Original subtitles

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.