Water occupies 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 scientific debate.
The puzzle was solved a step closer Thursday after a French team reported in the magazine Science they had identified which space rocks were responsible, and suggested that our planet had been wet since they formed.
Cosmochemist Laurette Piani, who led the study, told AFP that the findings contradicted the prevailing theory that water was brought to Earth by a distant comet as asteroids.
According to early models for how the solar system came to be, the large disks of gas and dust that revolved around the sun and eventually formed the inner planets were too hot to sustain 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.
Scientists therefore theorized that the water came after, and the main suspects were meteorites known as carbonaceous chondrites rich in aqueous minerals.
But the problem was that their chemical composition did not closely match the rocks of our planet.
The carbonaceous chondrites were also formed in the outer solar system, making it less likely that they could have peeled the early Earth.
Planetary building blocks
Another group of meteorites, called enstatite chondrites, are a much closer chemical similarity, with similar isotopes (species) oxygen, titanium and calcium.
This indicates that they were the building blocks of the earth and the other inner planets.
However, because these rocks formed close to the sun, they were thought to be too dry to account for the Earth’s rich reservoirs.
To test whether this was true, Piani and her 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, making up only about two percent of known meteorites in collections, and it is difficult to find them in the impure, unpolluted state.
The team found that the rocks contained enough hydrogen to supply the earth at least three times the water mass of its oceans – and possibly much more.
They also measure two isotopes of hydrogen, because their relative proportion is very different from one celestial object to another.
“We found the hydrogen isotopic composition of enstatite chondrites comparable to that of the water stored in the 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 the enstatite chondrites – more evidence that these were responsible for most of the earth’s water.
The authors further found that the nitrogen isotopes of the enstatite chondrites are similar to Earth – and suggested that these rocks may also be the source of the most abundant component of our atmosphere.
Piani added that research does not rule out subsequent addition of water by other sources such as comets, but indicates that enstatite chondrites contributed significantly to the Earth’s water budget at the time it formed.
The work “brings a crucial and elegant element to this puzzle,” wrote Anne Peslier, a planetary scientist for NASA, in an accompanying editorial.
© Agence France-Presse
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