Did Earth’s Water Come From Space?

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 https://carnegiescience.edu/dr-michael-l-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 https://www.npr.org/series/g-s1-3299/short-wave-space-camp series and our episode on whether life started on the ocean floor https://www.npr.org/2025/08/11/nx-s1-5495449/life-space-moon-ocean-hydrothermal-vents.. 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 [email protected] mailto:[email protected]. Listen 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 http://plus.npr.org/shortwave. See pcm.adswizz.com https://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 https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:00Speaker 00

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• 0:18Regina Barber?

  You're listening to Short Wave from NPR.

• 0:23Regina Barber?

  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.

• 0:33Speaker 02

  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.

• 0:41Regina Barber?

  That's Michael Wong, an astrobiologist and planetary scientist at Carnegie Science here in Washington, DC.

• 0:47Speaker 02

  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.

• 1:06Regina Barber?

  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.

• 1:15Speaker 02

  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.

• 1:28Regina Barber?

  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.

• 1:40Speaker 02

  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.

• 2:05Regina Barber?

  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 Short Wave, the science podcast from NPR.

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• 3:51Regina Barber?

  OK, 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?

• 4:15Speaker 02

  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, um, 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, um, there's also a debate over where the water came from in the first place and how much.

• 4:48Regina Barber?

  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.

• 4:55Speaker 02

  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 coagulate 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. Um, 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.

• 5:21Regina Barber?

  This is this planetary disk?

• 5:22Speaker 02

  This is this planetary disk, right? The, the starting materials, all the building blocks of what will eventually form planets, has this temperature gradient, hot near the sun, colder farther out.

• 5:32Regina Barber?

  Yeah.

• 5:32Speaker 02

  And there's this point where all of a sudden you can condense H2O. Water can form ice, and we call that the snow line. 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 snow line was.

• 5:51Regina Barber?

  Where is the snow line, like, if we're thinking about, like, where planets are right now?

• 5:56Speaker 02

  Yeah, so, uh, the snow line is somewhere between, say, the orbits of Mars and Jupiter, roughly where the asteroid built it.

• 6:03Regina Barber?

  Where the asteroid built. That's where all those planetesimals just, like, ended up.

• 6:06Speaker 02

  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, those asteroids that are left over from, from star formation.

• 6:17Regina Barber?

  Okay.

• 6:18Speaker 02

  Uh, 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?

• 6:34Regina Barber?

  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?

• 6:46Speaker 02

  Yeah, so I think early on, people just assumed that it was comets. They are big, dirty snowballs.

• 6:52Regina Barber?

  Yeah.

• 6:52Speaker 02

  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 gonna 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.

• 7:22Regina Barber?

  And deuterium is what?

• 7:23Speaker 02

  It's a heavy form of hydrogen, right?

• 7:25Regina Barber?

  Yeah.

• 7:26Speaker 02

  So it's got a proton and a neutron. Regular hydrogen just has a proton.

• 7:29Regina Barber?

  Just has a proton, yeah.

• 7:30Speaker 02

  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 noticed that comets and asteroids have very different ratios of deuterium 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.

• 8:06Regina Barber?

  What?

• 8:07Speaker 02

  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.

• 8:20Regina Barber?

  Where do these asteroids come from? Do they come from these like drier, bone dry, like you said, before that snow line?

• 8:25Speaker 02

  So these asteroids would have formed beyond that snow line.

• 8:29Regina Barber?

  Okay.

• 8:30Speaker 02

  And so basically, you'd have to come up with a mechanism to shuffle around 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.

• 8:46Regina Barber?

  So you mean like Jupiter or Saturn?

• 8:48Speaker 02

  And Uranus and Neptune.

• 8:49Regina Barber?

  Oh.

• 8:50Speaker 02

  Well, Jupiter and Saturn do most of the heavy lifting.

• 8:52Regina Barber?

  Yeah.

• 8:52Speaker 02

  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 of the planets early on in the solar system.

• 9:03Regina Barber?

  Yeah, in a computer, they let it run.

• 9:04Speaker 02

  They let it run in the computer and they put a little point, 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.

• 9:23Regina Barber?

  Yeah.

• 9:23Speaker 02

  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.

• 9:34Regina Barber?

  So when is this happening in planetary science where they're just like, maybe asteroids? This is way more compelling than we thought.

• 9:42Speaker 02

  Hmm, 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 cometary material gets scattered in. I mean, it's just inherently harder to scatter something from what would become the, you know, the Oort Cloud or the Kuiper Belt all the way in toward Earth.

• 10:04Regina Barber?

  And those, the Oort Cloud, Kuiper Belt, this is past like Pluto.

• 10:08Speaker 02

  Yeah, exactly.

• 10:09Regina Barber?

  Or where Pluto is.

• 10:10Speaker 02

  Yeah, yeah. Pluto is a Kuiper Belt object. So things like that, you know?

• 10:14Regina Barber?

  Yeah.

• 10:15Speaker 02

  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 cometary material, according to these dynamical simulations, but most of it actually comes from the asteroids.

• 10:33Regina Barber?

  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, 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.

• 10:52Speaker 02

  Yeah, the, the, 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.

• 11:08Regina Barber?

  We didn't know about anything else.

• 11:09Speaker 02

  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.

• 11:31Regina Barber?

  How?

• 11:32Speaker 02

  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, you're either swimming in magma or, um, no, you, you have to be swimming in magma.

• 11:53Regina Barber?

  Or you're in a plane.

• 11:54Speaker 02

  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 protostellar 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 the theory went,

• 12:37Regina Barber?

  Uh huh.

• 12:37Speaker 02

  To basically rip the oxygen off of the iron oxide and deposit it in hydrogen, therefore creating water.

• 12:45Regina Barber?

  Okay, so Mike, you're saying that like you have this thick atmosphere and you have this magma ocean and somehow those two things can make water?

• 12:52Speaker 02

  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.

• 13:08Regina Barber?

  Have they have they done this in like the lab? Like, how do we? How do we know this works?

• 13:11Speaker 02

  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.

• 13:33Regina Barber?

  That sounds intense. Okay. Tell me about this diamond anvil cell.

• 13:37Speaker 02

  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.

• 13:48Regina Barber?

  It is, yeah.

• 13:49Speaker 02

  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.

• 13:57Regina Barber?

  Yeah.

• 13:58Speaker 02

  And then, because diamonds are transparent, you can beam a laser through the diamond and heat whatever you are squeezing between their those tips of diamonds to extraordinary temperatures.

• 14:11Regina Barber?

  This sounds so fun. I would I would have loved to see this.

• 14:15Speaker 02

  It's pretty amazing.

• 14:16Regina Barber?

  What kind of data do we need more of to figure this mystery out?

• 14:19Speaker 02

  Oh, 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 we can test this hypothesis, basically. Technically, 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.

• 15:01Regina Barber?

  Mike, thank you so much for talking with us.

• 15:04Speaker 02

  It's been an absolute joy. Thanks.

• 15:06Regina Barber?

  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 Burleigh McCoy and edited by our showrunner Rebecca Ramirez. Tyler Jones checked the facts. Jimmy Keeley was the audio engineer. Beth Donovan is our vice president for podcasting. I'm Regina Barber. Thank you for listening to Short Wave from NPR.

• 15:45Speaker 03

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