Where Did Earth’s Water Come From?

Our solar system formed around 4.6 billion years ago, and evidence suggests that abundant liquid water appeared about 4.3 billion years ago, so pretty quickly.

There have been many theories on how solar systems form. And as our discoveries of more and more planets around other stars continue, the theories must be modified—sometimes the distances from the star of the new planets don’t fit with our models.

The prevailing theory links formation of the solar system bodies (e.g., planets, dwarf planets, asteroids) with star formation, in our case, the formation of the Sun. According to this theory, the Sun formed within a huge, messy sphere of gas and dust grains. Everything in space spins, so the sphere kept spinning and eventually gravity caused most of the gas and dust grains to coalesce into a pancake-like disc with the Sun at the centre. Over time and through countless collisions of gas and dust particles, bodies formed and became the planets.

An artist’s conception of the swirling gas and dust during the formation of the solar system. Credit: NASA/JPL-Caltech

Having planets form from gas and dust grains doesn’t scream water. So how did our planet come to be so water-rich? For a long time, the major theory was that Earth’s water came as a result of collisions of comets with Earth. Comets are basically dirty snowballs; they contain different types of ice, but about 80% is water ice. And we know that Earth has been hit by all kinds of debris—including comets and asteroids—throughout its 4.6 billion-year history.

A more recent theory suggests that asteroid collisions with Earth may have been responsible for the abundance of water on our planet. Through analysis of different asteroids, scientists discovered that a certain class of asteroid, called carbonaceous chondrites, has levels of hydrogen and nitrogen isotopes that are similar to those found in ocean water on Earth. (Isotopes are variations of atoms.) And carbonaceous chondrites are among the oldest types of asteroids in the solar system.

But now there is a new theory for where the water came from. The theory is from several researchers from around the world, including Michel Fich, of the University of Waterloo.

Professor Fich and colleagues used data from the Herschel Space Observatory to study regions in space where stars are forming. Turns out that water molecules form in regions like this. The hot water molecules get lost during star formation—they get pushed out into space. But the cooler water molecules, both cool water and ice, stay behind. The icy molecules form layers around the dust grains. Then, as outlined in the formation of the solar system above, innumerable collisions cause all the particles to coalesce and eventually form planets and other bodies.

So, the water has been there since the beginning! In theory, new solar systems contain enough water that their planets could become habitable.

The European Space Agency’s Herschel Space Observatory studied star-forming regions areas in the infrared. Credit: ESA

Professor Fich and graduate student Mollie Conrad contributed to this research by studying Herschel data from the nebula called NGC 7129, which is a star-forming region in the constellation called Cepheus. Credit: SIRTF/NASA