All language subtitles for How The Universe Works S04E07 Secret History Of The Moon_ENG_Subtitles01

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Imagine an alien world,

a scorched, barren landscape,
hot enough to boil water.

Where mysterious holes
on the battle-scarred surface

host a labyrinth of tunnels,

each wide enough to house a city.

This world is dead,

yet it breathes life into
its nearest planetary neighbour.

This extraterrestrial place is
closer than you might imagine.

This is our moon.

Look at our solar system from
an alien visitor's perspective.

What would surprise you?

What would make you look twice?

The intriguing answer -

our oversized moon.

If you look at our moon

and compare it to all of
the other moons in our solar system,

it's actually quite odd.

It's huge.

Way bigger than any other
moon in the solar system

compared to its planet.

It's a giant moon.

It's almost like
we're a binary planet,

two objects orbiting around
each other to some degree.

Our planet-sized moon
is unique in the solar system.

And this battle-scarred giant
serves as our guardian angel.

Its immense gravity raises the tides

that breathe life
into the Earth's oceans.

The Moon stabilises
the tilt of our planet,

regulating the climate and seasons.

Without the Moon,
humans may never have evolved.

We owe everything to our moon,

yet its formation is
one of the greatest

unsolved mysteries
of planetary science.

There are many theories for
the origin of our supersized moon,

but they all start the same way,

with the formation of the inner
solar system 4.6 billion years ago.

When the solar system first formed,

it would've looked very different
from the way it looks now.

Instead of having a few planets
and mostly empty space

surrounding the sun,
you would've had a disc.

And this disc would've been thick,

it would've been composed of
gas and dust and rocky bodies.

The infant sun sparks into life,

blowing away the clouds of gas
that are closest to it.

Over time, the rocky fragments
that are left behind

clump together to form
dozens of new planets,

many more than we now see today.

These fledgling worlds
jostle for position,

crossing paths as they struggle
to find stable orbits.

Is it possible that
one of these planets

was destined to become our moon?

There have been a lot of ideas about

where our oversized moon came from.

One idea was just that
it was wandering around

in our solar system
and we captured it.

According to the capture theory,

a wayward planet passed a little
too close to the early Earth

and our planet's immense gravity
seized hold of it.

The planet then settled
into orbit around the Earth

and became the Moon we see today.

This theory seemed
to tick all the boxes...

..but scientists needed proof.

If it was really true that
the Moon was a captured planet,

its constituents should
be different from the Earth.

It was something that formed in
a different place from the Earth

and then got captured,

so you would expect them
to be made of different stuff.

To prove the theory,
scientists needed to compare

the Moon's earliest rocks with
similar samples found on Earth.

The best rock for comparison
is anorthosite,

a volcanic rock that
could only have formed

when the newly born Moon
was still molten.

On Earth, an anorthosite forms
in highly geologically active places,

like Iceland.

This is what we want.

You see this anorthosite,
how white it is.

Anorthosite forms in a different
way than normal basalt.

Um...you can almost think
of it like the white foam

on the head of a dark beer,
sort of floating up to the top.

So in a magma environment,

this would form and then just
float up to the top of a...

of a magma sea.

Getting a sample of anorthosite
from the Moon was crucial,

because darker rocks
could've had their chemistry

altered by asteroid impacts.

We really had to have
a piece of this anorthosite rock,

because we couldn't learn
about the origin of the Moon

from the dark materials.

We had to have, you know,
that genesis rock

to tell us about
the origin of the Moon.

In the 1970s,
Apollo astronauts collected samples

and brought them
back to Earth for comparison.

The results shocked
the scientific world.

What they found was that
the composition of the Moon

was almost exactly the same

as the composition
of the crust of the Earth.

So this idea that the Moon
was a captured planet

from elsewhere
in the solar system was out.

There was no way that was true.

PHIL: The rocks looked
very Earth-like in many respects

and that was a puzzle.

With the captured-planet
theory blown out of the water,

planetary scientists went
back to the drawing board.

Scientists said, "Look, we've got to
get together and figure this out."

And they went to a conference
together in Kona, Hawaii,

all the top scientists
in planetary science,

and they hammered out
all of the leading ideas.

A lot of people think scientists
don't have an imagination,

we're just robots looking at things
and analysing. It's not like that.

