The idea challenges a long-standing assumption that Earth’s water had to be imported. For decades, the dominant narrative held that the inner Solar System was too hot for water to condense during planet formation, so our planet’s oceans must have been delivered later by water-rich asteroids or comets scattered inward from beyond the orbit of Jupiter. That picture, developed in models such as those by Raymond and Izidoro in 2017, treats water as a precious cargo shipped from the cold outer reaches. The new experimental work instead points to an in situ production line, powered by nothing more exotic than rock, hydrogen gas, and heat.

The chemistry at the heart of the study is deceptively simple. Ordinary chondritic material, the kind of primitive silicate stuff thought to have built the terrestrial planets, contains a substantial fraction of iron oxide. When such oxidised rock is exposed to hydrogen gas at high temperature, hydrogen acts as a reducing agent: it strips oxygen from the iron oxide, converting ferric and ferrous iron toward metallic iron while the liberated oxygen pairs with remaining hydrogen to form water. In effect, the reaction FeO plus H2 yields Fe plus H2O. Every atom of oxygen pulled out of the rock becomes a molecule of water vapour.

What makes the new work compelling is that the researchers did not merely balance equations. They ran high-temperature experiments in streams of pure hydrogen gas, simulating the conditions that falling pebbles would have experienced as they plunged through the thick, hydrogen-dominated envelope that surrounded the growing Earth. In these experiments, the reduction of iron oxide in the silicate starting material proceeded rapidly, producing measurable water vapour on timescales of minutes to hours. Speed matters here: a planet grows fast, and any water-making mechanism must keep pace with the torrent of incoming material.

The context for this mechanism is the emerging paradigm of pebble accretion. Modern models of planet formation, reviewed extensively by Ormel and developed for the terrestrial planets by Johansen and colleagues in 2021, argue that planets like Earth grew largely by sweeping up millimetre-to-centimetre-sized pebbles drifting inward through the protoplanetary disc, rather than solely by collisions of large planetesimals. Garai, Olson and Sharp have recently argued in Geochimica et Cosmochimica Acta that building Earth from chondritic pebbles matches the planet’s major-element composition, and Olson and Sharp’s 2023 work on the hafnium–tungsten isotopic system constrains how quickly such pebble accretion must have proceeded. In this framework, the young Earth was bathed in a steady drizzle of dry rock.

Crucially, a growing embryo massive enough to hold onto gas is wrapped in a primordial atmosphere of hydrogen, captured directly from the nebula. Pebbles falling through this envelope encounter a region that is both hot, because of the gravitational energy released by accretion, and chemically reducing, because hydrogen dominates. That combination is precisely what the experiments show is needed to convert the iron oxide in the pebbles into water vapour. The water thus generated would join the atmosphere, and as the planet grew and the hydrogen envelope was eventually lost to space or incorporated into the interior, the newly made water could be retained, condensed, and delivered to the surface and mantle.

The implications scale up beyond Earth. If dry pebbles falling through hydrogen atmospheres routinely manufacture water, then the process should operate on any Earth-sized planet forming in a similar way, regardless of where it sits in its disc or how far it lies from the snow line. The authors suggest that water production on Earth-sized exoplanets should be widespread, which would help explain a growing observational puzzle: rocky exoplanets appear to be common in the habitable zones of their stars, yet the delivery mechanisms for their water have remained uncertain. A built-in production mechanism would make watery rocky worlds a natural by-product of planet formation rather than a lucky accident of volatile delivery.

The finding also reframes a debate about isotopic fingerprints. Earth’s water has a deuterium-to-hydrogen ratio broadly similar to that of carbonaceous chondrites, which has often been cited as evidence for an outer-Solar-System source. A hydrogen-based production mechanism raises new questions about how isotopic signatures are set, since the hydrogen involved would come from the nebular gas itself. Reconciling the measured isotopic composition of terrestrial water with an endogenous origin will be a key test for the pebble-rain hypothesis as it is developed further, and it is an area where cosmochemists and modellers are likely to focus intense attention.

There are also consequences for what lies deep beneath our feet. Reduction of iron oxide in accreting material does more than make water; it produces metallic iron, and the figure accompanying the research contrasts iron droplets with iron crystals, hinting at the different forms the reduced metal can take depending on the conditions. A planet built from partially reduced pebbles would carry a record of that chemistry in its core formation history and in the oxidation state of its mantle. Earth’s mantle is more oxidised than pure equilibrium with metallic iron would predict, and any model of water production via reduction must ultimately fit alongside the oxygen fugacity recorded in ancient mantle rocks.

