Why do we have a moon?

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It seems like everyone is going to the moon right now.

A couple weeks ago, India launched a mission to the moon, and if the probe lands safely, it's going to be the first time a country besides the US, China, or Russia has landed a ship safely on the moon.

But India is just the start.

Japan has a moon launch scheduled for next month, Russia is going back, and the US has three moon missions with private companies on target for later this year.

It seems like slowly but surely the moon is becoming more accessible, and our relationship with it might start to look different pretty soon.

So we thought it would be a great time to revisit an episode we made on why we even have a moon in the first place.

It was part of our Lost World series we did last year all about ancient mysteries.

So if you missed it, now would be a great time to binge the whole thing.

The moon's always been my favorite thing in the sky.

Science writer Rebecca Boyle has a soft spot for the moon.

I think for a lot of people, it's like taken for granted.

It's this sort of humdrum thing, and galaxies and nebula and, you know, stars and planets are more intriguing for people, but the moon is very weird.

In all our searching of the cosmos, we've seen other planets with moons, but we've never seen another moon quite like ours.

The other rocky planets don't even really have moons.

Mercury has no moon.

Venus has no moon.

Mars does have moons, but they're nothing like our big, white, round planet of a moon.

They're not even really round.

They're like potatoes.

And they're tiny, like they're just rocks.

The gas giants, like Jupiter and Saturn, those guys, they have tons of moons.

Big, proper, planet-sized moons.

But they're much, much smaller than their host worlds, relatively speaking.

Our moon?

Our moon is huge compared to the size of the Earth.

Like almost a quarter of the size of our planet.

That makes our our moon the fifth biggest moon in the entire solar system.

And our giant weird moon is one of the biggest mysteries in planetary science.

We have no idea why the moon is here and it's the moon.

It's not like this is some strange faraway thing.

It's the moon.

The mystery of how we got our moon unfolds into an epic tale.

A strange, world-bending apocalypse of mythic proportions.

I'm Meredith Hodnot, and this is Lost Worlds, a new series from Unexplainable, exploring scientific mysteries buried in the deep, deep past.

This week,

how did we get our moon?

You're looking at a kind of world that hasn't existed for millions of years.

It hasn't existed just in millions and millions.

Millions and millions.

It's almost as if time forgot this place.

Is there life on Mars?

There's a whole universe out there, Steve.

Beyond anyone's comprehension.

In the 1950s and 60s, scientists thought that the moon probably formed like a lot of the other celestial bodies in our solar system.

Scientists believe the moon formed from the gravitational attraction of small particles floating in space.

Billions of years ago, the solar system was a chaotic and honestly pretty messy place.

Debris and dust were orbiting around our newly born star and slowly smushing together to form planets and moons.

It was kind of thought to be this stage of planetary formation that created the Earth and Mars and Mercury and Venus and all the gas giants, where it was made from these crumbs left over from the sun's formation.

But when the Apollo missions landed on the moon, we were actually able to test these theories for the first time.

Apollo 15 opened the scientific phase of moon exploration.

These Apollo scientists were fascinated with the moon because it's a time capsule.

The Earth buries its ancient past with plate tectonics and erosion, but the moon is a world frozen in time.

Almost any rock the astronauts stumble over has not been altered and quite likely has not been moved since long before the appearance of man on Earth, before the record of any fossil in the deepest rock on Earth.

The Apollo astronauts scoured the moon's surface, looking for clues to unlock this celestial time capsule.

Guess what we just found?

I think we found what we came for.

That one crystalline rock, and they found it atop a large brown rock sitting there by itself like it had been waiting for them.

On a pedestal.

And a pedestal had been there millions of years.

They went to the right place.

In total, the Apollo missions brought down like 850 pounds of rocks from the moon.

And a few of these lunar samples made their way to a lab in in MIT in 1979, where Darby Dyer was an undergraduate lab assistant.

Mostly I was, you guessed it, grinding rocks and picking minerals out of rocks.

One day, Darby asked her professor what he was working on.

And he said, oh, I'm just grinding up a lunar sample.

Would you like to hold it?

And I went, uh.

He said, here, you can hold it.

And my hand shook.

I couldn't believe.

that I was really holding a lunar sample.

