Universe Today
The origin of Earth's water is one of those foundational issues that explains our planet's habitability. The idea that water was delivered by comets and asteroids has been on the minds of planetary scientists for a long time. Both types of bodies, and especially comets, are known to have water, and both types are known to have crashed into Earth in large numbers during the planet's early ages.
But there's also a growing understanding that rocky planets form with a primordial envelope rich in hydrogen and helium that they acquire from their solar nebulae. Since water is—obviously—H2O, then Earth was born with substantial amounts of hydrogen, and all that's required was oxygen. Did that need to be delivered to Earth somehow?
Sort of, but not exactly, and new research shows why. It's titled "Dry pebble rain could have delivered water to Earth through hydrogen reduction of iron oxide," and it's published in Nature Geoscience. The lead authors are Susmita Garai, a graduate student in Earth and Planetary Sciences at the University of New Mexico, and Zachary Sharp, professor and director of Earth and Planetary Sciences at UNM.
"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.
"Water is vital to planet habitability. However, the sources of water for terrestrial planets and timing of delivery remain controversial," the authors write. There are timeline constraints for how this water could have been delivered by comets and asteriods. It has to take into account the giant impact that created the Moon, the Late Heavy Bombardment, and the abundance of certain elements in the Earth's mantle. It's difficult to fit delivery by comets and asteroids in with these constraints.
This illustration shows a young planet in its magma ocean phase. Terrestrial planets like Earth were entirely molten when they formed. It's possible that water was delivered to young planets by comets and asteroids during the magma ocean phase. Image Credit: Merikanto via Wikimedia Commons, CC BY-SA 4.0 https://commons.wikimedia.org/wiki/File:Volcanic_planet_4_r_1.png
If Earth's water didn't come from an external source and was generated locally, then it sidesteps these constraints. "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," the authors write.
“Previous ideas about the source of water to Earth rely on an external source, such as comets or meteorites. Here we show that virtually any Earth-sized planet should have water by this newly identified mechanism,” co-author Sharp explained in a press release. “In order to evaluate the possibility of ‘pebble’ rain’ being a source of water, we needed to construct and execute some very challenging high temperature experiments. Dr. Garai’s efforts paid off mightily, as she found that water is produced in just minutes in the high temperature environment of the Early Earth’s atmosphere.”
When a protoplanet's mass exceeds about 0.3 Earth masses, chondrules and pebbles fall through the atmosphere slowly. For hours or even days, these pebbles are above their melting point and experience extensive FeO reduction. This removes the oxygen and releases it into the atmosphere to form water, leaving metallic iron. For this to happen, there needs to be a powerful reducing agent at high temperatures, and in this case, it's the heated hydrogen in the atmosphere.
"The gravitational potential energy released by settling pebbles directly heats the atmosphere, resulting in a steep atmospheric thermal gradient, and for large planets, very high temperatures at the surface." These temperatures can exceed 3,000 K (1650 C) and the pressure can exceed 200 bars (~197 sea-level pressure). A magma ocean also heats everything up from below.
"Together, these processes result in molten pebble rain onto the protoplanet surface, volatile loss from pebbles in the atmosphere and ingassing of hydrogen, helium and other volatiles into the magma ocean," the researchers write.
This paints a dramatic picture unlike anything a human being has ever witnessed. At the heart of it is a very young Earth in its magma ocean phase. It's a ball of molten rock, to which an atmosphere nothing like the modern atmosphere clings. Falling through that atmosphere is a rain of molten pebbles, material that's coalesced out of the solar nebula, drawn by Earth's increasing mass to accrete onto the planet's hellish surface.
This figures show how molten pebbles rained onto Earth's surface, falling through the atmosphere and off-gassing volatiles, hydrogen and helium among them. Some of the hydrogen and helium escapes, and some in-gasses into the magma ocean. Image Credit: Garai et al. 2026. NatGeo
The researchers tested this scenario by running some high-temperature laboratory experiments that came as close to the hellish conditions in Earth's Hadean eon as possible. "To quantify the amounts and the kinetics of iron reduction in pebbles settling in through a nebular atmosphere, we conducted a series of experiments in which mafic and ultramafic silicates were melted in He and H2 gas streams," the authors explain. Mafic and ultramafic are the two main categories of igneous rocks, both rich in iron and magnesium.
This image illustrates some of the lab work in the study. It shows an experimental apparatus heating silicate rock samples. Garai et al. 2026. NatGeo
The researchers heated silicate rock samples to between 1,200 and 1,400 degrees Celsius in streams of hydrogen, and separately in streams of helium. The experiments lasted between 15 minutes and three hours. The conditions mimicked the time in Earth's growth where it was massive enough to attract a rain of pebbles and was also massive enough to have a hydrogen-rich atmosphere, super-heated and highly pressurized. The moving streams of hydrogen and helium mimicked the pebbles movement through that atmosphere.
The results showed that in the hydrogen, almost 80% of the iron oxide was converted to metallic iron, producing water vapour as a result. In the helium, the same reaction didn't occur.
The efficiency of the reduction depends on the metal fraction in the pebbles, which the researchers calculated. "Partial reduction of pebble FeO at these efficiencies yields prodigious amounts of water," they write. "At peak efficiency, several hundred ocean masses of water are produced from the A composition, and tens of ocean masses from the B composition." The A and B compositions come from previous research.
These panels show how many oceans worth of water is produced by pebbles with with different compositions and different iron concentrations. Image Credit: Garai et al. 2026. NatGeo
"We had suspected and hoped that we would produce iron metal and water in our reduction experiments, but we really had no way of knowing how fast and how much. We were quite surprised to find that extrapolation of our work suggests that hundreds of oceans worth of water were likely produced in this way. 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,” said Sharp.
The idea that Earth's water came from comets and asteroids was always a little shaky, though still compelling. This research shows a different way that Earth could've acquired its life-giving water. The results may also extend to other worlds around other stars, and even to our neighbour Mars.
“The idea of pebble accretion is still relatively new. It’s only been around for the last decade. But it seems to solve many problems that exist with the existing paradigm for planet formation,” Sharp said. “And now we see that it also can explain why our planet is habitable. By extension, it is reasonable to expect that other Earth-sized bodies circling other stars would also have water and possibly life.”
This work is early proof that the basic mechanism is possible. But as for how much of Earth's water actually stemmed from this, that needs more research. "The net water gain depends strongly on the amount of produced water extracted by exhaust to the nebula," the researchers write. "Although subsequent water loss by atmospheric escape is possible, water produced during pebble rain represents an early and fundamental step in the evolution of a habitable planet."
It's still possible, for instance, that other processes also played a role, and even that comets and asteroids did deliver some of our planet's water.
The source of Earth's water will remain an ongoing subject of research. Over time, the picture will become clearer.
