Water From a Rain of Dry Pebbles: Hydrogen May Have Made Earth’s Oceans

One of the oldest puzzles in planetary science may have just taken a surprising turn. A new study published in Nature Geoscience suggests that the water sloshing in Earth’s oceans, locked in its mantle, and cycling through its atmosphere could have been manufactured on the spot, from materials that arrived at the young planet bone dry. According to the research, a relentless rain of silicate pebbles falling through a hot, hydrogen-rich primordial atmosphere could have generated water vapour in abundance, through a chemical reaction that runs to completion in minutes to hours at the right temperatures.

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