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AFP – Water covers 70 percent of the Earth’s surface and is crucial to life as we know it, but how it got here has been a long-standing scientific debate.
The puzzle came one step closer to being solved Thursday after a French team reported in the journal Science that they had identified which space rocks were responsible and suggested that our planet has been wet since it formed.
Cosmochemist Laurette Piani, who led the research, told AFP that the findings contradicted the prevailing theory that water was carried to an initially dry Earth by powerful comets or asteroids.
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Asteroids are rocky celestial objects that orbit the sun; comets are made of ice and dust, and they also orbit the sun; and meteoroids are much smaller objects, mostly fragments of asteroids and comets.
According to early models of how the Solar System came into being, the large disks of gas and dust that swirled around the Sun and eventually formed the inner planets were too hot to hold the ice.
This would explain the barren conditions on Mercury, Venus, and Mars, but not our blue planet, with its vast oceans, humid atmosphere, and well-hydrated geology.
Therefore, the scientists theorized that the water came later, and the main suspects were meteorites known as carbonaceous chondrites that are rich in hydrated minerals.
But the problem is that its chemical composition does not match the rocks on our planet.
Carbonaceous chondrites also formed in the outer Solar System, making it less likely that they could have thrown away at the early Earth.
Stars move in the night sky during the Perseid meteor shower on Lake Pineios, near the town of Velanidi, Peloponnese, Greece, on August 13, 2020 (AP Photo / Petros Giannakouris)
Planetary Building Blocks
Another group of meteorites, called enstatite chondrites, are a much closer chemical combination, containing isotopes or similar types of oxygen, titanium, and calcium.
This indicates that they were the building blocks of Earth and the other inner planets.
However, because these rocks formed close to the Sun, it was assumed that they were too dry to account for Earth’s rich water reserves.
To test whether this was really true, Piani and his colleagues at the Center de Recherches Petrographiques et Geochimiques (CRPG, CNRS / Universite de Lorraine) used a technique called mass spectrometry to measure the hydrogen content in 13 enstatite chondrites.
The rocks are now quite rare, accounting for only about two percent of known meteorites in collections, and difficult to find in pristine, uncontaminated condition.
The team found that the rocks contained enough hydrogen to provide Earth with at least three times the mass of water in its oceans, and possibly much more.
They also measured two isotopes of hydrogen, because the relative proportion of these is very different from one celestial object to another.
“We found that the hydrogen isotopic composition of enstatite chondrites is similar to that of water stored in the Earth’s mantle,” Piani said, comparing it to a DNA match.
The isotopic composition of the oceans was found to be consistent with a mixture containing 95 percent water from enstatite chondrites, further proof that enstatite chondrites were responsible for most of the water on Earth.
The authors further found that the nitrogen isotopes of enstatite chondrites are similar to those on Earth, and they proposed that these rocks could also be the source of the most abundant component of our atmosphere.
Piani added that the research does not exclude the subsequent addition of water by other sources such as comets, but does indicate that enstatite chondrites contributed significantly to Earth’s water budget at the time the planet formed.
The work “brings a crucial and elegant element to this puzzle,” wrote NASA planetary scientist Anne Peslier in an accompanying editorial.
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