Where did Earth’s water come from?

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1:05Caroline Steel:You're listening to CrowdScience from the BBC World Service. I'm Caroline Steele, and I'm stood on the banks of the watery heart of London, the River Thames. It's fast-flowing. It's really busy. There's a lot of boats, a lot of people. It's very wide and long, and I imagine pretty deep. There's a lot of water. And this episode's listener lives near an even bigger body of water, an ocean. And that's got him marvelling too. I've thought about this a lot because I'm not sure why we call Earth. Earth, we should call it water because there's so much water. Hello, CrowdScience. This is Bill from Los Angeles.

1:43I was wondering, where did all the water come from? There's plenty of it here. I live in Los Angeles on the west side near the ocean. and every time I drive down the hill in the neighborhood and I look at the ocean, I go, where did all that water come from? I mean, I've thought about this a lot. I mean, 70 % of the surface is water, and water is, you know, maybe an average of 500 or 600 feet. That's a lot of water.

2:08Caroline Steel:Yeah, it is vast. So what are you expecting us to find? Have you got any sort of inklings about where all the water might have come from? Well, I've heard that it came from the comets. or meteorites or something like that, which is technically feasible, but considering the amount of water that's here, that would have to be a lot of comets because comets aren't that big. And even that big one that wiped out the dinosaurs wasn't that big. I mean, you know, it just doesn't seem feasible. Listener Bill makes a great point. Our oceans are vast. They contain 1.3 billion cubic kilometres of water, and it must have come from somewhere.

2:49Caroline Steel:But is that everything? The Earth has a volume of 1.1 trillion cubic kilometres and surely some of that is water. So do scientists know exactly how much water we're trying to account for? This is Professor Richard Greenwood, a meteorite researcher at The Open University. Do scientists know exactly how much water there is on Earth? Yeah, that's a really good question, actually. And the answer is no, they don't. We obviously know how much water there is in the oceans. And by virtue of that, the units of water, because we're talking about a lot of water, are called a global ocean unit. So we take the oceans as being one unit.

3:31Okay, so all the water in all the oceans, that makes one. And what we don't know is how much water there is in the interior of the Earth. And so it could be, some estimates are there could be up to 12 oceans on our planet.

3:45Caroline Steel:Wait, wait, wait. So 12 times as much water as there is in our oceans on the inside of our planet? Yes. So that's an upper estimate. It could go down to 1.4 in total. And there aren't so many people who believe that. I mean, that's huge. Is it sort of, I don't know, would it be stored in watery pockets? Not really, no. It's trapped in minerals. So you've got to remember that we live on a planet with plate tectonics. Water is actually taken back. So we have the material on the surface of the Earth, which goes down into the interior. And of course, it drags in there. It drags in water. So water has continually been taken from the surface back down into the planet itself.

4:29Caroline Steel:And I guess that makes it harder to sort of work out conclusively where all the water came from, if you don't know how much water you're trying to account for in the first place. It is, and that is where the problem lies, really. OK, so we're trying to work out where all the water came from without knowing how much water there actually is. We'll come back to Richard later on in the show. But for now, to unpick this problem, we need to go back to the very beginning. We need to talk to someone who knows a lot about a very long time ago, the beginning of our universe. OK, my name is Muhammad Latif.

5:04I'm an associate professor at United Arab Emirates University, physics department, College of Science. And I'm basically an astrophysicist. I'm interested in, you know, understanding the formation of stars, black holes, planets, and particularly the first billion years of the universe, you know, how things started to begin, how our building blocks of our galaxies and stars were built.

5:26Caroline Steel:Where the ingredients for our galaxies and stars came from is incredibly interesting. In fact, it's so interesting that we've made a whole crowd science on it. Search for how old are the elements wherever you get your BBC podcasts. But in this episode, we're just going to be focusing on water. So water is basically made of hydrogen and oxygen. So according to our understanding of the universe, initially there were only two elements, I would say. Hydrogen and helium were present. The rest, everything was cooked inside stars. So the oxygen was not present. So we're getting a recipe. where new elements are cooked inside stars.

