Earth’s core may contain vast hidden reserves of hydrogen, reshaping theories about planet’s water origins. Beneath our feet lies a hidden reservoir that could dwarf all of Earth’s oceans. The discovery could transform our understanding of how Earth formed and where its water came from.
Deep beneath the crust and mantle, at depths far beyond the reach of any drilling technology, Earth’s core stands as one of the planet’s most inaccessible realms; however, emerging research indicates that this hidden, extreme environment might conceal a remarkable secret: an immense reserve of hydrogen that could surpass the total volume of all the water in Earth’s oceans several times over. Scientists have recently suggested that the core may contain at least the equivalent of nine global oceans of hydrogen, with estimates potentially rising to as many as 45, a finding that, if validated, would position the core as Earth’s largest hydrogen reservoir and profoundly alter current ideas about the planet’s early evolution and the origins of its water.
Hydrogen, the lightest and most abundant element in the universe, stands as a fundamental component in the chemistry of life and the evolution of planets. On Earth’s surface, it is most commonly encountered combined with oxygen in water. Yet, recent assessments suggest that large reserves of hydrogen could be sequestered deep within the metallic core, representing about 0.36% to 0.7% of its total mass. While that share might seem small, the core’s extraordinary scale and density ensure that even a tiny proportion corresponds to a vast amount of hydrogen.
These findings carry significant implications for understanding when and how Earth acquired its water. A long-standing scientific debate centers on whether most of the planet’s water arrived after its formation through impacts from comets and water-rich asteroids, or whether hydrogen was already incorporated into Earth’s building materials during its earliest stages. The new research lends support to the latter possibility, suggesting that hydrogen was present as the planet formed and became integrated into the core during its earliest phases.
Rethinking the origins of Earth’s water
Over 4.6 billion years ago, the early solar system existed as a chaotic realm of swirling gas, dust and rocky fragments encircling a youthful sun, and over time these elements collided repeatedly and slowly merged, giving rise to increasingly larger bodies that ultimately became the terrestrial planets, including Earth. As this process unfolded, the planet underwent differentiation, with its dense metallic core descending to the interior while lighter substances spread outward to create the mantle and the crust above.
For hydrogen to be present in the core today, it must have been available during this critical window of planetary growth. As molten metal separated from silicate material and descended inward, hydrogen would have needed to dissolve into the liquid iron alloy that became the core. This process could only occur if hydrogen was already incorporated into the planet’s building blocks or delivered early enough to participate in core formation.
If the majority of Earth’s hydrogen existed from the outset, it indicates that water and volatile elements were likely not just late arrivals brought by cosmic collisions. Rather, they may have formed essential ingredients of the primordial materials that came together to build the planet. In this view, the core would have drawn in a substantial share of the hydrogen within the first million years of Earth’s evolution, well before stable surface oceans emerged.
This interpretation challenges models that rely heavily on cometary bombardment as the primary source of Earth’s water. While impacts from icy bodies likely contributed some water and volatile elements, the new estimates imply that a substantial fraction of hydrogen was already embedded within the planet’s interior during its earliest stages.
Exploring a frontier long beyond reach
Studying the makeup of Earth’s core poses immense difficulties, as it starts about 3,000 kilometers below the surface and reaches the planet’s center, a realm where sun‑like temperatures and pressures millions of times greater than those at the surface prevail. Because direct sampling remains beyond today’s technological capabilities, scientists must depend on indirect investigative techniques and controlled laboratory experiments.
Hydrogen poses a particularly difficult measurement problem. Because it is the smallest and lightest element, it can easily escape from materials during experiments. Its tiny atomic size also makes it challenging to detect with conventional analytical tools. For decades, researchers attempted to infer the presence of hydrogen in the core by examining the density of iron under high pressures. The core’s density is slightly lower than that of pure iron and nickel, indicating that lighter elements must be present. Silicon and oxygen have long been considered leading candidates, but hydrogen has also been suspected.
Previous experimental strategies frequently depended on X-ray diffraction to examine how iron’s crystal lattice responds when hydrogen becomes embedded within it. As hydrogen diffuses into the atomic framework, the lattice expands in detectable ways. Yet the interpretation of these shifts has produced highly inconsistent estimates, spanning from minimal traces to exceptionally large quantities comparable to more than 100 ocean volumes. These discrepancies arose from methodological constraints and the inherent challenges of accurately reproducing genuine core conditions.
