Short Wave - Did Earth’s Water Come From Space?

Episode Date: January 5, 2026

Choose your fighter for the origin of water on Earth! Was it always here or did it come to this planet from somewhere else in space? And, either way, what does this mean for other water worlds in our galaxy? To find out, we talk with Michael Wong, an astrobiologist and planetary scientist at Carnegie Science. He gets into scientists’ strongest candidates for the ways water could have come to our planet many, many years ago – including whether it could have been made here. Buckle up: This is a hot debate in astrobiology right now.If you enjoyed this episode, check out our Space Camp series and our episode on whether life started on the ocean floor..Interested in more space science and more unresolved hypotheses about how the universe came to be how it is today? Email us your question at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Earth is a water planet. But where did it all come from? Currently, planetary scientists are taught that water wasn't really present when our planet was forming. We thought that Earth was like pretty bone dry to begin with. So it had to be delivered from somewhere else in the solar system. That's Michael Wong, an astrobiologist and planetary scientist at Carnegie Science here in Washington, D.C. I was taught that basically the rocky materials that formed where Earth is now at its distance from the sun, those materials were very dry.

They didn't have a lot of water in them because they were too close to the sun and too hot to retain any H2O. But the thinking goes, somewhere farther out in our solar system, there are objects with water in the form of ice. And that could have hauled ice to us at some point. And so there was kind of this. debate over whether or not it was mostly asteroids or mostly comets that were responsible for delivering, shipping Earth's water late on in the game. But that's not the only hypothesis in the race. For years, some scientists have been disagreeing in a healthy way with each other about whether another hypothesis could be the true one.

I think people had published these very theoretical papers about planets just being naturally imbued with water or generating their water themselves through reactions between hydrogen atmosphere and the metals in the planet. But I don't think anybody really took that very seriously, at least until now. Today on the show, choose your fighter for the origin of water on Earth. Where did it come from? Was it always here? And what does that mean for other water worlds in our galaxy?

I'm Regina Barber and you're listening to Shortwave, the science podcast from NPR. Okay, Mike, you kind of blew my mind last time when you came on the show and you told me that how life formed on Earth, that was still a huge debated question in science. And now I brought you back to tell me about another very, very hot topic, the origin of water on Earth. How long do you think this debate on, like, where did water come from? How long do you think that has been going on? Oh, my goodness. I don't know. As long as planetary science existed, I guess, as long as we have wondered where we came from in the universe and why Earth is special. You know, as soon as you look, you know, left and right in our cosmic neighborhood, you look at Venus, oops, bone dry. You look at Mars, oops, also bone dry. And so, you know, a lot of investigation has gone into once you have some water on a planet, what happens to it? Do you lose it or do you not lose it? But there's also a debate over. where the water came from in the first place and how much. Okay, so let's just take a step back and go over how planets form and when that water would have come into the picture. Right. Okay. So we know that planets form in what's called a protoplanetary disk, this swirling disk of gas and dust in which little dust bunnies collagulate together and form pebbles and those pebbles crash into each other and form planetesimals. And then the planetesimals crash into each other and form big, round planets.

And the temperature in this disk sort of starts out really hot near the sun and gets colder and colder the farther you go out. This is this planetary disk. This is this planetary disk, right? The starting materials, all the building blocks of what will eventually form planets has this temperature gradient. Hot near the sun, colder, farther out. Yeah. And there's this point where all of a sudden you can condense H2O. Water can form ice.

And we call that the snowline. Interior to the snow line, we think that that material would have been pretty bone dry. And Earth right now, where we are, is interior to where this primordial snowline was. Where is the snow line, like, if we're thinking about, like, where planets are right now? Yeah. So the snow line is somewhere between, say, the orbits of Mars and Jupiter. Roughly where the asteroid built. That's where all those planetesimals just, like, ended up.

Yeah, it's where they ended up. So those are the remnants of the early solar system. And we actually see a gradient and a diversity of amounts of water in those planetesimals. There's asteroids that are left over from star formation and planet formation. So it was thought that Earth formed dry and then had to have its water delivered from outside the snow line into Earth. And there was a big debate over, okay, was it asteroids that delivered that water or was it comets that delivered the water? Okay. So you're an undergrad and they're saying it could have been comets, which are these big dirty snowballs in space, or it could have been asteroids, which are rockier but do still have ice. what was their reasoning? Like, what's the debate?

Yeah. So I think early on, people just assumed that it was comets. They are big dirty snowballs and snow is ice and ice is water. And so it's like, okay, well, look at the iciest thing in space that would eventually hit a planet. It's going to be that. That's what delivered the water. But then people started to look for clues. And the biggest clue comes in what's called the deuterium to hydrogen ratio of the water. So water is made of two hydrogen. atoms and an oxygen atom, right, H2O. And you can substitute every once in a while a deuterium for

the hydrogen. And it's a deteriorium is what can be? It's a heavy form of hydrogen, right? So it's, it's got a proton and a neutron, regular hydrogen just as a proton. And so instead of H2O, you get HDO. And the amount of HDO that you have in water kind of is a clue to where it came from. And we notice that comets and asteroids have very different ratios of deterium to hydrogen or different amounts of HDO mixed in the regular H2O. And you can compare that to the amount of HDO that we find here on Earth. You get a rough sort of estimate for like what the D to H ratio is for the Earth. You compare that to the D to H ratios for comets.

