Earth shaped by primordial H$_2$ atmospheres
Authors: Edward D. Young, Anat Shahar, Hilke E. Schlichting
AI-generated Key Points
- Earth's water, intrinsic oxidation state, and metal core density can be attributed to the equilibrium between H$_2$-rich primary atmospheres and underlying magma oceans in its progenitor planetary embryos.
- Earth's fundamental chemical features can be explained by reactions between hydrogen-rich atmospheres and magma oceans during planet formation.
- Rocky exoplanets commonly formed with hydrogen-rich envelopes that were lost over time, leading to the hypothesis that Earth may have also formed from bodies with H$_2$-rich primary atmospheres.
- 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.
- This study provides a simple explanation for fundamental features of Earth's geochemistry which is consistent with rocky planet formation across the galaxy.
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.
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