Origins: How did Earth get its water?
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Listen to our latest episode, Anna, wherever you get your podcasts. In 1972, three NASA astronauts were heading to the moon. Hello, you're Apollo 17. How do you read? Before they turned their attention away from Earth, they turned around. The Earth just fills up window five. And they looked out the window. I know we're not the first to discover this, but we'd like to confirm that the world is round. The astronauts caught a glimpse of a few continents...

But all that land paled in comparison to the rest of what they saw. The rest of it is all ocean. The Indian Ocean out into the Pacific Ocean and back into the Atlantic Ocean. So they decided to take a picture. It might be the most famous photo of Earth ever taken. You've probably seen it before. It's called the Blue Marble. I'll tell you, if there ever was a fragile appearing piece of blue in space, it's the Earth right now.

Over two-thirds of Earth's surface is ocean. And when you look at this photo, it's just obvious that water is the defining feature of our planet. But the fact that there's all this water here at all is kind of confusing. Because when you start thinking about the history of our planet...

Earth being blue doesn't really make any sense. The issue that scientists have always had is that when Earth formed really early during our solar system, Earth would have just been too hot to have any kind of condensation of liquid water. Lydia Hallis, planetary scientist, University of Glasgow. So it's always been a bit of a problem as how do you get so much water onto Earth?

So let's go back in time about four and a half billion years.

Earth is about to be born, and the solar system is a really different place. It's chaos. Pieces of dust and rock are floating everywhere. They're colliding into each other, making larger and larger piles of rock. And eventually we end up with planet-sized bodies. These collisions are explosive. They release tons of energy. So when these planets form, they're incredibly hot.

so hot that they actually melt. This happened to the early Earth, so early Earth completely melted. Which means Earth used to be less of a blue marble and more of a bright orange molten magma marble, which raises a pretty major question. How do you get so much liquid condensing onto the surface of a planet that should be really, really hot? How did Earth get its water? It's a pretty epic question in its own right,

But it leads to an even bigger one, maybe the biggest one there is. Why are we here at all? We only know life as it exists on Earth, and without water, life can't exist. I'm Noam Hassenfeld, and this is Origins, a new series from Unexplainable about the beginning of life on Earth and just how improbable all of this might be. But to even start answering that question, we need to figure out how Earth went from a bright orange marble to a blue one.

Where did Earth's water come from? Every marvelous living creature on our Earth is built of complex living cells. Life is made up of atoms and molecules and chemical reactions. But what makes them alive? How does this come about? What is life? Lydia wasn't always interested in enormous existential questions.

She just liked rocks. I'm just really interested in minerals. So I was just born that way. And she started her research pretty early on. Ever since I was little, I would just like digging around in the ground. As she grew up and became an actual adult scientist, Lydia started noticing how much everything she would dig up was constantly being shaped and reshaped by water. It really drives you.

every mechanism on a rocky planetary body from plate tectonics down to different types of minerals that Earth has. But water does more than just shape rocks. It's a perfect environment for the kinds of chemical reactions that actually create life. It acts as kind of a chemical soup to allow the reactions to start going. And once life is around, water is what allows it to stick around.

We just need the water to keep our cells intact, really, because cells dehydrate and they start to degrade. And then if you don't get water, you die. Without water, we just wouldn't be here. And Lydia wanted to know where all that water came from, especially given how hot Earth used to be.

So she started by checking out some of the ideas that had already been put forward. So the classical theory is that it was delivered after Earth cooled down. The idea is that millions of years after fireball Earth, magma hardened into rock and then poof, special delivery. Icy bodies from the outer solar system, so maybe comets would have delivered all the water that we need to Earth through impacting the Earth.

Comets are mostly made up of ice and they swing through the solar system all the time. They're essentially a consistent interstellar water pipeline. But this theory isn't perfect because scientists have a way to actually check. We can figure out, OK, that's the signature of comet water and this is the signature of Earth's water.

