Where did our moon come from?

Earth didn’t always have a moon. In the beginning of the solar system, when the planets were still forming, something happened that would change Earth’s night sky forever: The Moon was created! How did it happen? This episode, co-host Regina G. Barber searches for answers with planetary scientists. With co-host Emily Kwong, they discuss how the moon was likely made, how scientists know and what that might mean for where Earth’s water came from. Check out Regina's reporting on hydrothermal vents https://www.npr.org/2024/11/04/1211597144/life-space-moon-alien-oceans-hydrothermal-vents. Interested in more planetary science? 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. A previous version of this episode mischaracterized a scientific hypothesis. The episode incorrectly said the idea is that hydrothermal vents may have been the origin of water on Earth. In fact, the hypothesis is that life may have come from hydrothermal vents. 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

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:18Speaker 00

  You're listening to Short Wave from NPR.

• 0:23Emily Kwong?

  Hey, Shortwavers. Emily Kwong here, and I don't know about you, but nothing grounds me quite like looking at the moon, this beautiful, powerful companion to Earth. And for the first time in over 50 years, humans have traveled around the moon.

• 0:37Speaker 02

  Three, two, one, booster ignition and liftoff. We have a beautiful moonrise. We're headed right at it.

• 0:45Speaker 03

  We have reached the closest point of our destination to the moon.

• 0:49Emily Kwong?

  And back on the Artemis 2 mission.

• 0:53Speaker 02

  Integrity's flashdown sending post landing command now.

• 0:56Speaker 04

  A new chapter of the exploration of our celestial neighbor is complete. Integrity's astronauts, back on Earth.

• 1:03Emily Kwong?

  And all of this moon joy has got me thinking about everything the moon has been through.

• 1:08Regina Barber?

  Because she's been through a lot.

• 1:10Emily Kwong?

  Yes, Regina Barber, my Short Wave co-host and astrophysics queen.

• 1:13Regina Barber?

  Yes.

• 1:14Emily Kwong?

  Also our resident moon connoisseur, moonisaur, mistress of moons.

• 1:18Regina Barber?

  I like that one.

• 1:19Emily Kwong?

  OK, speaking of names, other moons have names, like Europa.

• 1:23Regina Barber?

  Yeah, so Europa is one of Jupiter's large moons. It's my favorite moon. But today's episode is all about Earth's moon. Some people call it Luna, but its name is the moon with a capital M.

• 1:35Emily Kwong?

  With a capital M. What is then the leading hypothesis on how she was born?

• 1:41Regina Barber?

  Through a collision.

• 1:43Speaker 06

  By a giant impact with a Mars-sized body named Theia and Earth that then shot out magma from the Earth and that balled up and formed the moon.

• 1:55Regina Barber?

  Wow. Yeah, that's Kelsey Prizel, a volcanic rock planetary chemist at Purdue University.

• 2:01Emily Kwong?

  I have so many questions. Like, where did Thea come from? Why did she come for us? And how did the moon come to stay in our orbit and be so round and so beautiful?

• 2:10Regina Barber?

  Em, I love your enthusiasm. I'm here for you. And actually, I had my own questions, too, so we'll tackle them all today.

• 2:18Emily Kwong?

  So today on the show, our moon. We try to answer one of the most enduring questions. How was the moon made? And how do we know?

• 2:26Regina Barber?

  And what does all of this have to do with water's origin story?

• 2:29Emily Kwong?

  You are listening to Short Wave, the science podcast from NPR.

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• 3:54Emily Kwong?

  All right, Gina, I know you love Europa, one of the moons of Jupiter.

• 3:57Regina Barber?

  Yes.

• 3:57Emily Kwong?

  But where does our homegrown moon rank for you?

• 3:59Regina Barber?

  It's my second favorite. Like, I actually have a phase of the moon tattoo. It's the first tattoo I've ever got.

• 4:04Emily Kwong?

  It's a cool tattoo, yeah.

• 4:05Regina Barber?

