for National Geographic News
A barrage of comets may have delivered Earth's oceans around 3.85 billion years ago, a new study suggests.
Scientists have long suspected that Earth and its near neighbors were walloped by tens of thousands of impactors during an ancient event known as the Late Heavy Bombardment.
This pummeling disfigured the moon, leaving behind massive craters that are still visible, preserved for millennia in the moon's airless environment. But it's been unclear whether the impactors were icy comets or rocky asteroids.
Now, based on levels of a certain metal in ancient Earth rocks, a team led by Uffe Jorgensen of the Niels Bohr Institute in Denmark says comets were the culprits.
Whether Earth had oceans before any comets arrived has been intensely debated, Jorgensen noted.
Some experts say enough water could have existed from the moment Earth formed, while others argue that the young planet's heat would have vaporized any liquids.
"It's the kind of subject that can make scientists fight physically with one another," Jorgensen said.
His team thinks early Earth was just too hot to retain large bodies of water. But by the time of the Late Heavy Bombardment, things had cooled down, allowing meltwater from the flurry of comets to become the world's first seas.
"We may sip a piece of the impactors every time we drink a glass of water," the study authors write in their paper, which will be published in an upcoming issue of the journal Icarus.
Jorgensen and colleagues arrived at this conclusion after measuring the levels of iridium in surface and near-surface rocks from Greenland—some of the oldest known rocks in the world, dating back to the time of the bombardment.
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