If you want to figure out how
something like the Moon came to be,

you have to have a wild imagination
and try all these crazy ideas,

but they have to be
constrained by reality.

They locked themselves
into a room together

and emerged from that meeting
and said, "Yes, this can work."

Scientists switched their attention

to the water contents
of the Moon rocks.

They were surprisingly dry.

Something must've heated up the Moon
to unimaginable temperatures,

and we were led to think that
it must be some sort of collision.

A new theory emerged
around the possibility

of a catastrophic collision,

4.5 billion years ago.

But it wasn't an
impact into the Moon...

..it was an impact into the Earth.

There was a crazy idea
that the Earth formed,

and while it was still young,
another planet-sized object,

something about the size of Mars,
came in and hit the Earth.

It blew off
a huge amount of material,

which then coalesced
and formed the Moon.

This is a pretty cool idea.

It was ground-breaking,
I guess, literally.

Scientists name
the Mars-sized planet Theia

and modelled how the impact
would've played out.

4.5 billion years ago,

Theia clips the Earth
with a glancing blow.

The impact throws
molten debris far into space,

forming a ring of burning rock
around the Earth.

If we could travel back in time

and somehow stand
on the surface of the Earth

when the Moon was forming,
it would've been amazing.

A bright ring of fire was
stretching across the sky.

And here in the sky would've
been a bright ball of magma,

glowing, with all kinds
of shrapnel and small rocks

being attracted to that point
because of the immense gravity.

This was the proto-Moon,

the thing that would
eventually become the Moon

we have in our sky today.

Our moon forms in under a year.

Its crust is almost
chemically identical to Earth's,

because they share a common origin.

This impact idea,
as weird as it sounds,

actually does the best job
explaining everything

that we see about the Moon.

The impact hypothesis
becomes the leading theory

for the origin of our oversized Moon.

But when space probes journey
to the far side of the Moon,

they discover something that
throws the theory into chaos.

The Moon has a dark secret...

..it was born with a twin.

*

*

Take a look at the Moon tonight

and you might see a face,
or perhaps a rabbit,

or a tree, depending on your culture.

Myths and legends
surround the patterns

etched on the surface of the Moon.

But what can these marks
tell us about the origins

of our oversized guardian?

What we call the Man in the Moon

is actually just a series
of light and dark patches

on the Moon's surface.

And of course, people are programmed
to recognise faces everywhere

so that's why we see this,
you know, face.

These dark patches appear
to be unique to the Moon.

Centuries ago, astronomers thought

they were seeing enormous
oceans of liquid water.

We now know the dark stains

are actually ancient floods
of volcanic lava.

Planetary geologist Jani Radebaugh

captures a bird's-eye view
of similar features on Hawaii,

using her research kite-cam.

These lava flows
we're standing on are identical

to the things we would see
if we were standing

on the dark patches of the Moon.

They are dark in colour,
they're made of basalt

and they flowed out in vast
lava flows across the surface.

It turns out the dark plains of lava
on the Moon are battle scars...

..evidence of a violent past.

Four billion years ago,
asteroids rained down

on the newly formed Moon,
tearing into the surface.

Where the fractured
crust is thinnest,

molten lava seeps
from the Moon's hot interior.

Spreading out in giant pools...

..and solidifying to form the scars
we see today from Earth.

Strangely, although the Moon rotates,
the Earth's gravity
holds onto it so tightly

that the same face points
towards us at all times,

making the features on the far side
of the Moon a complete mystery.

For all of history,
the far side of the Moon

was invisible to the Earth.

It's the backside, you can't see it.

In 1959,
the Soviet spacecraft Lunar 3

flew past the Moon's far side

and photographed it
for the first time.

Astronomers expected to see the same
familiar dark and light patches,

but they were in for a big surprise.

When scientists saw the pictures,
they were shocked.

The far side looked completely
different than the near side.

It's saturated with craters.
It just was such a huge dichotomy.

Nobody was expecting that at all.

The far side still had
massive impact craters,

but it was uniformly pale.

It seemed like no dark lava had
bled out onto the surface at all.

But why?

The only solution we
could come up with is

there's a difference in thickness
between the crust of the near side

and the crust of the far side.

That the backside must be so thick

that lavas are not able
to come up through them

and erupt out onto the surface.

Instead, on the front side,
where it's very thin,

lavas can easily
come up through cracks

and flow out onto the surface.