  Thank you. And, Em, you might think this love comes from, like, childhood, because it's, like, so deep. But kind of like Kelsey, the planetary scientist we heard from earlier, I learned a lot of what I know about the moon now when I was in college.

• 4:18Speaker 06

  There's been other impacts throughout Earth's history that often coincide with mass extinctions. And so when I was first learning about it, I was thinking of impacts are this crazy catastrophic thing that happens. Little did I know, they could form a brand-new planetary body.

• 4:36Emily Kwong?

  An impact that gave us our moon. This is the hypothesis I want to learn more about.

• 4:40Regina Barber?

  Okay.

• 4:42Emily Kwong?

  Before we get into it, though, are there other competing hypotheses?

• 4:45Regina Barber?

  Yeah, so there are ones that, like, don't involve a collision at all. So the moon could have just been made the same way Earth was made, in this planetary disk around the sun, all this rock and debris, like, clumping together. Think of the sun and having rings like Saturn.

• 4:59Emily Kwong?

  I've heard early solar system formation is like roller derby. It's just all these players, like, knocking each other around.

• 5:05Regina Barber?

  Colliding into each other, totally.

• 5:07Emily Kwong?

  Yeah.

• 5:07Regina Barber?

  So there's another hypothesis I talked about with Tab Priesel, a planetary scientist also at Purdue.

• 5:12Speaker 08

  This moon capture idea, that there was already a moon body somewhere in the solar system that sort of got captured up into Earth's gravitational orbit.

• 5:23Emily Kwong?

  Oh, I'm gonna call this the Jimmy Stewart theory. You know how Jimmy Stewart says to Mary in "It's a Wonderful Life," "I'm gonna lasso the moon"?

• 5:32Regina Barber?

  Yeah, yeah.

• 5:33Emily Kwong?

  Earth kind of was like, "Come here, I got you."

• 5:35Regina Barber?

  Very romantic.

• 5:36Emily Kwong?

  Yeah.

• 5:36Regina Barber?

  But Tab did add that this idea of moon capture has mostly been ruled out.

• 5:41Emily Kwong?

  Well, okay.

• 5:43Regina Barber?

  It's because there's all this evidence that the moon and the Earth are very similar, rather than being completely unique bodies that ended up orbiting the other by accident.

• 5:52Emily Kwong?

  Oh.

• 5:52Regina Barber?

  An early hint of this came from the Apollo missions from 1969 to 1972. They brought back samples of the lunar rock to be studied, and those samples revealed that the Earth and the moon have almost identical chemical and elemental compositions.

• 6:07Emily Kwong?

  Ah, yes, this does support the idea that something crashed into Earth and the moon is just shrapnel from that collision.

• 6:13Regina Barber?

  Yeah, yeah, and of course, how it happened exactly is still being studied. Here's Tab again.

• 6:19Speaker 08

  One thing that the Apollo samples have really done... Is exemplify the value of sample return missions that, you know, even after decades and decades of study, we're still learning new things about the moon and Earth-moon system and developing these alternative scenarios for the Earth-moon system forming impact.

• 6:39Emily Kwong?

  Okay, so among these other theories, there's the moon capture hypothesis, the Lasso one. You mentioned this idea that the moon just like clumped together from debris in Earth's planetary disk. Are there any other theories that scientists are kind of, I don't know, not sure of, but are out there?

• 6:55Regina Barber?

  Yeah, there's one other hypothesis that takes into account the very similar composition between Earth and the moon. This one suggests Earth spinning so quickly when it was forming that stuff just like flew off. So imagine like I'm spinning on ice or something and I'm spinning so fast that my arm just flies off.

• 7:12Emily Kwong?

  Ugh, and your arm would become your moon?

• 7:14Regina Barber?

  Exactly. And TAB is actually more willing to entertain this idea than the non-collision options.

• 7:20Emily Kwong?

  Hmm.

• 7:21Speaker 08

  I live in the realm that everything is still possible and it's really, really hard to completely objectively rule something out to 100% certainty. So I'm still happy with that.

• 7:30Regina Barber?

