When Apollo astronauts returned from visiting  the moon, they brought some of it back to Earth.

And when scientists analyzed those lunar  samples, they were surprised by what they found… Because, although these rocks  came from another world, in a way,   they were strangely similar to rocks from Earth.

And this wasn’t the first time we recognized   that some aspects of our moon  make it a bit of an oddball.

In fact, us having a moon at all is unusual.

Almost all of the moons in  our solar system orbit planets   beyond the asteroid belt, like Jupiter and Saturn.

And our moon is also big – both  absolutely and compared to Earth.

It’s almost the same size as Mercury,   and more similar in size to its parent planet  than any other moon in our solar system.

So, where did our unique moon come from?

It turns out those lunar rocks are  a clue, pointing to the origin of   our closest cosmic companion, an origin  even stranger than you might imagine… Because, the moon’s formation might have something  to do with that time the Earth ate a planet.

In the beginning - more than 4 and a half billion  years ago - our solar system was a busy place.

At its center, the newly formed Sun was  surrounded by a cloud of gas, dust, and debris.

And in this protoplanetary disk, small  rocky particles were being pulled together   by gravity – smashing into each other and  combining to form bigger chunks of material.

These violent collisions  continued until eventually,   entire planets took shape…including Earth.

But this early version of Earth was  missing something very familiar…its Moon.

Scientists think that the Earth was  probably “flying solo” for somewhere   between the first 60 to 250 million years  after the formation of the solar system.

And over the years, there have been  lots of ideas about the Moon’s origins.

Scientists once thought it might have  formed at the same time as Earth,   in that protoplanetary disk.

Or maybe, Earth just captured it from  somewhere else in the solar system,   like from one of our closest neighbors, Venus.

But none of those ideas explain all our Moon’s  quirks and those Apollo-collected rocks.

So what did scientists find in  those lunar samples, exactly?

Well, when they analyzed the samples, they  found that the amounts of the stable oxygen   isotopes O-16, O-17, and O-18 matched very  closely with what’s found in rocks from Earth.

And this similarity hinted  at where the moon came from.

See, isotopes are versions of an element  that have the same number of protons,   but different numbers of neutrons.

This gives them the same chemical  properties, but slightly different masses.

Now, isotopes that are radioactive  are unstable and break down over time,   but stable isotopes can remain  the same practically forever.

So, when it comes to rocks from beyond  Earth, the ratios of stable isotopes in   a sample are like unique fingerprints for how  and where in the solar system that rock formed.

And scientists found that it wasn’t  just oxygen isotopes that the Moon   and Earth had in common…other stable  isotopes were a close match as well.

They realized these similarities  were too much to just be coincidence.

The more likely explanation was that the Moon  had formed from part of the Earth itself.

But even though Earth and Moon rocks  resemble each other in some ways,   researchers noticed some differences, too.

Like, they discovered that  lunar rocks were lower in   volatiles – which are elements that  vaporize easily with intense heat.

And minerals in the Apollo rock samples hinted  that the Moon was once covered in a vast,   deep, magma ocean – which meant that it  must have been very hot when it formed.

Another core difference between our planet and   the Moon is that the Moon has  less iron overall than Earth.

Its tiny iron core makes up only 1 to  2% of its total mass.

That’s pretty   small compared to Earth’s core, which  accounts for a whopping 30% of its mass.

So, with evidence piling up,  scientists were beginning to   form a clearer picture of  where the Moon came from.

It seemed to have started out  hot and formed from iron-poor   material that likely came from Earth…but how?

Was there a way to explain the Moon’s  size, its isotopic similarities with   Earth, its small iron core, its lack of  volatiles, and its once-deep magma ocean?

The simple answer is…yes.

It started “with a bang!” See, in 2001, astrophysicists mathematically  modeling possible moon-forming events   landed on something interesting.

Through a model now called the “canonical   impact,” they could reach a reconstruction that  fit many of the observations about the moon.

In this model, the moon formed  from a giant impact…one where   a protoplanet the size of Mars  crashed into the early Earth.

The researchers estimated that the collision  would have released a huge amount of energy,   partially melting and vaporizing both planets.

And the impact would have  launched debris into space   that eventually formed a disk around the Earth.

Now, most of the debris in  the disk was too close to the   Earth to form big chunks without being  disintegrated by the planet’s gravity.

But material further away could  have escaped this destructive force,   allowing it to collect together to form the Moon.

Once the “dust settled,” the Earth was slightly  larger than it had been before and it had a Moon.

