Original subtitles

- [Narrator] The pale blue dot.

The only known place in the entire solar system

to cradle the exquisite gift of life.

Here, rolling oceans and sun soaked lands are nourished

by an oxygen rich atmosphere.

The perfectly unique conditions for organisms to thrive,

but this planet doesn't work alone.

At the edge of its horizon looms a cosmic companion,

The moon.

A beacon of both survival and wonder.

This luminous guardian has watched over Earth

since the dawn of living history.

From guiding biological cycles and patterns of migration,

to inspiring profound works of art and scientific thought.

It is deeply entwined with the evolution of Earth

and the beings that call it home.

This seemingly barren rock is an anchor,

the calm stabilizing force that has grounded Earth

through eons of chaos and devastation,

but it hasn't always been this way.

Beneath the face of our celestial neighbor lies a secret,

an origin story potentially forged in the fires

of unimaginable violence.

This humble moon bears the scars of an apocalyptic event.

Does Earth also have a history

that brought it to the brink of destruction?

(dramatic music)

(gentle music)

There are hundreds of moons in the solar system,

each a satellite floating in the gravitational embrace

of their parent planet.

The bulk of these belong to Saturn,

the gaseous ringed giant,

which holds a staggering 274 in its orbit.

Jupiter currently boasts 97, Uranus has 28.

Even the distant Neptune claims 16

and yet Earth has just one.

- We're so fortunate to live on this planet

with one really big moon and that's kind of special

in our solar system.

- The vast majority of the moons we see in the solar system

are around the outer planets beyond the asteroid belt,

around the gas giants like Jupiter and Saturn

and those ice giants Uranus, Neptune.

Mercury and Venus don't have any moons,

and the next terrestrial planet Mars

only has these two very small asteroid like moons.

- [Narrator] Earth's moon is an outlier

in more ways than one.

- The most striking distinction between the Earth

and the moon is probably its size.

It's about a quarter of the size of the Earth

and just over 1% of its mass.

So while that might seem small, the ratio of the moon's mass

to the Earth is the greatest between any combination

of moon planet In the solar system.

- Jupiter's moon, Saturn's moons are nowhere near as large

as their host planets.

- If you were to shrink down the Earth into the size

of a basketball, the moon would be a tennis ball

on that size scale,

but Jupiter was the size of that basketball.

Its largest moon Ganymede,

which is one and a half times larger than the moon

on that same scale would be the size of a marble.

So it's these comparisons between the planetary body

and its moon and that fractional difference

that makes the Earth moon system so unique.

- [Narrator] Perched a distance of around 380,000 kilometers

from Earth, the moon orbits the planet every 27 days.

Its shifting face casts a familiar glow across the night sky

enticing humans' earliest ancestors to gaze up

and ponder their place in the cosmos.

- The moon is the brightest object in the night sky.

It's been observed by humanity for millennia.

- Throughout human history.

We've used the moon for navigation, + for keeping track

of time, and we've also looked up and admired the beauty

of this nearest celestial object in the sky.

- The moon is this bright, interesting object

that is somewhat familiar to us.

We see it every night, but also still somewhat mysterious.

It's far away, except for a very small number of humans

we've never been able to visit it,

and I think people are inspired by trying to understand

the night sky in general and the moon in particular.

- The main thing we wonder with the moon

is how it formed in the first place.

It has a unique set of properties.

It seems very large relative to the Earth,

so how is it we wound up

with that system orbiting around us?

- When we look at the solar system as a whole

and we look at the different moons that orbit the planets,

they're easier to some degree to explain

like how that came to be.

When we look at our own moon, it's much more of a mystery,

which is actually quite ironic.

- [Narrator] Further into the solar system

is the gas behemoth Jupiter,

the fifth planet from the sun,

and besides the sun itself, the heavyweight champion

of our solar system.

Here we find dozens of satellites,

the most prominent being the Galilean moons.

Named after Galileo who discovered them in 1610.

These four moons, Io, Europa, Callisto and Ganymede

are thought to have emerged from the same ancient nursery

of gas and debris that created our solar system

4.5 billion years ago.

- Jupiter is the largest planet by far in our solar system,

which means it has lots of gravity

and it was able to attract a lot of material around it

when it was forming.

- Material would've been able to coalesce in that disc

into independent bodies and those bodies would've remained

in orbit around the planet.

- [Narrator] Is it possible then that Earth and moon emerged

from the same forces, a sibling born from the leftover disc

of planetary dust and debris?

- One theory for the formation of the moon

is this co-formation theory.

- Could the moon have formed at the same time

as the Earth out of the same material?

The theory struggles to explain that

because our planet Earth is not so big, it's not Jupiter.

It wouldn't have had such an extensive disc around it.

So how would you grow a moon that is so big?