All water comes with a particular signature, a kind of fingerprint. In the H2O, you just look at the H. Hydrogen is actually really useful for tracing where water came from because there are two types of hydrogen. There's normal hydrogen. And then there's something called deuterium, which is twice as heavy as a normal hydrogen atom.

Because this kind of hydrogen is a bit heavier, when it's part of H2O, scientists call it heavy water. And the closer you get to the sun, the less of it there is. So we can measure the ratio of normal hydrogen versus deuterium in any water from any part of the solar system. And we can kind of piece that together and try and figure out where Earth got its water from.

But that fingerprint in Earth's water, it doesn't match comets. We need to get that water from somewhere else. So Lydia came across another, even wilder explanation. Wandering planets. So people tend to think that the planets were born and they've been in the same place since they formed, but that's not true.

A lot of models suggest that Jupiter and Saturn moved right in actually towards where Earth is orbiting now, and then they went back out again. After the early Earth formed, Jupiter may have been pulled inward by the Sun's gravity. As it moved, it would have pulled tons of objects from the outer solar system along with it, including ice-covered asteroids that were far enough from the Sun that their ice hadn't already vaporized.

Lots of those water-rich asteroids were actually brought in to where Earth was orbiting around the Sun in the inner solar system. And it caused lots of collisions with Earth. So without Jupiter, Earth might not have had any water. But without Saturn, things could have been even worse. It seems as though our solar system is very special in that we had Jupiter, but we also had Saturn, which pulled Jupiter back.

When Jupiter wandered in, it might be that the only reason Earth didn't just fall into the Sun was gravity from Saturn. The masses are just correct and the distances between the planets are just correct so that you end up with Jupiter migrating back out again and leaving behind these four rocky planets that have formed in quite a special situation.

Without this improbable balancing act, if Jupiter and Saturn were even slightly different sizes, there wouldn't be an Earth. No water on it. No life as we know it. Scientists often talk about the Goldilocks zone, that Earth isn't too far from the sun for warmth and liquid water. It's also not too close that we'd burn up. But it turns out there's this whole different Goldilocks zone on the other side.

We're just the right distance from Jupiter and the sun that we got water. And then Jupiter is just the right distance from Saturn that the solar system didn't completely burn up. This whole thing almost reminds me of one of those Rube Goldberg type contraptions where you pull a string and launch a ball, which lands perfectly in a cup, which lights a match or something. One tiny thing goes wrong and the whole thing falls apart.

All of these things had to match up to make Earth the planet that it is today. And that seems like quite a rare and special situation. If the wandering Jupiter theory is true, the fact that we have the right conditions for life at all is essentially a miracle. Maybe a planet like ours is just rare and there isn't any other life out there. Maybe the universe is just kind of lonely.

But Lydia wasn't so sure. She thought this whole Rube Goldberg machine might not be necessary at all. So she went back to the very first assumption and she questioned it. Do you form a rocky planet completely dry and then wait for this kind of lottery of delivery from asteroids and the outer solar system to get water?

Or is there water there, actually primordial water that's in there, in the first building blocks of Earth? And to me, it seems improbable that you could actually form any mineral completely dry within the solar system. Lydia goes primordial water hunting. After the break. Support for Unexplainable comes from Greenlight.

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They'll live much, much longer in a vase full of ice, water. Lydia had assumed that because Earth was so hot when it formed, it must have gotten its water after it cooled down. Which would have meant that the only reason we have water at all is because we won this giant interstellar lottery of wandering planets balancing just right.

But then, about 10 years ago, just as Lydia was finishing up her PhD, scientists were examining moon rocks. And they found something shocking. They found that there was water content inside the rocks that were brought back by the Apollo missions. These rocks showed that there was likely water deep inside the moon. The last place that you would ever expect there to be water. And there is a measurable water content. Just like the Earth, the moon was incredibly hot when it formed.

