Earth’s water must have arrived here earlier than we thought

The arrival of water on our planet is shrouded in mystery. Our leading theory says icy meteorites brought it here after most of the planet and its core had formed, about 4.5 billion years ago. But now an analysis of isotopes from meteorites seems to imply that the wet stuff got here much sooner.

Mario Fisher-Gödde and Thorsten Kleine at the University of Münster, Germany, looked the Tagish Lake meteorites that fell in British Columbia, Canada, in January 2000. They compared the abundance of ruthenium isotopes in these meteorites with the abundance in Earth’s mantle.

“Meteorites impacted Earth during its formation and they can leave signatures,” says Katherine Bermingham at the University of Maryland. If this kind of icy meteorite brought water to Earth late in the planet’s formation, then the isotopes inside them should match the isotopes in Earth’s mantle.

“Ruthenium isotopes are stable. That means they can act as fingerprints,” says Bermingham.

But the team found that the ruthenium isotopes in the meteorites did not match those found in Earth’s mantle (Nature, DOI: 10.1038/nature21045). “We can exclude a late water delivery,” Fisher Gödde says.

“We can now rule out water arriving on icy comets after the planet was mostly formed”

“The ruthenium data suggest comets could not have played a large part in the late addition of material to Earth,” says Lydia Hallis at the University Of Glasgow, UK. Hallis previously used hydrogen isotope ratios in volcanic basalt rocks to conclude that Earth’s water may in fact have been part of the very dust cloud from which the planet first condensed.


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