- [Narrator] Co formation suggests that the moon and Earth

grew up like sisters born from the same disc

of gas and rock, but shouldn't siblings look alike?

And in this case, the moon is visibly the odd one out.

- The Earth today has these continents

and it's covered in these liquid water oceans.

If we look at the moon, it's extremely different.

There's no surface water.

The moon is this incredibly dry place.

- One way around this apparent discrepancy is if the moon

had started with a lot of water that was subsequently lost.

The way that this happens is you have to heat the water

to extreme temperatures

but leave the Earth relatively unscathed.

It's not clear whether or not this would've been possible

during the earliest stages of the Earth moon's formation,

- But not only the water is a weird discrepancy

with the moon and the Earth.

There are other elements like the iron for instance,

that is very abundant in the core of the Earth.

It's about 30% of its mass,

whereas for the moon it's only 2%.

- If the same gas and dust went into forming the Earth

and the moon at the same time,

you would expect the moon to have a comparable

amount of iron in its core and it doesn't.

It's very iron deficient compared to the Earth.

- [Narrator] With such striking differences.

It's unlikely that the Earth and its moon formed in unison,

but there are other ways to birth a companion.

Further out in the solar system,

the planet Neptune is the most distant planet from the sun.

It has 16 moons that we know of,

15 of them rotate in the same direction as Neptune,

but Triton orbits in the opposite direction

to its parent planet.

- In the co-formation theory, you would expect any moons

that form at the same time as their host planet,

to be orbiting in the same direction.

So the question is how did Triton

end up on this retrograde orbit?

- The one idea of how this moon may have come to be

is through a capture.

You can think of it as Neptune's gravity

sort of grabbing this moon as it was passing by

and keeping it in its orbit in this retrograde motion.

- What you need for that to happen

is something with quite a lot of gravity, like a big planet,

something else that's perhaps smaller,

less gravity and it's not moving too fast.

- [Narrator] It is believed that Triton

likely began its life in the outer region

of the solar system,

but was kidnapped by Neptune in a close flyby.

Other planets like Mars have also captured moons

in a similar fashion.

Perhaps then Earth's moon

with its smaller core, thinner crust

and distinctly different design was adopted by its planet

like Triton as it drifted through the dark abyss.

(dramatic music)

This cosmic adoption would explain the compositional

differences between the two bodies,

but there are some major hiccups in this tale.

- You can't say that the moon was some random space rock

that got too close, it's too big.

- The probability that the Earth could capture something

as large as the moon, very unlikely, and if it did,

it's very hard to imagine they wouldn't have just collided.

- When you have this massive object

and it's trying to capture a smaller thing,

it's easier for that massive object

to completely overwhelm the gravity of the smaller thing.

Where these larger objects as they're passing by,

they have a significant influence on the planet itself

and you would expect it to be in this very elliptical

and not very circular orbit like we see the moon today.

- [Narrator] If the moon wasn't born elsewhere

and taken by the Earth,

then perhaps the answer lies much closer to home.

Perhaps the mark of lunar origin is etched onto the surface

of the planet itself like a deep scar

running beneath its blue oceans.

- An alternative theory for moon formation

is a so-called fission theory.

- [Narrator] The fission theory suggests

that in Earth's infancy it was packed with momentum

and spinning at incredibly rapid speeds.

- And just because of the centrifugal force on its equator,

it can crack and some liquid material can be ejected out

like a massive volcanic eruption.

- And that material that's separated

continued to orbit around in that equator region

and it accumulated so much mass

that that mass then got incorporated into the moon

as we see it today.

- [Narrator] Could this sleepy satellite be a byproduct

of the Earth in its young tempestuous days?

- There was some possible evidence for this

in the form of for instance, the large fissure

in the Pacific Ocean, the Mariana Trench.

- [Narrator] The Mariana Trench is a deep scar,

a void of mass torn from the Earth in its early days,

- But that theory doesn't explain everything.

It doesn't explain for instance

why they're chemically different.

- We would expect the moon to be made up of

all the same material as we see on the Earth.

- But also there's no indication the Earth

was ever really spinning fast enough for that to happen.

- The resulting angular momentum that we see in the system

today is much too low for a very fast spinning Earth

that would've resulted in this material breaking apart

and forming the moon as it is today.

- [Narrator] To unravel this mystery,

we begin our journey at the source

on the moon's speckled face.

Where fragments of a turbulent past

are written into its shimmering terrain.

- We can actually see on the surface of the moon

that it's covered in craters, impact basins

and even mountain ranges, some bigger than Mount Everest.

- [Narrator] Perhaps the most telling feature

of the lunar visage is its pockmarked skin,

the craters and basins, ranging from mere kilometers

to hundreds more in diameter.

Could these be the fossilized remnants

of the solar systems chaotic youth?