Recent NASA missions confirm
the crust on the back of the Moon

is around 26 kilometres thicker
than the crust on the front.

The far side is thicker.

It's not like, "Oh, part of it is
and part of it isn't."

No, really, the other side of
the Moon has a thicker crust

than the near side. That's bizarre.

So one of the biggest mysteries
in planetary science

over the last about
50 years is, you know,

"Why is the crust of
the far side so thick?"

The impact theory
doesn't really cover that.

It just forms the Moon
but it doesn't say

why one side should be
so different than the other one,

unless something strange happened.

The new age of
supercomputer modelling brought about

the first credible explanation.

The impact of Theia could've
made two moons, not one.

And this double birth might also
explain our double-sided moon.

According to the theory,
4.5 billion years ago,

there were two moons
in the night sky.

The smaller moon chases
its larger sibling,

gradually getting closer.

Eventually, the two moons collide
and slowly mould together.

The smaller moon covers
the far side of the larger moon,

creating a new,
much thicker crust.

Not all impacts are high-speed,

super-violent events that
eject material everywhere.

Instead of just "Wham",
smacking into it,

it would've just merged with it,
just been pulled apart

and smeared out over the Moon.

Essentially, when you look
at one half of the Moon,

you see more of one body,
and when you look at the other side,

you see more of the other body.

They're kind of wrapped
around the first body

and so it's thicker on that side.

Two moons become one.

The evidence for this
cosmic collision seems to add up,

inside supercomputers, at least...

..but not all astronomers
are convinced.

Some believe there's
an alternative explanation.

And their proof lies
on an alien planet

that's being cooked alive.

480 light years from
our solar system,

a rocky, Earth-like planet
orbits a sun-like star...

..but this is no place for life.

COROT-7b is a sun-grazer.

This bizarre exoplanet orbits
60 times closer to its star

than the Earth does to the Sun.

Their close proximity
locks one face of the planet

to its fiery companion,

just like the Moon
is locked to the Earth.

The result is a cosmic barbecue.

Temperatures soar to over
1,600 degrees Celsius

on the near side of the planet,

reducing the rocky surface to
a boiling ocean of liquid magma.

By contrast, the shaded
far side of the planet

has a cool, solid crust.

This overcooked exoplanet
is the inspiration for a surprising

new theory about the formation
of our moon's lopsided crust.

It's the brainchild of
a pair of young researchers

with no background
in lunar science.

Because I work in exoplanets,
I knew very little

about lunar geology
when this whole thing started.

In 2011, Jason Wright was discussing

sun-grazing exoplanets
with colleagues,

when conversation
turned to the mystery
of the Moon's half-and-half crust.

And then I remembered that
the Moon, when it formed,

was very close to the Earth
and the Earth would've been

extremely hot after the impact
that formed the Moon.

And in fact, the geometry
is almost exactly the same

as the COROT-7b system.

That got me thinking,
"Well, maybe something similar

happened to the Moon."

Jason theorises that
the hot Earth cooked

the near side of the Moon,

just like stars cooked
sun-grazing planets.

So if you imagine that
the Moon is forming,

this big, hot ball of the Earth
is hanging in its sky,

on the far side of the Moon,
it would be quite cool.

The near side of the Moon
is kept hot by the molten Earth.

Vast clouds of mineral vapour
stream from the hot surface,

feeding a cloud that
surrounds the Moon.

On the cool side,
these minerals condense

and rain down,
building a thicker crust.

It's a nice idea,
but it only works

if the Moon locks one face
to the Earth straight after it forms.

Jason asked grad student Arpita Roy
to see if that was even possible.

When Jason first came
to me with the idea,

he was very excited about it
and he believed it was a big deal.

The calculation to check
the tidal-locking of the Moon

was pretty easy to do.

Um...the number was so small
that we were surprised.

It was on the order of 100 days,

which is very, very quick
in astronomical timescales.

Arpita's breakthrough means
that the near side of the Moon

locked to the Earth
while our planet was still molten,

supporting Jason's theory
of a cooked moon.

It's still very new,
but the theory is gaining momentum.

In science, if you have two ideas
that explain something equally well,

you kind of want to go with the one
that has more natural outcome.

This broiling-Earth idea
is a natural outcome

of what we know must have happened.

