The chemical composition of the outer core of the Earth has been a long-standing question in geoscience since 1952. This study constrained the outer core composition by first principles calculations by comparing calculated densities and bulk sound velocities of liquid iron alloyed with five candidates of light elements with seismic observations.

Background
The outer core of the Earth mainly consists of liquid iron. The Preliminary Reference Earth Model (PREM) deduced from seismology suggests, however, that the density and bulk sound velocity of the outer core is ~10% lower and ~5% faster than those of liquid pure iron, respectively. These differences indicate that the outer core includes substantial amounts of light elements such as silicon, oxygen, sulphur, carbon, and hydrogen, in addition to iron and nickel. This issue was pointed out in 1952 by Francis Birch, and still remains unsolved. A promising way to constrain the chemical composition of the outer core is to compare the density and bulk sound velocity of liquid iron alloys with the PREM under high pressure and temperature conditions corresponding to the outer core. Recent high-pressure experimental studies reported the density and velocity of liquid iron alloys up to ~100 GPa, which is still lower than the outer core pressure range, 136 GPa—330 GPa. Hence, first principles calculations are very useful to investigate liquid iron alloys under these extreme conditions.

Research outcome
This research presented first principles molecular dynamics calculations of the densities and bulk sound velocities of liquid iron alloyed with candidates of light elements, (Fe,Ni)x(H, Si, O, S, C)1-x, under outer core conditions. By comparing calculated densities and velocities with the PREM, the chemical composition of the outer core was constrained (Figure 1). While the range of possible outer core composition is rather wide, the authors searched for the ‘best’ estimate that is the most compatible with the PREM; in other words, that which gives the smallest deviation in density and velocity from the PREM (Table 1). The ‘best’ estimate depends on the inner-core boundary temperature (TICB); TICB is also not yet well known because it is related to the chemical composition. When TICB is not high (4,800 K – 5,400 K), hydrogen is the primary light element. If this is the case, a large amount of water should have been delivered to the Earth during its accretionary stage. On the other hand, oxygen is the most important light element if TICB = 6,000 K.

Future perspectives
This study showed the possible chemical composition of the outer core and the importance of hydrogen by comparing the density and sound velocity of liquid iron alloys between theoretical calculations and seismological observations. Other constraints besides the density and sound velocity will help narrow down the outer core composition further. Simultaneous solubilities of light elements in liquid iron are very important. If some of the candidates for light elements cannot be dissolved in liquid iron simultaneously, such a combination of light elements is excluded from the possible outer core composition. For example, it was experimentally reported that simultaneous solubilities of silicon and oxygen in liquid iron are limited. The melting temperature of iron alloys is another crucial constraint. Since the outer core is liquid, any composition which gives too high of a melting temperature is excluded. The precise determinations of the liquidus temperatures of iron alloys, both experimentally and computationally, will be the next important new step.