Materials under extreme conditions: applications to the study of the interior of solar and extrasolar planets

Abstract

Extrasolar planets are formed by accretion of the material available in the initial molecular cloud that forms the protoplanetary disk, which includes silicates, iron, graphite and polycyclic aromatic hydrocarbons. Since direct observation or measurement of the composition are not yet possible, several models for the internal structure of a planet have been built on the basis of measurements of its mass and radius, coupled with a possible range of compositions. These models rely on the knowledge about the properties of the materials that are part of the composition, which are known to be under very high pressures and temperatures in the different layers that form the planet. Hence, these models and our understanding of planetary evolution will be more accurate to the extent that we know the changes induced by pressure and temperature on these materials and the interactions carried out between them at the extreme conditions reached in their interiors. This thesis presents a succession of ab initio studies about materials relevant for planetary interiors, which include iron and silica, aimed to address different scenarios inside gas giants and super-Earths after its formation. The first study deals with the erosion of the rocky core of gas giants due to the presence of metallic hydrogen, where silica, one of the main candidates for the rocky-core composition, has been chosen as a representative material. In this study, the free energy of solvation of silica into metallic hydrogen was calculated, using the thermodynamic integration technique to get the Helmholtz free energy from molecular dynamics simulations. The study reveals that, for the thermodynamic conditions present in the core-mantle boundary of the gas giants of our solar system, erosion is energetically favored, in good agreement with calculations performed for other rocky core materials, like water ice, periclase and iron, which can also be dissolved by hydrogen. These results have major implications for the evolution of giant planets, since erosion, coupled with convection, may explain the enrichment in heavy elements in giant planet atmospheres. The second study addresses the melting properties of silica for pressures relevant to the core of giant planets and the mantle of super-Earths, where silica is also expected to be very abundant. The results of this study reveal an abrupt increase in the melting curve previously reported by other studies, and extend the curve up to 6000 GPa. The implications of this study modify the picture of stable rocky cores in giant planet interiors, since they may not only be dissolving, but they may also be melting. For our solar system's giants, this study concludes that the silica component is not molten at their cores, and the curve itself provides a constraint for planetary interior models, which allows more accurate predictions and, therefore, better understanding of their structure and evolution. The structure of iron at the center of Earth was also studied in this thesis. There have been persistent arguments that the stable phase of iron at Earth's inner-core conditions is bcc, but these studies suffered from misinterpretation of the requirements of mechanical instability and the use of classical many-body potentials, the accuracy of which is untested in studies of the mechanical instability of bcc iron. In this study, different distortions were performed over the system in order to calculate mechanical stability properties, like stress and shear anisotropies and mean square displacement of the atoms. The results point toward a close-packed crystalline structure for Earth's inner core (hcp and/or fcc), rather than bcc. The importance of this studies lies in the fact that it indicates that iron will melt in hcp phase because bcc structure is mechanically and thermodynamically unstable at 360 GPa to the temperatures of 7000 K.