Thankfully for us, the surface of
the Earth is a lot cooler today.

But the inside is still hot,

thanks to the radioactive
decay of metals,

deep inside the core of the Earth.

This rising heat drives the volcanism

and geological activity
we see on the surface today.

There are mountain ranges
being pushed up

and other mountain ranges
being subducted into the ocean.

The crust of the Earth
is continuously changing.

The Moon doesn't have
a radioactive core,

so scientists have always
assumed that its centre is cold

and its geology is dead.

But a series of unexplained
observations suggest that once again,

we're wrong about the Moon.

One of the big questions
we've been trying to answer
over the last few decades is,

"Is there any activity
on the Moon left at all?"

For hundreds of years,
astronomers have reported

strange bursts of light
coming from the Moon.

Others have witnessed
reddish glows that lasted

for minutes at a time.

Could these rare sightings show

that the Moon's geological heart
is still beating?

The Moon is small enough
that over four billion years,

it should've cooled
all the way through.

And yet there's still some
things that we see that, maybe,

kind of, sort of indicate
that there's still stuff

going on inside of it.

Aristarchus is a volcanic region

in the northwest
of the Moon's nearside.

Astronomers have reported
over 700 glowing lights here

in the last 50 years.

Even Neil Armstrong saw
something strange here

from the window
of the Apollo 11 lander.

(RADIO BEEPS) NEIL: Hey, Houston.

I'm looking north up
towards Aristarchus now

and uh...there's
an area there that is

considerably more illuminated
than...than the surroundings.

A slight amount of fluorescence
to it, quite bright.

(RADIO BEEPS)

Uh...roger, 11. We copy.
(RADIO BEEPS)

What's going on in Aristarchus?

Smoking volcanoes
like this one in Hawaii

have given planetary scientists
a potential answer.

Because where there's smoke,
there should be fire.

You know, one clue to how
these events are happening

on the Moon
is where they're found.

They're actually found in a very
special location on the Moon

and it's very much like what
we're standing on right now.

Aristarchus is covered
in vast plains of volcanic ash,

and some scientists
believe this fine material

is responsible for the strange
bursts of light we see from Earth.

The kind of ash that I'm holding
right here is a little bit damp,

because there's been
a recent rainfall in the region.

Whereas the material
we'd be holding on Aristarchus

would be extremely fine.

Gases escaping
from this active volcano

carry particles of fine volcanic ash
and steam into the air.

Sunlight brightens the plume,
making it visible for miles.

Perhaps something similar
could happen on the Moon.

There's gas that
comes out of the vent,

picks up the very fine lunar dust

and spreads it out
across the landscape

so that we can see it
illuminated by sunlight.

If the theory's true,

it means the Moon's volcanoes
are still active.

Something like this is just
kind of so mind-boggling that

it's hard for us to wrap our minds

around the fact that the Moon
actually could still be alive today.

I mean, we've thought for many years
that the Moon is cold and dead,

but maybe these are happening now

and that means that the Moon
is not dead, the Moon is alive.

That's very exciting.

We thought we understood the Moon...

..but each time scientists
peel back the layers,

they find another secret.

And perhaps the biggest of all...

..is that without the Moon,
we wouldn't be here.

*

*

The Moon and the Earth
were born together.

They have dramatically shaped
each other's evolution.

And we now think that
life, too, was given

a head-start by the presence
of our oversized moon.

This is the Earth
4.4 billion years ago,

around the time scientists
think life on Earth got started.

The newly formed Moon sits
just 24,000 kilometres away,

appearing much larger in
the sky than it does today.

Its gravity raises enormous tides
in the Earth's warm, young oceans.

In that era, the tides
were not measured in feet,

they were measured in miles.

You'd have these massive tsunamis

that would wash up on land
and then wash back into the sea.

The Moon's gravity creates tides

by drawing Earth's oceans
up towards it in a bulge of water.

And as the Earth spins, this bulge
washes onto land as a tide.

The closer the Moon,
the bigger the pull of gravity

and the stronger the tide.

Some scientists think
that the warm rock pools these

giant early tides left behind
formed the perfect mixing bowl

for the ingredients of life
to come together.

The good thing about a tidal pond
is that environments change.

Water comes in,
brings nutrients, goes away,

the nutrients concentrate.

