All language subtitles for Pluto Back From the Dead 1080p BBC Horizon 2020

3.5 billion miles from Earth,
beyond Jupiter,

Saturn, past Uranus and Neptune

hides Pluto and its five moons.

Discovered in 1930,

for decades we knew almost
nothing about it.

As a child, I learned that Pluto
was a mysterious grey rock.

So in 2006, NASA launched a rocket
carrying a lightweight probe.

All telemetry is nominal.

Copy that. We have a healthy
spacecraft.

The spacecraft returned
the first-ever close-up images

of Pluto's surface.

We've recorded data of the
Pluto system.

And sent our understanding
of this icy world into disarray.

The data returned has forced us
to rethink everything about Pluto.

This was not the lifeless rock we
once thought.

There is a heat source which must be
coming from inside,

but the details of that
are still very mysterious.

This small, unassuming little world
may actually have a liquid water

ocean beneath its icy surface.

Incredible new discoveries
about Pluto are even

changing our understanding
of life beyond Earth.

No-one in their right mind would
have thought Pluto had life,

but now it's a possibility

and that in itself is kind of
mind-blowing.

This is the story of how Pluto
came back from the dead.

In 2006, a visionary group
of scientists began

a ground-breaking mission.

To send a spaceship further
than ever before.

Its mission - to explore the far
reaches of our solar system and take

the first-ever close-up image
of Pluto,

before continuing into deep space.

The probe was called New Horizons.

And it would take more than nine
years to make the

3.5-billion-mile voyage to Pluto.

Keeping things on track was mission
operations manager Alice Bowman.

This is the best picture of Pluto
that we had before New Horizons

arrived, and this was taken

by the Hubble Space Telescope

and it was the picture that inspired
us to send a spacecraft to Pluto.

In 2015, ten days before
New Horizons' arrival,

Alice's team began uploading
computer commands to the probe,

telling the spacecraft what to do
when it reached Pluto.

That was a very anxious time
because we wanted to make sure

that those commands were received
and accepted by the spacecraft,

because if they weren't,
we wouldn't have a mission.

But as the final command was sent,
Alice's worst fears were realised.

We lost comm with the spacecraft.

Totally silent. And so you can
imagine when you're in any kind of

critical situation, the worst thing
is lack of any information.

And that's essentially what we had.

The team had lost all contact
with the probe.

Pluto Ace MOM on New Horizons,
Pluto One.

Could you advise why we are not
locking up to telemetry?

MOM, stand by. I'll check with the
station on the status of telemetry.

New Horizons was designed to
fly past Pluto, taking pictures

as it went.

If they didn't regain contact,

it would fly past without
recording a thing.

Station 43 Pluto Ace.

MOM on New Horizons Pluto One,
go ahead, Pluto Ace.

After a frantic hour, the team

received a weak signal from
the probe.

We found the spacecraft,
it was responding to us,

it was giving us information,
but it was in a state that could not

accomplish the encounter with Pluto.

We determined that we had asked the
processor on board the spacecraft

to do too many labour-intensive
things.

And the spacecraft switched over to
the redundant or backup computer.

New Horizons was back online,
but there was a problem.

When it had switched to the backup
computer, it had wiped every line

of command needed for its encounter
with Pluto.

All non-essential systems
had shut down,

including the cameras.

The question became, did we have
enough time to recover

the spacecraft and start
that flyby encounter on time?

So our plan was to switch back to
the main computer and then resend

the commands to the spacecraft.

And it took us three days to
do that.

Round the clock? Around the clock.

We slept in our offices.

No-one wanted to leave because we
had waited all this time

and there was no way that we were

going to let this opportunity
slip by.

Working through the night,

the team re-uploaded the
computer commands.

Once completed, all they could do
was wait.

It was a very anxious time.

Travelling past Pluto more than ten
times faster than a speeding bullet

meant every new line of command

needed to work with
meticulous precision.

There would be no second chance.

No way to go back.

And with New Horizons busy
collecting data,

it would be out of contact
with Earth.

Until the spacecraft beamed
back a signal,

the team would have no idea
if the new commands had worked.

Subsystems, please report
your status as you get enough data.

OK, we're in lock with carrier.

Stand by for telemetry.

The atmosphere was very intense.

We were all anxiously awaiting
that first bit of telemetry to tell

us that the spacecraft had survived.

