Was Earth born with its water?
Earth was born about 4.54 billion years ago, after our Sun formed from a collapsing cloud of gas and dust. Leftover material formed the rest of the Solar System, including our planet.
Was Earth simply born with everything needed to make the oceans, lakes, and rivers that are vital to our survival today? The problem is that the early inner Solar System was a very hot place. Any liquid water would have been vaporized and blown into space.
What about water locked inside rocks? A recent study showed that enstatite chondrites, a type of meteorite thought to be representative of the raw materials that formed Earth, “contain sufficient hydrogen to have delivered to Earth at least three times the mass of water in its oceans.” It’s not clear when these meteorites might have delivered their water, but they are a good match for the rocks found in Earth’s interior. If Earth started out with water trapped beneath its surface, volcanic activity could have released it as water vapor, which would have condensed and fell back to Earth as rain.
About 4.51 billion years ago, a Mars-sized world named Theia is believed to have plowed into Earth. Some of Earth’s mantle was melted in the process, and material leftover from the collision formed the Moon. Could Theia have brought water to Earth in the process? At least one study says yes.
Another theory is that Earth simply made its own water. Powerful telescopes have spotted baby exoplanets shrouded in molecular hydrogen. Modeling by one team of scientists suggests that this hydrogen could interact with magma oceans, forming copious amounts of water in the process.
Did asteroids bring water to Earth?
The inner Solar System was once a violent place. Around 4.0 to 3.8 billion years ago, the orbits of the outer planets shifted. Their gravitational jostling sent icy space rocks hurtling through the inner Solar System, in a theorized event known as the Late Heavy Bombardment.
Many of these worlds slammed into Earth, and could have brought water here. Comets were long suspected to be a major source of Earth’s water, but space missions including Giotto, which visited Halley’s Comet in 1986, and Rosetta, which visited comet Churyumov-Gerasimenko from 2014 to 2016, have found comet water with different chemical signatures than Earth’s water.
That leaves asteroids and meteorites as the culprit. An analysis of asteroid Ryugu samples returned to Earth by Hayabusa2 showed that water locked in the asteroid’s rocks matches the type of water found in Earth’s oceans. Ryugu’s composition matches a type of meteorite called CI chondrites, which could have delivered 30% of the total mass of water in Earth’s oceans.
NASA’s OSIRIS-REx spacecraft returned samples of asteroid Bennu to Earth in September 2023. A preliminary analysis of the samples found water and organics, another hint that asteroids may be responsible for much of Earth’s water. Ongoing analysis of the samples could uncover more clues about whether asteroids like Bennu brought water to our planet.