Physical properties of liquid Fe alloys at high pressure and their bearings on the nature of metallic planetary cores

Sulfur and silicon are among the expected alloying light elements in planetary liquid iron cores. Structural properties of Fe‐27 wt % S and Fe‐17 wt % Si liquid alloys at high pressure and high temperature (0–5 GPa/1400–2300 K) are measured by synchrotron X‐ray diffraction. Sulfur strongly modifies the local structure of liquid iron whereas silicon has only small structural effects. Fe‐27 wt % S melts are indeed poorly ordered which explains a higher compressibility compared to pure liquid Fe. These results point out the necessity to consider the strong effect of S on liquid Fe properties while modeling planetary interiors. They imply a low S content in the Earth's outer core, leaving Si as a strong candidate, and argue for a present‐day Martian solid core when combined with previous global chemical models.