MOM on Pluto One.

We have a healthy spacecraft.

We've recorded data
of the Pluto system.

And we're outbound from Pluto.

WHOOPING AND CHEERING

The flyby was a success.

And Pluto's surface came into focus
for the very first time.

It was amazing.

It just blew our minds that
something could be so beautiful.

It's like telling us, "Why the heck

"did you wait so long to
come visit us?"

What New Horizons sent back was
astonishing.

The probe had taken thousands
of photographs of Pluto's surface.

Multiple cameras had charted
the topography.

The spectrograph detected
what that surface was made from.

Revealing Pluto's secrets
for the first time.

And in spectacular detail.

Oh, there's my friend! Beautiful.

Leslie Young and Jeff Moore are two
of the project's leading scientists.

Their job - to decipher
this mysterious alien world.

It's amazing, you look at all
these exotic features.

Everything's so amazing
and so diverse and so complex.

I was in the main room when I first
saw this image.

We were on tenterhooks.

Pluto was ready for its close-up.

When these images arrived,
the team was struck

by Pluto's familiar appearance.

These mountains down here, they look
an awful lot like Earth mountains.

They're snow-capped.

These guys are 4km tall,
just like the Rocky Mountains are.

But unlike Earth, these mountains
were made from ice, not rock.

Pluto is so far from the sun,

the scientists were expecting
an almost featureless frozen sphere.

Instead, New Horizons had revealed

Pluto to be a world of puzzling
complexity.

There were mountains forged
from ice.

Craters of every size.

Gaping canyons and vast plains

covered in nitrogen frost
and methane snow.

All of it mysteriously coloured
from deep red...

..to bright white.

We knew Pluto was going to be
scientifically astonishing.

We didn't know it was going to be
so beautiful.

Nobody expected to see anything
like this.

Pluto's full glory had finally
been revealed,

85 years after it was discovered by
American astronomer Clyde Tombaugh.

Its distance from Earth has meant
that for much of the time

that we've known about it,
Pluto has remained as mysterious

as the stories that inspired
its name.

Pluto is the god of the underworld
in mythology.

He lives in the realm underground.

And this is the place where people
go when they die.

The first thing to know about Pluto
is that it's tiny.

And really far away.

We're standing on a lake bed,
a frozen lake in Colorado.

Let's pretend that this is the plane
of the solar system.

And over there, the ice resurfacing
machine,

we'll call that the sun.

That's where the sun is, then we're
roughly where Earth is.

But, of course, Earth is much
smaller. If I don't break it,

here is the Earth, roughly a
centimetre across,

something like that.

Now, Pluto, by comparison,
would be a grain of sand.

That grain of sand would not
be here, of course, in this part

of the solar system, it's in the
outer part.

And it would be roughly on that
mountaintop right over there.

So about 6km
away would be Pluto.

Pluto's extreme distance from Earth

meant that very little was known
about it before

New Horizons arrived.

Astronomers knew that it was
cold enough to be made from ice.

They knew it was small,

and that it had company.

Pluto is not alone.
In fact, it has five moons.

Tiny Pluto isn't one body,

but a whole system of icy worlds

all engaged in a strange
and complex dance.

And as New Horizons approached
Pluto, the scientists were able

to see them in detail
for the very first time.

Pluto's first and largest moon
is Charon.

The name comes from Greek mythology
as the name of the boatman

who would row people across
the river to get them

to the underworld.

Charon is remarkably large
compared to Pluto.

It's about half the size
and they're very close.

Sometimes we call them a binary
planet because, in fact, they orbit

together as if there were a rod
between them and they were two ends

of a lopsided dumbbell.

Pluto's moon Charon doesn't orbit
Pluto.

Unusually, they orbit each other...

..around a point in empty space.

Beyond Charon are four smaller
moons. There's Styx.

Styx is the river that the boatsman,
Charon, would take people across

once they died and entered
the underworld itself.

Styx is a very small body, it's just
kind of a lumpy, irregular object.

There's Nix.

Nix is about 50km across.

It is rotating in a very strange
way that we haven't quite

figured out yet.

Then there's Kerberos

and Hydra.

Hydra is spinning very, very fast
and we simply don't know why.

If you were standing on Hydra,
you might have to actually hold on.

Why does a tiny dwarf planet
have so many moons?

The peculiar dance of Charon
and Pluto are a clue.

