Baby Earth may have brewed hundreds of oceans’ worth of its own water from hot rock

Water vapor appeared within minutes when University of New Mexico researchers heated iron-bearing rock to temperatures between 1,200 and 1,400 degrees Celsius in a stream of hydrogen gas. Run the same chemistry on a growing planet, they argue, and the result could be hundreds of oceans’ worth of water.

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Graduate student Susmita Garai led the work, with Zachary Sharp and Peter Olson. If it holds, the standard picture of Earth’s water arriving from space needs a rival.

Pebble rain through a hydrogen-rich atmosphere

The prevailing story says icy material from the colder outer Solar System carried water inward. Sharp, a UNM professor and director of Earth and Planetary Sciences, said previous ideas about the source of Earth’s water rely on an external source, such as comets or meteorites. The University of New Mexico release proposes something different: Earth grew by pebble accretion, the buildup of small rock fragments, while wrapped in a dense, hydrogen-rich atmosphere far hotter and higher in pressure than today’s.

Pebbles falling through that atmosphere would react with the hydrogen. The hydrogen strips oxygen from iron oxide in the silicate rock, leaving metallic iron and releasing water vapor. Universe Today adds the conditions the authors envision: settling pebbles release gravitational energy, so the atmosphere could exceed 3,000 kelvin (about 1,650 degrees Celsius) and 200 bars of pressure, and pebbles would stay above their melting point for hours or days while falling. Rocky planets also pick up a hydrogen- and helium-rich envelope from the solar nebula, so the hydrogen was already there. The missing ingredient was oxygen, and the rock supplied it.

Almost 80 percent of the iron oxide converted

The paper, “Dry pebble rain could have delivered water to Earth through hydrogen reduction of iron oxide,” appeared in Nature Geoscience on Oct. 12, per ScienceDaily, which ran its summary on Nov. 12. The team heated silicate samples in flowing hydrogen, with helium runs for comparison, for 15 minutes to three hours. In hydrogen, almost 80 percent of the iron oxide was transformed into metallic iron and water vapor formed. Helium produced no comparable reaction, which the authors read as evidence that hydrogen drives the process.

The archived dataset in the Dryad repository lists the starting materials as anhydrous mid-ocean ridge basalt, picrobasalt and synthetic basalt, melted in hydrogen streams, and adds Anthony Gargano to the author list.

Hundreds of oceans’ worth, and where the estimate stops

At peak efficiency, Universe Today reports, pebbles of one assumed composition would yield several hundred ocean masses of water, while a second composition would yield tens of ocean masses; both compositions come from earlier research. The range matters because the phrase hundreds of oceans’ worth describes the favorable end of a calculation, not a central estimate, and it depends on how much iron oxide the pebbles carried to begin with.

The oceans figure is an extrapolation. Sharp said hundreds of oceans’ worth of water were likely produced this way, but that figure scales laboratory results up to early-Earth conditions and is not a measurement of how much water the planet actually received by this route. What the experiments do show is that the reaction works at these temperatures. The authors state that how much of today’s water came from the process needs further research, along with a comparison against comets, meteorites and other deliveries. Universe Today relays a further caveat from the paper: net water gain depends strongly on how much of the produced water was exhausted to the nebula, and later atmospheric escape could have trimmed the inventory. The authors argue that comet and asteroid delivery fits poorly with constraints from the Moon-forming impact, the Late Heavy Bombardment and the abundance of certain elements in Earth’s mantle, but they do not claim comets delivered nothing.

Olson, listed in the UNM release as a University of New Mexico researcher, shares credit for the framework. Sharp also noted that pebble accretion has only been discussed for roughly the last decade, so the surrounding theory is still young. In his words, virtually any Earth-sized planet should have water by this newly identified mechanism, a claim that reaches well beyond the Solar System.

Sharp added that it is reasonable to expect other Earth-sized bodies circling other stars would also have water and possibly life, as Astrobiology.com reprinted from the UNM release. Whether that holds depends on whether those planets built themselves from pebbles inside hydrogen-rich gas the way the experiment assumes, a condition no one has yet measured around another star.

The lab design explains why the team trusts the chemistry. Helium is chemically inert, so the helium runs act as a control: the same rock, the same temperatures, and no water. The Dryad abstract adds that the experiments used anhydrous samples, meaning the water that appeared could not have been released from moisture already trapped in the rock, and that iron reduction and water production occurred in a matter of hours at most.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.