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Pluto, for so long a mystery,

has finally been revealed.

And these astounding images have rocked science.

Everywhere we looked, it looked different.

And that shook us to the core.

Icy volcanoes, fast-flowing glaciers,

and mountain ranges as big as the rockies.

We thought pluto would be dead and boring.

We couldn't have been more wrong.

It was amazing.

I mean, every new world is just ... is just incredible.

But, like, we had no idea that pluto was so complex.

In 2015, pluto showed us its heart.

But does its greatest secret lie below?

Could tiny pluto harbor alien life forms

3 billion miles from the sun?

captions paid for by discovery communications

Robotic probes armed with scientific instruments

have been compiling a family photo album

of our solar system for over 40 years.

And these close-up portraits have taught us

almost everything we know

about our closest cosmic relatives.

The lesson of planetary science

is you have to go there

to really resolve things in detail

and to make the big discoveries.

But until recently,

one portrait had escaped our robotic eyes,

the dwarf planet pluto.

Pluto used to be our most distant planet

until astronomers discovered lots of other icy objects

orbiting alongside pluto.

And in 2006, poor pluto got demoted.

Pluto is tiny and so far away,

around 3 billion miles,

that our finest telescope can pick out

only a faint blob.

The very best images we've made of pluto were

with the hubble space telescope.

And the hubble space telescope could only get, yeah,

about 20 pixels across pluto.

We knew that there were areas that were bright and dark.

But we didn't know what they were.

And it wasn't a very clear view at all.

This fuzzy image was pretty much

all we had to work out what pluto was like.

We knew that it had nitrogen ice on it.

We knew it had a moon. And we knew these other things.

Um, but still you have to go there

because it is so far away,

3 billion miles or more.

We just had to go there to understand it.

Summer, 2015,

NASA's new horizons probe flies past pluto,

recording the first-ever close-up images

of this most mysterious of worlds.

Many scientists were expecting to find a flat,

dead world similar to our moon.

But, instead, the pictures

reveal a world of dizzying diversity.

High mountain ranges, winding glaciers,

and vast crater-free plains

that all hint at recent geological activity.

Pluto, it seemed, was alive.

When we saw the first truly high resolution images

that revealed pluto in all of its glory and complexity,

it was an amazing emotional release

for all of us on the mission team.

We suddenly got up this image that showed jagged mountains

and totally bizarre hills and smooth plains

and no craters at all on that particular image

showing that this really was a geologically youthful region.

Some of the features revealed by new horizons

defied simple explanation.

One of the most baffling was a pair of vast mounds

on the surface of pluto,

each topped by a deep depression.

These mounds look just like young volcanoes.

But how could that be possible on a planetary surface

made mostly from ice?

Now, if you were to see something like that on earth,

you'd say, "that's a volcano."

If you saw it on Mars, you'd say, "that's a volcano."

You see it on pluto, you think, "what is this?"

There's nothing that can drive volcanoes on pluto,

as far as we know ... so what are these things?

The mysterious mounds on pluto are huge,

over 100 miles wide

and around 3 miles high.

To make sense of these perplexing formations,

scientists turned to similar looking features here on earth.

Planetary geologists like jani radebaugh

call them shield volcanoes.

This volcano we're heading for is called skjaldbreidur

and it means "broad shield" in Icelandic.

So in some ways it's sort of the ... the, uh,

the eponymous shield volcano

that we're about to land on.

Shield volcanoes are formed by long,

drawn-out eruptions of runny lava.

And they're said to resemble the shape

of a warrior's shield lying on the ground.

This one erupted just 9,000 years ago.

Radebaugh: Can see a giant crater right here,

summit crater, that's typical of shield volcanoes.

And it's actually pretty deep right here,

deeper than I expected.

This is quite a young volcano.

And so you can still really clearly see that crater.

And, actually, we see the summit crater

on pluto's shields very clearly,

and that really kinda makes us

really think they could be a shield volcano

just because of the broad shape

and a summit crater just like this.

It's not unusual

to find shield volcanoes on other planets.

Mars, like earth, is mostly made from rock.