So the big question, of course,
is you have to get two bodies,

you have to get a binary planet
in the middle.

And the only way we really know to
do that is there was a proto-Pluto

out there very early in the history
of the solar system.

And something else hit it,
something very, very big.

Pluto kind of blew apart.

And eventually re-coalesced.

But all that material
that was floating around,

a lot of it formed into Charon.

And the two got kind of stuck
together as they evolved

into this perfect circular
dumbbell system.

Maybe there was some debris
left over that landed further out

and that became Hydra, and Kerberos,
and Nix and Styx.

New Horizons had given the
scientists a better understanding

of Pluto's peculiar moons.

But it was the pictures of Pluto's
surface that contained

the biggest surprises.

A smaller impact than the one
that created the moons

could be responsible for one of
Pluto's most mysterious features.

We think this was a big impact

crater just from the shape of it,
really.

This is probably an ancient crater
rim of a huge impact

that must have happened early on
when Pluto was still very young.

So this is about 1,000km across,

probably something about 200km
across came in and hit Pluto.

New Horizons was carrying a
spectrograph, an instrument capable

of analysing chemical composition.

It revealed that much of Pluto's
freezing surface was covered

by soft nitrogen ice.

Normally a gas on Earth, the
scientists believe that vast amounts

of it have flowed in to fill
this enormous crater.

We're pretty sure we understand
that what makes up the plain

is a vast reservoir of solid
nitrogen ice,

which even though it's solid,
is very soft.

It was like a glacier,
but made from nitrogen ice.

The area was informally dubbed
Pluto's heart

after its obvious resemblance.

But closer inspection left
the scientists with a puzzle.

It had all these weird forms
and textures on the surface.

Some stand up quite clearly,
some are very subtle.

An intricate and strangely organised
pattern appeared to be scribed

into the surface of the
nitrogen ice.

And there was something missing.

The other thing that struck us
was the fact there was no impact

craters anywhere in this
huge expanse.

Ancient planetary surfaces are
almost always covered in craters.

A result of being battered by
asteroids over billions of years.

And much of Pluto's surface was
heavily cratered,

but here there were none.

Nobody imagined there'd be
a huge region

that was many hundreds of kilometres
long and wide

that would be completely
devoid of craters.

That was mind-blowing.

What's forming this huge plain
has to be an ongoing process.

Jeff believed that whatever force
had created these shapes must

still be operating on Pluto today.

This isn't an extinct world,

this is an active world
in which geological processes

are operating today and they're

destroying any evidence
for past cratering.

And that was truly remarkable.

It was a monumental discovery.

This was not the dead world
that everyone thought.

Pluto's heart...

..appeared to be beating.

The scientists' job now was to find
out what was driving it.

All right, Mary. Commercial break.

We got 1:45 when we get to you.
1:45. OK. Coming out to you guys
here

in five, four, three...

Welcome to KPIX 5 News At Noon.

The New Horizons planetary
scientists were stumped.

Mysterious patterns appear
to be carved into the surface

of the nitrogen glacier.

..dry and quiet conditions,

milder weather expected Wednesday,
Thursday and especially for Friday.

Meteorologist Mary Lee was

not involved in the
New Horizons mission,

but to her the shapes are a
familiar sight.

That's going to do it for a
KPIX 5 News At Noon.

Have a wonderful afternoon.

I am the morning meteorologist

for the CBS station here
in the Bay Area, KPIX 5.

I wake up at 2:30 to start my day.

When I'm putting the forecast
together I'm really looking at

satellite data so we can determine
the clouds

that are over our region.

Mary is paying special attention
to the clouds forming

over the Pacific Ocean.

These are stratocumulus clouds.

So really you can see that puffy,
lumpy texture of those cloud tops.

These clouds are very common
on the west coast

of the Pacific Ocean.

So here in the Bay Area,
we see them all the time.

You can actually see the boundaries
of these clouds and it kind of forms

a bit of a hexagonal shape
across our atmosphere.

These repeating shapes are
very similar to what scientists see

in Pluto's heart.

They're a result of a unique way
that these clouds are formed.

I love looking at these
stratocumulus clouds.

They form over the Pacific Ocean.

And really what happens, you have
the sun hitting the surface,

that warm air rises and the liquid
droplets, they condense

and so they form these clouds.

And the top of these clouds,
it's very cold air.

So that cold air sinks.

