Some of Earth's water may have been made on the spot during the planet's formation instead of being brought in from space by comets or meteorites, according to a study in Nature Geoscience by researchers at The University of New Mexico.
Susmita Garai, a graduate student, headed the project. Her collaborators were Peter Olson and Zachary Sharp, a professor who leads the university's Earth and Planetary Sciences department. The team used laboratory experiments at high temperature to test how rocks containing iron behave when exposed to hydrogen, mimicking conditions of early planetary growth, the university said.
The proposed mechanism
The idea builds on pebble accretion, the process in which small rocky particles collect into a bigger body. Once the young Earth had enough mass, it could have held a hot, dense atmosphere rich in hydrogen. Silicate pebbles dropping through that gas would have had their iron oxide react with hydrogen, leaving metallic iron and releasing water vapor.
Laboratory results
The clearest signal came from samples heated in hydrogen: close to 80% of the iron oxide they contained was reduced to metallic iron, and water vapor was given off. When the team swapped hydrogen for helium as a control, that reaction failed to appear, which the researchers say confirms hydrogen's central role. According to the university, the silicate samples were held at 1,200 to 1,400 degrees Celsius in a flow of gas, with individual tests lasting anywhere from 15 minutes up to three hours.
Sharp said Garai found that water appears "in just minutes" under such hot conditions. The team had hoped to see iron metal and water emerge, he said, but could not predict the speed or the amount. Extrapolating the work to the infant planet, he said, suggests "hundreds of oceans worth of water were likely produced in this way."
Rival explanation and remaining uncertainty
Researchers have long debated where Earth's water came from. A leading view points to icy pebbles and other water-rich material that formed in colder parts of the early solar system and moved inward. According to the university, it is still unclear how so much water could have travelled such distances, and whether that route fits Earth's chemical makeup.
The experiments demonstrate a plausible pathway but do not show how much of today's water it supplied. Further research is required to judge its contribution.
Wider implications
If other rocky planets also develop hot, hydrogen-rich atmospheres while forming, the same chemistry could yield water on Earth-sized planets around other stars. Sharp said it is reasonable to expect such bodies to have water and possibly life. The paper is titled "Dry pebble rain could have delivered water to Earth through hydrogen reduction of iron oxide" (DOI 10.1038/s41561-026-02138-3).