And it's home to the monumental volcano,

Olympus mons,

built up from vast eruptions of hot,

runny lava millions of years ago.

But lava is melted rock and that's a problem

because pluto should be too small

and too far from the sun to have anything melted.

If its broad features really are volcanic,

they must be made something very different to lava.

Jani radebaugh believes the light color

of pluto's mysterious mounds offer a crucial clue.

Radebaugh: The kind of eruptions that make this shape of volcano

are, you know, hundreds or thousands of ...

of individual eruptions over time.

They ... they erupt out of ...

of the fissure and flow out across the surface.

And then they just pile up on top of each other

and continue to flow out

building up a broad shield over time.

The same thing has happened on pluto,

it's just the lava that's erupted from

pluto doesn't look anything like this.

It doesn't ... it's not dark in color.

It's not made of basalt.

Instead, it's made of water ice.

Unlike earth, pluto has a surface of frozen nitrogen

on top of a bedrock

and mantle of solid water ice.

Some of this water ice

must be melting to form

explosive flows of liquid slush

that spew out onto pluto's surface,

just like hot rocky lava does on earth.

The warm slush freezes rapidly on the surface,

building up to form broad icy shields,

just like the rocky shields we find on earth and Mars.

If the water under the surface

can be melted by something, some energy,

it can burst through the surface

and flow just like lava does here on earth.

So instead of volcanoes, like you have here,

you get cryovolcanoes, cold volcanoes,

on these more distant objects.

We have a myopic view of the universe thinking

everything has to be the way it is here.

Here on earth, volcanoes are hot.

But on pluto, volcanoes run cold, not hot.

But perhaps the most exciting thing

about pluto's volcanoes

is that they're virtually free from impact craters,

meaning the warm slush that formed the ice volcanoes

must have flowed out of them really recently.

These features are very, very young.

Pluto is active today.

There hasn't been enough time

for the bombardments of comets impacting

pluto to build up and build up a record cr...

uh, a record of craters on the surface.

Pluto is active today.

That's the headline.

The potential for active volcanoes

on pluto stunned scientists.

But they pale in comparison

when close-up images

of pluto's smooth heart are revealed

because this Texas-sized

basin of ice appears to be boiling.

Our solar system bears the scars of a violent past.

Impact craters litter the surface of rocky planets

and moons like our own.

Yet, on earth, these battle scars

are wiped away by lava, water and wind.

If you look at the moon now and you look at it

in 1,000 years, not gonna be very different.

However, if you look at the earth now

and wait an hour, right, we have weather,

we have volcanoes,

there are lakes that come and go.

The sea levels rise and fall over thousands

and millions of years.

So, uh, with, uh, all of this geologic activity,

we say that the earth is a living world.

Scientists expected pluto to be heavily cratered,

a geologically dead world similar to our moon.

But when new horizons flew past in 2015,

it revealed a vast, heart-shaped icy plain,

which appears much younger than the rest of the planet.

And the western part of this plain,

sputnik planum, was completely crater-free.

Of all the things that we saw coming back,

the heart, sputnik planum,

was the most surprising and the most interesting

and the most, sorta, "oh, look at that."

The broad, pale surface of sputnik planum

offers a window into pluto's unexpected activity

because it looks just like the icy glaciers do,

here on earth.

There's not a crater to be seen anywhere across that surface.

And that immediately tells us it's young.

Some of the estimates are maybe as young as 10 million years,

you know, only since that stuff

has been moved around and changed.

That's ... that's today in geologic terms.

There's got to be something actively making

that surface for it to be that smooth.

Something is regularly smoothing out

the surface of sputnik planum,

but what?

Scientists can rule out cryovolcanoes.

They make mountains, not plains.

So what else could it be?

The first clue comes from pale snaking lines

that appear to feed into the northern rim of the plain.

These sinuous features look just like

the rivers of ice

that flow from mountainous glaciers here on earth.

Radebaugh: What we're looking at here is,

uh, is a massive glacier pouring out across the valley

and emptying into a glacial lake.

One of the first things we noticed when we saw the ...

the close-up images from new horizons

is not only a vast field of kind of a flat glacier

but also tiny little feeder networks into the glacier,

like valley glaciers just like this one

emptying into the larger continental glacier.

