4.5 billion years ago, Earth  was off to a rocky start.

We'd just collided with another early planet  the size of Mars, a cataclysmic event that   melted both planets and spun off a sizable  chunk of lava that cooled into our moon.

This impact set the stage for the  Hadean Eon - named, of course,   for Hades, the Greek god of the underworld.

It was a name that matched Earth's  generally hellish vibes at that point   since our planet was so hot that our atmosphere  actually contained vaporized gaseous rock.

We’re talking temperatures  around 2000 degrees Celsius!

But as the Earth and the moon began to cool,  something new formed on Earth's surface:   liquid water.

Which seems standard - after all,   71% of the Earth is covered in our  estimated 366 trillion gallons of water.

Except, we shouldn't have that water  – we formed too close to the sun.

Mercury, Venus, and Mars are all  super low on water – so where did   ours come from and why do we have so much of it?

Well it's complicated and still debated,  but we think our water came from a few   unlikely sources: meteorites,  space dust, and even the sun.

Water itself isn't actually that uncommon  in our solar system - it's just further out.

For example, Jupiter's moon  Europa is a quarter of the   Earth's size but has twice as much water as we do.

And that has to do with the fact that  Europa formed farther away from the sun.

Because, in the beginning of our solar  system, we were all star-stuff.

Our   solar system is formed from a nebula -  the dusty remains of an exploded star.

As that stardust collided and collapsed it formed  our sun.

And the gravity of that young sun made   all the rest of the dust spin around it in a flat  plate-like cloud called a protoplanetary disk.

But our early sun was hot - and soon  that disk of dust began to differentiate.

Solar and magnetic winds blasted  light gasses away from the sun,   leaving mostly heavier elements  like iron and silica behind.

Which is why Earth is a rocky planet - that's what   was in our section of the disk,  lots of iron and lots of silica.

We also had some of the ingredients  for water, but not all.

Water is made   of hydrogen and oxygen, and oxygen is  pretty heavy so we have a lot of it.

In fact, though it makes up only  about 21% of our atmosphere,   oxygen binds so readily to silica and iron that  it's the most common element on our planet.

But oxygen is only part of what you need  for water - and hydrogen is really light,   so it got blown out towards Jupiter, Saturn, and  other planets, which now have a lot of water ice.

So if it didn't form here to start  with, where did we get that missing   ingredient?

Where did our hydrogen  come from, and when did it get here?

The easy answer is - we got pelted with it.

Water ice and hydrogen hang  out in the outer solar system,   but sometimes they come inwards in the form of  space pebbles, aka asteroids and meteorites.

And there is a specific type of meteorite  called a Chondrite that is full of water.

But I don’t mean full of water like a Gusher  - you can't crack open a meteorite like a   coconut and sip star water from it, as much as  I would like to, as much as we would like to do.

Instead the solo hydrogen and the  complete H2O water molecules have   become part of the chemical structure  of the minerals that make up chondrites.

These include minerals like serpentine, chlorite  and smectite, which hold onto their water tight.

This is actually how most of Earth's water is  stored today - while we have a bunch of liquid   water, we estimate that Earth's rocks hold maybe  18 times the amount of water that our oceans have!

So how do you get that water out of these  minerals?

Well, the easiest way is to melt it.

Which… isn't always going to happen just  by flinging those water-bearing minerals   at the Earth.

After all, we have solid chondrite  meteorites that hit our planet and didn't melt.

But that's because they hit today.

If they’d hit Earth in the Hadean, that  would’ve been a much different story… Back in the beginning of the Hadean,   anything that hit the surface would have  just melted straight into our magma oceans.

There, hydrogen met oxygen and the  resulting water superheated into gas.

Lighter than magma, the water vapor would  bubble up and out into our atmosphere.

But this process was a race against  the clock, because our magma surface   was cooling - and once a solid lid of  rock formed, no more water could escape.

The oldest traces of rocks we've found date  to about 4.4 billion years old - so that's   probably when the lid of solid rock formed.

But  those rocks have signs of something surprising:   they were exposed to a liquid ocean.

Yep!

As soon as the Earth cooled enough to  put a cap on the magma, we had an ocean!

That said… it was probably a little  different than the oceans we see today.

Earth's atmosphere in the early Hadean was  full of carbon dioxide and was very thick,   potentially as thick as 215 bars of pressure  – which is 215 times what it is now.