But somehow it still had water in its rocks. It just goes to show that water clings around in very high temperature environments. So Lydia had an idea. Maybe water was locked so deep inside Earth that it didn't evaporate. If that were true, it would mean Earth's water and all that life that ultimately came from it isn't as improbable as scientists thought.

So she set out to find some of the oldest, deepest, most undisturbed rock on the planet to see if she could find primordial water locked inside Earth's original building blocks. I'd heard about these lava flows near Baffin Island in Canada that were thought to be

from a source that is unaffected by any plate tectonic recycling or any surface effects for around about 4 billion years. These lava flows had hardened into rocks a long, long time ago. But they have large, what are called olivine crystals inside them. It's a green mineral. And these crystals are thought to crystallize really, really deep down. And my thinking was that if those contain water, then some of that water might be from

the Earth's earliest formation. So Lydia hooked up with a group of professional climbers and they headed to the Arctic Circle. It's an area called Palovik Island, which is off the southeast coast of Baffin Island, near a very isolated town called Kikitarovik, which you can only get a plane in and out of. There's no train, there's no road that goes there. So it's a very isolated place.

Not totally isolated. It's also infested with polar bears. So we had to get a boat out there and the boat captain actually pointed out that there were two polar bears overlooking where we were going to be dropped off. And I thought, oh, that means we're going to go somewhere else. And he was like, OK, then bye. And he just left us there with the polar bears. But they were...

Nice polar bears. Most of them just sit and stare at you and make sure that you don't go anywhere near them. And if you avoid them, they're quite happy to avoid you. The main problem was less polar bears and more just getting to the actual rocks. The lavas form these huge big cliffs.

So to actually sample the rocks is very difficult and quite dangerous because they're just in huge big cliffs that are kind of on this uninhabited island and that fall down into the sea. So I really appreciated the help of professional climbers.

She got ready to climb up and she grabbed some extremely technical scientific tools. I just had a hammer and I was just trying to get a fresh piece of the rock out of the cliffs. But the issue is they're also quite crumbly. So then if you start to climb the cliffs, there's big chunks just coming off in your hand and they're quite dangerous, really. Eventually, Lydia was able to climb and she managed to get a fresh piece of rock out of the cliffs.

She cracked it open, looked inside, and she found it. Primordial water. H2O. But it wasn't liquid. It was locked inside what looked almost like glass. Tiny little glass beads that are trapped within those big green crystals of olivine that I was looking for, that's been protected by those crystals ever since it erupted and has been sitting around on the surface of Earth.

Lydia was pretty sure these rocks were from the initial formation of Earth.

which meant this primordial water had been here the whole time. But she still needed to figure out where this water actually came from. We could measure Earth's very first primitive water and see what kind of area in the solar system that primitive water's hydrogen matches up to. Lydia's work is still ongoing. It involves not just these rocks, but other similar ones she's been collecting for years. But so far, she's found a big clue —

When she looks at hydrogen fingerprints in rocks like these, they don't match comets or the asteroids brought by Jupiter in the early solar system. Some of this water must have come from somewhere else. The only signature that matches in the solar system is the sun.

So it suggested that Earth has at least some of its hydrogen and water directly from the sun. Just think about that for a second, that some of Earth's water might have come from the sun. It's hard to wrap your head around exactly what that means, but here's how it could work.

The sun is constantly shooting out solar wind, this type of radiation mostly made up of hydrogen. And rocky bodies in our solar system are mostly made up of silicon and oxygen. And when you introduce hydrogen into that mix, sometimes the silicon-oxygen bond can be broken and hydrogen actually attaches itself to that oxygen. So you end up forming water inside the rock. You take hydrogen from the sun, you add it to oxygen in the rock,

and you get H2O. It's not liquid, it's not gas, and it's not ice. It's water that's actually entwined into the rock itself. Water is present as H2O molecules, but it's kind of attached within the crystal structure. So it's a solid part of the crystal. And so it's protected from being evaporated by high temperatures. Lydia thinks this could have happened in the very early solar system.

that even before Earth formed, solar wind would have hit the very first piles of debris floating around and baked water into those rocks. So when the Earth did form, that water could have stuck around through the blistering heat. And then, millions of years later, it could have made its way up through volcanoes. Where it would be churned out as gas onto the surface.