Ices must be forming in pluto's highlands

and flowing into sputnik planum,

just like outlet glaciers

flow from high mountains here on earth.

But on pluto it's too cold for water ice to flow.

So the outlets feeding sputnik planum must be filled

with a very different kind of ice.

So the glaciers have to be made of things like nitrogen.

Now, nitrogen is what makes up most of our atmosphere.

It's a gas here.

But at those temperatures it freezes and becomes an ice,

a glacier of nitrogen.

On pluto, nitrogen ice has the texture of toothpaste.

And it flows down mountains made of water ice.

Those glaciers are flowing.

And, what's more, there are no craters on them.

So they're active, they're young.

They're flowing right now today.

We now think sputnik planum

is a vast basin filled with frozen nitrogen

and fed by mountainous glaciers to the north.

This steady flow of fresh nitrogen explains

why the edges of the basin are so smooth.

But what about the regions

that sit beyond the reach of the glacial flow?

Something else must be wiping out craters

in the middle of the basin.

Jani radebaugh believes a weird pattern

on the surface of sputnik planum suggests

an astonishing possibility ...

pluto's surface could be boiling.

Welcome to sputnik planum.

I mean, really this is what it would look like

if we were standing on the surface of pluto.

You see a vast, flat glacier behind us,

really white extending off to the distance,

and behind us a ring of mountains.

When we look at the surface of sputnik planum,

we noticed that there ...

there are regularly spaced cells.

And they're a little bit rounded in the middle,

they taper off at the edges.

And they really have the appearance of something that ...

that formed by convection.

So if you think about a pot of oatmeal,

um, as it's ... as it's boiling away,

it's bringing material up to the surface

and then it's kind of pulling it back down

again at the sides, convecting over and over again.

The nitrogen ice of sputnik planum

could be boiling like a pot of oatmeal,

but so slowly

the roiling surface appears frozen at any time.

A similar, but much faster,

process makes patchwork patterns

in earth's most active lava lakes.

Hot globs of lava rise up,

forming rounded cells on the surface

and cool lava sinks down,

slipping between the gaps formed

by the hottest bubbles of molten rock.

As we look at the surface of sputnik planum,

what we're seeing is basically a lava lake in slow motion.

There's material rising up and then moving over

and descending again as it gets cool.

And, um, basically, if we could see that,

if we could speed it up over a few thousand years,

we would see individual cells

all across the surface roiling and churning

and boiling, moving, convecting nitrogen.

You really have to step back and realize

just how amazing and weird this is.

The temperature is hovering near 400 degrees below zero.

And things are bubbling up and moving around.

There's geology happening.

What you have to remember is

that it doesn't take a lot of energy

to make something as cold as nitrogen ice flow.

But it does take some energy.

And while pluto wears its magnificent heart on its sleeve,

a second, warmer heart must be deep below the surface.

But out here in the frozen expanse

of near outer space,

the question still remains,

where's the heat coming from?

In 2015, the new horizons probe

stuns mission scientists when it reveals

pluto as a dynamic, living world.

Its glaciers, mountain ranges,

and likely volcanoes are all evidence

of an active geology driven by heat.

But scientists are mystified

where that warmth is coming from.

Pluto is too small to have retained the heat

from its violent formation over 4 billion years ago.

And it sits so far from the sun

that sunlight hardly makes an impact on pluto.

Well, it's about 20 minutes before dawn right now.

And the light that we see is exactly the same

as the illumination

we see on the surface of pluto at midday,

when there is the maximum amount of sunlight on the surface.

And because pluto receives so little sunlight,

its surface is bone-chillingly cold,

as low as minus 400 degrees fahrenheit.

This tiny little world,

so far away from the sun, receiving so little sunlight,

I mean, we never dreamed it would be an active world.

If the heat that drives pluto's dynamic geology

isn't coming from sunlight,

it must be coming from below.

But from where?

The best guess is that

pluto has a deep subterranean ocean of liquid water

and somehow this ocean is driving heat up to the surface.

There's pretty good circumstantial evidence

that pluto has a massive water ocean in its interior.