- Astronomers believe that these impact craters

came from frequent collisions of asteroids

and other small bodies with the surface of the moon.

- Remarkably, what we can do is take lunar samples

and figure out when do we think these craters were formed

and the vast majority of them were formed

around 4 billion years ago.

- This would be when the inner solar system

was being completely bombarded from asteroids and comets

coming from the outer solar system

and being sent into the inner planets.

- The large outer planets and in particular Jupiter,

we believe have moved during the history of the solar system

and between the planet Mars and Jupiter lies a rich belt

of rocky material called the asteroid belt.

And if Jupiter moves in its orbit, it's really easy

for Jupiter to scatter a whole bunch of asteroids.

- [Narrator] Today, these remnants are scattered

across the lunar surface.

Like the evidence of a long forgotten crime scene.

- While we expect both the Earth and the moon

to have experienced these giant impacts

that would've cratered the surface.

Unlike the Earth,

the moon doesn't have a substantial atmosphere

that can smooth out its surface

and erase the history of those impacts.

- The Earth has tectonic plates and active geology.

We have water and weather systems, we have erosion.

Anyone who's ever built a retaining wall at their house

knows that it doesn't last forever.

- By comparison, the moon is absolutely still,

barren, lifeless, there's no wind,

there's no atmospheric pressure,

there's no water to erode things.

And as a result, whatever features formed on the surface

are preserved for some cases billions of years.

- If we go to the moon like we've done in the past,

humans have walked on the surface.

Our footprints are still there, preserved forever.

- And another important point is that these smaller objects

that might impact the moon and leave a crater

actually burn up in Earth's atmosphere

and never make its way down to the surface

and create these impact craters.

- [Narrator] Since its infancy,

the moon has continued to bear the brunt

of cosmic collisions.

Some powerful enough to send shock waves through its crust

and launch lunar debris into space.

These free floating fragments known as ejecta,

occasionally fall under Earth's gravitational pull,

carrying with them the clues of the moon's ancient past.

- A few things that are extremely mysterious

have been noticed.

Some elements such as potassium, phosphorus

and all sorts of very rare Earth element are present

and very well mixed together into the lunar dust.

It is strange to find these chemical composition altogether,

and a way to explain it is if the moon

was once completely melted,

it would've allowed those chemical elements

to mix very well before it then settle in.

- The modern snapshot of the moon also bears clues

of a molten past in the form of its dark basalt maria,

visible dry seabeds,

which would've once been filled with lava.

But what happened in the moon's past

that led to such a volatile landscape to form a molten sea,

a great heat producing event must have occurred

in order to melt solid rock.

One far more colossal

than the average crater forming impact.

And one that may have revolutionized planet Earth itself.

(gentle music)

The proto-Earth, around 4.5 billion years ago.

It's a place likely, entirely unrecognizable

next to its modern form.

A lone body growing amid the backdrop of cosmic carnage.

- Our early solar system was a dynamic place.

There was a lot happening.

(dramatic music)

We had this cloud of gas and dust

that started to collapse under gravity,

making the proto-Sun down in the center.

There was this disc of mostly gas

with some dust and some ices mixed in

that went into making all the planets and the moons

and this was all happening at once

in this messy disc of material.

- [Narrator] This is a primordial battleground

of fire and stone.

Under a relentless barrage of debris,

the early Earth is locked in a grand tug of war

with dozens of rival proto-planets,

each fighting for their survival

in the sun's unruly outskirts.

- During the great migration of the planets,

lots of proto-planets were shifting orbits

and colliding with each other.

- This would've been quite a violent environment.

Only the largest objects survive

- [Narrator] For tens of millions of years,

the proto-Earth blazes a solitary path

floating solo in its orbit around the sun.

But soon a rogue companion begins to trail close behind

not the moon or another wandering asteroid,

but an entire world known as Theia.

- At this time, Theia is thought to have been a planet

that is still undergoing formation.

It was probably about the size of Mars, about 10% the mass

of the Earth, and it was still in this very hot period

where the accretion of materials

as it was hitting the surface was delivering this energy

to the surface and keeping it as molten rock.

- [Narrator] Though no longer in existence today,

it's believed this hypothetical body emerges

from the same primordial forces

that have long been building Earth.

And though only a fraction of its neighbor's mass,

Theia is destined to reshape the cosmic corner

they both call home.

- In any system with multiple objects with gravity.

There are these special places called Lagrange points.

- These Lagrange points can be thought of as a seesaw

in their orbits, where if you weren't in the LaGrange point,

the seesaw might be pointed a bit towards the Earth,

where the Earth is sort of winning gravitationally

and pulling that body in closer.

And if you're tilted the other way,

you might be being pulled into the sun a little bit faster.

