Earth Shaped by Primordial H2 Atmospheres

Several aspects of the formation of the Earth remain poorly understood despite decades of study. One is the origin of Earth's water. Another is the origin of the oxidation state recorded by the bulk composition of the planet, and yet another is the identity of the light elements responsible for the ∼ 10% density deficit in the metal core. Studies of exoplanets may provide useful context for elucidating the causes of these fundamental features of our planet. Planet formation and evolution models demonstrate that the rocky super-Earths, among the most common planets we observe today, are consistent with having formed with hydrogen-rich envelopes that were lost over time by either photo-evaporation and/or core-powered mass loss, suggesting that the super-Earths and sub-Neptunes originally formed as one population with hydrogen envelopes.

Motivated by these findings, 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 H2-rich primary atmospheres and underlying magma oceans in progenitor planetary embryos (see Figure). Water is produced from dry inner Solar-System 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 at equilibrium, causing metal density deficits of 8% that are then enhanced by compression upon reaching a full Earth mass to the 10% deficit seen today. Oxidation of the silicate rocks from solar-like to Earth-like oxygen fugacities also ensues as Si enters the core along with H and O. Reactions with hydrogen atmospheres thus explain fundamental features of Earth's geochemistry and geophysics and place Earth's formation into the context of rocky exoplanets, many of which appear to have formed with H2-rich primary atmospheres.