New research from the University of New Mexico suggests Earth produced much of its water during its formation through chemical reactions between rocky material and a hydrogen- rich atmosphere, rather than (or in addition to) receiving it mainly from external sources like comets or meteorites.
The study, published in Nature Geoscience (around October 2026), was led by graduate student Susmita Garai with professors Zachary Sharp (director of Earth and Planetary Sciences) and Peter Olson.
It focuses on “pebble accretion”, in which small rocky particles (“pebbles”) build up a growing planet. Once the planet becomes massive enough, it can hold a hot, high- pressure, hydrogen- rich atmosphere. As iron- bearing silicate pebbles fall through this atmosphere (“pebble rain”), iron oxide in the rocks reacts with hydrogen to produce metallic iron and water vapor.
Researchers heated silicate rock samples to 1,200- 1,400°C in streams of hydrogen (and, for comparison, helium) for 15 minutes to 3 hours, simulating early planetary conditions.
In the hydrogen experiments, nearly 80% of the iron oxide converted to metallic iron, generating water vapor. The reaction did not occur in helium, confirming hydrogen’s essential role. Extrapolation of the results indicates that hundreds of oceans’ worth of water could have formed this way.
Sharp noted that previous ideas relied on external delivery, but “virtually any Earth- sized planet should have water by this newly identified mechanism”. Water forms rapidly (in minutes under these high- temperature conditions), and the process could help explain Earth’s habitability.
This mechanism does not require all of Earth’s water to arrive as ice or water-rich materials from farther out in the solar system. It could operate on other Earth- sized rocky exoplanets that form under similar conditions with hydrogen atmospheres, making water (and potentially life) more common.
Pebble accretion itself is a relatively recent idea (last decade or so) that addresses several issues in traditional planet- formation models.
The researchers emphasize that while the experiments demonstrate a viable pathway, further work is needed to determine how much of Earth’s actual water inventory came from this process versus other sources. This adds to ongoing scientific discussion about Earth’s water origins, which has included contributions from nebular hydrogen, magma- ocean interactions, and later deliveries.
The pebbles didn’t bring water; they made it chemically while falling through Earth’s early atmosphere.
Here’s the step- by- step process according to the 2026 Nature Geoscience experiments (Garai, Sharp, Olson & Gargano):
1. The setting
As the young Earth grew by pebble accretion, it pulled in millimetre- to centimetre- sized dry silicate pebbles (rocky particles rich in iron oxide, FeO). Once the planet became massive enough, it held onto a thick, hot, hydrogen-rich (H₂) atmosphere captured from the solar nebula. Temperatures near the base of this atmosphere were high enough to melt the falling pebbles (above the silicate melting point, roughly 1,200- 1,400 °C in the experiments).
2. The chemical reaction
As the molten pebbles settled through the hydrogen atmosphere, a redox reaction occurred:
FeO (in the silicate melt) + H₂ (gas) → Fe (metallic iron) + H₂O (water vapor)
- Hydrogen stripped oxygen from the iron oxide in the rock.
- That oxygen combined with more hydrogen to form water vapor.
- Metallic iron was left behind.
Lab experiments showed this reaction is fast and efficient: in just minutes to a few hours, up to ~70–80% of the iron oxide in the samples was converted into metal, producing measurable water vapor.
3. Scaling it up
Because vast numbers of pebbles rained onto the growing Earth over millions of years, the cumulative effect was enormous. Extrapolation of the experimental results suggests this process could have generated hundreds of oceans’ worth of water.
The newly formed water vapor could later condense into oceans (or dissolve into the magma ocean and be released later), while the metallic iron sank toward the core.
Key takeaway
Earth’s oceans weren’t delivered ready- made by icy comets or asteroids in this model. Instead, dry rocky pebbles and a hydrogen atmosphere and heat produced the water in place during the planet’s assembly. This makes water formation a natural byproduct of how Earth- sized rocky planets grow.