In the LaGrange point, the seesaw is stable

where there's an equal pull between both the Earth

and the sun, and that body can stay there relatively stable

for an extended period of time.

- Theia was once believed to be located

at one of these LaGrange points

- [Narrator] Anchored to this gravitational sweet spot.

Theia orbits the sun in tandem with Earth.

However, this precarious perch is vulnerable

even to the slightest disturbances in the solar system,

and soon instability brewing further afield

will send these two bodies on a dangerous collision course.

- This is likely from Jupiter in its migration

to the inner solar system disrupting things.

There was probably this perturbation

caused from Jupiter's gravity as it was coming in

and Venus may have began tugging on the planet as well,

and this would've thrown Theia off from this stable orbit

and may have allowed it to progress inwards

towards the Earth.

- [Narrator] A shudder in the outer solar system

suddenly jolts Theia from its comfortable LaGrange point,

sending it barreling towards Earth

at over 30,000 kilometers per hour,

- 30,000 kilometers per hour is relatively slow

on cosmic scales.

In comparison, asteroids when they're being sent barreling

into the inner solar system

during the late heavy bombardment,

were hitting the Earth at 70,000 kilometers per hour.

But it's the sheer size of Theia

that makes this such a dramatic collision.

- Thea is so massive that the amount of energy

that would've imparted onto the Earth

is equivalent to the force of billions

of megaton nuclear bombs.

- [Narrator] Theia slams into its target

in an impact so immense that it causes both bodies

to partially vaporize and melt.

Billions of tons of debris are ejected into the void

as Theia's dense iron core plunges into the heart of Earth.

- We have never seen anything like this.

Incredibly energetic, much more energetic than any weapon

that you could possibly imagine, just would've released

tons of light and tons of energy into space.

- [Narrator] The resulting explosion

rapidly hits the proto-Earth,

triggering a massive wave of molten rock

that surges across its surface like a fiery tsunami.

- The energy of this impact would result

in the surface of the Earth's being completely melted

to this molten lava ocean, which could be hundreds

to even thousands of kilometers deep.

- [Narrator] This theorized impact

is Earth's last growth spurt,

swelling it to the size observed today.

When the dust finally settles.

Theia is no more, its bones scattered into space

or swallowed by the Earth.

What remains is one world now fully grown

but forever changed.

After narrowly escaping total ruin,

Earth is left a bubbling wasteland of molten lava.

The atmosphere is a thick haze of toxic gas.

The surface, a scorching expanse of magma.

So how do the foundations of the modern world

take shape in such hellish conditions?

- This collision would've been completely catastrophic

for any sort of life that was beginning to evolve

on the surface and it would've completely sterilized

the Earth and made it impossible for life to evolve

for an extended period of time.

- This devastating crash into the Earth effectively hits

the reset button on the Earth's composition.

- It took a lot of time for that celestial body to cool down

and eventually form a crust around it.

The core remained hot.

The circulation of energy within the mantle

also enabled cracks to form on the crust

and form what we call today the tectonic plates

which are still active and moving today.

- Not all of the planets in the solar system

have plate tectonics.

Some of them have cooled to the point

where their crust is frozen.

The Earth is a geologically active planet

that is still in the process of cooling down

as it has been for the last 4 billion years.

- [Narrator] But before Earth gains the solid footing,

an astounding spectacle unfolds in the skies.

As the ghost of Theia looms above its remnants birth

a celestial companion closely bound to the planet's orbit,

- That lighter, rocky material

that gets blasted off into space

doesn't necessarily leave

the Earth's gravitational influence.

It ends up in an orbit around the Earth

before coalescing into forming a new rocky body

that is mostly rock and heavily deficient in iron.

- So this theory explain why the moon as lighter, element

is relatively less dense than the Earth,

and also why all of the elements present in its crust

and its dust are so well mixed

and should not have formed together in the first place.

- [Narrator] From the inferno of Theia's impact,

emerges Earth's moon, a hot molten body circling the planet.

- In Greek mythology, Theia is the mother of Selene,

the moon goddess.

So in this way we have this really poetic connection

between Theia's name and its service to our Earth.

- Following its explosive birth,

the moon is left to find its place

in the gravitational battlefield of the inner solar system.

Too close to the Earth

and it will be ripped to shreds by the planet's force.

Too far and it will float into the hazardous abyss of space.

Fortunately, the moon settles into the perfect zone.

- When a moon is in orbit around a planet,

it's not guaranteed that it will be able to survive

where it is.

The planet's gravity is stronger closer in

and a little weaker further out.

And that means there's actually a difference in the force

on the front and back side of the moon.

And if that difference in force is too large,

it can actually tear a body apart.

- The Roche limit is this safe distance from a planet

where an asteroid or a moon can survive

around that body without being pulled apart

from the gravity itself.

