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.
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."
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