Experimental simulation of water formation on Earth from dry pebble rain
“Experimental simulation of water formation on Earth from dry pebble rain” is a 2026 paper in Nature Geoscience by Susmita Garai, Zachary D. Sharp, Peter L. Olson, and Anthony M. Gargano (primarily University of New Mexico, with affiliation to the Lunar and Planetary Institute). DOI: 10.1038/s41561-026-02118-7.
It experimentally tests the idea that much of Earth’s water formed in situ during planetary growth via pebble accretion, rather than (or in addition to) being delivered later by water- rich asteroids or comets.
Core hypothesis
In the pebble accretion model, a growing proto-Earth accretes millimetre- to centimetre-sized silicate “pebbles.” Once the body is massive enough, it retains a thick, hot, H₂-rich primordial atmosphere captured from the solar nebula. Basal temperatures in this atmosphere exceed the silicate liquidus, so falling pebbles melt as they settle through it. Iron oxide (FeO) in the molten silicate reacts with atmospheric hydrogen:
FeO (in silicate) +H2 → Fe (metal) + H2O (vapor)
This produces water vapor while reducing iron oxide to metal. The process is rapid (minutes to hours under experimental conditions) and can generate large quantities of water endogenously.
Experimental methods and results
The team melted anhydrous (water- free) samples of mid- ocean ridge basalt (MORB), picrobasalt, and synthetic MORB in flowing hydrogen gas streams at 1,200- 1,400 °C (and control experiments in helium). Run durations ranged from 15 minutes to 3 hours.
Key outcomes:
- A substantial fraction of FeO (up to ~70- 80% under optimal conditions) was reduced to iron- rich metal, generating water vapor.
- Moderately volatile elements such as Na and S evaporated from the melt.
- The newly formed metal strongly sequestered phosphorus, stripping it from the silicate.
- No comparable reduction occurred in helium, confirming the role of hydrogen.
Extrapolation suggests this mechanism could produce hundreds of oceans’ worth of water during Earth’s formation. The metal would eventually sink to the core, helping explain Earth’s mantle Fe/Mg ratio and its relatively low phosphorus content compared with Mars.
Broader implications
- Provides a massive endogenous (internally generated) water source that does not require delivery of icy material from beyond the snow line.
- Aligns with pebble accretion solving several compositional and timing issues in terrestrial planet formation.
- Suggests Earth- sized exoplanets that form via pebble accretion in hydrogen-rich environments could routinely inherit substantial water inventories, aiding habitability.
The authors note that while the experiments demonstrate an efficient water-producing pathway, quantifying the exact contribution to Earth’s present- day water budget (surface oceans plus mantle reservoirs) requires further modeling and constraints (including isotopic considerations such as D/H ratios after later hydrogen loss). The work adds a chemically grounded alternative or complements to traditional exogenous delivery models.
Journal: Nature Geoscience, (2026)
DOI: 10.1038/s41561-026-02118-7
Provided: University of New Mexico
Published: 24 September 2026
Authors: Susmita Garai,
Zachary D. Sharp,
Peter L. Olson &
Anthony M. Gargano
Abstract
Water is vital to planet habitability. However, the sources of water for terrestrial planets and timing of delivery remain controversial. Here we show experimentally how oceans of water can be produced within the high-temperature H2-rich atmosphere above a terrestrial planet growing by capture of millimetre-to-centimetre-size silicate pebbles. We simulate this process by melting anhydrous mafic lithologies in H2 streams. Measurements show large amounts of water generated by hydrogen reduction of iron oxide to iron metal within several hours. We also find that the produced metal strongly sequesters phosphorus, stripping it from the silicate. Our experimental results imply that silicate pebbles settling through a hot H2-rich atmosphere experience volatile loss, iron reduction to iron metal and H2O vapour production. Transfer of this metal to the core offers an explanation of the low phosphorus content of Earth’s mantle relative to Mars as well as the low Fe/Mg ratio of Earth’s primitive mantle. Atmospheric reduction of iron oxide under pebble accretion represents an endogenous source of water, and Earth-sized exoplanets built from pebbles may inherit oceans of water this way, representing a step towards habitability.
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