So that may have been a process

that concentrated
the stuff life needed

in a way that led to life.

4.4 billion years ago,
onrushing tides stirred up

organic molecules from
the surface of the Earth.

As the tides receded,
these chemicals were left behind

in shallow rock pools, which then
evaporated in the heat of the sun,

concentrating their
chemical contents.

And perhaps the first life was born
inside this rich organic soup.

If we had tiny, little moons
around the Earth like Mars does,

then we never would've
had the massive tides that carry

materials and energy up
onto the beach environment,

where life really
might've gotten a foothold.

And so we wouldn't
have had the minerals,

we wouldn't have had the energy
and maybe we wouldn't have had life.

Did the Moon create life on Earth?

The jury's out,
but one thing is certain -

intelligent life
takes time to evolve.

At least four billion years
in our case.

We've gradually changed
from simple single cells

to the kind of organism that
can question its own origins.

And it's the Moon that's provided
the stability for life to evolve

by holding the Earth's axial tilt
steady for over four billion years.

The Moon locked that tilt into place
and has actually stabilised
our rotational axis

and made sure the seasons
are mostly the same,

century after century,
millennia after millennia.

When we look at
the tilt of the Earth,

it's been about the same
for the time that

complex life has existed,
and this is really important.

As we progress out of
single-cell, simple organisms

to much more complex organisms,

greater stability really helps that.

How different would things
have been without the Moon?

Just look at Mars to see
the devastating effects

of an unstable tilt.

Mars actually had a tilt
that is as far as 60 degrees.

That means that
there was actually...

it was cold at the equators on Mars
and it was warm around the poles.

And so that's really
a difficult environment

for life to arise and grow.

When we look at Mars, we see that
its axial tilt has swung around

to such a degree
that it's inhibited life.

If the Earth had
swung that chaotically,

ice ages would have
come and gone like seasons.

Life would have needed to start
from scratch over and over again,

never having time to evolve
into complex organisms.

Mars serves as an indicator
that our supersized moon

may have been instrumental

in the development
of intelligent life on Earth.

And increasingly, scientists
believe similar cosmic partnerships

are the key to finding other
intelligent life in the universe.

When we're looking at exoplanets
and we're wondering

which one of these could have
civilizations, advanced life forms,

and there's a bunch that had
a big moon and a bunch that didn't.

I would say, "Let's first
look at the ones with a moon,

if for no other reason than
we know that on this world

we have a big moon
and we have advanced life."

As we're looking out
beyond our solar system,

we're looking into the galaxy,
looking for exoplanets
that might be habitable,

maybe we should be looking
for an object that has

a supersized moon in
the right location around its star.

It might be the perfect place
to look for life.

Scientists have discovered
over 1,000 exoplanets

orbiting stars in
our galactic backyard...

..by measuring the dip in
brightness as the planet passes
in front of its parent star.

An exomoon should also cause
a tiny extra dip in brightness.

Current technology
can't pick out this double-dip,

but a future generation
of space telescopes

could potentially reveal
large moons in the Milky Way.

And perhaps then we'll be able
to narrow down our search

for a second Earth.

If we ever want to see these
distant exoplanets up close,

we'll need a cheap,
reliable route into space...

..and the Moon could be the key
to making this dream a reality.

One of the greatest barriers
to conquering space

is the enormous amount
of fuel required to escape

Earth's gravitational pull.

But the Moon's gravity
is six times weaker.

A lunar launchpad could
become a gateway to the stars.

So if we establish a base,
uh...on the Moon, for example,

now we need very little energy
to get off of the Moon and to go

and explore other bodies from there.

What would it take to build
a lunar Cape Canaveral?

(INDISTINCT RADIO CHATTER)

The biggest hurdle is keeping
the ground crew alive.

One of the biggest dangers
of being on the surface of the Moon

is you're not protected
by a magnetic field.

On the Moon, you don't have that.

If there's a big solar storm,
the flux of high-energy particles

would hit astronauts
with so much energy

they would break down the cells
and destroy our DNA.

The Moon is also hit with radiation

that originates from
far outside our solar system.

Distant supernovas
throw out charged particles

called cosmic rays,
at close to the speed of light.

On Earth, our atmosphere
blocks most of the incoming rays.

But on the Moon,
astronauts are in the firing line...

..and many have reported
seeing tiny flashes,

as cosmic rays smash
through their optic nerve.