Oops, they don't match at all. What? And then you go to the D to H ratio for these specific kinds of asteroids called carbonaceous chondrites, which are these asteroids that are rich in carbon, but also very rich in water. And you say, oh, that kind of looks the same. Where do these asteroids come from? Do they come from these like drier, bone dry, like you said, before that snow line? So these asteroids would have formed beyond that snowline. Okay. And so basically, you'd have to come up with a mechanism to shuffle around a material early on in the solar system

and fling stuff from the outer part of the solar system inward to crash onto Earth. And we think that the gas giants may have been actually responsible for this. So you mean like Jupiter or Saturn? And Uranus and Neptune. Oh. Well, Jupiter and Saturn do most of the heavy lifting. But sometimes Uranus and Neptune in these what we call dynamical simulations. Basically, somebody is like solving the equations of motion and gravity for all the planets

early on in the solar system. Yeah, in a computer. They let it run in the computer. and they put a bunch of point particles to represent the asteroids and the leftover material from planet formation. And Jupiter and Saturn just wreck havoc on the entire solar system.

Sometimes Uranus and Neptune actually sort of flip places. They like swap their order in the outer solar system. Yeah. This causes a lot of chaos. And we think that some of these, you know, tumultuous motions

of the gas giants could have actually sent this water-rich material into where Earth was forming. So when is, this happening in planetary science where they're just like, maybe asteroids, this is way more compelling than we thought? I would say, you know, it could be as much as 20 years ago. People started doing this and started realizing this is another point in favor of the asteroids

that actually not a lot of commentary material gets scattered in. I mean, it's just inherently harder to scatter something from what would become the, you know, the Orch Cloud or the Khyber Belt all the way in toward Earth. And those, the Orcloud, Khyber Belt, this is past like Pluto. Yeah, exactly. Or where Pluto is. Yeah, yeah. Pluto is a Kiper Belt's object. So things like that, you know, things that are orbiting right next to Pluto, getting that

all the way to Earth, having that intersect Earth orbit and having that crash and deliver a lot of water, you know, maybe as much as 10% of Earth's water tops comes from commentary material according to these dynamical simulations. But most of it actually comes from the asteroids. And then there's this like one final big suspect that kind of just came up recently. And that's that maybe water formed on Earth as Earth formed. Am I remembering right that maybe this is the original way scientists thought because we didn't, we really didn't know about comets and stuff until I would say 400 years ago? Yeah.

The idea that, okay, you form a terrestrial planet and along with that formation process, you create water may have been the original. way that people just thought water spawned on a planet because we didn't know about anything else. Yeah, I didn't know about asteroids and comets. And so, you know, this idea that the water had to be delivered from somewhere else only came about once we realized that, okay, the initial material of the earth was bone dry. It was just too hot here for there to be water intrinsically. But you could form that water shortly after the planet was created out of bone dry material. And here's how. How? So basically, if you go right to the beginning of planet Earth, you've got a magma ocean because the earth is so hot that all of its rocks are basically just molten.

So imagine, you know, there's nowhere to stand. You're either swimming in magma or, no, you have to be swimming in magma. Or you're in a plane. Oh, yeah. Right. And the plane is sort of the spacecraft of our imagination is soaring not in an atmosphere. of mostly nitrogen and oxygen, as we have today, but a hot, thick atmosphere of hydrogen gas. Why? Because this would have been the first gas that Earth would have gravitationally bound,

grabbed from this proto-stellar nebula. And so the combination of hydrogen gas, H2, and this magma ocean, which is full of what we call iron oxides, basically iron bonded to oxygen, those two components, iron oxides and hydrogen gas, can react together, or so the theory went, to basically rip the oxygen off of the iron oxide and deposit it in hydrogen, therefore creating water. Okay. So, Mike, you're saying that, like, you have this thick atmosphere and you has this magma ocean and somehow those two things can make water.

That's right. That's right. Yes. Yeah, it was a completely different world, right? scorching hot, no land, all the rocks are melted, and the atmosphere is this thick, oppressive layer of hydrogen gas. And those two things together contain all the ingredients you need to make water. Have they done this in like the lab? Like how do we know this works? So that's where this new paper from earlier this year comes into play. People have come up with this theoretical model

of taking iron oxides and hydrogen. And oh yeah, you should just be able to make water out of that. But this paper shows. that you actually can in the lab. And they used a very clever technique called a diamond anvil cell to do it. That sounds intense. Okay. Tell me about this diamond anvil cell. Yeah.

Okay. So a diamond anvil cell works by essentially squeezing material between two diamonds to very high pressures. Pressure, as you know, is force divided by area. It is. Yeah. So if you take two diamonds and you squeeze something between the tips of those diamonds, the area is very small. so the pressure shoots way up. Yeah.

And then because diamonds are transparent, you can beam a laser through the diamond and heat whatever you are squeezing between those tips of diamonds to extraordinary temperatures. This sounds so fun. I would love to see this. It's pretty amazing. What kind of data do we need more of to figure this mystery out?

Yeah. So many things. I mean, from getting better measurements of the, isotopic ratios of asteroids and comets, you know, to looking for signs of water on exoplanets outside, you know, because we can test this hypothesis, basically. We should be able to see signs of watery worlds out there with future telescopes. And if we do, then we can sort of distinguish between some of these hypotheses, whether or not they had, you know, the conditions right for delivery from outside, from farther out in their solar

system versus if they all just seem to be intrinsically full of water. Mike, thank you so much for talking with us. It's been an absolute joy. Thanks. If you like this episode, follow us on the NPR app or wherever you listen to podcasts. Also, check out our space camp series and our episode on whether life started on the ocean floor. We'll link to them in our show notes. This episode was produced by Burley McCoy and edited by a showrunner Rebecca Ramirez. Tyler Jones checked the facts.

Jimmy Keely was the audio engineer. Beth Donovan is our vice president for podcasting. I'm Regina Barber. Thank you for listening to Shortwave from NPR.