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Light elements (e.g., Si, C, O) play a pivotal role in defining the chemical make-up of planetary cores, their internal structure, and consequently their dynamic properties. The latter are of fundamental importance as the presence of a core-driven dynamo can determine the onset and sustenance of a magnetic field. This in turn can protect the planet from strong solar winds and prevent atmospheric escape. For these reasons, the presence of a magnetic field is often considered a requirement to sustain life at the surface of a planet. The discovery of planets outside our solar system, orbiting stars with different compositions from the Sun, has led to questions of planetary diversity and habitability. How diverse are (exo)planetary cores relative to Earth s core? Which cores can possibly sustain a dynamo and magnetic field?

To begin answering these questions, this work focused on the Fe-Si-C system. Along with silicon and other light elements, only a small portion of carbon is considered to be present in the Earth s core. Studying Fe-Si-bearing alloys with higher carbon contents offers a view based on a chemical system that hasn t been investigated at high pressure and temperature yet. Thus, the results provide insights into the potential composition and structure of cores accreting in more reducing conditions and a more holistic view of planetary interiors.

To effectively investigate the ternary Fe-Si-C system, angle-dispersive X-ray diffraction (XRD) experiments on four different compositions were performed at the ID27 beamline combining diamond anvil cell and laser heating

techniques. The high flux at 33 KeV and small focus of the X-ray beam (3x3 µm2) permitted experiments to be carried out at pressures up to 200 GPa and temperatures up to 4000 K. The scope of the experiments was to define the phase assemblages and melting relations at different pressures and temperatures, identifying eutectic temperatures and the solid phases stable in the melting interval, including the liquidus phase. The latter represents the last phase to melt upon heating and accordingly the first phase to cool from a liquid; hence, it represents the first phase to crystallise in the core. The physical properties (i.e., density) of the first crystallising phase with respect to those of the remaining liquid will control the crystallisation style of the core and its potential to start a dynamo.

The results obtained in the present study made it possible to characterise the Fe-Si-C ternary system at extreme conditions. Each bulk starting composition exhibited different stable phase assemblages with increasing pressure and temperature. An FeSi phase and two different iron carbides (i.e., Fe3C and Fe7C3) were the main phases observed, along with carbon (as diamond) that displayed an extensive stability field. C was observed to be largely soluble into FeSi; conversely, Si does not partition into the carbides. In the investigated pressure range, samples with lower light element contents were observed to melt at lower temperatures relative to other compositions, suggesting the presence of two different invariant points in the phase diagram (Figure 5). When applied to planetary cores, these observations highlight how a difference in light element content can lead to extremely different

Fig. 5: Temperature-composition surfaces portraying the first phase to crystallise for each portion of the Fe-Si-C ternary phase diagram. Insets: Different crystallisation scenarios at three different times during cooling.