Earth's Oceans Formed Early When Our Planet Collected Meteorites

Earth is known for its vast bodies of water. In fact, our planet's oceans are what are largely responsible for sustaining life. Yet how did this water first form? Scientists have now found evidence that our Earth's oceans were created not when asteroids slammed into the planet, but far earlier.

Previously, scientists believed that planets originally formed dry, due to the high-energy impact processes of planet formation. Water then came later from sources such as comets or "wet" asteroids, which are largely composed of ices and gases. These objects fell to the planet and then released their sources of water there.

"The answer to one of the basic questions is that our oceans were always here," said Adam Sarafian, the lead author of the new paper, in a news release. "We didn't get them from a late process, as was previously thought."

In this case, the scientists examined another potential source of Earth's water: carbonaceous chondites. These are the most primitive known meteorites and were formed in the same swirl of dust, grit, ice and gasses that eventually created the sun about 4.6 billion years ago, well before planets were formed.

"These primitive meteorites resemble the bulk solar system composition," said Sune Nielsen, co-author of the new study. "They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth's water."

The scientists measured the ratio between two stable isotopes of hydrogen, deuterium and hydrogen. Knowing the ratio for carbonaceous chondrites, the researchers compared them to an object that was known to crystallize while Earth was actively accreting. This, in turn, allowed them to find out when water first appeared on Earth.

"The study shows that Earth's water most likely accreted at the same time as the rock," said Horst Marschall, one of the researchers. "The planet formed as a wet planet with water on the surface."

The findings reveal a bit more about the origins of Earth's water, and shows that while it's possible that meteorites that struck later could have added to this water, the original source probably arrived far earlier.

The findings are published in the journal Science.