Presence of primordial Mg explains the seismic low-velocity layer in the Earth’s outermost outer core

The exact composition of Earth’s liquid outer core, which is believed to consist primarily of Fe-Ni alloys with minor light elements such as Si, O, C, S, and H, has long been debated. Available models incorporating these light elements cannot explain the seismically identified low velocity layer in the uppermost few hundred kilometers of the outer core, known as the E¢ layer. Here, we propose that the presence of primordial Mg, a potential light element that could have entered the outermost outer core following the Moon-forming giant impact, may provide a viable explanation for the formation of the E¢ layer. Employing first-principles molecular dynamics simulations, we determine the equation of state (pressure-density-temperature relations) and sound velocity ( V _P ) of Fe-Mg alloying liquids under outer core conditions, which were unknown previously. Our results suggest that the presence of Mg can slightly decrease the V _P of liquid Fe, in contrast to the enhancing effects of other light elements. Using seismically observed density and V _P as constraints, we find that 0.5-1.79 wt% Mg is required to match core properties, depending on core composition and seismic models. The amount of primordial Mg in the outer core constitutes a significant portion of the total Mg budget in the bulk Earth, partly explaining its slight depletion in the bulk silicate Earth relative to chondritic meteorites.