Their simulations produced an Earth and a  Moon of the correct size and iron content.

They also explained why the  Moon started out so hot.

And,   importantly, how it could be partially  made of material ejected from Earth.

Scientists named this ancient protoplanet  that collided with Earth “Theia”...after   the Greek Titan who was the mother  of the personification of the Moon.

But there was still one problem.

Canonical impact simulations predict  the Moon-forming disk would be mostly   made of material from Theia – not Earth.

And remember, lunar rocks have a  stable isotope fingerprint similar   to Earth’s - too similar for  them to be made mostly of Theia.

It’s possible that, by chance, Theia  was Earth-like isotopically – but that   seems unlikely, because the stable isotope  ratios of other inner solar system bodies,   like asteroids and Mars, are not Earth-like.

Researchers call this mismatch between the model  and the observations the “Isotopic Crisis.” So, for years they’ve been adjusting  these models to potentially explain   why there aren’t clear isotopic  traces of Theia in lunar rocks.

Like, maybe the Earth was  initially spinning faster.

Or maybe it was more molten when it was hit.

They also tried changing the size of  Theia and modeling many little impacts,   instead of one big one.

Or it could be that Theia was moving  so fast that the impact was a “hit   and run” – where Theia smacked into  Earth, and then continued on its way.

Another possibility is that the material  from Theia and Earth somehow mixed together   really thoroughly after they collided,  creating the isotopic similarities.

Some of these ideas are promising, but they  either rely on scenarios that seem far-fetched,   don’t match all of our observations,  or need more data to support them.

For years, none seemed to  provide the perfect solution.

But a new impact model published  in 2022 might solve the crisis.

In this study, the researchers simulated  Moon-forming impacts at a higher resolution.

Instead of only using 100,000 to  1 million simulation particles to   model the impact like most studies,  they tested different scenarios using   up to 100 million particles – the  highest resolution ever attempted.

And in their simulations, the Earth is  struck by Theia and then a giant blob   of material is immediately  flung away from the impact.

Shortly after, a smaller chunk of material  slingshots off the far end of the blob to   become the proto-moon, and what’s left of the  main blob swings back and re-impacts the Earth.

The proto-moon then settles into a safe,  stable orbit where it eventually becomes   the full fledged Moon.

And all of this  takes place within a matter of 35…hours.

Their simulations predicted that the  outer part of the Moon could be made   of up to 60% material from the Earth,  with more of Theia beneath the surface.

This could solve the isotopic crisis by  explaining where some traces of Theia are hiding.

But …where is the rest of it?

How  could an entire planet just disappear?

Well, in 2023, scientists published another new   paper suggesting the answer may be a  lot closer to home than we thought.

See, there are a couple of anomalies deep in  Earth’s mantle at the boundary between the   mantle and core where shear waves produced  by earthquakes slow down significantly.

These mysterious areas are called  “large low velocity provinces” or LLVPs,   and the changes in the shear wave  speed tell scientists that these   continent-sized LLVPs are made of something  different than the surrounding mantle.

We know this because seismic  waves change speeds when they   encounter materials of different densities.

And because Theia was likely a fully  formed protoplanet when it hit the Earth,   with its own mantle and iron core,  the scientists argued that these LLVPs   are leftover chunks of Theia still inside Earth.

Remember, in models of the  Moon-forming impact some of   Theia’s mantle was flung into space  to become part of the Moon.

But its   core and the rest of the mantle would  have been “swallowed up” by the Earth.

In other words, Earth ate most of the planet.

The researchers argued that Theia’s mantle  would’ve had more iron, and been denser, than   Earth’s, and that it would have mixed into Earth’s  lower mantle during the Moon-forming impact.

Their models predict these denser,   iron-rich Theia bits would sink downward  through the mantle over time and collect   together in two large areas at the edge of Earth’s  core…right where the LLVPs are found today.

If the researchers are correct,   that could mean there's direct evidence of  Theia still inside Earth.

But the authors   admit they can’t completely rule out that  the LLVPs aren’t something else entirely.

So, in the end, what does all this  tell us about how our Moon came to be?

Well, scientists agree that the best explanation  for the origin of the Moon seems to be a giant   impact between Earth and Theia, but  the exact details are far from settled.

And hopefully a combination  of methods - from modeling   to analysis of lunar rocks - can help  unravel the Moon’s mysterious origins.

But for now, we’re still searching for the  missing pieces of the puzzle, and for “solid