Interactions Between Planetary Embryo Magma Oceans and H₂-Rich Atmospheres Could Account for Earth’s Water Abundance

Illustration of how Earth’s abundance of water and overall oxidized state could be due to interactions between planetary embryos and an H₂ atmosphere. Credit: Edward Young (UCLA) and Katherine Cain (Carnegie Institution for Science).

The origin of Earth’s signature geochemical features, including its abundant water, oxidized mantle, and core density that is lower than that of pure iron metal, has continued to puzzle scientists. Water could have been delivered to Earth by accretion of cometary ice, primitive, volatile-rich meteorites such as carbonaceous chondrites, or various differentiated meteorites. However, exoplanet research over the past decade has provided a new perspective by showing that it is common for young planets to have hydrogen-rich atmospheres during their first several million years of growth before these hydrogen envelopes dissipate. Interactions between primordial atmospheres on planetary embryos and their magma oceans could have an important effect on the initial composition of a larger planet that incorporates those smaller planetary embryos and could have a cascading effect on overall planetary characteristics.

A new study by Edward Young (UCLA), Anat Shahar (Carnegie Institution for Science), and Hilke Schlichting (UCLA) reports the results of thermodynamic modeling that shows that Earth’s water, low core density, and overall oxidized state can all be produced by interactions between H2-rich atmospheres and magma oceans within planetary embryos. The authors suggest that at least one of the planetary embryos that came together to form the Earth grew faster than previously thought, so it was massive enough to retain a large hydrogen atmosphere while still being molten. Due to interactions between the hydrogen gas and the magma ocean, hydrogen could enter the core, leaving behind iron oxides in the mantle. As iron oxides evaporated, water was produced by atmospheric oxidation. This model can simultaneously explain Earth’s total surface water abundance, the mantle’s oxidation state, and the density of the core, which otherwise might be thought to require separate explanations. The authors have demonstrated that early interactions between atmospheres and magma oceans are important in understanding Earth’s unique, life-sustaining composition and exoplanets where JWST can now observe these early stages of planetary formation. READ MORE