WASHINGTON, UNITED STATES -- Water covers 70 per cent of the Earth's surface and is crucial to life as we know it, but how it got here has been a longstanding scientific debate.
The puzzle was a step closer to resolution Thursday after a team of French scientists reported in the they had identified which space rocks may have been responsible.
Cosmochemist Laurette Piani, who led the research, told AFP that, contrary to prevailing theories, Earth's water may have already been contained in its building blocks.
According to early models for how the Solar System formed, the large disks of gas and dust that swirled around the Sun and eventually formed the inner planets were too hot to form ice.
This would explain the barren conditions on Mercury, Venus and Mars -- but not our blue planet, with its vast oceans, a humid atmosphere and well-hydrated geology.
The most common idea is that water was brought later by extra-terrestrial objects, and the prime suspect was water-rich meteorites known as carbonaceous chondrites.
But the problem was that their chemical composition doesn't closely match our planet's rocks.
They also formed in the outer Solar System, making it less likely they could have pelted the early Earth.
Another type of meteorite, called enstatite chondrites (ECs), is a much closer chemical match, indicating these were Earth's and the other inner planets' building blocks.
However, because these rocks formed close to the Sun, they had been assumed to be too dry to account for Earth's rich reservoirs of water.
To test whether this was really true, Piani and her colleagues at the Universite de Lorraine used a technique called mass spectrometry to measure the hydrogen content in 13 enstatite chondrites.
They found that the rocks contained enough hydrogen in them to provide Earth with at least the three times the water mass of its oceans.
They also measured the two types of hydrogen, known as isotopes, because the relative proportion of these is very different from one Solar System object to another.
"We found the hydrogen isotopic composition of enstatite chondrites to be similar to the one of the water stored in the terrestrial mantle," said Piani, comparing it to a DNA match.
She added that research doesn't exclude later addition of water by other sources like comets, but indicates that enstatite chondrites contributed significantly to 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.