- In the case of the moon,

it would naturally only have been able to form

outside that limit.

So there's a natural explanation to where it started off.

- After this giant impact,

the moon is left spinning at a very rapid pace

and it's so rapid that it's getting stretched out

into this rugby ball type shape.

And because of this shape,

the Earth's pulling on either side of the moon

in a different amount of force

and it's causing this torque in its motion

that's slowing down its spin.

The Earth pulls on the rotation of the moon so much

that eventually the moon ends up in this position,

which we call tidally locked

- The moon orbits at the same rate around the Earth

as it spins.

So it takes about a month for it to spin around

and it takes about a month for it to orbit around the Earth.

- And the result of that is that we always see the same face

of the moon as it orbits around us,

and there's one face that we never see.

- [Narrator] Together the Earth and the moon

will draw an elliptical orbit around the sun.

And this intimate face-to-face encounter

will also play a significant role

in how the lunar body cools and matures.

(dramatic music)

Before aging into the gray expanse we observe today,

the moon burns bright red in the night sky.

Similar to the effects on Earth.

Theia's collision gives the moon a vast ocean of magma

believed to be hundreds if not thousands of kilometers deep.

- Rather than this dry desolate view of the moon surface

that we have today.

Shortly after its formation, it would've been more

of a magma ocean covered in molten rock,

the remnants of which we can see today

by looking at the different types of minerals

on the moon's surface.

- [Narrator] Within just one millennium of its birth,

A mere moment in cosmic timescales,

the angsty moon begins to cool its surface quickly

solidifying into smoky rock.

Yet unlike Earth's fragmented tectonic plates,

the moon's molten sea will harden into a singular,

unbroken crust.

(dramatic music)

Coming of age in the solar system

fraught with stray asteroids,

the young moon endures a relentless onslaught

of cosmic debris, pummeling its fragile new crust.

All the while hot lava left churning in the moon's core

surges to the surface erupting from the deep scars

in its battered skin.

As time passes, these molten flows fill the cratered basins

left by comets and asteroids,

creating the dark basaltic maria

that now covers approximately 16% of the lunar surface.

- So we can look at the age of rocks

on the surface of the moon and date them.

Some of these rocks that we know are produced

by vulcanism are about a billion years old or so.

But there is still some evidence

that there were small eruptions producing rocks

as young as 120 million years old.

Still about the age of the dinosaurs on Earth.

- [Narrator] Now locked in place,

the moon is set to play a pivotal role in the evolution

of its planetary partner.

Though measuring just over 1% of Earth's mass,

this lunar satellite will wield its profound influence

in more ways than one.

- The Earth's rotation axis is not perfectly aligned

with its orbit.

It's actually tilted by about 23 degrees

and that in the absence of a moon

could be an unstable configuration.

- [Narrator] Like the head of a spinning top,

Earth's tilt could be pulled in all directions.

Vulnerable to the pull of other large bodies

in the solar system.

- But the mutual gravitational interaction

between the Earth and the moon system

actually helps to stabilize the Earth's axial tilt

and prevent it from wobbling excessively.

- Without the moon, the Earth's tilt could vary wildly,

which would mean our weather could vary wildly,

which might have meant the Earth was not as great a place

for life to evolve on.

- [Narrator] But could Earth's magnificent oceans,

lush landscapes, and even intelligent life

really be accredited to an ancient hypothetical collision?

(dramatic music)

- It would be wonderful to have direct evidence say a movie

of this event we think formed the moon.

We don't obviously have that.

The closest we come to having direct evidence

is from the isotopic ratios of rocks and minerals

on the Earth and the moon.

- So those samples from the moon that were collected

starting in 1969 and through the '70's

as we visited the moon, have taught us an enormous amount

about the moon's composition and its history.

- [Narrator] Locked within samples of lunar rocks

and meteorites are chemical timestamps,

concealing the secrets of the solar systems infancy.

And these atomic clues, known as isotopes,

may just be the smoking gun

of the moon's elusive origin.

- So an isotope is a version of the same element

that is slightly heavier or less heavy.

It has slightly different number of neutron

inside its nucleus.

(gentle music)

- Oxygen, Earth's most abundant element

is present here in three stable forms.

Oxygen 16, oxygen 17, and oxygen 18.

More than 99% of the blue planet's reservoir is oxygen 16.

With the remaining forms existing in trace amounts.

This specific ratio is like a fingerprint,

unique to Earth's identity.

- These isotopes are present because of different processes,

the environment in which they were created

and what they were exposed to.

When we look at the Mars isotopes,

it is strikingly different than the one on Earth.

But if you look at the ones on the moon and on Earth,

they're really, really similar.

- Which is an indication they came from the same material.

