Unraveling the composition and structure of the Earth's outer core

The structure of Earth's crust, mantle, and core holds clues to its thermal state and chemical composition, and, in turn, its origin and evolution. Geophysical techniques, and seismology in particular, have proved successful at probing Earth's deep interior and have done much to advance our understanding of its inner workings from mantle convection to crystallization and solidification of Earth’s liquid core. As the outer core cools and solidifies, light elements, such as Si, S, C, O and H, preferentially partition in the fluid outer core. However, the exact composition and thermal state of the outer core remains unknown. Traditionally, the composition of the core has been determined by performing theoretical ab initio calculations on candidate compositions and comparing the results for Vp, Vs and density to seismic reference models (e.g., PREM). Instead, we determine structure, composition and thermal state of Earth's outer core by inverting a plethora of short- and long-period seismic and astronomic-geodetic data in combination with new density functional theory calculations that are fit to a novel Gaussian Process Regression (GPR) equation of state (EoS). The GPR-EoS allows us to self-consistently compute thermo-elastic properties of liquid multi-component mixing models in the Fe-Ni-Si-S-C-O-H system along outer-core adiabats and across its entire pressure and temperature range. By mapping out the thermo-chemical model space of Earth’s outer core that match the seismic and geophysical data within uncertainties, we find two families of solutions characterised by: 1) Si (~4 wt%) and negligible amounts of H and C and 2) C and H (both 0.5 wt%) and smaller amounts of Si (<1 wt%). A correlation between H content and outer-core thermal structure is apparent, such that solutions with little-to-no H correspond to relatively high CMB and ICB temperatures (4100--4400~K and 5750–6000 K, respectively), whereas models with large amounts of H are characterised by lower CMB and ICB temperatures (~3600 K and 4750 K).