© Nicolle Rager-Fuller, NSFA comet slams into what is now Chesapeake Bay in an artist's conception.
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.

Comets' Metal

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.

Iridium is a scarce metal on Earth, but it's relatively common in comets and asteroids.

According to the team's calculations, iridium levels in the rocks around an asteroid impact should be about 18,000 parts per trillion.

A comet impact, meanwhile, should leave behind only about 130 parts per trillion. That's because comets would carry less metal, since they're mostly made of loosely packed water ice with some rocky debris.

Comets also strike Earth at higher speeds, because of their longer orbits around the sun.

As a result, "the explosion formed by a comet is more violent than from an asteroid, and the amount of material - including iridium - thrown back into space is larger," Jorgensen said.

The team found that the Greenland rocks contained about 150 parts per trillion of iridium, supporting the idea that comets were the main players in the Late Heavy Bombardment.

All that ice from the comet swarm then thawed to create a global ocean more than half a mile (about a kilometer) deep, the team calculates.

The moon, meanwhile, lacks an ocean because its gravity is much weaker than Earth's, so most if not all of the debris from a comet strike would be thrown back into space, Jorgensen said.

But Nicolas Dauphas, a geophysicist at the University of Chicago, isn't yet convinced that the bombardment featured comets, not asteroids.

The new study, he said, relies on too many estimates - such as the predicted amount of iridium deposited following an impact.

"I am afraid [they have] stretched their conclusions too far," Dauphas said.

Accidental Life?

Chandra Wickramasinghe, an astrobiologist at Cardiff University in the U.K. not involved in the new study, also supports the theory of an ancient comet bombardment.

And he thinks it's possible that comets seeded Earth not only with water but with life.

According to some controversial studies, the oldest evidence for life on Earth dates back to about 3.85 billion years ago, around the time of the Late Heavy Bombardment, he noted.

"It could be a coincidence, but to me it would be a remarkable coincidence," Wickramasinghe said.

Study co-author Jorgensen is inclined to agree.

"The [Late Heavy Bombardment] was an accident," he said. "If it had not happened, there would have been no water on Earth, and no life."