National Geographic News
Comet Hale-Bopp shines over southern Paris on March 10, 1997.

Comet Hale-Bopp shines over southern Paris on March 10, 1997.

Photograph by Christian Tatot, AP

Ker Than

National Geographic News

Published December 11, 2009

The gases that make up Earth's atmosphere came from a swarm of comets, not from bubbling volcanoes as long thought, a new study says.

The new theory came about after scientists discovered that pristine samples of the elements krypton and xenon, recently collected from deep within the Earth, have the same chemical makeup as ancient meteorites.

The discovery has squelched the volcano theory, said project leader Chris Ballentine of the U.K.'s University of Manchester.

Most of the gases in the air we breathe originated in the solar nebula, the cloud of gas and dust that formed the sun and planets, the study says.

The gases became gravitationally bound to a young Earth and were then transported in the Earth's interior—leaking out over the eons through volcanic belches and cracks in the Earth's crust.

It's still true that volcanoes spewed out some gases, "but [that] contribution was insignificant" for the creation of Earth's atmosphere," Ballentine said.

Comet Bombardment

Ballentine and colleagues studied krypton and xenon because they're noble gases, so called because they don't mix chemically with most other elements.

As a result, most of Earth's krypton has remained unchanged since its arrival on our planet—allowing scientists to precisely study the conditions of early Earth.

Based on their research, Ballentine and colleagues claim that our atmosphere likely formed when gas and water-rich comets bombarded Earth, shortly after its formation 4.54 billion years ago.

This may or may not have coincided with a period of heavy cometary pounding in the inner solar system known as the Late Heavy Bombardment.

Scientists have already discovered that the comet barrage likely formed Earth's oceans.

Premature Conclusions?

The new findings, to be published December 11 in the journal Science, also suggest that comets can bring life-giving substances to habitable worlds elsewhere in the universe, experts say.

"What our study shows is there are orbital processes that bring icy bodies [such as comets] from the edges [of a star system] to the inner rocky planets, which don't have much water or volatiles [or substances that vaporize quickly]," Ballentine said.

"We can see a process that is mixing these two planetary regions much more efficiently."

Robert Pepin, an atmospheric scientist at the University of Minnesota, called the team's data "superb." But their conclusions are premature, added Pepin, who was not involved in the study.

For example, even if early Earth did amass its atmosphere from the solar nebula, he said, it's not a given that the planet was able to concentrate any of those gases in its interior.

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