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Asteroid sample prompts reexamination of where Earth got its water

Scientists must sometimes, as William Blake put it, “see the world in a grain of sand.”

Now, researchers analyzing a sample returned 13 years ago from the peanut-shaped asteroid Itokawa have “seen” a possible source of Earth’s water in a crystal of salt.

“The discovery of sodium chloride in a sample from an S-type asteroid means that this asteroid could contain abundant water during its formation and evolutionary history,” said lead author Shaofan Che, a postdoc at University of Arizona's Lunar and Planetary Laboratory (LPL).

S-type asteroids are a primary source of ordinary chondrites — silica-rich space rocks that a make up more than four-fifths of meteorites that fall to Earth. They are generally thought to be made up of water-free minerals.

But analysis published in the journal Nature Astronomy shows some samples gathered by Japan’s first Hayabusa mission contain tiny salt crystals that could only have formed in the presence of liquid water.

“And if that's the case, I think it adds another dimension to the possibility that these types of asteroids could have also been sources of water that was delivered to the Earth,” said senior author Thomas Zega, professor of planetary sciences at LPL.

Prior to the discovery, experts believed water reached ancient Earth aboard comets or on a different class of asteroid called a C-type. C-types are packed with carbon-filled compounds like organic molecules, hydrated minerals and clays. They are thought to have originated in the frigid outer solar system and therefore to have more readily held onto volatile substances like water.

Che says the discovery shows that new analyses can still turn up surprises in old samples.

“This also motivates the planetary science community to search for more evidence,” he said. “Like, for example, other minerals that could provide evidence for presence of water in other types of asteroids.”

Sometimes there’s just no substitute for slipping the surly bonds of Earth and sampling a space rock.

If scientists relied solely on telescopes, they might still think asteroid Bennu is a rubble-pile instead of an electrostatic dust-bunny.

And if they were limited to space rocks falling from the sky, experts might never have learned that seemingly “dry” silicate asteroids might have delivered water to an infant Earth.

“The discovery, I think, prompts some renewed thinking about how these other types of meteorites may have played a role, perhaps even a significant role, in delivering Earth’s water,” said Zega.

Nicholas Gerbis was a senior field correspondent for KJZZ from 2016 to 2024.