The Cosmic Origins of Earth's Water

Seventy percent of Earth is covered in water.

That's every last drop in the oceans, in clouds, in icecaps, in your body.

And even less of that is fresh water.

All that water together would fill a sphere only 860 miles (about 1,385 kilometers) in diameter.

Water is one of the things that makes Earth so special and without it, life as we know it couldn't exist.

Was our home always a waterpark?

If not, how did it all get here?

What do we really know about Earth's H-two-Origins?

[INTRO] The solar system formed when a cloud of gas and dust collapsed into a swirling disk.

And at least some of that cloud was water.

In the orbit where Earth would one day be, the young sun's intense heat prevented water vapor from condensing on its own.

But tiny grains of dust acted as 'condensation nuclei' to let droplets form.

These tiny debris particles coalesced into our young planet along with the water they carried.

As Earth cooled, rocks in the crust reacted with chemicals in the early atmosphere to create more water molecules.

Water existed inside Earth's mantle, but not as independent H-two-O molecules, rather it separated into H's and OH's in the rock.

We don't know exactly how much water is still locked inside Earth today, but there may be several oceans' worth of primordial H2O embedded within minerals, or even as ice inside diamonds deep underground.

This explains some of Earth's water, but much-if not most-of the water on the surface today was imported from farther out in the solar system.

One type of asteroid called a 'carbonaceous chondrite' contains water molecules, and also their raw ingredients, oxygen and hydrogen.

During the early years of the solar system, we think Jupiter may have orbited closer to the Sun than it does today.

Its gravity would've flung thousands of these watery asteroids toward the young Earth.

But comets also hold a lot of water ice, and there are billions of them zipping around the solar system.

If they crashed into early Earth they could have brought water too.

So how can we tell if most of our water came from asteroids or comets?

Every water molecule contains two hydrogen atoms and one oxygen.

But a tiny fraction of the hydrogen atoms in water have an extra neutron in their nucleus.

Comet water and asteroid water have different ratios of these heavy hydrogens to normal hydrogens.

The atomic fingerprint of the water we find on Earth doesn't quite match what we see in comets, but the water in asteroids is a closer match.

Like many things in science, we can't be 100% sure yet, but so far it seems that most of our water came from asteroids.

So Earth probably got its water in multiple ways.

Some was built into the planet.

Earth made a bit on its own.

And the rest-was delivered by space rocks.

But where did all this water come from... before the solar system?

Astronomers have detected water all over the cosmos.

They've seen it in clouds of gas between stars and in other planet-forming disks.

Around one supermassive black hole, they've even detected a reservoir of water 140 trillion times heavier than Earth's oceans.

This space fog is far enough away that it, and its water, must have existed at least 12 billion years ago, and maybe earlier.

So how old is the universe's oldest water?

All of the universe's hydrogen was made during the Big Bang, but oxygen is only produced inside the nuclear furnace of stars.

So water could have formed as soon as the very first stars in the universe ignited, about 12.7 billion years ago at the earliest.

A tiny fraction of the air we breathe-and the water we drink today-contains some of these very first oxygen atoms.

Why do we care so much about the history of something as seemingly boring as a glass of water?

The earlier there was water in the universe, the earlier there could've been watery planets.

And the earlier there were watery planets, the more time there could be for life to evolve.

That's a refreshing thought.

Stay curious.