Light elements in the Earth's outer core: A critical review

There is little doubt that densities for the Earth's outer core, inferred from seismology, require that it is constituted of an alloy of liquid iron and light elements. However, the nature of the light alloying elements is still uncertain as it depends in a large measure on the conditions of accretion of the Earth and formation of the core. The arguments brought forward for or against silicon, oxygen, sulphur, hydrogen and carbon are critically reviewed. There is no reason to consider that only one element is present in the outer core. Experimentally determined and/or calculated ternary and quaternary phase diagrams are needed to provide constraints on the nature of the light elements.