You know, at that point, the memory of the Apollo landings was fresh in my mind.

We interrupt our regularly scheduled programming in order to bring you the following special report on the Apollo 15 space mission.

My elementary school had one television.

It was in the library, and they would take the entire elementary school and sit us down in front of this little TV.

And here's the first astronaut out of the land.

Watching the astronauts splashing down in the ocean, watching them get out of the capsule with these little things that looked like igloo coolers, which were the things that they were carrying the lunar samples in and here comes the second astronaut isn't it amazing that all of that culminated in these samples and here i darby dyer am holding one of these samples i just never thought i would be in this situation and here i am so how incredibly lucky am i

darby and the rest of the planetary science community were totally seduced by these rocks from another world.

And as the preliminary research came in, these rocks started to challenge everything we thought we knew about the moon.

In 1984, a bunch of Darby's grad school friends went to a conference.

And they came back just totally lit up.

And I remember them coming back and saying, you're just not going to believe this.

Geologists had found that the moon was covered in a special kind of rock called a northrosite.

Glittery, bright, reflective.

This is the rock that makes the moon shine white in the night sky.

And at the time, it was thought this rock could only be formed in a very specific way.

Magma.

And since the moon is absolutely covered in this stuff, the vast majority of the surface of the moon is an orthocyte.

Scientists thought, at one point, the moon could have been a giant magma ocean.

And in order for the moon to have been this floating magma ocean in space, there needed to have been some sort of cataclysm, something that poured so much energy into the moon that it literally melted.

This and other evidence meant that the moon couldn't have been created by the slow smooshing of dust and crumbs over millions of years like scientists originally thought.

There had to have been some epic, violent event.

It's always neat when a paradigm gets challenged and crushed and suddenly all the possibilities of the new idea occur to you.

Then you go racing after that idea.

It's really neat.

neat.

So scientists started to piece together a new story about the formation of the moon, the giant impact hypothesis.

Science writer Rebecca Boyle is going to help me walk through this story.

So once upon a time, some 4 billion years ago, there is a small rocky planet, which scientists named Theia, after Greek mythology.

Theia is the mother of the Greek goddess of the moon.

Theia was probably the size of Mars, a little smaller than Earth and floating in the same general orbit.

But that's not a big enough location for two planets.

And so gravity would have brought them together.

This collision was inevitable and

it would have been the worst day in the history of Earth.

If you were standing on Earth and could somehow witness this without being obliterated, you know, a few days before the collision, you would have seen like what we have now, a moon, another planet in the sky at night.

It would have been pretty bright, lit by the sun, but it would have been getting closer.

You would feel it through gravity.

You would feel the Earth shaking.

There would be, you know, horrendous earthquakes down from Earth's mantle.

There would be...

crazy winds, crazy tides.

The whole planet's being torn apart, essentially.

Earth would be kind of melting from within at this point.

All of a sudden, the whole sky would kind of close off.

It's a whole planet sort of like coming through space towards you.

Thea sort of glances off Earth.

Doesn't hit head-on, and both worlds blew apart, but Theia blew apart more and the moon is left over from that.

Like it's the core of this planet that was destroyed.

These skeletal remains of Theia burned magma hot and then cooled into our bright white moon.

Since the 80s, the giant impact hypothesis has become the story of moon formation formation among the planetary science community.

And as our examination of the Apollo moon rocks got deeper, scientists hope to get a better understanding of Theia because planets, depending on where and when they form in the solar system, will have different chemical signatures.

So Mars rocks will look different from Earth rocks that look different from Venus rocks.

Because they formed in different locations around the Sun at slightly different times, they look different enough chemically that you you could tell them apart.

So if the moon formed from the molten core of Theia, then the rocks from the moon should look like rocks from another planet.

The moon should look like Theia, you know, chemically speaking.

If the moon is the remains of this world Theia, it should look like that world, and it doesn't.

It looks like Earth.

There seems to be no trace of Theia.

The geology and the physics of how the moon moves suggest a giant impact, something huge, that created a lot of heat and a lot of energy to form the moon.

But what hit us, we really don't know.

It's like seeing a completely smashed and totaled car on a quiet, empty street.