This process is called convection.

You have this cycle of warm air
rising and cold air sinking, and
that's

a convective cell.

A plume of warm air rises,

forms a cloud, cools, then sinks.

And when you have many of these
convective clouds,

an hexagonal pattern is produced
in the sky.

So these clouds can only exist
when you have heating from below.

Convection currents can happen
whenever hot meets cold.

We see them in clouds.

On the surface of the sun.

And even in Mary's coffee.

An understanding of how convection
currents work has led the scientists

to realise that the soft nitrogen
ice on Pluto is slowly churning.

We know that the convection rate
at least takes place over hundreds

of thousands of Earth years.

This is a slow process in terms
of human lifetime.

It's a very quick process
in terms of geological time.

The images sent back by New Horizons
show that fresh material

is being brought to Pluto's surface
driven by convection currents.

But convection needs heat.

How could that be possible
on an ice world that's supposed

to be frozen solid?

The data returned from New Horizons
has forced us to rethink

everything about Pluto.

The surface of Pluto
has been repaved.

The images are telling us
that there's some sort of geological

processing going on.

How could that be powered?

On Earth, the heat that powers
geological activity doesn't come

from the sun, but from radioactive
elements deep within Earth's mantle.

Often when we think about
radioactive decay, we think

about places like this, a nuclear
power plant.

But there's a different kind of
radioactive decay

that is constantly occurring
within heavy elements

that are trapped within rocks,
rocks that make up planets.

A tiny fraction of those radioactive
elements are harnessed as fuel

for nuclear power stations
like this.

But there's enough of them in
Earth's rocky interior

to continually produce
44 trillion watts of power.

RUMBLING

And that power creates enormous
amounts of heat.

Radioactive decay within the Earth
provides enough heat to produce

all of the volcanoes that we see,

the tectonic activity that leads
to the uplift of mountains...

..the earthquakes and all of the

geology that we see here
on planet Earth.

Radioactive rock accounts for most
of the Earth's internal heat,

heat which powers the geological
activity we see on the surface.

But tiny Pluto, being mostly made
of ice, was thought to be different.

Prior to the arrival of
New Horizons,

the conventional wisdom was that
it's probably too small

and too low in density to have
enough rock in its interior

to drive enough radiogenic decay,
enough heating.

But once New Horizons started
returning images and data

about Pluto, that all changed.

Could Pluto be heated from within
after all?

The images sent back by New Horizons

had allowed the scientists to see
Pluto's surface

in unprecedented detail.

Comparing multiple pictures allowed

them to work out the height
of the mountains.

And among them, Jeff spotted

something that should be impossible
on a frozen world.

We're looking at a large mountain,

which is about 5km high,

it's about 150km across,

and in the centre is a large pit

and the pit goes down to about the
same depth as the mountain is high.

And when we first saw this we were
really surprised and puzzled

as to what it might be.

Nothing like this had ever been seen
on another world.

And for Jeff, there was only
one explanation.

A volcano.

Its striking resemblance to a
volcano was so shocking

that, in fact, I made the decision
not to discuss it

in our initial press conference

just because making such claims are
somewhat extraordinary

and I didn't want to say anything
till we had more evidence.

Once you see it as a volcano,
you can't unsee it.

But a volcano didn't make sense.

There is no lava on Pluto.

A spectrograph on board the probe

confirmed that Pluto
was made from ice.

It has to be made of water ice.
Mm-hm.

And water ice is the most difficult
thing to mobilise on Pluto.

So it's a real challenge to imagine
how water ice could erupt

under the surface the same way lava

erupts up to the surface
on the Earth.

The question was, how could ice
flow like lava

on a world that's
230 degrees below zero?

Space volcanologist Kelsi Singer
is on a mission.

I would love to go to Pluto.

Being able to stand on the surface

and actually survey these features
like a geologist would be amazing.

She wants to find out how
on a frozen world like Pluto,

enormous volcanoes can exist.

A lot of the landforms that you see,
like the mountains or the ice

volcanoes, are probably mostly
made of water ice.

The volcano on Pluto looks
remarkably similar

to volcanoes on Earth...

..where thick lava flows out over
huge areas to produce

shallow mountain slopes.

But Pluto is made from ice,
and ice doesn't behave like lava.

If you have a feature that you think
formed from the flow of ice, that's

very difficult to understand
how that would occur on Pluto.