For now, the study stands as a striking demonstration that a planet’s most essential ingredient may not need to be imported at all. The same dry, unremarkable pebbles that assembled Earth, falling through the hydrogen blanket of the early planet, could have brewed the oceans in a chemical reaction fast enough to keep up with a world under construction. If the mechanism proves robust across the full range of accretion conditions, the search for habitable worlds becomes at once broader and more hopeful: the recipe for water may be written into the standard ingredients of planet formation, waiting wherever rock, hydrogen, and heat come together.

Subject of Research: Water production on early Earth via hydrogen reduction of iron oxide in accreting pebbles

Article Title: Dry pebble rain could have delivered water to Earth through hydrogen reduction of iron oxide

Article References: Dry pebble rain could have delivered water to Earth through hydrogen reduction of iron oxide. (n.d.). https://doi.org/10.1038/s41561-026-02138-3

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02138-3

Keywords: Earth formation, water origin, pebble accretion, hydrogen atmosphere, iron oxide reduction, primordial atmosphere, chondritic material, planet formation, exoplanets, geochemistry, Nature Geoscience, early Solar System

The research team, led by Haiyang S. Wang of the University of Copenhagen together with planet-formation theorist Anders Johansen and colleagues across Europe and the United States, combined models of pebble accretion with collisional growth from volatile-depleted planetesimals. Pebble accretion is the process by which young planets sweep up millimetre-to-centimetre-sized particles of dust and ice drifting through the protoplanetary disk, a mechanism known to be extraordinarily fast. Planetesimal accretion, by contrast, is the slower, more chaotic route in which larger rocky bodies collide and merge over millions of years. Rather than pitting these two theories against each other, the team asked whether both could have contributed, and whether the resulting mixture could be read directly from the planets’ chemistry.

The key insight is that each growth channel leaves a distinct thermal signature. Pebbles falling onto a growing protoplanet must pass through a hot hydrogen and helium gas envelope that surrounds the embryo. Using a one-dimensional model of this envelope, the researchers calculated what happens to elements of different volatility during the descent. Elements whose sublimation temperatures fall below the temperature at the bottom of the envelope are vaporized, and once vaporized they are efficiently carried away. The team validated this assumption with three-dimensional hydrodynamical simulations of the convective envelope, tracking nearly a million tracer particles. Within a single year only about thirty percent of the released volatiles remained in the envelope, and after five years less than three percent survived, because large-scale recycling flows continuously exchange envelope gas with the surrounding disk.

Crucially, this thermal processing is selective. Refractory elements such as aluminium, calcium and titanium, along with the moderately refractory magnesium and silicon, are protected. When silicates sublimate deep in the envelope, they form a dense vapour atmosphere in equilibrium with the underlying magma ocean, and an inner radiative zone where silicate clouds form strongly suppresses convection, preventing these vapours from diffusing upward and escaping. The moderately volatile lithophile elements, by contrast, are lost almost completely once the growing protoplanet becomes massive enough for its envelope base to exceed their sublimation temperatures. The result is a characteristic volatility pattern: a sharp drop in abundance for elements more volatile than roughly the lithium and manganese threshold, with refractory elements essentially untouched.

The second ingredient in the hybrid recipe comes from meteorites. The asteroid Vesta, often regarded as a surviving planetary embryo, shows strong depletion of elements with sublimation temperatures below about 1,200 kelvin, a pattern likely produced when short-lived radioactive isotopes such as aluminium-26 melted early-formed planetesimals and boiled away their volatiles. The angrite meteorite group shows an even more extreme depletion, losing even silicon and magnesium, which suggests that thermal processing of the first generation of planetesimals was widespread. The team adopted Vesta’s composition as representative of this population of baked planetesimals, which then contributed their volatile-poor material to the growing planets through collisions.

With these building blocks characterized, the researchers turned to Bayesian inference, a statistical framework that weighs competing models by how well they explain the data while penalizing unnecessary complexity. For Earth, the best-fitting model describes a three-component mixture: a proto-Earth and a giant impactor, both grown by pebble accretion and therefore stripped of their moderately volatile elements, plus a contribution from Vesta-like planetesimals. The analysis indicates that at least about seventy-five percent of Earth’s mass came from the two pebble-grown bodies, with up to roughly twenty-five percent from the baked planetesimals. If the planetesimal population is allowed to have a hypothetical depletion pattern not observed among known meteorite parent bodies, its contribution could rise to around forty percent. Models lacking the pebble-grown components were disfavoured decisively, and adding volatile-rich, carbonaceous-chondrite-like material improved nothing, contributing no more than ten percent of Earth’s mass.

