The Earth's early history is a captivating tale of water's journey through our planet's formation. A recent study in Science challenges our understanding of this narrative, suggesting that a significant portion of the Earth's water may have been locked away deep within the mantle during its molten phase. This revelation adds a fascinating layer to the story of our planet's evolution.
The research, conducted by Wenhua Lu and colleagues, focuses on bridgmanite, a mineral that dominates the lower mantle, the Earth's largest rocky layer. This mineral, under immense pressure, has the potential to hold a substantial amount of water, possibly as much as an ocean's worth. The study's experiments, conducted at extreme pressures and temperatures, revealed that bridgmanite can incorporate water concentrations reaching up to 0.2% by weight, which is a significant finding.
What makes this discovery even more intriguing is the context in which it was found. The young Earth, during its formation, was a molten inferno, lacking a solid surface and a blue ocean. Water, present in the magma ocean, was dissolved in molten silicate, creating a complex scenario. As the planet cooled, the question arose: did the water stay with the melt or move towards the atmosphere and surface? The study's experiments and models suggest that a substantial amount of water remained with the solid mantle, potentially as much as one present-day ocean.
However, this is not a straightforward story of water retention. The study highlights the importance of the partition coefficient, a measure of water concentration in crystals compared to the melt. Previous experiments had suggested bridgmanite was nearly dry, but the new findings indicate a sharp increase in water partitioning with temperature. This means that the mineral, under the conditions of a magma ocean, could hold a significant amount of water, even if it preferred the melt.
The implications of this research are profound. It suggests that the early Earth may have divided its water between the atmosphere and the solid mantle. Some water moved towards the atmosphere, eventually forming the surface ocean we know today, while another fraction entered the solid mantle, potentially influencing the planet's dynamics and evolution. This internal water reservoir could have played a crucial role in the development of plate tectonics and life.
The study also raises questions about the survival of this early water reservoir. Over billions of years, mantle convection, plumes, and volcanic activity have stirred and released water. The question remains: could any of that primordial water still be hidden deep within the mantle? The research provides a starting point for further exploration, as it describes the distribution of water after magma-ocean crystallization and suggests that early stored water could have influenced mantle dynamics.
Additionally, the study's findings are supported by independent evidence. A 2026 Nature study found neodymium isotope patterns in lavas from the Comoros archipelago, indicating a contribution from bridgmanite-rich material formed during the Hadean magma-ocean solidification. This further strengthens the argument for the survival of Hadean bridgmanite-rich material, even if its hydrogen inventory has been altered over time.
In conclusion, this research offers a captivating glimpse into the Earth's early history, revealing a complex interplay between water, minerals, and the planet's evolution. It challenges our understanding of how a planet becomes habitable and highlights the dynamic nature of water's journey through Earth's formation.