With all that pressure and heat,  Earth's surface was… weird.

Today, water turns to gas when  it reaches 100 degrees Celsius.

But if you change the pressure, you  can change when water turns to gas.

You can even do this by walking up a mountain -  at the top of Mt.

Everest, where air pressure is   a third of what we see at sea level, water  turns to gas at only 68 degrees Celsius.

And in the Hadean, the much thicker atmosphere  meant that the reverse was happening.

It was so thick that even when the  surface was 230 degrees Celsius,   water didn't boil!

So our  first ocean was… superheated.

Fortunately for us, that  superheated ocean didn't last.

By the end of the Hadean, 4 billion years ago,   the surface of the earth was very similar  to what it's like today - a rocky crust,   an ocean of liquid, but not superheated, water and  an atmosphere of about the same pressure as today.

So from meteorites to magma  to air to superheated oceans,   that should be the story of Earth's water.

But  there's one really big problem with this story.

Turns out, the chemical composition of the  water and hydrogen in chondrite meteorites   doesn't actually match the chemical  composition of most of the water on Earth…   the water that is in our rocks.

When we say chemical composition, what we  mean is isotopes, or types of hydrogen.

There are two really important  isotopes of hydrogen - the regular   brand, with one proton and one electron, and  what's known as Deuterium, with a proton and   neutron.

The addition of a neutron makes  Deuterium "heavier" than regular hydrogen.

Most of the deuterium in our solar system  was actually formed in the Big Bang.

And it's a huge component of chondrite meteorites.

They're "heavy" with this old type of hydrogen.

The oceans are pretty heavy too - but our modern   oceans aren't actually a great  representation of our early water.

For one, they're not 230 degrees Celsius  - and for another, they've been sitting   on our surface for 4.4 billion years, so  we know they've undergone some changes.

Strangely, if we want to understand what our  liquid water looked like 4.4 billion years ago,   we actually have to look at the water  that's contained within our rocks.

When Earth's magma oceans cooled  enough to form a seal of hard rock,   not all the water in the magma  escaped into the atmosphere.

A   lot of it was trapped below the surface  in a layer of Earth known as the Mantle.

Over time, pieces of the mantle have been  shoved to the surface through plate tectonics   and we've been able to look at the  hydrogen the rocks of the mantle contain.

And it's a lot lighter than the  hydrogen we see in the oceans.

Which is part of why the source of Earth's  water is still somewhat debated - how could   we have such light water in our rocks  when chondrites have such heavy water?

This problem has spurred a lot  of research in the last decade.

Scientists did find a special meteorite  called an enstatite that has lighter hydrogen,   but most of them think there's not enough of  these meteorites to make up the difference.

Then, in 2021, scientists reported  on some interesting samples that were   collected from an asteroid and  maybe came up with the answer.

They found that the samples had a uniquely  high level of light hydrogen.

But to confirm   their growing suspicions of where that hydrogen  came from, they had to replicate the process.

So they measured the hydrogen and water  content of olivine crystals before   and after they exposed them to  the equivalent of solar wind.

When they looked at them afterwards, they  found that, just like the asteroid samples,   they had a crust of accumulated  water built with light hydrogen.

So where did this light water come from?

Well, it's literally light water  because it came from the sun!

Solar winds from our sun shoot out a  lot of particles, including protons.

When those protons hit dust in  their path they can sometimes   steal an electron.

And a proton  plus an electron is light hydrogen.

Slam that light hydrogen into a rock with some  oxygen in it and boom, you've made light water!

Or more realistically, slap that hydrogen ion  into some localized space dust or a meteorite.

When that falls to Earth, it carries  down light water into our magma ocean,   which is then trapped in our mantle over time.

So the story of water on Earth is… complicated.

During the Hadean, we accumulated light water and  stored it in the rocks that became our mantle.

But   meteorites continued to rain down, and our oceans  eventually became full of much heavier water.

So the water that you drink today  has had quite a journey to get here…   and it isn't all from the same source.

It came from space as a combination  of meteorites and sun-burnt dust.

It melted into our magma, then bubbled out  and rained down into superheated puddles.

Eventually it cooled enough  to let life form.

So while   the story of where Earth's water  came from is incredibly complex,   what it ultimately means is that your existence  boils down to space dust and sky pebbles.