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 (VP) and shear (VS) 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 VP of body‐centered cubic (bcc) Fe by 0.074(15) km/s and decrease the VS by 0.024(9) km/s with each weight percent Si at 300 K. However, 3 wt.% Si has minimal effect on the VP 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 VP by 0.59 km/s compared with that of fcc‐Fe. Based on the Apollo seismic observation and VP 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 VP 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 VP profiles would be 6.17–6.85 km/s. Plain Language Summary As typical bodies within the solar system, the Moon and Mercury share similar internal structures with the Earth, consisting of a crust, mantle, and core. Their solid metallic cores are considered to be mainly composed of Fe alloyed with a few weight percent Si. Therefore, the sound velocities of body‐centered cubic (bcc) and face‐centered cubic (fcc)‐Fe and Fe‐Si have been obtained under the core conditions of the Moon and Mercury in this study. Our results show that the incorporation of Si could increase the VP of fcc‐Fe. The incorporation of light element is expected to be lower the sound velocity of fcc‐Fe to reconcile the seismic observation of the lunar core. Consequently, the Si is unlikely to be the primary light element in the lunar core. For Mercury, the solid inner core may be in the fcc phase or mixture phases of bcc and fcc depending on the Si content. The corresponding VP‐ρ profiles have been constrained. Key Points The effect of 3 wt.% and 6 wt.% Si on the sound velocity of Fe has been constrained Lunar core would be S