6:04Caroline Steel:At the beginning, the only ingredients we have are hydrogen and helium. So what happens is that you have this hydrogen and helium. We expect like grandparents of our sun, what we call the primordial star or first generation of stars in simple words, they farm purely from hydrogen and helium. They are expected to be a bit more massive than our sun and they were short-lived and they ended their lives and all elements were produced, heavier elements, what we call them, were produced inside the stars. So once those stars died, the hydrogen was there, oxygen was there and then, of course, basic ingredients to farm the water were there.

6:42Caroline Steel:So now we have the ingredients we need for water. Hydrogen was always there and thanks to the death of the first generation of stars, we now have oxygen too. But previously people thought that universe might take a bit longer, like one billion years. that things cool down and then hydrogen and oxygen can combine and the water can form. But Mohamed was part of a team that used computer models to simulate the birth and death of the first stars in a realistic context. And their results suggest something different. But what our study showed that it was actually much, much earlier than previously thought.

7:17It's just a few hundred million years after the Big Bang. And this is our result that shows that water could actually form much earlier than before galaxies started to form. And these stars enriched that hydrogen, helium medium with those oxygen and other elements. And we see that water was formed in the aftermath of kind of death of those stars.

7:40Caroline Steel:So that's our recipe for producing the first water a couple of million years after the Big Bang. And talking of recipes, I've come to producer Marnie's house to follow one. Let's fast forward to a mere 5 billion years ago to a corner of our universe that was forming into our solar system.

8:03Caroline Steel:Hello. Welcome, welcome. Thank you. I'll take your shoes off. You're a good guest. All good? You happy? Yeah, I've got some things for you downstairs.

8:27Caroline Steel:Right, welcome to the kitchen. Thank you. It's a kitchen where we are going to make a planet. Right, okay. It's not that big of a kitchen, but I'm interested to see what you have in store.

8:45Caroline Steel:Okay, so first we're going to need to prep our conditions. So we're going back to the formation of the sun, a yellow dwarf star. It's hot. It's a glowing ball of hydrogen and helium at the centre of our solar system. And surrounding it, a cloud of dust and water vapour that will make the Earth. So our version is this. And by this, you mean your oven? Yep. Cool. OK, so we've got your oven to be the sun. Oh, there you go. Turning it on, firing it up. what are we going to do for the sort of dust cloud that the earth was forming from so dust cloud was dust gases and metal elements so take your bowl grab yourself some dust okay so this is flour i mean elemental dust how many spoons of flour um i'm going quite a small earth so probably probably one should do it okay now you're digging into your food waste bin what are you pulling out of the food coffee grounds oh awful right right sprinkling of that great okay so we've got flour with bin coffee grounds or heavy elements and dust nice should I stir them together yep does one of us have to eat this at the end maybe depends how well you do and we're missing a crucial ingredient which is water vapor so get some water please okay over to the tap should i stir that in this is really quite you can keep the lumps i feel like it needs sieving but i guess probably wasn't a sieve around in the early earth okay so now we have a sort of brownie liquid with lumps of floating coffee and lumps of floating flour and this is basically simulating the conditions at the very start of our earth forming is that right this is simulating the elements what we haven't done and what we can't do is simulate the conditions because these things would clump together because of gravity pulling them all together and we haven't really got enough stuff to emulate a planetary level of gravity but what What I do have is a freezer.

11:05Caroline Steel:Okay, and how is a freezer a substitute for gravity? Okay, well, it's cheating slightly. Rather than sticking together because of gravity, our proto-Earth is going to stick together because I froze it in the freezer. Here is your bowl of proto-Earth ingredients, and I've always wanted to say this, here is one I made earlier. Okay, Marnie's gone over to the freezer. You can see you've got a little container with, oh my goodness me, the most revolting looking ice cube I have ever seen. Yeah. It's brown. There you go. Hold that in your hand. That is your proto-earth. Okay. So it's basically the same as what we've made here.

11:45Caroline Steel:Yes. It's all of the elements all together, but it's missing one crucial element, which is the effect of the sun. And we're going to put this block in the oven and see what happens. I have a hunch what's going to happen.

12:04Caroline Steel:It's going to stink, isn't it? It's going to stink, yeah. Whilst our kitchen-made Earth is cooking, let's talk to an actual planetary scientist about what was going on when our real Earth was cooking. So my name is Sarah Russell and I'm a planetary scientist here at the Natural History Museum. So could you give us a sort of a nice summary of our early solar system? So where's the sort of traditional picture of where all the water was when Earth formed? Well, so our solar system formed from a cloud of dust and gas from which the sun accreted and eventually the planets accreted. What does accreting mean?