Incoming space rocks are
another major hazard on the Moon.

On the Moon, there's no atmosphere,

so something the size
of a grain of sand

is moving at speeds multiple
times the speed of a bullet.

And if that hits your colony, it can
put a pretty good hole in the wall.

Yet another obstacle
to long-term survival on the Moon

is the extremes of temperature.

One of the things
we don't really think about

about the air around us,
is it actually redistributes heat.

On the Moon, you have no air.

So if you have your hand

and half of it is in sunlight
and half is in shadow,

you've got a 400-degree
difference there.

The best protection from all
of these long-term problems

is a thick layer of rock.

But a recent discovery suggests

future astronauts won't need to dig
to build their shelters underground.

*

*

This remarkable hole
on the surface of the Moon,

called a skylight,
is the size of a football pitch.

We are seeing big openings
to massive cave systems

that might form the perfect base
for lunar exploration.

Skylights are the entrances
to a network of vast tunnels

which potentially run for miles
under the surface of the Moon.

There are already
underground caverns there.

And they're not carved by water,
these are carved by lava.

Similar natural tunnels
called lava tubes

exist on Earth in
volcanic regions like Iceland.

So we're in the bottom
of a hole that's formed

uh...from a collapsed lava tube.

Now, we know that on
the surface of the Moon,

there are holes like this.

And if all indications are correct,

they also lead to giant lava tubes,
just like this one.

Lava tubes form when
flowing rivers of molten rock

start to cool.

The surface solidifies first,
insulating the hot lava

below that continues to flow,
carving out long underground tunnels.

Living in a tube like this
actually wouldn't be that crazy.

In fact, this is a big tube,
it's quite large,

but the ones on the Moon
are 10 times bigger than this.

On the Moon, lava tubes run beneath
hundreds of metres of solid rock,

protecting the tunnels
from radiation,

micrometeorites
and keeping the temperature stable.

A lava tube like this on the Moon
would be nice and stable,

and perhaps never get much colder
than the coldest caves on Earth.

Astronauts on the Moon
could seal themselves inside

tubes like this by adding airlocks.

If we were able to colonise
a lava tube under the lunar surface,

we could adjust the environment.

We could pump in air,
we could make the temperature right.

You could get out and walk around
in this vast subterranean tunnel.

You could have
an entire city down there,

lighting up the walls
of the lunar terrain.

A lava tube would be
a great first lunar station.

I mean, it's like
a ready-made home for us.

In the future, when we
have colonies on the Moon,

they may very well be
inside of these lava tubes.

It'll be dark all the time,
you're in a cavern,

it'll have to be lit.

But you don't have to worry about

enclosing your domes
or anything like that,

it's already a sealed,
self-sufficient environment.

You fill it with air
and you can live in it, outside.

It'd be pretty amazing.
Uh...you're in one-sixth gravity,

you don't necessarily
need a spacesuit.

That would be fantastic.

If you want to go out on the
surface, which you would have to,

yeah, you have to wear a spacesuit
and be able to protect yourself.

But I think living in
one of these colonies

would be really astonishing.

You could... you could fly.

You could have wings. You could
make wings and flap and fly.

There have been science-fiction
novels written about this.

I would love to see this someday.

I think that would be an amazing
future to look forward to.

Scientists think that
these lunar space stations

could be self-sufficient.

There are craters
that are always shaded

and they contain water ice.

And so that is really exciting,
because we didn't realise

that there was actually
still water on the Moon.

We thought the Moon
was just bone-dry for many years,

that was the mantra.

And now we're finding out

that contained in soil,
contained inside of crater walls

that are permanently shadowed,
there is lots and lots of water ice.

Water is the perfect resource
for a lunar launchpad.

As well as drinking it,
you can split water with electricity

to create oxygen to breathe
and hydrogen to use as a fuel.

When you think about it,

rocket fuel is made of
liquid hydrogen and liquid oxygen,

and, hey, those are
the components of water.

The Moon's craters hold around
six billion tons of water ice.

That's enough to launch
20 rockets into space every day

for over 100 years.

For over four billion years,
the Moon has driven our evolution,

shaped our climate,
and in the future,

its resources will
allow us to conquer space.

But it's not going
to be around forever.

Sometimes when we talk about
things that are reliable,

we say there's nothing as reliable
as the rising of the sun, right?

