Where did our moon come from?

Summary

Short Wave (NPR) — Episode Summary

Episode Title: Where did our moon come from?
Date: April 22, 2026
Hosts: Emily Kwong, Regina Barber
Guests: Kelsey Preisel (planetary chemist, Purdue), Tab Preisel (planetary scientist, Purdue), Timo Hopp (planetary scientist & geochemist, Germany)

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Overview

In this engaging and lively episode, hosts Emily Kwong and Regina Barber explore the origin story of Earth’s Moon. Inspired by recent lunar missions and personal moon fascination, they unpack the leading scientific hypotheses, discuss how new lunar samples are reshaping what we know, and delight in the mysteries the Moon still holds—especially regarding the formation of water and the deeper history of both Earth and its celestial companion.

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Key Discussion Points & Insights

1. The Moon’s Significance and Recent Human Missions

• Emily expresses how the Moon “grounds” her and notes the excitement of recent lunar missions (00:16).
• Regina shares that Earth’s Moon is her second favorite, after Jupiter's Europa. She even has a moon phase tattoo, signaling deep personal connection (04:06).

2. Leading Scientific Hypotheses for the Moon’s Origin

The Giant Impact Hypothesis

• Summary: The dominant theory is that a Mars-sized proto-planet named Theia collided with a young Earth, ejecting debris that coalesced to form the Moon.
• Quote:
  • “Through a collision 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.”
    — Kelsey Preisel (01:39)
• Regina and Emily delight in the vividness (and wildness) of this catastrophic event (07:58):
  • “It amazes and terrifies me... another body coming closer and closer and then smashing into Earth... Can you imagine what that looked like from the volcanoes here? They were probably like, ‘Ah, what is that?’” — Emily Kwong (07:58)

Competing Hypotheses

• Co-accretion: The Moon formed alongside Earth from the same disk of material around the Sun.
• Capture Theory: The Moon formed elsewhere and was pulled into Earth’s orbit ("the Jimmy Stewart theory") — now largely ruled out due to similarity in Moon and Earth composition.
  • “Earth kind of was like, come here, I got you.” — Emily Kwong (05:30)
• Fission Hypothesis: Earth spun so fast during its formation that part of it broke off to form the Moon. Most computer simulations don’t support this possibility.

3. Evidence from Lunar Samples

• The Apollo missions brought back rocks showing the Moon and Earth are nearly chemically identical.
  • “Those samples revealed that the Earth and the Moon have almost identical chemical and elemental composition.” — Regina Barber (05:49)
• Sample analysis has been invaluable, showing the importance of sample return missions:
  • “Even after decades and decades of study, we're still learning new things about the Moon and Earth-Moon system...” — Tab Preisel (06:25)

4. How the Giant Impact Shaped the Moon (and Earth)

• Theia probably exploded on impact, with debris forming a disk around Earth that eventually became the Moon.
• The early Moon likely had a molten “magma ocean” that cooled to form its crust.
  • “Instead of icebergs, there were like these giant chunks of rock... rockbergs floating in this magma ocean.” — Tab Preisel (10:17)

5. Surprising Lessons About Water

• Recent research indicates Theia came from the inner, drier part of the solar system, meaning it likely didn't deliver Earth’s water.
  • “Theia could not have brought a lot of water to the Earth.” — Timo Hopp (11:11)

6. The Moon as an Archive of Solar System History

• The Moon preserves a geological record Earth can't: Earth's active surface has erased much of its early history.
  • “The surface of the Moon is this archive of deep time that we have just completely lost.” — Tab Preisel (11:50)
• The Moon helps us study the ancient Sun, cosmic rays, and “galactic processes,” since its surface records events Earth lost to erosion and tectonics.

7. The Need for Further Exploration

• Despite many missions, only a tiny fraction of the lunar surface has been explored—comparable to a commute from Providence to Boston (13:45).
• New missions and far side samples (recently brought by China) could unravel more about the Moon’s formation, particularly the difference between its far and near sides.

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Notable Quotes & Memorable Moments

• On lunar tattoos and moon love:
  • “It’s my second favorite. ...I actually have a phase of the moon tattoo. It's the first tattoo I ever got.” — Regina Barber (04:06)
• On the wildness of impacts:
  • “Impacts are this crazy catastrophic thing that happens. Little did I know they could form a brand new planetary body...” — Kelsey Preisel (04:26)
• On the poetic, emotional connection:
  • “I don't know about you, but nothing grounds me quite like looking at the moon. This beautiful, beautiful, powerful companion to Earth.” — Emily Kwong (00:16)
• On Theia’s role in forming the Moon (and not bringing water):
  • “Now we find that Theia most likely comes more from the inner solar system, which was assumed to be a more dry place... That means Theia could not have brought a lot of water to the Earth.” — Timo Hopp (10:57)

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Timestamps for Important Segments

• [00:16] — The hosts’ love for the Moon and launch into the episode’s theme
• [01:39] — Kelsey Preisel explains the Giant Impact Hypothesis
• [04:06] — Regina shares her moon tattoo and affection for the Moon
• [05:13] — Overview of competing hypotheses (co-accretion, capture, fission)
• [05:49] — Evidence from Apollo samples: Earth and Moon composition
• [06:46] — Value of sample return and ongoing discoveries (Tab Preisel)
• [07:58] — Breaking down the aftermath of the Theia collision
• [09:59] — “Rockbergs” and the Moon’s magma ocean
• [10:39] — The mystery of water’s origins and Theia’s dryness (Timo Hopp)
• [11:41] — Why studying the Moon matters for understanding Earth’s history
• [13:11] — Far side vs. near side of the Moon; potential from new samples
• [13:45] — How little of the Moon has actually been explored

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Conclusion

The episode wraps up with the hosts marveling at how the Moon serves as a time capsule for the entire solar system—and a testament to the dynamism of early planetary formation. While much has been learned from returned samples and fresh data, many questions remain—not just about the Moon, but Earth's own deep past. The show leaves listeners with a sense of wonder, playfulness, and anticipation for what the next era of lunar exploration will reveal.