And over time as the planet cools,

the water must be freezing.

And as water freezes, it releases, uh,

a form of energy called latent heat.

And that release of energy from the freezing ocean

could very well be

what's powering a lot of pluto's geology.

Could freezing water really make a planet hotter?

It sounds crazy.

But jani radebaugh can demonstrate

this weird quirk of physics

using an ice bath

to represent pluto's frozen upper crust

and a cup of hot water

to represent pluto's hidden liquid ocean.

At first, the ice bath causes

the temperature of water to drop.

But as soon as the water freezes,

the temperature inside of the glass

does something amazing:

It rises. 'Kay.

It's at 30, so it's definitely ready to freeze.

So I just wanna give it a little bit of a helping hand

and see if we can start it to freeze.

Here we go. Just a little stir.

Oh, there it goes.

Uh, now it's completely frozen and the temperature

is dramatically climbing,

like, just seconds after it froze.

It has climbed ... climbed 8 degrees.

Look at this. It was 30.

Now all of a sudden it's 38, just seconds after it froze.

It might seem kind of counterintuitive,

but freezing water

actually gives off energy to its surroundings.

All of that kinetic energy, the motion of the water

molecules in the liquid state,

um, has to be given up

for those water molecules

to enter a solid crystalline structure.

That released energy is called the latent heat of fusion.

It actually can warm the surroundings as water

is freezing into ice.

This is how scientists believe latent heat in an underground

ocean could be driving volcanism on pluto.

The upper most layers of the internal ocean freeze

where liquid water meets the hard frozen ice above.

This act of freezing releases latent heat,

which then travels up through pluto's crust,

driving active geology on the surface.

Radebaugh: I mean, this seems like just a little bit of heat

that's forming from ... from this freezing process.

But if there's a ... a whole layer of ocean

that's freezing up against the ice crust,

that's a tremendous amount of heat,

maybe it could bring up cryovolcanoes driven

simply by the heat from freezing in parts

of the liquid water ocean, just amazing.

Incredibly, just a few degrees

of heat leaking from pluto's ocean

may be all it takes to drive the dynamic surface of pluto,

to raise mountain ranges,

to fire up watery volcanoes

and to set nitrogen glaciers

racing down icy slopes.

We humans are prejudiced. We live on this warm planet.

If you go down a few miles under the surface of the earth,

the rock there is ... is almost molten.

It's incredibly hot.

And so we think you need a huge amount of energy

to drive volcanoes

and geysers and plate tectonics and all that.

But that's not really the case.

It's just that rock takes

a lot of energy to melt and do all that.

Ice doesn't take nearly that much energy.

So on pluto, if something was able to heat it up

even a little bit and get that ...

that ... that water ice to melt,

that can drive the same sort of surface geology

that we have here on earth.

And if our galaxy is filled with frozen worlds

like pluto, the universe

just got a whole lot more interesting.

And so it could be that most of the worlds in the ...

in the galaxy are ... are cold worlds.

And it might be that most of the volcanism in the galaxy

is actually the running cold volcanism

that we see on pluto.

And pluto is actually going to end up changing the way

we think of any sort of solid planet.

Somehow, it's still alive when it shouldn't be.

New horizons has shown us

that pluto most likely has both heat

and liquid water.

And that simple fact

raises an astonishing possibility:

Could tiny pluto,

sitting 3 billion miles from the sun,

provide a secret haven for life?

In all the vastness of space,

we only know of one planet that's home to life:

Our planet, the earth.

On earth, we have all the ingredients

we needed for life to arise,

clearly, we're here.

But those three things are basically a source of energy,

liquid water, and organic molecules ...

carbon-based molecules.

We used to think earth was the only world

with all three ingredients.

But when robotic probes visited Jupiter and saturn,

they found multiple active moons with underground

oceans warmed by hot volcanic vents.

And these worlds also appeared to be spiked

with the basic chemical ingredients for life.

So if Jupiter's moon, europa,

and saturn's moon, Enceladus,

are considered potential havens for life,

why not the subterranean oceans of pluto too?

Well, we think there's liquid water possibly on pluto.

