The Martian Core: ab initio calculations and high P/T experiments on iron alloys

Our solar system is composed of a rich variety of bodies, each with a complex and diverse evolutionary history; understanding their evolution presents one of the major challenges in Earth and planetary sciences. Critical understanding of the basic physical processes that govern planetary interiors is now possible because the solar system is accessible to highlydetailed remote-sensing and in-situ measurements with ever-more detailed data being returned from current (and future) space missions. Processes occurring deep in planetary interiors are reflected in their surface morphologies as, for example, volcanic and tectonic features. The last decade has also seen rapid advances in the techniques and facilities necessary to investigate both the materials from which the planets and their moons are composed, and the structure, dynamics and evolution of these bodies. Parallel computing now allows simulations at length-scales from the atomic to planetary, while laboratory experiments can now measure a wide range of physical properties under the relevant conditions of pressure and temperature; accurately determined material properties at the relevant planetary conditions are an essential pre-requisite for large-scale planetary modelling.

Core formation and evolution are products of the planetary accretionary process, and the role of metallic cores is important in understanding planetary internal structure and magnetic field generation. If we are to understand planetary evolution, geophysical modelling of planetary interiors - the outputs of which can be tested against data from spacecraft and ground-based observations - is essential. However, if these geophysical models are to be reliable, the physical properties of the constituent core-forming materials (essentially iron and iron alloys) must be accurately known in the pressure and temperature range of relevance. At present,
this is often not the case for the conditions expected in the interior of Mars. Key deliverables of the project will be the determination of the phase diagrams and physical properties of iron alloys, especially ternary systems such as Fe-Ni-Si and Fe-S-Si, at pressures up to ~40 GPa. Observations from spacecraft are providing increasingly detailed information about the composition, internal structure and magnetic field of Mars. Our knowledge of its deep interior will be greatly enhanced in the near future by the geophysical observations to be made during the InSight mission (arriving November 2018). Although Mars is thought to have evolved in a
similar way to the Earth, its smaller size suggests that it may have cooled more rapidly than the Earth and the combination of different cooling rates and a bulk composition differing substantially from Earth, gives Mars a distinct chemistry and evolutionary history. To
understand the evolution of Mars, geophysical models of its interior must be constructed, the outputs of which can then be tested against data from spacecraft and ground-based observations. If these models are to be accurate, however, it is essential that the physical properties of the core-forming materials (essentially iron and iron alloys) are known in the pressure (P) and temperature (T) range of relevance. These properties include the phase diagrams, equations of state and transport properties of the relevant liquid and solid phases for T≤~2500 K and P ≤~40 GPa. For example, we don’t even know the crystallisation regime of Mars (see figure below), which is dependent on the relative gradients of the adiabat and the liquidus throughout its evolution; these in turn are dependent on fundamental physical properties of the core-forming phases.