Water in the Deep Earth: Budget, Distribution, and Accretion

Water in Earth’s interior exerts a profound influence on mineral and melt rheology, phase stability, and mass transport, with far-reaching implications for mantle dynamics, core evolution, and long-term planetary habitability. Constraining the budget, distribution, and accretion history of water in the deep Earth is therefore fundamental, yet remains challenging due to the lack of direct samples from the lower mantle and core. Here, we present constraints on Earth’s deep water budget, distribution, and accretion processes by integrating ab initio calculations with experimental and numerical studies. Recent advances highlight the unique role of hydrogen in governing the physical and chemical properties of Earth’s core, enabling improved constraints on hydrogen storage based on core–mantle partitioning and elasticity studies. Building on these constraints, the amount and distribution of water retained in the early mantle can be inferred from mineral–melt partitioning data. This framework allows estimation of the water inventory acquired prior to late veneer addition and, through comparison with present-day water budgets, reconstruction of Earth’s water accretion history. Our results indicate that substantial amounts of water were incorporated into the core and the basal magma ocean, with significant consequences for core–mantle interactions, mantle convection, and the thermal and chemical evolution of the planet.