- [Narrator] These remnants of planetary DNA on the moon

are tangible evidence of its tragic beginnings,

but there remains one glaring problem.

How can the moon be encoded with Earth genetics

without bearing similar traces

of its supposed mother planet Theia.

Imagine the aftermath of the accident.

The smaller Theia plows into proto-Earth,

its shattered remains spewing out into space.

The carnage left behind coalesces to form the moon,

a body that should be composed mainly of Theia's leftovers,

but today they are nowhere to be found.

- So there are a few possible explanations.

One is that Theia was an icy body

and that when it hit the Earth it immediately vaporized.

Then we lost most of that light material.

It blew then part of the Earth's mantle into orbit

around the Earth, and that's what formed the moon.

The second possibility would be that the collision

was so fast and so energetic.

This is sometimes called the hit and run model,

that the collider would've gone through,

disrupted the proto-Earth,

but kept on going and taken most of its own material with it

and that that material would no longer be left

in the Earth's moon system.

- But what if the one component of this cosmic crash

that holds the answer is simply time.

Perhaps the chemical kinship between Earth and its moon

is the result of a collision unfolding

at an incredible pace, not years or even months,

but mere hours.

- Recent simulations have shown

that when a collision happened with two objects

of about the size of Theia and the Earth,

instead of having a mix of the two

and then some debris is sent around to form the moon,

they actually fuse and then a chunk of the material

mostly from the Earth's mantle is then flung into space

and that material is what created the moon.

(dramatic music)

- [Narrator] Yet even if this high speed sequence

did in fact sweep through the early solar system,

it doesn't fully explain Theia's fate,

namely where its remnants can be found today.

How could an entire Mars sized proto-planet

simply disappear?

Where are the remains of this ancient wanderer?

- At the present, we don't have direct evidence

of this planet Theia because that material

likely sunk to the center of what is now the Earth.

However, geologists who study the interior of the Earth

are currently looking for evidence for this planet Theia.

- To access the structure of the Earth's interior,

scientists use seismic waves to map out

how pressure waves propagate through the Earth's interior.

It helps us paint a picture of what the different layers

of Earth's interior actually are.

And we can use that information to look for signatures

of a past impact like Theia.

- [Narrator] Deep beneath the basin of the Pacific Ocean

and the vast continent of Africa,

two enigmatic giants lay hidden.

For billions of years,

these abnormally dense formations have laid nestled

in Earth's lower mantle, but their origin is unclear.

It is possible they may be the lost

geological puzzle pieces, the bones of the proto-planet

that created our companion.

While a potentially crucial clue remains sealed

deep below Earth's surface,

the hints written on the lunar surface

are equally challenging to read.

- The moon is roughly 400,000 kilometers away from us,

which makes it a very long journey to go get a sample.

- So because of this tidally locked nature of the moon,

we only ever see one face

and we sometimes call the far side of the moon,

the dark side of the moon but this is a misconception

because it isn't inherently dark.

It does see sunlight in the phases

where we don't see the face of the moon

because it's dark, the far side is being illuminated.

- [Narrator] Tidally locked in a vis-a-vis embrace,

the moon conceals around 40% of its surface

from the Earth's gaze.

Beyond reach of the planet's strongest gravitational tug,

the far side of the moon has evolved into a starkly

different world since the dawn of its violent birth.

- So the hidden side is characterized

by these thousands of craters all over the place,

whereas the near side has these large lakes or ocean

of frozen magma from the past.

Not only that, but when you study the structure

of the surface of the moon, you notice that the near side

as it crosses that is actually 150 kilometers thick,

while the other side is only 70 kilometers thick.

That's extremely intriguing. Why are they so different?

- Although there has now been a landing

on the far side of the moon,

that side remains relatively unexplored.

So it's possible there are regions of the surface there

that have quite a different isotopic composition

that could correspond to fragments of Theia.

- [Narrator] The lopsided moon may be the ultimate monument

to Earth's near death experience.

Its hidden hemisphere possibly holding lost indications

of Theia's crash.

Though while this side may forever be obscured from view,

the moon's luminous face will continue to shine over Earth,

bearing witness to the grand unfolding of life.

Like training wheels, the moon helps steady the Earth

as it navigates its orbit around the sun.

- Contrary to some belief when summer is when the Earth

is closest to the sun and winter when it's further away,

it's actually the tilt of the planet.

And when the Earth is tilted towards the sun,

that hemisphere is experiencing summer

because it has more direct sunlight.

And when it's pointing away, it experiences winter

because there's less direct sunlight.

- [Narrator] This relatively constant tilt

gives the planet a predictable cycle

of four gently transitioning seasons.

A rare and precious phenomenon within the solar system.

- By comparison, some of our planetary neighbors

in the solar system can have quite extreme tilts,

ones that result in extreme seasonality

or effectively no seasons at all.