We can think of the Moon
in the same way.

It goes through its phases.
It's there night after night,

year after year.

But it turns out, the Moon is
actually moving away from the Earth,

and that's due to
the interaction of the Moon

and the Earth's tidal bulge.

The bulge of water
pulled up by gravity

sits slightly ahead of the Moon

because the Earth spins
faster than the Moon orbits.

The Moon pulls, by gravity,
on that bulge

and slows the Earth's rotation.

Over billions of years, that has
slowed the Earth's rotation a lot.

We used to be spinning
a lot more rapidly,

probably more than twice
as fast as we do now.

The Moon's attraction to the bulge

has the opposite effect
on its own orbit,

speeding it up.

This increased speed
makes the Moon's orbit wider,

pushing it further and further away.

It's a very small amount,
so it's only about 3.8 centimetres,

which is about
an inch-and-a-half a year.

Over billions of years,
the Moon will shrink to a dot

in the night sky and the Earth's
spin will become so slow

that the Moon will appear
to freeze above our heads.

There will come a time where
the Earth is actually locked.

One side of the Earth faces
one side of the Moon

and the two of them
will go around in lockstep.

So there'll be one place on Earth
where you can see the Moon.

So you might imagine you'd
have to go on some kind of uh...

vacation to actually see
the Moon at that point in time,

but that's going
to be a long from now.

So...so, you know, um...
I wouldn't start booking

your tickets quite yet. (CHUCKLING)

Is this the long-term
future of our moon?

Some scientists envision
a more dramatic ending -

a death by fire that will destroy
the Moon and, quite possibly,

all life on Earth, too.

The process begins with
the expansion of the sun.

The actual future history
of the Earth-Moon system

will depend upon the sun...

..and it could produce
remarkable effects.

As the sun gets older, it expands,

filling the inner solar system
with a dense solar wind.

This wind will impede the Moon.

So as the Moon orbits around
the Earth, there'll be drag,

there'll just be more stuff in space
for the Moon to push against.

So the Moon has been
moving away from the Earth

for billions of years, maybe at
that point, it'll start coming back.

This new inward trajectory
is a death spiral.

The Moon eventually is going
to spiral closer and closer.

And then, because of
the gravitational forces,

the tidal forces are
gonna be so strong,

it's going to essentially explode.

18,000 kilometres above
the surface of the Earth,

the Moon reaches
a point of no return...

..the gravitational pull of
the Earth finally overwhelms it.

And you'll see it shaking,
you'll see it quaking.

It'll be stretched way out
and it'll be stretched sort of

toward us and away from us,
it'll be hard to tell.

But eventually, you'll see
that actually breaking apart,

an entire world being shattered
by the gravity of Earth.

The fractured remains of the Moon

create a Saturn-like ring
of rocky debris.

Having a ring around the Earth
would be a phenomenal sight,

I would love to see that.

You would look up and you
would be able to see the ring,

it would be at
an angle to the Earth.

If you were at
the right place on the Earth,

you'd be able to see it broad,
stretching across the sky.

I don't know if you'd be able to
see it during the day, but at night,

it would be one of the most
spectacular sights I can imagine.

But the beauty soon turns to terror,

as pieces of the ring
rain down on Earth.

I mean, it's going to be an
awesome sight, a terrifying sight.

I mean, the whole sky
is going to be filled with

raining meteors just
showering through the sky

and they're going to be huge.

Eventually, all that of material
will be incorporated into the Earth,

and now these two siblings,
separated at birth,

now are finally again one body.

From the fiery inferno,
a new Earth is born...

..but this world is sterile.

The Moon's presence allowed
life to arise on the Earth,

and the Moon's presence
will also destroy life on Earth.

The Moon giveth
and the Moon taketh away.

So that's kind of a neat storyline,

although maybe not so great for us.
(CHUCKLES)

It's pretty sad to imagine
the Earth without the Moon.

We're partners, we've affected
each other's development.

Seeing the Moon in the sky
is something that gives me joy

every single time,
that has never gotten old.

We can't help
but show affection towards it.

It's there every night
staring at us and, of course,

romances have been written
about it and will continue

to be written about it.

Life on Earth may not have
existed without the Moon,

but certainly,
without it, even if it did,

it'd be much less romantic.

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