12:43So by accreting I mean the solar system began as just dust and gas but the dust slowly stuck to other bits of dust come bigger and bigger eventually snowballing into the bigger bodies that we see around us today. But there's a bunch of small bodies that are smaller than the planets that were formed at the same time and never got to grow big enough to become a planet. Some of these are rocky and contain very little water. Some of them contain a mixture of water and rock. And some of them, the comets, are made with huge amounts of water in them. And we think when the planets first formed, well, the terrestrial planets, the inner planets first formed, they would have got incredibly hot.

13:26And especially, for example, the Earth had this dramatic event in its early history when a planet-sized body the size of Mars smashed into it, which eventually formed our moon. but one of the side effects of that was that it melted the whole exterior of the earth and that would have meant that any water that was around at the time would have evaporated away so the water on the earth and the other terrestrial planets must have come later on by being seeded probably by these collisions with these minor bodies the asteroids and the comets.

13:58Caroline Steel:Okay so there might have been water on earth initially but it would have then evaporated and had to come back again. Yes Yes, exactly. It's probably the Earth did form out of the same materials that the asteroids and the comets are made of. And so that probably would have had some water. But most of it would have been removed by these dramatic processes and the heating that the Earth experienced. Oh, that's our planet ready. Opening up the oven. Got my oven gloves on. Oh, yum, yum, yum, yum. Mum! Okay, what do you see? What has happened? It looks like a brownie. I think I need something to poke it with.

14:40Caroline Steel:I'm going to grab a fork. We put in all the ingredients we need for our planet. What's come out the other side is just a lot of dry dust. So I'm doing a bit of a sort of poke of the dust with a fork and it's just, yeah, crumbling. It's very dry. That's the core of our mystery of where did all the water come from? because if you look at this planet, guess what? Loads of water. Right. And according to our kitchen sink efforts to recreate a planet, there shouldn't be. OK, so this doesn't work. We need another way that water could have arrived on Earth. So despite our earliest stars working hard, or should I say exploding hard, to make us oxygen for water, Earth probably lost it all.

15:30Caroline Steel:And our meteorite scientist Richard has some iron-clad evidence that our planet was dry at this point. What you've got to remember is that the Earth as a planet has a huge iron core. So it's wonderful, we've got this metal core, but you can't have a metal core if you've got a lot of water floating around. Of course it will form iron oxide, you know, like the rusting. If you stick a piece of iron metal in your garden, it will rust. As I say, if there had been any significant amount of water, then it would have been a really serious problem for forming a metal core. And we've got an amazing metal core, and it's still liquid, and we are very happy about that, because as I say, that's what creates this dynamo, this magnetic field, which keeps us all safe.

16:08Caroline Steel:So really, I feel pretty grateful that we probably lost our early water, because that allowed us to have an iron core, and our iron core gives us our magnetic field, which we rely on to be alive and to make this show. So where did all the water come from? That's what listener Bill wants to know, and we're about to find out.

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17:30Caroline Steel:You're listening to CrowdScience from the BBC World Service, where we answer your science questions. Listener Bill has heard that the water on Earth may have come from comets or asteroids. Here's planetary scientist Sarah Russell to tell us the difference. Asteroids are made mostly of rock, and they formed either in the inner solar system or beyond Jupiter, whereas comets formed way out, probably out towards Neptune and beyond, and they contain even more ice because the further you get away from the sun, the more ice was available as solid material to become part of these bodies. Okay, and some of these bodies might have landed on Earth and bought all the water that we need?

18:15Pretty much. You're saying that like it's an extraordinary thing to say. Yeah, but it is. But yeah, that's exactly right. So some meteorites that fall to Earth can have 10 % or more of water inside them. and the earth although two-thirds of the earth's surface is covered in oceans that's really right on the surface the actual bulk amount of water in the earth is very very small and you can easily get the water that we see on earth just from impacts from meteorites that came from asteroids mostly and maybe some comets as well.

