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.
Far below the crust and mantle, at depths unreachable by drilling technology, Earth’s core remains one of the least accessible regions of our planet. Yet new scientific findings suggest that this remote and extreme environment may hold an extraordinary secret: a vast store of hydrogen potentially equivalent to several times the volume contained in all of Earth’s oceans. Researchers recently proposed that the core could harbor the equivalent of at least nine global oceans’ worth of hydrogen, and possibly as many as 45. If confirmed, this would make the core the largest hydrogen reservoir on Earth and significantly reshape prevailing theories about the planet’s early development and the origin 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 hold far-reaching consequences for interpreting when and by what processes Earth obtained its water, and they touch on a long-running debate over whether most of the planet’s water was delivered after its formation by impacts from comets and water-rich asteroids or whether hydrogen had already been built into Earth’s initial materials. The new research favors this second scenario, indicating that hydrogen existed as the planet was taking shape and became incorporated into the core during its earliest developmental stages.
Reevaluating how Earth’s water first came into existence
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 most of Earth’s hydrogen was present from the beginning, it suggests that water and volatile elements were not merely late additions delivered by cosmic impacts. Instead, they may have been fundamental components of the materials that assembled into the planet. Under this scenario, the core would have sequestered a large portion of the available hydrogen within the first million years of Earth’s history, long before the surface oceans stabilized.
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 approaches often relied on X-ray diffraction to analyze changes in the crystal structure of iron when hydrogen is incorporated. When hydrogen enters iron’s atomic lattice, it causes measurable expansion. However, interpreting these changes has led to widely varying estimates, ranging from trace amounts to extremely high concentrations equivalent to more than 100 ocean volumes. The uncertainty stemmed from the limitations of the techniques and the difficulty of replicating true 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 the samples cooled, scientists turned to atom probe tomography, a technique capable of producing near-atomic-resolution three-dimensional images and detailed chemical profiles. The materials were crafted into extremely fine, needle-shaped specimens measuring only a few dozen nanometers across. Through the use of precisely regulated voltage pulses, individual atoms were ionized and captured sequentially, allowing researchers to directly quantify hydrogen and map its distribution alongside elements like 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 combining this atomic-scale data with independent geophysical estimates of how much silicon resides in the core, the researchers calculated a new range for hydrogen content. Their results suggest that hydrogen accounts for between 0.36% and 0.7% of the core’s mass, translating into multiple ocean equivalents when expressed in familiar terms.
Consequences for the magnetic field and the potential for 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.
The interplay between hydrogen, silicon and oxygen in the core could affect how heat was transferred from the core to the mantle in the planet’s early history. The distribution of light elements influences density gradients, phase transitions and the dynamics of core convection. If hydrogen played a significant role in these processes, it may have contributed to establishing the long-lived magnetic field that made Earth more hospitable to life.
Understanding the distribution of volatile elements such as hydrogen also informs broader models of planetary formation. Hydrogen, along with carbon, nitrogen, oxygen, sulfur and phosphorus, belongs to a group of elements considered essential for life. Their behavior during planetary accretion determines whether a world develops surface water, an atmosphere and the chemical ingredients necessary 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 can also offer indirect boundaries, as seismic wave analyses that uncover the core’s density and elastic behavior make it possible to assess whether suggested hydrogen levels align with recorded data, and combining laboratory findings with seismic modeling will be essential for forming a fuller understanding of the core’s overall makeup.
A deeper perspective on Earth’s formation
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.
The idea that the core contains the majority of Earth’s hydrogen also reframes how scientists think about the distribution of water within the planet. While oceans dominate the surface visually and biologically, they may represent only a small fraction of Earth’s total hydrogen budget. The mantle likely holds more, and the core could contain the largest share of all.
Earth’s profound interior is portrayed not as a fixed base lying under the crust but as a dynamic force shaping the planet’s chemical and thermal development, with the events set in motion during Earth’s earliest million years still molding its internal architecture, its magnetic field and its ability to sustain 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.
