Sound Velocities of Fe‐Si Alloys at Conditions of the Cores of Moon and Mercury

The species and contents of the light elements within the metallic cores play a crucial role in understanding the structure, thermal state, geodynamo, and evolution of the terrestrial planets and satellites. While silicon (Si) has been considered as a primary candidate light element in the cores of small terrestrial bodies, the sound velocities of face‐centered cubic (fcc) Fe‐Si are rarely investigated. Here we measured the compressional ( V P ) and shear ( V S ) wave velocities of Fe alloyed with 3 wt.% and 6 wt.% Si up to 6.3 GPa and 1,273 K using pulse‐echo overlap technique coupled with energy‐dispersive X‐ray diffraction. The incorporation of Si could increase the V P of body‐centered cubic (bcc) Fe by 0.074(15) km/s and decrease the V S by 0.024(9) km/s with each weight percent Si at 300 K. However, 3 wt.% Si has minimal effect on the V P of fcc‐Fe if not negligible. The incorporation of 6 wt.% Si stabilizes the Fe in mixture bcc and fcc phases at high temperature and increases the V P by 0.59 km/s compared with that of fcc‐Fe. Based on the Apollo seismic observation and V P of Fe and Fe‐Si alloys, Si is unlikely to be the sole light element in the lunar core. The solid core of the Mercury would be in the fcc phase if the Si content is less than 3 wt.% and the V P profiles is 5.65–6.34 km/s. While 6 wt.% Si would stabilize the Mercury's core in mixed bcc and fcc phases, the V P profiles would be 6.17–6.85 km/s.