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, powerful companion to Earth.
And for the first time in over 50 years, humans have traveled around the moon.
Three, two, one, booster ignition and liftoff.
And we have a beautiful moon rise.
We're headed right at it.
We have reached the closest point of our destination to the moon.
And back on the Artemis II mission.
A new chapter of the exploration of our celestial neighbor is complete.
Integrity's astronauts.
Back on Earth.
And all of this moon joy has got me thinking about everything the moon has been through.
Because she's been through a lot.
Yes.
Regina Barber, my shortwave co-host and astrophysics queen.
Yes.
Also our resident moon connoisseur.
True.
Mistress of moons.
I like that one.
Okay.
Speaking of names.
Other moons have names, like Europa.
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.
With a capital M.
What is then the leading hypothesis on how she was born?
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.
Wow.
Yeah, that's Kelsey Preisel, a volcanic rock planetary chemist at Purdue University.
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?
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.
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?
And what does all of this have to do with water's origin story?
You are listening to Shortwave, the science podcast from NPR.
All right, Gina, I know you love Europa, one of the moons of Jupiter.
Yes.
But where does our homegrown moon rank for you?
It's my second favorite.
I actually have a phase of the moon tattoo.
It's the first tattoo I ever got.
It's a cool tattoo.
Thank you.
Yeah.
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 mood now when I was in college.
There's been other impacts throughout life 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 that gave us our moon.
This is the hypothesis I want to learn more about.
Okay.
Um, Before we get into it, though, are there other competing hypotheses?
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 clumping together.
Think of the sun and having rings like Saturn.
I've heard early solar system formation is like roller derby.
It's just all these players knocking each other around.
Colliding into each other, totally.
So there's another hypothesis I talked about with Tab Preisel, a planetary scientist also at Purdue.
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.
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?
Yeah, yeah.
Earth kind of was like, come here.
I got you.
Very romantic.
Yeah.
But Tab did add that this idea of moon capture has mostly been ruled out.
Well, okay.
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 compositions.
Wow.
Ah, yes.
This does support the idea that something crashed into Earth and the moon is just shrapnel from that collision.
Yeah, yeah.
And of course, how it happened exactly is still being studied.
Here's Tab again.
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.
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?
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.
And your arm would become your moon.
Exactly.
And Tab is actually more willing to entertain this idea than the non-collision options.
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.
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 the Earth-Moon system we have today, and it just doesn't fit.
So this brings us to why scientists think the most likely Ceres events was that Theia, this early proto-planet, smashes into Earth and makes the Moon.
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 to And then smashing into Earth.
Can you imagine what that looked like from the volcanoes here?
They were probably like, ah, what is that?
Well, I mean, we were still a proto-planet ourself.
It was still that roller derby.
All these things were happening.
We were getting pummeled by asteroids everywhere.
I don't know.
So baby Earth was like, no, just more of the same.
Sounds about right.
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 in at?
More than Theia?
Like Theia's?
Many Theia's?
Yeah.
Could have come for us?
Well, instead of like a Mars size object, like a whole bunch of like smaller things.
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.
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.
Yeah.
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.
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 Tab again.
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.
But I haven't even told you, like, in my opinion, the coolest thing yet.
What's that?
The came from the inner solar system.
And this is like a huge clue for how water came to Earth.
I kind of remember you talking about this before.
Tell me again.
Yes.
OK, 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.
Now we find that they are 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.
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 Theia might have come from.
Okay, so Theia is not our water bearer.
No.
I love learning about this, this full comprehensive history of Mama Moon.
Yes.
And we just went there, right?
Humans looped the moon together. 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?
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.
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 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.
Wait, why?
Why isn't Earth as good of a record for these galactic changes?
Earth's surface is active.
It's constantly changing.
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.
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?
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.
This is a dichotomy that we've recognized before. 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.
And Kelsey says all this can tell us even more about the origin story of the moon.
And TAP 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.
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 it's 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.
Gina, thank you for telling us the story of the moon.
You're welcome, Em.
I had a great time.
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.
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.
This episode was produced by Brilly McCoy and Hannah Chin.
It was edited by our showrunner, Rebecca Ramirez.
Tyler Jones checked the facts.
Kweisi Lee was the audio engineer.
I'm Emily Kwong.
And I'm Regina Barber.
Thank you for listening to Shortwave, the science podcast from NPR.