The thing that hit the early Earth and made the moon is nowhere to be found.

We can't interrogate this planet.

We can't find out what happened to it.

We don't know what it did, where it went.

It's a ghost now.

If Theia didn't exist, then what could have formed the moon with so much force?

And if it did exist, then where is it now?

That's after the break.

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Hey, uh, is it just me, or is the moon getting closer?

Time to introduce

It's unexplainable.

We're back.

I'm Meredith Hodnott, and we're exploring the mysterious origins of the moon.

As scientists looked deeper and deeper into the Apollo lunar samples, these rocks revealed one mystery after another.

First, the rock's geology told us a story of two worlds colliding, an impact between the early Earth and another planet altogether, Theia.

But when scientists went looking for evidence of Theia and the chemical composition of these moon rocks, they found that the moon and the Earth, they look the same.

They look very much like the same stuff.

Like you just kind of had one clump of material and then you pulled off part of it.

Science writer Rebecca Boyle has been fascinated by this missing planet.

the lost world of Theia.

And she's talked to scientists from around the world who've been trying to come up with new theories of how we got our moon and what happened to Theia.

One idea is that the giant impact was much more epic than we ever could have imagined.

So let's rewind the tape of the giant impact hypothesis.

Once upon a time, there is a rocky planet called Thea.

Theia is the mother of the Greek goddess of the moon.

Theia was a planet about the size of Mars in the same orbit around the sun as the Earth.

But But that's not a big enough location for two planets, and so gravity would have brought them together.

Wild gravity earthquakes, the sky closes off.

It's a whole planet sort of like coming through space towards you.

But instead of Theia glancing off, forming the moon, and leaving the Earth devastated but largely intact, this new theory says that both worlds, the early Earth and Theia, just completely melted together.

And ended up in this like bagel-shaped, rotating

hellfire disc of molten rock droplets.

Earth, Theia, Moon, everything was combined into one giant magma bagel.

All the magma on the inside swirled together and combined to form the Earth anew.

while the magma on the outside cooled and made the moon.

But they were all mixed together when they first collided.

So in this theory, we can't find evidence of Theia because it's so thoroughly mixed into what we know as the Earth and the moon today.

But my favorite theory, though it's not peer-reviewed yet, is that Theia, it's still with us.

Just not up in space, but deep, deep underground.

In the Earth's mantle, the thick layer of molten rock around the core of the planet, there are two places that are

just weird.

One is in the mantle near Africa, and one is in the mantle in the Pacific.

They're denser than the rest of the mantle.

We can see that by measuring seismic waves.

And they're massive.

They're like larger than continents.

They bracket the core almost like earmuffs.

And no one knows what these are.

They're these weird blobs.

And there's a new theory that argues that these weird ear muff blobs might be Theia.

That these blobs are like the remains of Theia

that was basically consumed by Earth.

Earth swallowed Theia, and now parts of Theia are inside Earth.

inside its mantle, beneath its crust, forever.

So in this theory, the Earth swallowed Theia, and the moon is an orb of melted earth flung into space from the collision.

But, you know, this theory, it's gonna be hard to prove.

It's a little tricky to get samples from the mantle, you know, thousands of miles underground.

Like we can through volcanic rock.

And people are doing that, looking at volcanic rocks from Earth to see if we can maybe sniff out the fingerprints of Theia consumed by our planet early in its history.

Looking for evidence of a lost alien world beneath our feet.

The truth is, we may never find Theia or get the full story of how we got our mysterious, gigantic moon.

But scientists continue searching because these epic moon creation myths, they lead to even deeper, more epic questions about our moon and about us.

I just, I would like to know why.

Like, why do we get it?

Why is it here?

And why does it not exist somewhere else?

And how rare is that?

How rare is this collision between a Theia and an early Earth that leads to this particular moon and this particular planet we live on?

It might be really, really uncommon.

And that might be a unique situation in the entire broader cosmos.

Getting our giant moon might be the luckiest thing to have ever happened to this planet.

It's shaped every age of the Earth.

And it's even thought that without the moon, there would be no life on Earth as we know it.

Don't you want to know?

What led to us?

Like, why here?

Why now?

Why nowhere else?

