Earth’s water, intrinsic oxidation state and metal core density are fundamental chemical features of our planet. Studies of exoplanets provide a useful context for elucidating the source of these chemical traits. Planet formation and evolution models demonstrate that rocky exoplanets commonly formed with hydrogen-rich envelopes that were lost over time1. These findings suggest that Earth may also have formed from bodies with hydrogen-rich primary atmospheres. Here we use a self-consistent thermodynamic model to show that Earth’s water, core density and overall oxidation state can all be sourced to equilibrium between hydrogen-rich primary atmospheres and underlying magma oceans in its progenitor planetary embryos. Water is produced from dry starting materials resembling enstatite chondrites as oxygen from magma oceans reacts with hydrogen. Hydrogen derived from the atmosphere enters the magma ocean and eventually the metal core at equilibrium, causing metal density deficits matching that of Earth. Oxidation of the silicate rocks from solar-like to Earth-like oxygen fugacities also ensues as silicon, along with hydrogen and oxygen, alloys with iron in the cores. Reaction with hydrogen atmospheres and metal–silicate equilibrium thus provides a simple explanation for fundamental features of Earth’s geochemistry that is consistent with rocky planet formation across the Galaxy.
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Example model results are available as Supplementary tables.
The Python code used for the models shown in Fig. 1 in this study is available at GitHub: https://github.com/eyoungucla/chems/blob/main/Exoplanet_atmosphere_model_vMCMC_coreT_3000K_dist.py.
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This AEThER publication is funded in part by the Alfred P. Sloan Foundation under grant G202114194. H.E.S. gratefully acknowledges NASA grant 80NSSC18K0828 for financial support during preparation and submission of the work.
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Extended data figures and tables
Extended Data Fig. 1 Plot of surface temperatures that allow for a gravitationally bound primary H2 atmosphere versus mass of planetary embryos19.
The region in temperature-mass space where a primary atmosphere is possible is shaded. The approximate solidus for silicate melt is overlain as the horizontal line.
Extended Data Fig. 2 Summary of equilibrium calculations for Si in metal in embryos with masses of 0.5M⊕ as a function of metal–silicate equilibration temperature (Tcore–mantle) and mass fraction of initial primary H2-rich atmosphere relative to the planet.
Arrow illustrates values for models that satisfy the required density deficit in the core but where H is scarce or absent.
Extended Data Fig. 3 Pressure vs. radius for 0.3M⊕ (left) and 0.5M⊕ (right) embryos.
Breaks in slope mark the core–mantle boundaries.
Extended Data Fig. 4 Iron isotope ratios of bulk silicate for model embryos compared with recent estimates for bulk Earth (grey bar) and chondrites showing that the model reproduces the offset between initial materials (chondrites, δ57Fe = 0) and Earth.
Black filled symbols are for E chondrites while white symbols are for all other chondrite groups. The multi-colour contours are probability densities for the chondrite δ57Fe values with an average indistinguishable from δ57Fe = 0.0. The y axis values are assigned randomly to each datum in equal spacings for clarity, with E chondrites confined to the lower quarter of the ordinate.
Extended Data Fig. 5 Summary of D/H ratios for Solar System materials from a variety of literature sources.
The circle + symbol labelled E denotes bulk Earth76. Black/white symbol labelled M refers to lunar highland apatites95. Blue symbols refer to calculated values for original water based on measured asteroidal rock values84. U, N, J, and S refer to Uranus, Neptune, Jupiter, and Saturn, respectively96,97. Model values for Earth’s water, primordial hydrogen atmosphere, and metal described here are indicated within the grey box, where the model is assigned the terrestrial value.
This file contains Supplementary Tables 1–4.
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Young, E.D., Shahar, A. & Schlichting, H.E. Earth shaped by primordial H2 atmospheres. Nature 616, 306–311 (2023). https://doi.org/10.1038/s41586-023-05823-0
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