Sulfur was the key to the first water on Earth

“These two papers reinforce each other tremendously, and I think their story is becoming really compelling,” says Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory in Nice, France, who was not part of either research team.

The four planets closest to the sun—Mercury, Venus, Earth, and Mars—all formed in the interior of the solar nebula, the disk of gas and dust that orbited the newborn sun. The inner region of the solar nebula was so dense that friction heated it enormously, drying it out. Therefore, many researchers have proposed that the Earth only got its water after icy asteroids and comets born far from the Sun hit the Earth.

However, in 2020, researchers reported a surprise: Hydrogen exists in rare meteorites known as enstatite chondrites, which resemble the building blocks of our planet (SN: 27.8.20). The discovery suggested that Earth’s building blocks possessed plenty of hydrogen from the start, discovered cosmochemist Laurette Piani of the University of Lorraine in Vandœuver-lès-Nancy, France, and colleagues.

But some scientists doubted the result. They feared that the water on today’s Earth had contaminated meteorites with hydrogen.

Last year, researchers in France reported that hydrogen in enstatite chondrites is bonded to sulfur. Now another team has found that most of the hydrogen is locked inside pyrrhotite, a bronze-colored iron sulfide mineral, Thomas Barrett of the University of Oxford and his colleagues report in a paper submitted to arXiv.org on June 19. .

“Their arguments about spectroscopic characterization of where hydrogen lives in rock are good,” UCLA cosmochemist Edward Young says of the latest work. This means that the hydrogen is native to the meteorite and not the result of terrestrial contamination.

Morbidelli agrees. “This explains why enstatite chondrites have hydrogen,” he says, calling the discoveries over the past four years a paradigm shift. “You don’t collect water. You collect hydrogen and oxygen separately in different minerals, and then they fuse together.”

This is easy to do because the early Earth was hot and molten, covered in a magma ocean. “You can think of a magma ocean as a big ball of hot oxygen,” says Young, because oxygen outnumbered all the other elements of the crust combined. Just add hydrogen from Earth’s building blocks and you have H₂O.

But Young questions whether Earth’s building blocks actually supplied most of the hydrogen in our planet’s water. He thinks the hydrogen also came directly from the solar nebula, which was mostly made up of molecular hydrogen, or H₂gas. And even more hydrogen, in the form of water, arrived when icy objects hit Earth.

“From an exobiological perspective, this study of the origin of water from enstatite chondrites is really important,” says Morbidelli. Sulfur is common – the tenth most abundant element in the cosmos – so even in solar systems lacking asteroids and icy comets, rocky planets should be able to take hydrogen and turn it into water, setting the stage for the development possible life in these worlds.