18:53Caroline Steel:That just it feels really hard to wrap my head around because is, you know, in my head, asteroids and comets aren't that big. And if they're only 10 % water, or some of them are only 10 % water, the Atlantic's huge. I don't know, just sort of, it feels intuitively weird. Okay. Do you see what I mean? Yeah, yeah, I guess I see what you mean. But actually, there's loads of extraterrestrial material that comes to Earth. So we think maybe 40 ,000 tonnes of extraterrestrial material comes down to Earth, most of it in the form of dust, not as big meteorites, but still the dust will also bring water and all sorts of volatile things with them.

19:36And in the early solar system, this impact rate would have been even higher because the solar system would have been a much more chaotic place with lots of things flying around in very strange orbits, and so there would have been many more collisions then. And so actually there's plenty of opportunities for the water to be brought that way.

19:56Caroline Steel:It sounds like our early Earth was far from what we experience today. It was called the Hayden era, as in Hades, hell. Earth had a molten surface, constant volcanic eruptions, and when taking out the bins, you'd have to watch your head for a comet or an asteroid. But which brought the water? OK, we are back in Marnie's kitchen. Can you open the freezer door? Yeah.

20:24Caroline Steel:So, Caroline, I have got for you an asteroid and a comet. I'm going to get these out. I'm going to give you one of each. There you go. There is your comet. Delicious looking ice cube, thanks Moni. And that one is your asteroid. The comet basically looks like a normal ice cube, but a bit dirty. The asteroid looks more like a frozen block of dirt. So if I were to ask you which one you reckon is a better bet for the candidate for bringing water to Earth, which would you pick? I mean, easy peasy comet, because it is literally a block of ice. whereas I can't imagine the asteroid brings that much water to be that useful.

21:17Caroline Steel:So yeah, going comet. That's what we used to think. Back to Sarah. There's an old idea that it's probably comets because they tend to be made mostly of ice and so that makes it sort of logical that they would bring a lot of water. But actually our more recent work suggests that asteroids were much more important in bringing water And that's because the ratio of deuterium to hydrogen in asteroids closely matches that of the Earth, whereas for comets, they've got too much deuterium in them. What exactly is deuterium? So deuterium is actually a type of hydrogen. So the vast majority of hydrogen is very simple.

21:59It's just one proton. But there's a tiny bit of hydrogen that's composed of a proton and a neutron, and that's deuterium. So it has the same chemical properties as hydrogen, but it's twice as heavy.

22:11Caroline Steel:Okay, and how is deuterium a clue about where the water on Earth has come from? Yeah, so we can use the ratio of deuterium to hydrogen as a kind of fingerprint of where the water may have come from. So we can look at this fingerprint in asteroids and in comets. And what we find is that asteroids tend to be a better match for the Earth's water, whereas in comets the deuterium to hydrogen ratio is too high. Just outside Sarah's office is an asteroid collected by a NASA mission. She's compared its water fingerprint, so its ratio of deuterium to hydrogen, to the water here on Earth. So this is our nitrogen glove box.

22:55So we use it to keep our meteorites pristine because that will make sure that all the samples inside don't react with the oxygen and the water in the atmosphere and they can stay pristine. Okay, so is Bennu somewhere in this? Yes, Bennu's somewhere in here. You've got to prepare to be very underwhelmed.

23:15Caroline Steel:You're sticking your hand in the glove. I'm sticking my fingers in the glove box. In that Petri dish, you see those little black dots? Yes, okay, so there's a little Petri dish filled with what kind of looks like some pepper. Yes. As in salt and pepper. Yes, but each of those little bits of pepper are actually bits of asteroid Bennu that were returned by the spacecraft. So it brought this material back to Earth in September 2023. And we were really lucky to be part of the science team that's been analysing it and trying to work out what it's made of. That's so cool. So how did Bennu arrive? Did you sort of get this black dust in the post?

23:58Caroline Steel:um i'm actually not allowed to tell really how we got it oh my gosh because of nasa security oh fair enough but yes we just got it how do you go about studying this black dust yes so what we been doing at the museum is been we've been picking out these individual bits of black dust one by one we ct scan them to see what they may look like on the inside and we do x-ray diffraction to find out what minerals they're made of and we use electron microscopes to find out more about their chemistry and then we work with other people to find out things like their isotope composition as well which are these fingerprints about that tell us where it came from.