This project is a theoretical study of molecular and atomic interactions at the high pressures characteristic of the interiors of giant planets. Two different first-principles computational approaches---density functional molecular dynamics and path integral Monte Carlo---will be used, to (1) simulate interfaces between ice and metallic hydrogen to understand their stability, particularly as related to giant planet formation and core growth; (2) simulate hydrogen-helium mixtures to understand helium rain and its effect of the cooling and radiated luminosity of Saturn; (3) predict the degree to which neon is depleted in Saturn's atmosphere through the process of neon sequestration by helium rain; (4) calculate equations of state for giant planet interiors, with an eye toward explaining the surprisingly large sizes of "hot Jupiters" and refining the temperature profile of Jupiter itself; and (5) calculate the electrical conductivity of H-He mixtures to better understand magnetic field generation and ohmic dissipation of tidal perturbations in giant planets. A graduate student will be supported and trained in this project, which will also produce high-accuracy tables of equations of state for exoplanet modeling. These equations of state will be made available to the research community. The PI and collaborators will also produce an interactive website called "Planet Builder" where students and the public can see how interior composition and physical conditions influence the overall characteristics of planets, in comparison with those in the Solar System.
The goal of proposal was to study the interiors of solar and extrasolar giant planets by applying and developing the atomistic computer simulation techniques. While the extreme temperature-pressure conditions (10000 Kelvin and 10 million atmospheres) prevalent in giant planets cannot yet be probed experimentally, they are readily accessible with first-principles simulations that characterize the properties of matter by simulating the motion of electrons and ions on parallel supercomputers. These simulations enabled us to determine the density of a different material in the interior conditions of giant planets. This information was entered into large-scale models for the interior of Jupiter. Predictions for the gravity field were compared with spacecraft observations. Model parameters like the size of Jupiter’s rocky core and the amount of heavier elements in the planet’s hydrogen-helium envelope were adjusted until a good match was found. We derived a comprehensive equation of state table for hydrogen-helium mixtures and we made it available on the internet: http://militzer.berkeley.edu/HHe-EOS so that it can be incorporated in future giant planet interior models. Computer simulations were also be employed to calculate Gibbs free energies, which allowed us to predict whether materials mix or phase-separate at high temperature and pressure. We used this free energy approach to predict the following processes in the interiors of giant planets: 1) We predicted helium rain to occur on Jupiter by combing the data from the NASA’s Galileo entry probe with results from our computer simulations. The probe had measured a drastic depletion of the noble gas neon compared to concentrations in the sun. These findings had remained unexplained for 15 years until our simulations determined that neon atoms preferentially dissolve into forming helium droplets. The helium rain then sequesters the neon atoms into the deeper interior of Jupiter. 2) We demonstrated that rocky cores in giant planets are not stable over the gigayear lifetime of a planet. After the gas has been accreted onto a core of rock and ice, the pressure and the temperature at the core-mantle boundary increase drastically. With computer simulations, we predicted all the typical core materials, water ice, rocky materials like magnesium oxides (MgO) and silica (SiO2) as well as iron, dissolves into the layer of metallic hydrogen. This established that core erosion needs to be considered when the models for the evolution of giant planets are constructed. 3) We predicted water to assume a new superionic form in the interiors of Uranus and Neptune. The superionic phase occurs at intermediate temperature in between the solid and the liquid states. The oxygen atoms remain stationary like in a solid while the much smaller hydrogen atom diffuse about the crystal liquid a fluid. Our simulations predicted that water assume a much denser form where the oxygen atom are arranged in a face-centered cubic lattice while previous work had been assumed a body-centered lattice of oxygen atoms. Our predictions have implications for the interiors of Uranus and Neptune and are in the process of being verified with laboratory experiments.