Pluto's surface is far too cold
for liquid water to flow

over any distance.

Instead, it would freeze
in an instant.

I am in an ice castle in Colorado.

It's about minus 12, so it's
pretty chilly.

And how they form this is they have
sprinklers embedded in the ice.

The water comes out at night and
then it freezes in these icicles.

It's pretty cool!

Temperatures here reach minus
20 degrees Celsius at night.

Water from the sprinklers
freezes in seconds.

And it looks nothing like
the volcano seen on Pluto.

Hidden in New Horizons' images,

Kelsi spotted the missing
ingredient.

This fracture is called
Virgil Fossae.

It's a couple of hundred kilometres
long and at least 5km high.

And it's got a unique red colouring.

And that red colouring turned out
to be associated with ammonia.

The spectrograph on board
New Horizons had detected ammonia

on Pluto's surface.

The detection of ammonia on Pluto
was very important because it lowers

the melting temperature of water.

In fact, ammonia can lower
the freezing point of water

by up to 100 degrees Celsius.

I'm going to try to recreate the
effect of ammonia on Pluto's ice.

But first, let's see what happens
with water just by itself.

So I'm pouring out some water here.

And then I'm going to add
the liquid nitrogen.

At around minus 200 Celsius

liquid nitrogen allows Kelsi
to simulate the extremely low

temperatures on Pluto's surface.

And we can see it's starting
to freeze here very quickly,

turning into very cold ice.

The ice on Pluto is about
minus 230 degrees Celsius.

It acts more like rock
because it's so cold.

Sort of like the ice that we have
recreated here.

At this temperature water ice
is as hard as granite.

This time we're going to try
adding ammonia.

So here I have water and ammonia.

And now I'm going to add
the liquid nitrogen.

Ammonia lowers the freezing point
of water by physically blocking

the water molecules, stopping them
from sticking together.

Getting thicker.

So now we have the water and ammonia
mixed together and it's forming

a very thick and goopy,

almost toothpaste-like fluid here,

and we think that might be how
the volcanoes on Pluto formed,

coming out from the subsurface
onto the surface

and building up a volcano over time.

Kelsi believes that this thick,
freezing liquid has erupted

through Pluto's crust, flowed
across the surface like lava

before freezing solid to form
the ice volcano that we see today.

If you were standing on one
and watching a flow of icy lava,

it would probably be actually quite

slow going down the slope
of the volcano.

Pluto has turned out to be
a geologically active world.

Ice can flow like lava
thanks to ammonia.

But volcanic eruptions still
need heat,

and heat on icy Pluto would mean
something incredible.

The fact that we see something
that could be an icy volcano,

that had to form from flow
of material, means that the interior

of Pluto must be somewhat warm.

And that means that we could have

a subsurface liquid water ocean
on Pluto.

Liquid water has not been proven
to exist anywhere beyond Earth.

This small, unassuming little world
may actually have a liquid water

ocean beneath its icy surface.

And the evidence from the
New Horizons mission...

..makes it quite hard to explain
what we're seeing without invoking

an ocean as an explanation.

The realisation that this icy world
could be hiding a liquid ocean

has forced scientists to rethink
everything about Pluto.

But there was a mystery.

Pluto's size.

The problem with Pluto in terms of
its heat from the interior

is that it probably doesn't have
enough of the raw materials.

Uh, there's not enough radiogenic
decay to keep it warm from within

for a very long period of time.

And yet the evidence is that
something is driving

this geological processing, so there
must be another piece of the puzzle.

There must be something else
on or within Pluto that is helping

it retain the small amount of heat
that it gets from the interior,

from radiogenic decay.

A small amount of radioactive rock
in tiny Pluto's core

may provide some heat.

But for Pluto to hang on to it,
something must be keeping it in.

And this could be down to a special
type of snow called a gas hydrate.

Scientists at the National
Oceanography Centre in Southampton

make it in their lab.

It looks like ordinary snow.

It's freezing cold, like
ordinary snow.

MATCH STRIKES

But take a match to it and it
immediately bursts into flames.

This, in my hand, looks like ice,
but it is actually burning.

And that flame is actually
methane gas that's been coming off

the ice as it melts.

It burns because flammable gas,
like methane, gets trapped

within the crystal structure of ice.

Gas hydrates form naturally on Earth
under very particular conditions.

