Where Exactly Did Earth’s Water Come From?

The origins of water, the essence of life on earth.

5 min readSep 11

Photo by NEOM on Unsplash

Water is the source and essence of life on Earth. Without it, humans, animals, or plants cannot thrive. Luckily for us, we have tons of it. Nearly 70% of the Earth’s surface is covered in water, which is far greater than any other rocky planet in our solar system. However, the exact origins of our plentiful water supply are a bit of a scientific enigma.

Scientists and researchers have their theories and ideas, but pinning down an exact source or reaching a definite answer is proving to be complicated. Let’s explore some of these theories and see what seems most plausible.

One of the more popular theories is that the planet’s water originated from water-rich objects called planetesimals. These water-abundant planetesimals would probably have been asteroids. Although the concept may seem puzzling at first thought — one would probably not consider a flaming rock blazing through Earth’s atmosphere to bring water; there is evidence that this theory has some merit to it.

Basically, there are two classes of asteroids: chondrites and achondrites. Achondrites are stony meteorites that lack chondrules (igneous rock). They were formed through the heat released by the decay of the early solar system’s radioactive isotopes. Research suggests that these bodies contain little to no water, as any water within them was turned into vapor upon formation.

Chondrites Can Be Up to 20% Water

As achondrites were likely not the source of water on Earth, this leaves behind only one plausible culprit: chondrites. They are the most abundant class of meteorites, constituting nearly 85% of all meteorite falls.

Chondrites are stony meteorites containing chondrules. Their composition is such that they primarily contain iron, magnesium, silicon, and oxygen. This makes them likely to bear water as scientist Sune Nielsen from the Woods Hole Oceanographic Institution has hypothesized that the water was locked up in minerals on these meteorites that decomposed once they reached Earth.

During the inception of our solar system, chondrites emerged from cold regions of the galaxy beyond Jupiter’s orbit, where…


I am a History Educator and a Lifelong Learner with a Masters in Global History.