And there's clearly a source of energy.

Could it have that third ingredient?

And, if so, is it that bizarre to wonder

if life could arise on pluto?

The third ingredient is organic chemistry,

the carbon-based molecules

that build the machinery of life.

Surprisingly, astronomers believe these complex chemicals

can form spontaneously in the cold of space.

All you need are simple carbon-based gases

and a source of energy, usually light.

The most active natural laboratory

we know of for this kind of organic chemistry

is the atmosphere of saturn's giant moon, titan.

When you look at titan, it has a very thick atmosphere,

thicker than earth's.

And it's surrounded by this haze.

And that haze is basically made of organic molecules

that are created from the more basic compounds

that have been zapped by ultraviolet light

and built up to ...

to make these more complex organic compounds.

Titan's atmosphere is thick with simple gases:

Methane, carbon monoxide, and ammonia.

And when sunlight strikes titan's cloud tops,

these gases join together

to form complex organic molecules,

which hang in the atmosphere,

forming dense layers of haze.

Scientists call these naturally occurring organic molecules

"tholins."

It's a generic term, it's a blanket term

for any sort of complex,

reddish colored organic molecule

that is formed from the splitting apart

and reassembling of simpler,

uh, starting, uh, molecules.

These organic molecules, these tholins,

you can think of them as, like, the lego

building blocks for life.

On earth, tholin-like chemicals

make up the working machinery of cells,

the DNA and proteins that make life tick.

If, like titan, pluto has an atmosphere rich in carbon,

could it too be producing tholins ... and therefore,

perhaps a shot at life?

When new horizons passed pluto in 2015,

the spaceship turned to take an image of the dwarf planet

backlit against the sun.

Scientists hoped to capture

the unmistakable haze of active organic chemistry.

The results were far more spectacular

than I would've expected.

Absolutely we saw that atmosphere,

a beautiful blue ring of nitrogen gas

surrounding pluto,

just like what we see on the earth.

These beautiful blue-haze layers are ...

are chemical factories

manufacturing tholins,

complex organic molecules,

from the simpler building blocks in the atmosphere of pluto,

the carbon monoxide, the methane, the nitrogen,

all these things that are in the atmosphere of

pluto are being broken apart by sunlight and reassembled

in these hazes into these just wonderful,

delicious organic molecules.

On titan, newly produced tholins rain down

from the atmosphere onto the surface of the moon,

producing a thick orange-brown carpet of organic gunk.

Scientists hope to see something similar happening on pluto.

But they had to wait for new horizons

to send back its first full-color images

of the dwarf planet.

We'd been getting a lot of

black and white images from pluto.

And so I was very excited to see the color ones.

And when they finally came in

and I saw these rich red, orange,

brown features on the surface,

my first thought was, "oh, my gosh.

Organic chemistry."

WoThere are areas where things look

almost kind of reddish-brown, even sort of pinkish.

And when you see a color like that,

you immediately think organics.

And, in fact, that's exactly what this is:

Pink organic snow.

Life as we know it

couldn't exist on the surface of pluto.

It's too cold.

But if tholins are getting cycled down

into the guts of the planet,

they could be transported to areas

where ice melts into liquid water.

And it's possible the chemical evolution

of these simple organic molecules

could continue there,

driven by pluto's meager heat.

And perhaps, just perhaps,

the machinery for simple life could assemble

in the subsurface oceans of pluto,

just like it probably did in the deep

oceans of earth 4 billion years ago.

I'm intrigued by the possibilities

that the sphere of life

around our sun may extend f...

much further away from the sun

than we ever dreamed possible.

So, you know, could there be life on pluto or ...

or under the surface of pluto?

I couldn't say no to that at the moment.

Um, it's an intriguing possibility.

Could there be life on pluto?

I don't think so.

Uh, that's my own personal opinion.

You know what, you can take flour

and butter and eggs and chocolate chips

and shake 'em all up and throw 'em in an oven.

You may not get a cookie.

What you're probably gonna get is a glob.

You're gonna get a mess.

While scientists debate

the stunning possibility of life

3 billion miles from the sun,

new horizons turns its attention

to pluto's tiny moons.