It occurs in very large quantities
in places like the Arctic.

And, in fact, in the Antarctic as
well.

So both polar regions where
it's very cold.

Huge amounts of gas hydrates exist
in the very coldest places on Earth,

especially where methane is present.

The same conditions that produce
gas hydrates on Earth also

exist on Pluto.

So it's possible that they form
on Pluto, too.

In order to explain what we see
on Pluto's surface,

we really do need heat.

We see activity currently going on,
which was exciting and unexpected

and wonderful, but now it

comes down to the science of
explaining it.

Dr Carly Howett designed the cameras
on the New Horizons probe.

She believes that it's the hydrates'
ability to trap gas

that could be the answer
to Pluto's heat problem.

The water ice forms a hexagon
almost like a cage.

And then inside is gas.

In this case, we think it's
probably methane.

These are incredibly small.

You'd end up with millions
upon millions of very,

very small cages, each holding
its own little methane gas bubble.

The gas bubbles on Pluto don't
ignite, but act like a thermal

insulator, trapping heat.

This gas hydrate is very insulating
and it could be kind of forming

a warm blanket that's enabling
Pluto to hold its heat in,

enabling Pluto to remain warm
for billions of years.

Pluto's surface temperature
is a frigid 230 degrees below zero.

And scientists now believe
that a thick layer of gas hydrate

snow could be insulating Pluto's
warm radioactive rock

from those freezing surface
temperatures.

Keeping in enough heat
to melt the ice.

So the heat comes from the rocky
core and it can't escape

through this insulating layer
and maybe it's warm enough

so that you end up being able
to sustain liquid water, form

and sustain liquid water, and maybe
even in the form of a sort of

global ocean, which is incredible.

Pluto is a long way from the sun.

That amount of heat is unexpected.

The scientists had found a way that
liquid water could exist on Pluto.

Everything they thought about this
icy world had been turned

on its head, which has led some
scientists to think the impossible.

So if Pluto is warm enough
to sustain a liquid water ocean,

this has huge implications
for astrobiology.

One of the things that we know
about life is it likes liquid water.

Liquid water is one of the

requirements for having life,
as far as we know.

And so if we were to find a
subsurface liquid ocean on Pluto,

that would give us more reason
to believe that there could

potentially have been in the past
or even currently be some kind

of life forms in the ocean on
Pluto.

If we've learned anything from our
study of life on Earth,

it's that where you find liquid
water, you generally find life.

We look for liquid water as the
sort of

canary in the coal mine
for habitability.

If you've got liquid water,
perhaps you could also harbour life.

On Earth, almost everywhere you find
liquid water, you find life.

And the new pictures of Pluto seemed
to suggest that it was hiding

a vast underground ocean.

Pluto had been propelled from being
an icy relic to a place

where alien life could exist.

But there was still something
missing,

if biology was to have any hope.

As well as heat and liquid water,

life requires a complex array
of chemical ingredients.

Could they exist on tiny Pluto?

The first clue about Pluto's
chemistry came in 1988,

long before New Horizons...

..when images like this were
all that scientists had.

So here is a picture you can take

from the ground-based telescope
like this.

And there are a lot of beautiful
background stars here,

but there's one particular
important one we are talking about

today, is Pluto here, in the middle
of this picture.

It's not much, right?
It's only a few pixels.

From Earth, Pluto was nothing
but a tiny white dot.

So astronomers waited for a rare
event called an occultation.

So, look at those background
stars here,

they are relatively stationary,

but Pluto is moving in the sky.

As Pluto passed in front
of a background star,

rather than blocking the starlight
abruptly, the starlight

was slowly dimmed.

This could only mean one thing.

That Pluto had an atmosphere.

Here dry ice.

OK. Look at this atmosphere

we just made in the laboratory.

And if there is a light source
behind it, the light source will

get dimmed because of the absorption
and a scattering of the atmosphere.

And that's similar to what happens
in Pluto's occultation.

Here, dry ice vapour scatters
the torchlight in the same way

that Pluto scattered the starlight.

It was proof that Pluto
had an atmosphere.

This was a complete surprise,

a tantalising clue that Pluto
was much more than a frozen relic.

But the question remained,
what was Pluto's

atmosphere made from?

The New Horizons probe was sent
to find out.

And when it returned this picture,
the scientists were able to see

the atmosphere in detail
for the very first time.

This is such a beautiful image.