The impactor component is particularly intriguing because it connects naturally to the Moon-forming giant impact. The inferred mass fraction of the impactor, between eight and forty-three percent of Earth’s mass, comfortably brackets both the canonical Theia mass of about ten to fifteen percent and the larger masses required in scenarios where a giant impact disrupts an early resonant chain of planets. The team also tested scenarios with two or three impactors. A second, small impactor remains statistically plausible, but a third is disfavoured, and its mass would be negligible. Notably, the preferred number of giant impacts is lower than the average of about three seen in classical N-body simulations, which the authors attribute to pebble accretion needing fewer planetary seeds to build a terrestrial-mass world, and therefore producing fewer embryos to collide.

Mars tells a complementary story. Because isotopic evidence indicates that Mars formed and differentiated its core rapidly, within the lifetime of the protoplanetary disk, the researchers modelled it as a combination of pebbles and planetesimals accreted early. The Bayesian solution assigns twenty-seven plus or minus five percent of Mars’s mass to pebble accretion and seventy-three plus or minus five percent to Vesta-like planetesimals, a result consistent with independent silicon isotope constraints. Allowing the planetesimal composition to vary freely reveals that an even more volatile-depleted population, more extreme than Vesta, could also fit the data. In every scenario tested, planetesimals dominate Mars’s budget while pebble accretion never falls below about ten percent, reinforcing that even the smaller terrestrial planet grew by hybrid means. The contrast with Earth, where pebble accretion was dominant, suggests Mars’s pebble supply was comparatively sluggish, perhaps because gravitational stirring by larger neighbouring protoplanets perturbed its orbit.

The implications extend well beyond the two planets we know best. The hybrid-accretion framework makes testable predictions about core chemistry: because moderately volatile siderophile elements such as sulfur can hide in the iron cores of planetesimals, the model yields sulfur abundances for bulk Earth and Mars that align with recent geophysical estimates of core composition rather than with older, lower values inferred from volatility trends alone. The model also dovetails with hafnium-tungsten chronometry, since a three-component Earth with substantial planetesimal input naturally explains our planet’s low tungsten-182 excess in the mantle. And because the same thermal physics operates around other stars, the framework offers a general tool for interpreting the volatile depletion observed in polluted white dwarf atmospheres and in Sun-like stars that have ingested planetary material, evidence that devolatilization may be a universal feature of rocky planet formation.

Perhaps most profoundly, the study reframes the origin of the ingredients for life. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur would have been largely stripped away during the pebble accretion phase, even for planets growing just outside the water ice line. Their delivery to rocky worlds must therefore have come through other channels: early accretion of volatile-rich pebbles before the protoplanet grew hot enough to expel them, impacts with smaller differentiated bodies, and late arrival of primitive planetesimals that escaped thermal processing. Reading the volatility patterns of rocky planets as accretion histories in this way transforms planetary chemistry from a static inventory into a dynamic record, one that astronomers can now begin to apply to the growing catalogue of rocky exoplanets as they assess which distant worlds might have retained the volatile inventory that habitability demands.

Subject of Research: Volatile element depletion in Earth and Mars as a record of hybrid pebble and planetesimal accretion

Article Title: Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion

Article References: Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion. (n.d.). https://doi.org/10.1038/s41550-026-02984-6

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02984-6

Keywords: pebble accretion, planetesimals, volatile elements, Earth formation, Mars, protoplanetary disk, Bayesian inference, Vesta, giant impact, cosmochemistry, rocky exoplanets, planetary differentiation

The idea hinges on a growing consensus in planet-formation theory that terrestrial planets may not have grown primarily through violent collisions between large rocky bodies. Instead, many models now favour pebble accretion, a process in which a growing planetary embryo sweeps up a continuous stream of small, pebble-sized particles drifting through the gas of the protoplanetary disk. As these pebbles rain down onto the young Earth, they pass through a dense primordial atmosphere rich in hydrogen, captured directly from the nebula. That atmosphere, the researchers argue, was not a passive bystander. It was a chemical reactor.

To test this hypothesis, the team did something deceptively simple but technically demanding: they melted dry, anhydrous mafic rocks, including a mid-ocean-ridge basalt composition representing typical silicate planetary material, in streams of hydrogen gas under controlled laboratory conditions. The setup simulates what a molten pebble would experience as it plunged through the hot, hydrogen-rich envelope above a growing terrestrial planet. The temperatures were extreme, and the chemistry was deliberately reducing, meaning the hydrogen gas was poised to strip oxygen from any oxide compounds it encountered.

The results were striking. Within several hours, the experiments produced large amounts of water. The mechanism is a well-understood redox reaction with profound planetary consequences: hydrogen molecules react with iron oxide dissolved in the silicate melt, reducing ferric and ferrous iron to metallic iron while the liberated oxygen combines with hydrogen to form water vapour. In chemical terms, FeO in the silicate plus molecular hydrogen yields metallic iron plus H2O. Every unit of iron oxide reduced is a unit of water born. The measurements showed that the reduction process is efficient, converting a substantial fraction of the iron oxide in the starting material into metal on timescales of hours, which is geologically instantaneous.