Mission scientists knew

that pluto most likely had five companion worlds.

But nobody could have predicted

just how plain weird

these moons would turn out to be.

When new horizons arrived at pluto,

scientists held their breath,

eager to see what pluto's moons would be like.

And we knew nothing about them.

So everything, literally,

that we found out about these moons

was going to be for the first time.

And it was going to be a surprise.

Three years before new horizons arrived at pluto,

the hubble space telescope

imaged five moons around the dwarf planet,

one oversized moon called charon

and another four, much smaller icy companions.

New horizons is reprogrammed to study these worlds

and to discover any additional moons

too small for hubble to pick out.

You know, before the fly by one of the things

that we surveyed people about was

how many more moons would we find?

And some guessed dozens.

And some guessed a few more.

Uh, and one of our top team members,

uh, guessed zero.

And I remember going to him and saying,

"I'll take that bet.

I'll bet you a dinner anywhere in the United States

that we will find at least one."

Stern lost his bet ...

new horizons confirmed

hubble had seen all there was to see.

But what incredible worlds

pluto's five moons turned out to be.

Oversized charon is revealed

alongside styx, nix,

kerberos, and hydra,

four tiny, icy moons,

each less than 25 miles across.

And they're unlike any moons astronomers had seen before.

These objects are spinning crazy fast compared

to what we expected.

Um, not only that but

instead of having their poles aligned

with pluto's pole like this,

they're actually spinning on their sides.

So they're sort of going around like this,

totally crazy behavior

that we had no expectation of.

To understand these crazy orbits,

scientists find clues

in the early history of the earth.

In the early days of

the formation of the kuiper belt,

these icy bodies were colliding with each other quite often.

And just like here at the earth, uh,

where a large Mars-sized protoplanet

actually collided with the earth to form our earth/moon system,

that's probably what happened at pluto as well.

Pluto most likely took a giant hit in its youth

and its moons are the leftovers from this mammoth collision.

Giant charon settled into a close spinning orbit

with pluto, and, farther out,

the four tiny moons orbit around both bodies

like tumbling shards of icy shrapnel.

The first images of oversized charon

show a gray, dead world pockmarked

with the scars of its violent past.

The biggest scar is huge.

It appears to almost split the moon in two.

Charon is, uh, a mess.

The first thing I thought of

when I looked at that picture

that we got in high resolution is, like,

"this is Frankenstein's moon."

This looks like a moon

that somebody basically took a hammer and chisel

to and broke it completely apart

and then just mashed it back together.

But what has the power to split a Texas-sized moon

like charon apart

in the frozen wastelands of the outer solar system?

Scientists are left with only one option:

Water.

Four billion years ago,

charon had an internal liquid ocean just like pluto today.

But because charon is smaller,

it cooled more quickly.

The entire ocean turned to ice

and poor charon burst at the seams.

If you take a soda can and put it in the freezer,

when the water in that soda freezes, it expands.

But the metal of the can won't.

And so what happens is it gets under more

and more stress, more and more pressure,

until it ruptures and it splits open

and that stuff oozes out.

This ... the ... the ... the system of ...

of canyons and faults across, uh,

the equator of charon really sorta represent that ...

that last gasp of geologic activity as it ...

it sorta, you know, spilled its guts out

as the interior ice froze, um, and ...

and, you know, on its way to becoming a dead world.

So, it was charon's last gasp, if you will.

Charon's red-stained polar cap

also catches the eye of mission scientist.

They name it mordor.

And this barren red-stained terrain presents the team

with perhaps their greatest challenge.

Of all the surprising things on charon

and, uh, it's a long list,

probably the most surprising thing is mordor.

I love that.

This is the north polar region of charon.

And for some reason it is red.

It's covered in some sort of red substance.

We think this is probably a tholin.

It's a complex organic molecule.

Mordor appears covered with tholins, but how?

The frozen world doesn't have the ingredients

or the atmosphere

to make complex organic molecules.

The tholins must be coming to charon premade,

but from where?

The only logical answer

is from pluto, over 10,000 miles away.

How would it get from being made on

pluto to getting to charon ... and, really, nobody's sure.

