GPa i.e. 100 thousand atmospheres), the water ice is known to adopt exotic behaviors so-called ionic and superionic. In the superionic phase, water ice has properties of both a crystalline solid (lattice formed of oxygen atoms) and a liquid (the hydrogen protons diffuse in the lattice). These studies demonstrated the existence of three new phases in the superionic state and the associated phase transitions. The high conductivity associated to the superionic state could explain the observed magnetic fields of Uranus and Neptune. From this work, the team of the Tokyo Institute of Technology will try to experimentally observe the new computed phase transitions.

The project led by Dr. Caracas was granted access to GENCI HPC resources, with 11 million compute core hours awarded in 2016 on supercomputers Ada, Occigen and Curie. These works resulted in six publications in 2016 in peer-reviewed journals such as Science and Physical Review Letters.

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