For example, Uranus, which is tilted by 98 degrees,

has seasons that occur on a 20 year timescale.

- [Narrator] These monumental seasonal shifts

trigger massive storms across the entire planet.

(dramatic music)

But Earth's tilt is not its only savior.

In fact, a world plagued by violent super storms

is not far off from what early Earth itself resembles.

Following the great impact,

the planet is a powerhouse, rotating at unfathomable speeds.

- This collision not only imparted a lot of energy,

but it imparted a lot of what we call angular momentum.

And so this spun up the Earth.

- The Earth was rotating much faster than today.

Instead of rotating on its axis once every 24 hours,

it was doing so in only a few hours.

(dramatic music)

- This energetic spin fosters a hostile environment

with hyper charged winds and the volatile flux

of temperatures.

This frenzied snapshot of Earth seems an impossible place

for life to materialize,

and it may well have been the case

if not for a helping hand.

Enter the moon.

Upon its initial formation,

it's positioned just 25,000 kilometers away,

17 times closer than its current distance.

This proximity gives the moon immense gravitational power

over Earth, kneading the planet's crust

and manifesting its mighty tides.

- The gravitational influence of the moon

on the Earth causes the Earth to bulge ever so slightly,

actually pulling the ocean on the near side of the moon

closer to the moon than on the opposing side of the Earth.

This actually stretches the Earth

into something like a football shape.

As this football shape Earth tries to rotate

once on its axis,

these tidal bulges are pulled back to towards the moon,

which gives rise to the high and low tides twice a day.

- So towards the moon,

the ocean is attracted making it a high tide.

And this high tide is ultimately following the moon

and the sky as we rotate

and also as the moon revolve around us.

- [Narrator] This perpetual ebb and flow,

the tidal pulse of the planet,

creates friction effectively dissipating

Earth's excess energy and calming its rotation.

- If the Earth is spinning and you've got this little tug

that's more on one side than the other,

it will slow down the spin over time.

We call that process tidal breaking.

- This tidal breaking that has worked

to slow the Earth's rotation

since the moon forming event, has increased the length

of a day by about two milliseconds every century.

- [Narrator] Beyond surface tides,

the moon also creates tides within Earth's outer core.

These forces stir currents of liquid iron,

generating a magnetic shield that envelops

and defends the planet from cosmic radiation and debris.

- We know this magnetic field has been protecting us

from the sun.

When we look at Mars today,

we know that in the past it lost its ocean

because there's no magnetic field to protect it.

And so would the fate of the Earth been similar

to Mars if the moon wasn't there?

If Theia has not been inside our core

generating this magnetic field that protects us,

that's very possible.

- [Narrator] The moon's gravitational spell

has both stabilized and protected the Earth

for billions of years,

allowing it to quiet and cool over time.

And remarkably, this steady relationship

forged from an ancient wreck,

may have even laid the groundwork

for the planet's most extraordinary future to emerge.

As Earth immense surface of molten rock cools.

The first droplets of liquid water mysteriously appear.

Marking the beginning of a new era.

- It may have come from icy comets

that delivered the water from afar,

or it could have been sequestered into the Earth's interior

when it first formed and was slowly released over time

as the Earth cooled.

- As this precious liquid resource spreads across the globe

filling deep oceanic basins, the moon's gravity continues

to pull on the planet's crust,

soon churning out powerful currents and monstrous waves.

(waves crash)

- When you look at today's tides,

some of the largest ones are in Canada,

in the Bay of Fundy.

Where you can see high tides that are 16 meter high.

But in the early phase of the Earth moon system,

the moon was actually closer

and the tides were up to a 1,000 times larger than that.

(dramatic music)

- It's possible that oceans actually sloshed over

whole continents and would have drained

all that material back into the ocean basin.

That kind of mixing would've produced

a much more mineral rich environment for life to form.

- [Narrator] Without this lunar stir,

tidal mixing and perhaps the only known nursery of life,

may have never existed at all.

- Our planet would be really strikingly different for humans

and also for other species if we didn't have the moon.

Maybe we wouldn't have humans without the moon.

We really don't know.

- [Narrator] Within 1 billion years

of Theia's deadly encounter,

life erupts on planet Earth.

(uplifting music)

Overhead, the moon stands watch,

maintaining harmony in the planet's cycles

and shepherding its many inhabitants

on their journey around the sun.

- The moon is the brightest object

that we see in the night sky.

- [Narrator] In fact, it's several thousand times brighter

than the next brightest object, Venus.

- We can think about the circadian rhythm of 24 hours

and how it affects our life,

just the cycle of day and night,

but the circalunar cycle,

the phases of the moon also affects

all sorts of living being on Earth.

First come to mind the sea turtle,

for which they're hatching happens during a full moon

helping the turtle to find their way back to the sea.