  But he also said that there's been a lot of computer simulations that just don't support that our Earth would have been spinning that fast enough to do that and still have like the Earth-moon system we have today. And it just doesn't fit.

• 7:42Emily Kwong?

  Yeah.

• 7:42Regina Barber?

  So this brings us to why scientists think the most likely series of events was that Thea, this early protoplanet, smashes into Earth and makes the moon.

• 7:51Emily Kwong?

  This is the part of the story that, I don't know why, it just amazes and terrifies me. It's like this idea of another body coming closer and closer and then smashing into Earth. Like, can you imagine what that looked like from the volcanoes here? They were probably like, ah, what is that?

• 8:09Regina Barber?

  Well, I mean, we were still a protoplanet ourself, you know? Like, it was still that roller derby. All these things were happening. We were getting pummeled by like asteroids. I don't know.

• 8:19Emily Kwong?

  So baby Earth was like, no, just more of the same. Sounds about right.

• 8:22Regina Barber?

  Same old, same old.

• 8:23Emily Kwong?

  I can take it. Yeah.

• 8:24Regina Barber?

  Yeah, so scientists think that Thea was possibly Mars size, due again to computer simulations. But of course, there's like debate over the exact size, the speed, if there were more objects than Thea, what angle Thea, or maybe these other objects came in at.

• 8:39Emily Kwong?

  More than Thea, like Theas? Many Theas could have come for us?

• 8:44Regina Barber?

  Yeah, well, instead of like a Mars-sized object, like a whole bunch of like smaller things.

• 8:48Emily Kwong?

  Okay. Going, though, with the leading idea that it was one object.

• 8:53Regina Barber?

  Yeah.

• 8:53Emily Kwong?

  What happened after the collision? Like, break down for me the lore and how it resulted in the moon.

• 8:59Regina Barber?

  So Thea is just flying around near the sun, like, with all that other debris making all the planets, because they're all still forming. And it comes towards Proto-Earth, and it just smashes into it. And the result of this collision is tons of material just flying off of Proto-Earth, and Thea probably exploded.

• 9:18Emily Kwong?

  Yeah.

• 9:19Regina Barber?

  So all of this debris starts to, like, orbit Earth, kind of like that planetary disk that made all the planets orbiting the sun. But now this disk is around the Earth. And the disk starts to clump. So, like, over time, gravity is pushing in all directions. It's making the clumps turn into things that are more spherical. And that's actually one of the criteria to be a planet. You have to be spherical. But remember, these processes are really hot. So in this hypothesis, when the moon does coalesce, it also has this magma ocean covering the entire surface.

• 9:52Speaker 08

  That magma ocean concept was really developed through Apollo missions and sample return. And it is a new perspective that we've added to kind of all the rocky planets in our solar system. The Earth may have had a magma ocean, Mars, Mercury, Venus.

• 10:09Regina Barber?

  That's Tab again, and he said instead of icebergs, they were like these giant chunks of rock. Hmm. He called them rockbergs, floating in this magma ocean, and that it eventually cooled and made this crust over the entire moon. And this has been verified with those sample return missions.

• 10:26Emily Kwong?

  Yeah.

• 10:27Regina Barber?

  But, Em, I haven't even told you, like, in my opinion, the coolest thing yet.

• 10:32Emily Kwong?

  What's that?

• 10:33Regina Barber?

  Thea came from the inner solar system. And this is like a huge clue for how water came to Earth.

• 10:39Emily Kwong?

  I kind of remember you talking about this before. Tell me again.

• 10:42Regina Barber?

  Yes. Okay, so there are some hypotheses that water came to Earth by objects from space.

• 10:47Emily Kwong?

  Mm-hmm.

• 10:47Regina Barber?

  They were, like, carrying water, and then they smashed into Earth. But here's the thing.

• 10:51Speaker 09

  Now we find that Thia most likely comes more from the inner solar system, which was assumed to be a more dry place. Generally, closer to the sun, it's hotter, so there's less water in the bodies. That means Thia could not have brought a lot of water to the Earth.

• 11:05Regina Barber?

