Hydrogen, Meet Magma
When astronomers look at exoplanets — worlds outside our solar system — they see a diversity of atmospheres. But when they simulate the ways the planets took shape, scientists find that many of them could have started out brimming with hydrogen. Could Earth’s formative years have been similar?
Scientists used to think that the early Earth had little hydrogen. They reached this conclusion after examining meteorites called enstatite chondrites that have a suspiciously similar chemical makeup to Earth. Because of this similarity, scientists think the two probably formed from the same material, Bryson said. These meteorites seemed to lack hydrogen, so scientists thought the same went for our planet.
But some studies, including one co-authored by Bryson, found that there was hydrogen in the meteorites all along. It was just hidden in their organic molecules, silicate glasses, and sulfur compounds. Perhaps, then, Earth was also awash in hydrogen in its early days.
Earth’s ocean of magma was full of oxygen. In a paper published in 2023, three scientists wondered what might happen if the hydrogen in a planet’s atmosphere and the oxygen in its magma were to mix — somehow. Hydrogen doesn’t just spontaneously bind to oxygen, so they aren’t the most willing chemical partners. Still, the researchers concluded that such a process would let a planet make its own water; they just weren’t sure how much.
Two years later, they were thrown a lifeline by an ambitious set of experiments built by the researchers Harrison Horn, a physicist at Lawrence Livermore National Laboratory; S.-H. Dan Shim, a geophysicist at Arizona State University; and others.
Among other things, they wanted to know how sub-Neptunes, commonplace exoplanets two to four times the diameter of Earth, can have atmospheres rich in water, as telescopic observations suggest, even when they hew close to their scorching-hot host stars. Could a reaction between a hydrogen atmosphere and a magma ocean be enough?
They suspected it could, but only if a huge amount of hydrogen put the magma under a sufficient amount of pressure. “That higher pressure is a big part of what facilitates the water production,” Horn said. “It actually enhances the chemical reactions.”
To test their model, the team wanted to re-create the extreme (and extremely dangerous) conditions present on adolescent sub-Neptunes. They needed to put hydrogen, a highly flammable gas, under intense pressure using special tools called diamond anvils, and then combine it with rock samples melted with lasers. It took them five years to develop the techniques they needed to conduct these experiments safely and effectively. “We broke a lot of diamonds,” Shim said. “It was an exciting journey.”
They had hoped the hydrogen and oxygen would react to make water. And that’s what happened, to the extreme: The reaction of high-pressure hydrogen and laser-melted rock was so efficient that it made up to 1,000 times more water than scientists predicted. (A second laboratory study, published around the same time, reported similar results.) “It doesn’t seem unreasonable [that you could] produce a huge amount of water quite quickly,” said Paul Byrne, a planetary scientist at Washington University in St. Louis. And “this is all homegrown, indigenous water”— no comets or asteroids required.
Does that mean Earth created its own oceans? This is where the waters get a little murky. “The paper doesn’t make strong claims about Earth,” Horn said. But both he and Shim think it’s a valid link to make. “It could happen,” Shim said.
Other scientists agree that some amount of water could have formed on Earth — but perhaps not nearly enough to produce its oceans. “I’d say it’s certainly possible that some water could be generated by reaction with hydrogen early on,” said Quentin Williams, an experimental geophysicist at the University of California, Santa Cruz. “How much might be generated is, however, pretty enigmatic.”
The issue is that nobody knows if there was enough hydrogen in Earth’s early atmosphere to create the pressure the reaction seems to need. Sub-Neptunes are far more massive than Earth, and their intense gravity is better at holding on to hydrogen. “Earth is right on the edge of where that kind of thing can start happening,” Horn said.
Some scientists don’t think Earth had the heft to manufacture its own water at scale. “I’m a little bit doubtful whether you can have this for an Earth-mass planet,” said Anders Johansen, an astrophysicist at the University of Copenhagen in Denmark and at Lund University in Sweden. But, Byrne said, the sub-Neptune experiments suggest that the reaction wouldn’t need to last long to create an enormous quantity of water. Earth might have been a water factory for only a moment, but that moment may have been enough to forge oceans.
If that’s the case, then the implications stretch far beyond our own solar system. Perhaps countless planets meet what may be a necessary condition for hosting life because, as Byrne said, they “are born water-rich.”
Drowning in a Sea of Possibilities
It’s possible that at least some of Earth’s water came from processes on the planet. But that’s not the end of the story: Comets are making a comeback.
By the time Rosetta met the duck-shaped comet 67P in 2014, scientists had studied the water of 11 other comets. All had D/H ratios unlike Earth’s — except one. In 2011, ESA’s Herschel Space Observatory found that the water in the comet Hartley 2 had a much more Earth-like signature.