Researchers at the University of New Mexico published a study showing that Earth likely generated much of its own water during planetary formation 4.5 billion years ago, as silicate pebbles reacted with a hydrogen-rich atmosphere to produce massive quantities of water vapor in minutes.
For decades, planetary science pointed outward for the origin of Earth’s oceans. The prevailing model assumed that icy comets and volatile-rich asteroids delivered the vast majority of our water through intense bombardments during the planet’s youth. That traditional asteroid-and-comet narrative has always carried heavy chronological baggage. It struggles to fit cleanly with the immense heat of the Moon-forming impact, the timeline of the Late Heavy Bombardment, and specific chemical fingerprint anomalies tucked inside the terrestrial mantle.
University of New Mexico Laboratory Experiments
Seeking an alternative to the external delivery model, a research team led by graduate student Susmita Garai and Earth and Planetary Sciences professor Zachary Sharp designed a high-temperature laboratory setup to test chemical reactions under primordial conditions. The findings were published in the journal Nature Geoscience.
In the experiments, scientists heated silicate rock samples to temperatures ranging between 1,200°C and 1,400°C while exposing them to flowing hydrogen gas streams. Control tests were run alongside these using helium gas. When exposed to heated hydrogen, nearly 80% of the iron oxide within the rocky samples was reduced to metallic iron, releasing water vapor as a direct by-product in just minutes. The helium streams produced no such reaction.
Dry pebble rain could have delivered water to Earth through hydrogen reduction of iron oxide.
Susmita Garai and Zachary Sharp, University of New Mexico
The setup replicated the physical state of the young Earth when it grew large enough to capture both an inward drift of millimeter-to-centimeter silicate pebbles and a thick, superheated atmosphere of hydrogen and helium. As those solid particles fell slowly through the high-pressure gas envelope, the intense thermal environment stripped oxygen from iron oxides in the rock, binding it with ambient hydrogen.
Scaling Up From Millimeter Pebbles to Planetary Oceans
This internal water generation operates through pebble accretion, a model of planet formation where small solid debris accumulates to build terrestrial worlds. Once a protoplanet crosses a threshold of roughly 0.3 Earth masses, its gravity holds onto a heavy primordial envelope of hydrogen and helium.
University of New Mexico: Research Suggests Earth May Have Formed Much of Its Water as the Planet Grew https://t.co/gr67x3fFaK
As rocky pebbles drifted down through that furnace-like atmosphere for hours or days, they melted and underwent extreme chemical reduction. The process yielded two essential planetary ingredients simultaneously: pure metallic iron to feed the growing core, and vast volumes of water vapor.
Zachary Sharp, professor and director of Earth and Planetary Sciences at UNM, stated that the team had suspected and hoped they would produce metallic iron and water in their reduction experiments, though they had no way of knowing the speed or quantity. He noted they were quite surprised to find that the extrapolation of their work suggests hundreds of oceans of water were probably produced in this manner.
Co-author Peter Olson joined Garai and Sharp in analyzing how this endogenous manufacturing sidesteps the strict timing hurdles of the old impact models. Rather than relying exclusively on rare collisions with distant icy wanderers, the planet brewed its own liquid base locally.
Implications For Habitable Worlds Beyond the Solar System
Because pebble accretion is a mechanism of planet building, the chemistry demonstrated in the New Mexico laboratory may apply to rocky exoplanets everywhere. Any terrestrial world that accumulates a hydrogen envelope and sweeps up silicate debris during its growth phase should experience similar iron reduction reactions.
Despite the staggering quantities of water vapor generated in the tests, researchers caution that calculating the exact net gain on Earth remains complex. Final water retention depends heavily on how much vapor escaped through exhaust venting into the nebula, as well as subsequent atmospheric stripping over time. Comets and asteroids may still have contributed a share of the planetary inventory, leaving the absolute proportions open to further investigation.
Looking out over a large body of water, it is hard to imagine that that water may have been produced 4 ½ billion years ago in a hell-like atmosphere, but it very well may have.
Zachary Sharp, University of New Mexico