An innovative approach crafted at the atomic scale
Researchers refined these estimates by employing a technique that allows materials to be examined at the atomic scale; in controlled laboratory settings, they reproduced the immense pressures and temperatures thought to prevail in Earth’s deep interior, using a diamond anvil cell to squeeze iron samples to staggering pressures and then heating them with lasers until they liquefied, effectively simulating the molten metal of the planet’s early core.
After cooling the samples, scientists employed atom probe tomography, a method that allows for three-dimensional imaging and chemical analysis at near-atomic resolution. The samples were shaped into ultrafine needle-like structures, only tens of nanometers in diameter. By applying controlled voltage pulses, individual atoms were ionized and detected one by one, enabling researchers to directly measure the presence and distribution of hydrogen alongside other elements such as silicon and oxygen.
This method stands apart from previous techniques by directly tallying atoms instead of deducing hydrogen levels from structural variations. The experiments showed that hydrogen closely associates with both silicon and oxygen inside iron when subjected to high pressure, and the measured hydrogen-to-silicon ratio in the samples was found to be roughly one to one.
By integrating this atomic-scale data with separate geophysical assessments of how much silicon is present in the core, the researchers derived a revised interval for hydrogen abundance, and their findings indicate that hydrogen comprises roughly 0.36% to 0.7% of the core’s mass, an amount that equates to several ocean volumes when described in more familiar terms.
Implications for the magnetic field and planetary habitability
The presence of hydrogen in the core does more than reshape theories of water delivery. It may also influence how scientists understand the evolution of Earth’s magnetic field. The core’s outer layer consists of molten metal that convects as heat escapes from the interior. This movement generates the geomagnetic field, which shields the planet from harmful solar and cosmic radiation.
Interactions among hydrogen, silicon, and oxygen within the core may have shaped how heat moved from the core to the mantle during the planet’s early evolution, and the way these lighter elements are arranged can alter density layers, phase changes, and the behavior of core convection. Should hydrogen have exerted a notable influence on these mechanisms, it might have helped lay the groundwork for the enduring magnetic field that made Earth a more life-friendly world.
Understanding how volatile elements like hydrogen are distributed also shapes wider models of planetary formation, and hydrogen — together with carbon, nitrogen, oxygen, sulfur, and phosphorus — is classified among the elements vital for life. The way these elements behave during planetary accretion dictates whether a planet acquires surface water, an atmosphere, and the chemical building blocks required for biology.
Assessing unknowns and exploring potential paths ahead
Despite the sophistication of the new experimental methods, uncertainties remain. Laboratory simulations can approximate but not perfectly replicate the conditions of Earth’s deep interior. Additionally, some hydrogen may escape from samples during decompression, potentially leading to underestimates. Other chemical interactions within the core, not fully captured in the experiments, could also alter hydrogen concentrations.
Some researchers point out that independent analyses have yielded hydrogen estimates in a comparable range, sometimes trending higher. Variations in experimental frameworks, assumptions regarding core makeup, and approaches to accounting for hydrogen loss can produce shifts in the resulting calculations. As analytical methods progress, upcoming studies may sharpen these estimates and further reduce existing uncertainties.
Geophysical observations may also provide indirect constraints. Seismic wave measurements, which reveal density and elastic properties of the core, can help test whether proposed hydrogen concentrations are consistent with observed data. Integrating laboratory results with seismic models will be crucial for building a comprehensive picture of the core’s composition.
An expanded view of Earth’s origins
If the proposed hydrogen levels are accurate, they reinforce the view that Earth’s volatile inventory was established early and distributed throughout its interior. Rather than being a late veneer delivered solely by icy impactors, hydrogen may have been present in the primordial materials that assembled into the planet. Gas from the solar nebula, along with contributions from asteroids and comets, likely played roles of varying importance.
Scientists now reconsider how water is distributed inside the planet, as the notion that the core holds most of Earth’s hydrogen reshapes this understanding. Although oceans visually and biologically dominate the surface, they might account for only a minor portion of Earth’s overall hydrogen reserves. The mantle is thought to store more, and the core may contain the greatest amount of all.
This perspective emphasizes that Earth’s deep interior is not merely a static foundation beneath the crust but an active participant in the planet’s chemical and thermal evolution. The processes that unfolded during the first million years of Earth’s existence continue to influence its structure, magnetic field and capacity to support life.
As research progresses, the emerging picture is one of a planet whose defining characteristics were shaped from the inside out. By peering into the atomic architecture of iron under extreme conditions, scientists are gradually revealing how the smallest element in the periodic table may have played an outsized role in shaping Earth’s destiny.
