There is roughly 1.39 billion cubic kilometers of water on Earth’s surface, though how and when water first got here has been up for considerable debate. A new study suggests that primitive meteorites called carbonaceous chondrites brought water to Earth while the planet was still forming. Adam Sarafian of the Woods Hole Oceanographic Institution is lead author of the paper, which was published in Science.

Many have previously suggested that water came to Earth only after the planet had finished forming by way of collisions with comets and asteroids with high ice or gas content. After all, planetary formation is a fairly turbulent time, and any water that might have existed on the planet during this time likely would not have fared well. 

"With giant asteroids and meteors colliding, there's a lot of destruction," co-author Horst Marschall said in a press release. "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.”

At the same time that the early solar system was bursting with activity and the proto-Earth was getting bombarded, carbonaceous chondrites were already in existence. In fact, these primitive meteors formed at the same time—and from the same material—as our sun, if not earlier. Because they are some of the oldest objects in the solar system, their composition can be compared to other objects that also formed early. 

Deuterium is a stable isotope of hydrogen that has a neutron in the nucleus, whereas ordinary hydrogen does not. The ratio between the hydrogen isotopes in carbonaceous chondrites matched that of asteroid 4-Vesta, which formed 14 million years later while Earth was forming in the same region.

"These [carbonaceous chondrites] resemble the bulk solar system composition," explained co-author Sune Nielsen. "They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth's water."

Ultimately, the researchers found that the ratio of hydrogen isotopes between carbonaceous chondrites, asteroid 4-Vesta, and Earth were roughly the same. That is, the hydrogen in our planet’s water is consistent with hydrogen from sources early in the planet’s formation.

"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," Marschall continued.

Of course, this doesn’t mean that some water couldn’t have also been brought to the planet after it had formed, but ratios indicate that most of the water was brought here sooner rather than later. Since liquid water is essential for life as we know it, early water also may have meant an earlier start to life.

"An implication of that is that life on our planet could have started to begin very early," added Nielsen. "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.”

In September, a PhD candidate from the University of Michigan used deuterium isotopes to reason that a considerable portion of water on Earth may have even originated outside of the solar system, before the formation of the sun.