You look at this and you think
blue skies, just like Earth.

This image says we were really there
because this is the kind of image

you can only get after you pass
by Pluto.

This picture was taken looking back
toward the sun and Earth,

and we're seeing Pluto backlit
by the sun.

The onboard spectrograph revealed
that Pluto's atmosphere was made

mostly of nitrogen gas,
just like Earth.

As New Horizons continued away
from Pluto, the long-range camera

took one of the most important
photographs of the entire mission.

And it contained something that the
scientists did not predict.

Like everybody, I was surprised
at this beautiful structured

atmosphere, all these layers
were wholly unexpected.

There is so much going on in this
picture.

We can actually see these layers
in the haze in Pluto's atmosphere.

They're about 20km or so between
each one and we see

the haze going up for 200km
or more in this image.

The layers in Pluto's atmosphere
were a complete mystery.

Invisible nitrogen gas alone
couldn't explain the layers.

Planetary scientist Xi Zhang thinks
it could be caused by sunlight,

because even on distant Pluto,

ultraviolet rays have a profound
effect on atmospheric gases.

It happens every day on Earth
and can even be seen

above downtown Los Angeles.

You can see the air quality
is not good.

Smogs, hazes.

That's because we have three million
cars crisscross the city every day

and those cars emit a lot
of air pollution.

The air pollution is mostly
volatile organic compounds

like hydrocarbons, soot particles.

Large soot particles hang
in the air above the city.

They're created by a
chemical reaction.

Exhaust fumes from cars are baked
by the sun, causing them to

break down and re-form into larger
particles.

Those smogs are actually pretty
similar to what we see on Pluto

in terms of chemical processes.

But there are no cars on Pluto,

so why is its atmosphere
choking with smog?

When New Horizons analysed Pluto's
atmosphere, it discovered

that it wasn't just made
from nitrogen.

There were also tiny amounts
of other gases like methane

and carbon monoxide.

When those air molecules hit
by the UV light from the sun,

the chemical bonds are broken
and those coagulate to make big

particles, it's similar to the
soot particles we have here.

Just like the exhaust fumes
above Los Angeles, the invisible gas

molecules in Pluto's atmosphere were
being broken apart by the sun...

..before re-forming into large
soot particles

and settling out into layers.

Those haze particles are produced
in the upper atmosphere of Pluto

and gradually settle down due
to gravity.

The soot particles were a clue

that something special was happening
on Pluto.

But exactly what chemicals were
being created?

Ready? High voltage down.

The probe had told scientists what
gases exist in Pluto's atmosphere.

The MSCs are off.

Which allowed them to do something
that had never been done before.

Cylinders are closed.

Recreate Pluto's atmosphere
here on Earth.

What this machine allows to do
is to simulate space environment.

In the case of Pluto, we can
simulate the atmosphere of Pluto.

OK?

Doing this experiment will allow us

to get a better idea of the type

of chemical processes that are
occurring in the atmosphere.

Pressure, eight torr.

Yeah, that's the base pressure,
that's good.

The base pressure is eight so
it's very good.

Pluto's atmospheric gases
enter this machine

at minus 170 degrees centigrade...

..where an electrical discharge
simulates the sun's rays.

After ten hours in the machine,
the gases are transformed

into an entirely new substance.

Tiny grains of dust.

These are these grains that we make
in the gas and that get deposited

onto a window and piled on top
of each other until we have a thin

layer of these dust grains.

And, as you can see, the colour
is a dark brown.

These are the same particles
that cause the layers

in Pluto's atmosphere.

Eventually they grow so big
that they fall to the surface

like rain, painting Pluto in a broad
spectrum of browns and reds.

So we can think that this could be

one of the contributions
for the colour on Pluto's surface.

A chemical reaction in Pluto's
atmosphere was causing

particles to rain down onto
the surface.

So this is an enhanced colour image
and we can see that this area

is really red and quite dark,

and much redder than the rest
of Pluto's surface.

Areas that receive more sunlight
appear much redder than elsewhere.

The next job for the scientists
was to find out exactly

what this red substance was.

Dale Cruikshank studies the surface
of small objects in space.

And since New Horizons,
he has turned his attention

to the chemical reactions
occurring on Pluto.

Pluto has a very thin atmosphere,
and we know that the basic

components are nitrogen,
methane and carbon monoxide.