You know, we have yet to find in our very, very detailed, desperate searching anything else like us, anywhere else like this.

And

I think the moon is a huge reason why.

So let's start in the early primordial oceans of Earth.

The moon's massive size means its gravity pulls our oceans towards the sky and gives us the tides.

The water bulges out to follow the orbit of the moon.

If life began in the oceans, which is most people's theory, in some

hydrothermal vent, deep ocean fissure in the Earth's crust, then the moon's tides would have mixed the water in such a way that it would have eventually dragged those early primitive life forms around and moved them and pushed them and brought them closer to continental shelves where they would have colonized the sea floor.

More millions of years and the life in the moon's tides reached the shore.

So the ancestors of all land animals, everything that walks with a backbone, began as fish.

And the moon probably played a role in bringing them to shore.

Tide going out would have stranded them on beaches and eventually they would have evolved to breathe air and to walk on land.

And the moon probably is one reason why they walked on land in the first place.

And beyond the tides, the earth owes a lot to the moon.

Its climate, its atmosphere, its temperature gradients, its seasons.

All of these things that we take for granted are largely sculpted by the moon.

The earth is tilted on its axis.

23 and a half degrees.

This tilt is what makes the seasons.

In the summer, the days are longer and warmer because you're on the hemisphere that's tilted 23.5 degrees towards the sun.

And in the winter, that same tilt away from the sun leads to cold, short days.

Which is like just enough to make a difference, but it's pretty calm as planets go.

The moon's mass and momentum as it orbits the earth plays a big role in keeping that tilt steady.

The moon really stabilizes Earth in its orbit around the sun and in its tilt on its axis as it spins.

And that keeps our climate pretty stable over millennia.

This stability is crucial.

And to see why, just look over at Mars.

Mars kind of wobbles like a top.

It's very, it goes wildly back and forth on its axis.

And it creates really dramatic changes in its seasons and its winds.

If the Earth wobbled like Mars.

It would make a huge difference to life here.

The temperature would be so different.

The climate would be the Arctic one millennia and then Florida the next, you know.

And so it's not really enough time for things to evolve to handle such extremes.

That could have had a huge effect on the way life evolved here, whether it did at all.

And as we look out into the universe for life on alien worlds, we might need to look for alien moons as well.

There are basically infinite planets, you know, and infinite places to look for life, but you might need a moon.

You really might need this huge anchoring force

right next to you.

All these grand stories and cosmic questions were contained in just a few hundred pounds of moon rocks brought down by the Apollo missions decades ago.

I feel like rocks are

they have a whole history.

They tell such involved stories and you just have to ask them the right questions.

People's entire careers are spent answering one question like this on one rock, you know, which I think is really fascinating.

Do you have a favorite Apollo to get samples from?

No, they're all amazing.

And, you know, my favorite thing actually, so occasionally you can see my office has this beautiful window.

And I often teach classes at night.

And every now and then I'll come back from class at night and this building and the campus is quiet and the moon will be looking in my window.

And, you know, every now and then I get the samples,

I go get the samples out of the safe and I stand here and hold the sample in my hand and look at the moon.

There's just something amazing about living in an age where you can hold a piece of the moon in your hand and look up at the sky and also see it.

Put your hand out, Meredith.

Oh man.

There you go.

Holy cow.

You've touched the moon.

This is incredible.

This episode was reported and produced by Meredith Hodnat.

It was edited by Catherine Wells with help from Brian Resnick and Noam Hasenfeld, who scored the episode with Meredith.

Sound design from Afim Shapiro and Christian Ayala, fact-checking from Richard Seema, and Bird Pinkerton and Manding Wen are running up that hill, parentheses, deal with God, and parentheses.

To find more of Rebecca Boyle's mind-blowing science writing, check out her website, rebeccaboyle.com.

And special thanks to Darby Dyer for welcoming Meredith into her lab at Mount Holyoke College and for letting her touch the moon.

To read more about some of the topics we cover on our show or to find episode transcripts, check out our site at vox.com/slash unexplainable.

And if you have thoughts about the show, you can always email us at unexplainable at vox.com.

Or you could leave us a review or a rating, which we would love to.

Unexplainable is part of the Vox Media Podcast Network, and we'll be back next week.

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