Earth shaped by primordial H${}_2$ atmospheres

Abstract

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 time. These findings suggest that Earth may also have formed from bodies with H${}_2$-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 H${}_2$-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 Si, along with H and O, alloys with Fe 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.

3 main figures, 5 auxiliary figures

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• Water versus land on temperate rocky planets
• Water solubility in silicate melts: The effects of melt composition under reducing conditions and implications for nebular ingassing on rocky planets
• Crash Chronicles: relative contribution from comets and carbonaceous asteroids to Earth's volatile budget in the context of an Early Instability
• Possible Evidence for the Presence of Volatiles on the Warm Super-Earth TOI-270 b
• An impact-free mechanism to deliver water to terrestrial planets and exoplanets
• Discovery of an icy and nitrogen-rich extrasolar planetesimal
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• The atmospheric entry of cometary impactors
• Diversity of Exoplanets
• On the importance of laboratory experiments for interpreting exoplanet observations
• Deep Mantle-Atmosphere Coupling and Carbonaceous Bombardment: Options for Biomolecule Formation on an Oxidized Early Earth