Sunlight coming in, in the
ultraviolet part of the spectrum,

has enough energy to break the
energetic bonds

that hold the atoms in the
molecules together.

These simple gases re-form randomly
into more complex molecules.

Like that.

And experiments to recreate this

have produced something
extraordinary.

Eventually, these materials form
what we call precursor molecules

to molecules that are of biological
interest,

including what are called
nucleobases.

There are many different
nucleobases, but five of them

are of biological interest
because these are the principal

components of DNA and RNA.

Nucleobases are the main ingredient
of DNA.

All life on Earth uses DNA.

From these complex molecules

life is just a few steps away.

And some scientists believe
it's possible

that they're being produced
on Pluto.

The New Horizons mission has given
us these fantastic images

from which we now deduce
that the colours are in fact

from organic material,
complex organics that have fallen

from the sky, literally.

We are making precursors to the
things that eventually

we think on Earth resulted
in the origin of life.

And the surface isn't the only place
where pre-life organic molecules

could be being made.

Complex organics are also
being produced in water

and have emerged onto the surface
through cracks in the crust

in a process that we call
cryovolcanism.

The red colour of the ice around
Virgil Fossae is evidence, not

only of ammonia, but also
organic material erupting

from an underground sea.

There may be habitable zones
on Pluto.

For example, in the interior,
if there is still a liquid

and it's full of organic materials,

this may be a habitat where life
could exist.

So, if we were trying to survey the
entire solar system for potential

habitable zones, habitable regions,

you'd have to include Pluto
in that list, I think.

The complexities of just how organic
molecules spawn life is not

yet fully understood.

But Dale's experiments are evidence
that Pluto has the raw ingredients

to produce some of the molecules
needed for life.

Within Pluto, you might have liquid
water, mixed with ammonia, mixed

with methane and any number
of different kinds of organic

compounds, carbon compounds
that life could utilise.

And so along with Pluto potentially
having this liquid water environment

that could be conducive for life,

it might have some of the elements
and energy that life

as we know it needs.

No-one in their right mind
would have thought Pluto had life,

but now it's a possibility
and that in itself is kind

of mind-blowing.

What might life on Pluto look like?

Who knows? But...I would be happy
with even the tiniest of

single-celled microbial organism

somewhere out there in our
solar system and beyond.

And if on Pluto,
the most unlikely of places,

then why not elsewhere?

We used to think that life could
only exist on a planet like Earth,

with ocean on the outside,
an atmosphere,

sunlight falling in and so on.

Now we learned there are liquid

oceans elsewhere in the
solar system.

They are around Jupiter,
they are inside Pluto.

And it changes our concept
of what the Goldilocks Zone is,

the zone in which life could
potentially form.

If places like Pluto, really small,
very distant from a star,

are able to have liquid water,

it could mean that Pluto
maybe has life.

But also, as we move into other
solar systems and we understand

more about other solar systems,

maybe life could be more abundant
than we thought.

My view is that the chemistry
of life, in so far as we understand

it, is so widespread through
the solar system, through our galaxy

and elsewhere in the universe,

that it is highly likely that
self-replicating metabolising

entities have originated.

And if that's life, then the answer
is yes, there is life elsewhere.

Long viewed as a world of
little importance...

..Pluto has been revealed
to be a place

of unimaginable complexity.

Mountains carved from ice...

..rival the most dramatic
landscapes on Earth.

A frozen volcano like nothing else
in the solar system.

A churning nitrogen glacier puzzles
science's greatest minds.

It's a place where something,
extraterrestrial life, might exist.

And could tell us where else
in the universe we might find it.

This great mystery on the very edge
of our solar system has led

scientists to call for another
mission.

The only way to really study
the surface of Pluto in detail

is to go back to Pluto.

And that's why I'm leading a concept
study to figure out how we would

go about doing that.

They plan to send another probe.

We need another mission
to go back to Pluto,

and this time orbit Pluto.

And we can bring different
instruments that can get

us different answers.

And peer beneath Pluto's icy crust.

If we committed ourselves
to exploring these ocean worlds,

it could be that within the next few
decades we might finally

have an answer to that primordial
age-old question of whether or not

we are alone in the universe.

To the Romans, Pluto was the
god of the afterlife.

But this tiny dwarf planet may,

in fact, be the gatekeeper
to life itself.

Pluto is well and truly
back from the dead.