But the thinking is that somehow this stuff

is getting into the atmosphere of

pluto and being transferred that way through space

to basically be deposited on the north pole of charon.

How that works is something,

uh, I don't think anybody understands.

Uh, and that's ... that's cool

because there are a lot of mysteries left

and a lot of studying to be done, uh,

even with all of the ... the data we have.

There are still many questions to answer.

But we now have a handle on how pluto works.

And it's allowed scientists

to imagine the future of this frozen world.

Incredibly, pluto may be about to rise from a deep, cold sleep.

2015, a tiny robotic probe shoots past

pluto and fires its photographic shutters.

The pin-sharp images stun the world.

Glaciers, smooth young plains,

and even the hint of active cryovolcanoes.

But one set of images

at first appeared impossible.

As we looked down on this brand-new world

to us, we saw things that,

at first, were very hard to explain.

There seemed to be these depressions and channels

that looked like lakes or even little rivers.

These flow features are made from nitrogen.

And although they're now frozen,

they appear to have been liquid

in the recent geological past.

But how?

Pluto is too cold

and its atmosphere too thin

to support liquids on the surface.

In the entire universe,

it is so difficult to get liquid.

Everything seems to wanna be a solid or a gas.

You have to have just the right circumstances

for that liquid phase of matter.

So even for something like nitrogen,

you not only have to just turn up the temperature

and get nitrogen warmer,

there also has to be some atmospheric pressure,

a thicker atmosphere is needed

to maintain liquid nitrogen on the surface.

Scientists deduce that in the last million years,

pluto must have warmed,

evaporating glaciers

and filling the atmosphere with nitrogen.

But what could've caused such an extreme climate shift?

Scientists first look to pluto's elongated orbit around the sun.

Once every 248 earth years,

pluto's elliptical orbit brings it close to the sun.

But this brief warming

isn't sufficient to thicken pluto's atmosphere.

When it comes in close to the sun,

it goes faster and faster.

And as it moves away from the sun,

it goes much slower.

So on pluto, summer doesn't last anywhere

near as long as winter.

But from time to time,

one half of pluto gets a super summer

thanks to the dwarf planet's unstable tilt.

We know from the way pluto's north

and south pole wobble over time

that that top that represents the spinning sphere

of pluto is not nearly as stable as the earth.

Um, our planet is tipped over at about 23 1/2 degrees

and sits that way for, you know, for ...

for millennia at a time.

Pluto, on the other hand, is much less stable.

And its poles wander over time.

Once every million years or so,

pluto tips completely on its side,

exposing the northern hemisphere

to constant sunlight

during its close pass of the sun.

And the warmth from this orbital shift transforms

the surface of the planet.

Picture pluto as it warms,

the atmosphere thickens and fills with clouds,

a warm wind sweeps across the plains

and pluto's glaciers begin to melt.

Like an animal emerging from hibernation,

pluto comes to life.

It's almost as if

it's undergoing these periodic changes

where it's warmer and colder and warmer

and colder in the past

and maybe again in the future.

Will pluto have liquid on its surface, even ...

even if only temporarily?

What would that mean? What would that even look like?

800,000 years from now,

pluto will awake once again.

It will tilt over completely to face the sun

and its mountains will fill

with the sound of running nitrogen.

WoYou begin to actually see flowing rivers of nitrogen.

You could stand on the shore of a nitrogen lake.

There'd be pink snow

with organic molecules coming down on you.

What a truly magical world.

The legacy of new horizons

goes far beyond just pluto.

It changes the way we view frozen worlds

across our galaxy

and the possibilities these far-off worlds offer.

It tells us that there's this huge stage

for the play of life out there in the galaxy,

um, a huge variety of worlds.

You image a world, it probably exists out there someplace.

That's kind of the lesson that pluto has for us.

It ... it tells us that ... that things can be

a lot more interesting out there

than you can possibly imagine.

Pluto has really inspired me.

It's shown me just how wrong we can be.

We thought we understood what pluto would be

and it was entirely different.

And as a scientist, you gotta think,

how much more out there do we have to discover?

If nothing else, pluto taught us that.