Presence of primordial Mg can explain the seismic low-velocity layer in the Earth's outermost outer core

The composition of Earth's Fe-rich liquid outer core has long been debated. Available models incorporating light elements, such as Si, O, C, S, and H, cannot explain the seismically low velocity layer in the uppermost outer core (E' layer). Here we employ first-principles molecular dynamics simulations to determine the density and sound velocity ( $V_P$ ) of Fe-Mg liquids under outer core conditions, which were unknown previously. Results show that the presence of Mg slightly decreases the $V_P$ of liquid Fe, in contrast to the enhancing effects of other light elements. Our modeling suggests that 0.5-1.79 wt% Mg is required to match seismically observed core densities and velocities. Such amount of Mg could have entered the outermost outer core following the Moon-forming giant impact, thereby providing a viable explanation for the formation of the E' layer and contributing to the slight Mg depletion in the bulk silicate Earth relative to chondritic meteorites.