  So that's Timo Hoppe, a planetary scientist and geochemist in Germany. And he was part of the team that's gotten the closest to finding out where Thia might have come from.

• 11:14Emily Kwong?

  OK, so Thia is not our water bearer.

• 11:16Regina Barber?

  No.

• 11:17Emily Kwong?

  I love learning about this, this full, comprehensive history of Mama Moon.

• 11:22Regina Barber?

  Yes.

• 11:23Emily Kwong?

  And we just went there, right? Humans looped the moon, brought back a ton of data that NASA is going through right now. Why do this? What can scientists learn from studying the moon in even greater detail now?

• 11:35Regina Barber?

  Yeah, this is the clincher that I really, really liked after talking to these researchers, that studying the moon can tell us something about the history of Earth.

• 11:43Speaker 08

  The surface of the moon is this archive of deep time that we have just completely lost. If we want to study the ancient sun, cosmic rays and, you know, this galactic processes, they're being recorded on the lunar surface and in the lunar surface materials. On Earth, anything that was happening, you know, four and a half billion years ago is gone. We just do not have that rock record.

• 12:09Emily Kwong?

  Wait, why? Why isn't Earth as good of a record for these galactic changes?

• 12:16Regina Barber?

  Earth's surface is active. It's constantly changing.

• 12:19Speaker 06

  Whatever was going on back then has been recycled by plate tectonics, by erosion, but from our atmosphere, from our oceans. We use the moon as this archive of everything that has happened in the solar system. And we've visited it and we've sampled it. And we've been able to analyze those things in our lab, which is something we haven't done for any other planetary body.

• 12:42Emily Kwong?

  The moon is essential. NASA plans to go back and study it more. What did your sources say about what they want to learn on future moon missions?

• 12:51Regina Barber?

  Well, Kelsey says it's all about that difference between the far side and the near side of the moon. And remember, like, the near side is always facing Earth because of how it's formed. And she says that there's just this big difference between them and what we know about them.

• 13:05Speaker 06

  This is a dichotomy that we've recognized for a really long time. And with Recent sample return missions from the Chinese Space Agency, we now have a sample from the far side to start to compare to the near side.

• 13:21Regina Barber?

  And Kelsey says all this can tell us even more about the origin story of the moon. And Tab gave me some perspective on how much lunar surface area humans have actually explored. Because even though we sent so many missions to the moon, there's still so much to explore. Here's Tab again.

• 13:38Speaker 08

  When you think back to the amount of surface we've been able to explore through the Apollo missions, it sums to essentially like a commute from Providence, Rhode Island to Boston, Massachusetts. So just like 45 minutes. And I don't say that to de-emphasize the immense value of the Apollo missions, but to really highlight how very little of the lunar surface we've been able to explore.

• 14:03Emily Kwong?

  Gina, thank you for telling us the story of the moon.

• 14:05Regina Barber?

  You're welcome, Em. I had a great time.

• 14:08Emily Kwong?

  If you like this episode and you want more space and moon joy in your life, check out our Space Camp series. We'll put a link to that in our show notes. It's really good.

• 14:16Regina Barber?

  Yeah, and maybe you know someone who doesn't know about the moon's origin story. And if so, share this episode with them. We would really appreciate it.

• 14:24Emily Kwong?

  This episode was produced by Brulee McCoy and Hannah Chin. It was edited by our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Kwasi Lee was the audio engineer. I'm Emily Kwong.

• 14:34Regina Barber?

  And I'm Regina Barber. Thank you for listening to Short Wave, the science podcast from NPR.

• 14:46Speaker 07

  Stay tuned for the following announcements and messages from our sponsors.

• 14:50Speaker 00

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• 15:03Speaker 07

  Support comes from Credit Union of Colorado. Supporting members throughout Colorado for over 90 years with personalized customer service, access to over 30,000 fee-free ATMs nationwide, and a seamless mobile app. Credit Union of Colorado provides not-for-profit banking with a focus on building community. They are dedicated to offering the services of a big bank with a heart. Credit Union of Colorado, honestly good. Learn more at honestlygood.org.