Stratification in planetary cores by liquid immiscibility in Fe-S-H

Model predictions of global geologic hydrogen resources

Geologic hydrogen could be a low-carbon primary energy resource; however, the magnitude of Earth's subsurface endowment has not yet been assessed. Knowledge of the occurrence and behavior of natural hydrogen on Earth has been combined with information from geologic analogs to construct a mass balance model to predict the resource potential. Given the associated uncertainty, stochastic model results predict a wide range of values for the potential in-place hydrogen resource [103 to 1010 million metric tons (Mt)] with the most probable value of ~5.6 × 106 Mt. Although most of this hydrogen is likely to be impractical to recover, a small fraction (e.g., 1 × 105 Mt) would supply the projected hydrogen needed to reach net-zero carbon emissions for ~200 years. This amount of hydrogen contains more energy (~1.4 × 1016 MJ) than all proven natural gas reserves on Earth (~8.4 × 1015 MJ). Study results demonstrate that further research into understanding the potential for geologic hydrogen resources is merited.

A thermally conductive Martian core and implications for its dynamo cessation

Mars experienced a dynamo process that generated a global magnetic field ~4.3 (or earlier) to 3.6 billion years ago (Ga). The cessation of this dynamo strongly affected Mars' history and is expected to be linked to thermochemical evolution of Mars' iron-rich liquid core, which is strongly influenced by its thermal conductivity. Here, we directly measured thermal conductivities of solid iron-sulfur alloys to pressures relevant to the Martian core and temperatures to 1023 Kelvin. Our results show that a Martian core with 16 weight % sulfur has a thermal conductivity of ~19 to 32 Watt meter-1 Kelvin-1 from its top to the center, much higher than previously inferred from electrical resistivity measurements. Our modeled thermal conductivity profile throughout the Martian deep-mantle and core indicates a ~4- to 6-fold discontinuity across the core-mantle boundary. The core's efficient cooling resulting from the depth-dependent, high conductivity diminishes thermal convection and forms thermal stratification, substantially contributing to cessation of Martian dynamo.

Melting phase relations in Fe-Si-H at high pressure and implications for Earth's inner core crystallization

Hydrogen could be an important light element in planetary cores, but its effect on phase diagrams of iron alloys is not well known because the solubility of H in Fe is minimal at ambient pressure and high-pressure experiments on H-bearing systems have been challenging. Considering that silicon can be another major light element in planetary cores, here we performed melting experiments on the Fe–Si–H system at ~ 50 GPa and obtained the ternary liquidus phase relations and the solid/liquid partition coefficient, D of Si and H based on in-situ high-pressure X-ray diffraction measurements and ex-situ chemical and textural characterizations on recovered samples. Liquid crystallized hexagonal close-packed (hcp) (Fe_0.93Si_0.07)H_0.25, which explains the observed density and velocities of the Earth’s solid inner core. The relatively high D_Si = 0.94(4) and D_H = 0.70(12) suggest that in addition to Si and H, the liquid outer core includes other light elements such as O, which is least partitioned into solid Fe and can thus explain the density difference between the outer and inner core. H and O, as well as Si, are likely to be major core light elements, supporting the sequestration of a large amount of water in the Earth’s core.