This single reaction may solve several long-standing puzzles about Earth’s composition at once. The first concerns water itself. If pebbles settling through a hydrogen atmosphere were repeatedly melted and reduced, the water vapour generated would accumulate in the planet’s envelope and eventually be incorporated into the growing planet or retained after the nebular gas dissipated. The authors calculate that this process could produce oceans’ worth of water, making it a genuinely endogenous source, manufactured from the planet’s own raw ingredients rather than imported from beyond the frost line. That would sidestep the timing problems that plague delivery models, in which water must arrive late enough to survive but early enough to participate in Earth’s subsequent evolution.

The second puzzle involves phosphorus. Earth’s mantle is strikingly poor in phosphorus compared with the mantle of Mars, and geochemists have struggled to explain why two neighbouring planets, built from broadly similar solar-system materials, should differ so much in this biologically essential element. The experiments revealed that the metallic iron produced by hydrogen reduction strongly sequesters phosphorus, pulling it out of the silicate melt and into the metal. Because the dense metallic liquid would subsequently sink through the magma ocean to join the growing core, any phosphorus it carried would be efficiently removed from the mantle. Atmospheric reduction during pebble accretion therefore offers a natural mechanism for stripping phosphorus from Earth’s silicate reservoir before the core finished forming.

The third puzzle is the iron-to-magnesium ratio of Earth’s primitive mantle, which is lower than chondritic expectations. Removing iron from the silicate fraction, by reducing FeO to metal and sending that metal to the core, lowers the Fe/Mg ratio of the residual mantle in exactly the direction observed. In this picture, the distinctive chemical fingerprint of Earth’s mantle is not an accident of which building blocks happened to arrive, but the direct consequence of the atmosphere through which those building blocks fell.

The experimental evidence is documented in detail across the study’s figures, which map the conditions under which iron oxide reduction occurs in settling pebbles, track the compositions of the run products against the initial basaltic starting material, and image the quenched melt surfaces with scanning electron microscopy. The reduction efficiency, expressed as the percentage of FeO converted to metallic iron, rises with both time and temperature in the hydrogen streams, and the team quantified the water produced alongside the metal generated. The underlying experimental data and the input data for their pebble-accretion model of Earth are publicly archived in the Dryad repository, allowing other researchers to scrutinise and extend the calculations. The work was supported by the United States National Science Foundation.

The implications extend well beyond our own planet. If Earth-sized worlds can acquire oceans by growing through pebble accretion inside hydrogen-rich envelopes, then the same chemistry should operate on rocky exoplanets forming by the same route, which current models suggest may be the dominant pathway for building planets across the galaxy. Water vapour has already been detected in the atmospheres of hydrogen-rich sub-Neptune exoplanets, and atmospheric reduction of iron oxide provides a physically grounded mechanism linking such observations to surface habitability. A planet does not need to be born wet or bombarded by comets; it needs only dry silicate pebbles, a hydrogen atmosphere, and time. Habitability, in this view, is not a lucky delivery but a step in the construction sequence.

There remain questions to resolve, as with any laboratory simulation of planetary-scale processes. The experiments were conducted at specific temperatures, pressures and hydrogen flow rates, and scaling the results to the full range of conditions in a primordial planetary envelope will require further modelling and measurement. The isotopic composition of Earth’s water, particularly its deuterium-to-hydrogen ratio, will continue to constrain how much water could plausibly have come from nebular hydrogen versus other sources, since water made from nebular hydrogen carries a distinctive isotopic signature. Yet the core result stands on its own: dry rock plus hydrogen gas plus heat equals water, and the young Earth had all three in abundance. The rain of pebbles that built our planet may have carried, invisibly, the recipe for the oceans that made it habitable.

Subject of Research: Experimental simulation of water production by hydrogen reduction of iron oxide during pebble accretion on forming terrestrial planets

Article Title: Experimental simulation of water formation on Earth from dry pebble rain

Article References: Garai, S., Sharp, Z. D., Olson, P. L., & Gargano, A. M. (2026). Experimental simulation of water formation on Earth from dry pebble rain. Nature Geoscience. https://doi.org/10.1038/s41561-026-02118-7

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02118-7

Keywords: water origin, pebble accretion, hydrogen atmosphere, iron oxide reduction, Earth formation, Nature Geoscience, planetary habitability, phosphorus depletion, magma ocean, exoplanets, geochemistry, protoplanetary disk