What is the problem/issue being addressed?
It is known that the Earth’s core is less dense than pure iron by about 7%, which is due to the presence of a light element(s) such as Si, S, C, O, and H. The goal of this project is to construct a thermodynamic model of the Earth’s central core. A particular focus is on the identification of the light element because the inclusion of these elements in iron liquid depends on the pressure, temperature, and chemical environment and hence provides us invaluable information about the origin and evolution of the solid Earth. We are examining phase relations and density of phases in Fe-light element systems by conducting high-pressure and -temperature experiments and employing thermodynamic calculations based on the experimental data.
Why is it important for society?
One of the important implications of the project is that the behaviour of the light element in the core should be linked to the origin of the magnetic field through geodynamo and hence the habitability of a planet. The convecting liquid iron core generates magnetic fields, which are protecting life on the surface from the harmful solar wind. The liquid convection is driven mostly by the preferential partitioning of the light element into the liquid outer core over the solid inner core. Therefore, for a planet, just being at an appropriate distance from the central star which stabilises liquid H2O is not the sufficient condition to be habitable; presence of light elements in the core is likely another key condition. This project will bring society a better understanding of Earth’s stable magnetic fields which have protected life since its birth.
What are the overall objectives?
The key research questions that the project is trying to address are:
1. What are the thermodynamic properties of iron and iron-light elements alloy under core pressure and temperature conditions?
2. What are the light elements dissolved in the Earth’s core?
3. How was/is the origin, current state, and evolution of the Earth’s core?
The conclusions of the action
We have established a thermodynamic model including liquids for each Fe-light element system, which can be applicable to Earth’s core pressure-temperature conditions. Under core conditions, Fe-Si liquids can be treated as ideal solutions as well as Fe-O. In contrast, Fe-S and Fe-C liquids remain nonideal to the shallow and bottom parts of the outer core respectively, which is contrary to theoretical predictions. None of the studied binary systems alone cannot explain the seismic properties of the core, while Fe-6wt%Si-1wt%O gives the density and P-wave velocity of liquids close to those of the outer core. Although the density of the crystallising phase from the Fe-Si-O liquid is not perfectly consistent with that of the inner core, we conclude that the Earth’s core should be Si-rich, which implies that core formation should have occurred under reducing conditions.
We also determine phase relations of Fe-Ni-Si alloy and found that the addition of Ni and Si simultaneously results in unexpected phase relations Fe, which enlarges the fcc stability field. This suggests that future studies should simultaneously include Ni and light elements for more realistic core systems.