If you enjoyed this episode, check out Short Wave's Space Camp series (link in show notes).

Transcript

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[Emily Kwong]

You're listening to Shortwave from NPR. Hey, Shortwavers. Emily Kwong here and I don't know about you, but nothing grounds me quite like looking at the moon. This beautiful, beautiful, powerful companion to Earth. And for the first time in over 50 years, humans have traveled around the moon. 3, 2, 1. Booster ignition and lift off. And we have a beautiful moon rise.

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We're heading right at it.

[Emily Kwong]

We have reached the closest point of our destination to the moon on the Artemis 2.

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Sending post landing command now.

[Tab Preisel]

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

[Emily Kwong]

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

[Regina Barber]

Because she's been through a lot.

[Emily Kwong]

Yes. Regina Barber, my short wave co host and astrophysics queen.

[Regina Barber]

Yes.

[Emily Kwong]

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

[Regina Barber]

I like that one.

[Emily Kwong]

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

[Regina 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 sun. Some people call it Luna, but its name is the moon with a capital

[Emily Kwong]

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

[Kelsey Preisel]

Through a collision 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.

[Regina Barber]

Wow. Yeah. That's Kelsey Preisel, a volcanic rock planetary chemist at Purdue University.

[Emily Kwong]

I have so many questions like where did Theia 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?

[Regina 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.

[Emily 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?

[Regina Barber]

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

[Emily Kwong]

You are listening to Shortwave, the science podcast from npr.

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[Emily Kwong]

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

[Regina Barber]

Yes.

[Emily Kwong]

But where does our homegrown moon rank for you?

[Regina Barber]

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

[Emily Kwong]

It's a cool tattoo. Yeah.

[Regina 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, there's been

[Kelsey Preisel]

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, an impact

[Emily Kwong]

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

[Regina Barber]

Okay.

[Emily Kwong]

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

[Regina 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.

[Emily Kwong]

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

[Regina Barber]

each other around, sliding into each other. Totally.

[Emily Kwong]

Ye.

[Regina Barber]

Another hypothesis I talked about with Tab Preisel, a planetary scientist also at Purdue,

[Tab Preisel]

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.

[Emily Kwong]

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

[Regina Barber]

Yeah, yeah.

[Emily Kwong]

Earth kind of was like, come here, I got you.

[Regina Barber]

Very romantic.

[Emily Kwong]

Yeah.

[Regina Barber]

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

[Emily Kwong]

Well, okay.

[Regina 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. 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 composition.

[Emily Kwong]

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

[Regina Barber]

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

[Tab Preisel]

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.

[Emily 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?

[Regina 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 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.

[Emily Kwong]

And your arm would become your moon.

[Regina Barber]

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

[Tab Preisel]

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.

[Regina 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. So this brings us to why scientists think the most likely series events was that Theia, this early proto planet, smashes into Earth and makes the Moon this

[Emily Kwong]

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?

[Regina Barber]

Well, I mean we were still a proto planet 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.

[Emily Kwong]

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

[Regina Barber]

Just sounds like rice. Same old, same old. I can take it. Yeah, yeah. So scientists think that THEIA 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 Theia, what angle Theia, or maybe these other objects came

[Emily Kwong]

in at more than Thea, like Theias, many Theias that have come for us.

[Regina Barber]

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

[Emily Kwong]

Okay. Going though with the leading idea that it was one object, what happened after the collision like break down for me the lore and how it resulted in the Moon.

[Regina Barber]

So THEIA 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 Theia probably exploded.

[Emily Kwong]

Yeah.

[Regina 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.

[Tab Preisel]

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, that's TABB again.

[Regina Barber]

And he said instead of icebergs they were like these giant chunks of rock. 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. Yeah, but, em, I haven't even told you, like, in my opinion, the coolest thing yet.

[Emily Kwong]

What's up?

[Regina Barber]

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

[Emily Kwong]

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

[Regina Barber]

Yes. Okay, so there are some hypotheses that water came to Earth by objects from space. They were, like, carrying water, and then they smashed into Earth. But here's the thing.

[Tab Preisel]

Now we find that Theia 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. In the bodies. That means Theia could not have brought a lot of water to the Earth.

[Regina Barber]

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

[Emily Kwong]

Okay, so Thea is not our water bearer.

[Regina Barber]

No.

[Emily Kwong]

I love learning about this. This full, comprehensive history of Mama Moon. And we just went there, right? Humans looped the Moon, brought back a ton of data that NASA's going through right now. Why do this? What can scientists learn from studying the Moon in even greater detail now?

[Regina 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.

[Tab Preisel]

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.

[Emily Kwong]

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

[Regina Barber]

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

[Kelsey Preisel]

Whatever was going on back then has been recycled by plate tectonics, by erosion 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.

[Emily 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?

[Regina 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.

[Kelsey Preisel]

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.

[Regina 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 like 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.

[Tab Preisel]

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, you know, 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.

[Emily Kwong]

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

[Regina Barber]

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

[Emily 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.

[Regina 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.

[Emily Kwong]

This episode was Produced by Burleigh McCoy and Hannah Chen. It was edited by our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Kwesi Lee was the audio engineer.

[Regina Barber]

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

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