The Earth is a very wet place. On the surface, 70% of the “Blue Planet” is covered in water. We also learned recently that there are oceans deep beneath the Earth that contain more water than all of the surface oceans and lakes combined. Where and when all that water arrived however is a matter of some debate.

We know that during the Earth’s early formative years, things were very hot. This has led many to believe that any early water on the planet evaporated or was lost to space. This created the hypothesis that water came late to the Earth from collisions with comets and “wet” asteroids, composed primarily of ice and gases.

“With giant asteroids and meteors colliding, there’s a lot of destruction. Some people have argued that any water molecules that were present as the planets were forming would have evaporated or been blown off into space, and that surface water as it exists on our planet today, must have come much, much later—hundreds of millions of years later,” said Horst Marschall, a geologist at Woods Hole Oceanographic Institution (WHOI) in a statement.

Marshall is coauthor of a paper, published Oct. 31 edition of the journal Science, from researchers at the Massachusetts Institute of Technology (MIT) and WHOI. The study refutes the idea that water was a late arrival. It significantly alters the timeline for the creation of the Earth’s water resources.

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

The researchers looked at carbonaceous chondrites, primitive meteorites formed at about about the same time as the sun 4.6 billion years ago.

“These primitive meteorites resemble the bulk solar system composition. They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth’s water,” said WHOI geologist and coauthor Sune Nielsen.
Different regions of the solar system are characterized by variations in the rations of deuterium and hydrogen, two stable isotopes of hydrogen. Scientists measure these variations to determine the source of water in planetary bodies.

The researchers compared the deuterium and hydrogen ratio in carbonaceous chondrites to crystallized samples from the 4-Vesta asteroid, which formed at about the same time and in the same region of the solar system as the Earth. The measurements showed the same hydrogen isotopic composition in the 4-Vesta samples that is found in carbonaceous chondrites.

“The study shows that Earth’s water most likely accreted at the same time as the rock. The planet formed as a wet planet with water on the surface,” said Marschall.

The researchers stress that it is possible that additional water was added at later dates but the early presence of water on Earth may have a bearing on other factors in Earth’s development.

“An implication of that is that life on our planet could have started to begin very early. Knowing that water came early to the inner solar system also means that the other inner planets could have been wet early and evolved life before they became the harsh environments they are today,” said Nielsen.