- We've observed that coral populations

can synchronize their spawning events with full moons

because those full moon events

actually lead to what we call slack tides,

where there's minimal water movement

in between high and low tide,

and that allows a somewhat stable environment

for the reproductive cells of these species

to be able to mingle together and fertilization to occur.

This is an example of how species on Earth

can actually use the moon phases to amplify the reproductive

and biological processes.

(gentle music)

- [Narrator] The moon speaks a language

that life has evolved to understand.

More than just a gravitational guide,

it has become a lighthouse, a compass,

a timekeeper, and for many a source of cosmic contemplation.

(gentle music)

- The moon means a lot to us as a species.

We've always looked up to it,

whether it be for forms of navigation,

for keeping track of time, but it's also been this muse

for not just scientific thinking, but also forms of art.

And it's the only surface in the solar system

that we've been able to step foot on

other than the planet that we were born on.

- It still blows my mind that we were able to send humans

safely to the moon and back in 1969,

remarkable the growth of technology that enabled that.

- The moon in a way provides a sort of stepping stone

for the rest of our solar system.

And we talk now about having people go and explore

other bodies in the solar system.

That would've been a complete fantasy

if it hadn't been for lunar exploration.

It's still a much, much harder thing to do

than getting to the moon,

but the moon at least shows us

some of what we're capable of.

(gentle music)

- [Narrator] The synchronous relationship between the moon

and its life bearing planet

has endured for 4.5 billion years.

A time span almost impossible to imagine

compared to life cycles on Earth.

- Humans have only been on this planet

for a brief little moment in the history of our planet.

For billions of years, there was no complex life like us.

- Yet even on this scale, change is inevitable.

Since the moon's creation,

it has slowly but steadily been retreating

from Earth's reach.

- The tidal force that's working

to slow the Earth's rotation rate down

is also causing the moon's orbit to expand,

albeit very slowly at only about four centimeters per year.

- So over the last number of billions of years,

the Earth's spin has been slowing down,

and that energy has been given to the moon,

and the moon is moving further away from us over time.

- [Narrator] However, this slow separation

also has implications for the future.

Might this be the final act of the moon's dance with Earth,

gradually losing its grasp until it slips away entirely.

- It would be hard to imagine the Earth without the moon.

'Cause everything around us has been somehow shaped by it.

- Without the moon's gravitational influence,

the Earth's axial tilt, oceanic patterns,

climate patterns would be completely out of sync.

The absence of these stabilizing effects

can have really drastic consequences on the Earth,

including destabilizing ecosystems,

triggering mass extinctions

and even jeopardizing life itself.

But the rate at which the moon is moving away

depends on its separation from the Earth.

And as it gets further away,

the rate at which that continues actually slows down.

So we're not really at risk of losing our moon, thankfully.

- But there's one really interesting implication

of the moon getting further away.

Today, the moon is just exactly the same size

as the sun in the sky,

which means every once in a while

you could be really, really lucky

and experience a total solar eclipse.

- It is truly a cosmic coincidence that these two objects,

despite being very different in size and reality,

they appear the same size on our sky.

Now, if the moon is moving away from us over time,

that means that its perceived size on our sky

is also decreasing slowly with time.

And at some point, we will actually lose the ability

to experience total solar eclipses on Earth.

- [Narrator] In around 50 billion years,

the two bodies are expected to stop moving apart entirely.

Yet across such vast timescales,

the solar system could sooner befall numerous other fates.

(dramatic music)

Until then, the moon maintains its steady circuit

around the blue planet, potentially the sole artifact

of a planetary interloper that would rewrite the fate

of planet Earth.

- Astronomers and planetary scientists

want to understand the world around them.

Where did we come from? How did we get here?

What are the pathways that led to the solar system

that we see today?

And the biggest questions are often around the planet Earth,

our home and the moon, the one natural satellite

that orbits around our planet, the Earth.

- It being our closest companion,

its own origin story is incredibly important to us.

- We've only just begun to unravel our understanding

of the origin of our moon,

and understanding the interaction between Theia

and the Earth that went into forming the moon

might help us to understand the early dynamical evolution

of the solar system, as well as the conditions

that may have led to the emergence of life on Earth.

- We know the moon is pivotal to understand the evolution

of the Earth and the evolution of life on the Earth.

So understanding how it came to be is essential

'cause we need to know if that could have happened elsewhere

in the universe.

Is a moon such as ours absolutely necessary,

or is there other ways for life to find its way?

- [Narrator] Today the beings of Earth may be the inheritors

of the solar system's greatest collision.

Each tide, each seasonal shift, and each night sky

that's lit by this lunar orphan,

a testament to one inherent truth,

throughout the cosmos,

chaos almost always precedes creation.

(bright music)