Inner core composition paradox revealed by sound velocities of Fe and Fe-Si alloy.

Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (V S ) and high Poisson's ratio (σ) in the solid inner core. To date, neither V S nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present V S and σ derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to ~230 GPa and ~5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low V S and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease V S and increase σ. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.