So you would actually entrap that water into the atmosphere and then potentially you get rain, you get rivers, lakes and eventually oceans.

That's why Lydia was so excited that the hydrogen signature she found in the rocks matched the sun. And that supports the fact that early on in the solar system, those first solid minerals that formed in the solar system were irradiated by the sun and then were incorporated into the earth. So earth wasn't formed completely dry.

So it's possible that even without that improbable water delivery from a wandering Jupiter, Earth could have become a blue marble. We don't think it's too long after Earth formed that there's liquid water present on Earth. And there is chemical evidence to suggest that the very earliest Earth may have had oceans.

Not everyone is sold on this whole idea. There's a chance the hydrogen signature could have changed over time. So Lydia is analyzing other elements in the rocks to make sure the hydrogen really did come from the sun.

But she's pretty sure that at least some of Earth's water was here the whole time. That's not to say that asteroids didn't hit Earth later on and deliver some water, because I'm sure they did. But the initial Earth that formed wasn't 100% dry. It did have an initial water content. Lydia's not sure exactly how much water was here from the start or how much water was delivered later by asteroids along with this wandering Jupiter situation, but...

But if she's right, and even any water was here from the start, if any water survived that early fireball Earth phase, it has big implications. Maybe it's just that water is everywhere and that it sticks around in the rocks. And so wherever you form a rocky planet, you're likely to have some water content on that planet. Maybe our water isn't as rare as it seems. And I think that's the most important thing to kind of come from this

theory is that if you don't need delivery from the outer solar system of water and Earth formed initially with its water or at least some of its water, then anywhere that you form a rocky planet in any solar system, you may end up with a water content on that planet, which could be supporting life. And if that's true, it means the chances of finding water on other planets goes up dramatically.

It means the baseline conditions for life may not be all that unlikely.

I think if you'd have spoken to me or someone else sort of 30, 40 years ago, we would have said that Earth is really special because it's the only water-rich planet. But the more we look at extraterrestrial materials from other planets and other solar system bodies, the more we realize that there's water everywhere, sort of even in the outer solar system. The moons of Jupiter, some moons of Saturn have got maybe liquid oceans beneath their ice crusts and they're very water-rich.

So it's something that's everywhere. And if that's important for life, then it's kind of revolutionary, really, to know that water is so common. But water is just the first step. You need something extra than just a habitable environment. So you need something extra than just liquid water and a friendly environment for life. We don't know what that is. We don't know what that is.

Next week on Unexplainable, the origin of life itself. If you asked any two origin of life researchers what they think the very first life looked like, you will almost certainly get different answers.

This episode was reported and produced by me, Noam Hassenfeld. It was edited by Brian Resnick and Catherine Wells. We had mixing and sound design from Christian Ayala, with help from Meredith Hodnot, who did a ton of work on the vision of this whole Origins series with Brian. We had fact-checking from Zoe Mullock, and some of the best vibes around from Manding Nguyen.

Bird Pinkerton slowly walked over to the wall and picked up that can of soda. She opened up the top, but instead of fizz, a small man hopped out. "The name's Pepper," he said. "Dr. Pepper." Special thanks this week to Pan Conrad for her help wrapping my head around all of this stuff.

And if you have thoughts about this episode or ideas for the show, please email us. We're unexplainable at Vox.com. And we'd also love it if you left us a review or a rating. Unexplainable is part of the Vox Media Podcast Network, and we've got more episodes of Origins coming your way the next two weeks. Stick around.