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NARRATOR: Throughout our solar system,

we find worlds shaped by ice.

SABINE STANLEY: These ice worlds

are so unexpected and surprising.

There is definitely more ice out there

than we really expected or anticipated.

NARRATOR: Today, spacecraft are bringing us closer than ever...

...to revealing their secrets.

We've got carbon dioxide snow

on Mars.

KELSI SINGER: On Pluto,

we have these nitrogen ice glaciers

that no one would have predicted.

Deep in the planet of Uranus,

you get this extremely dense black hot ice.

MICHAEL WONG: The data has truly transformed

the way that we understand how ice behaves

so far away in the depths of space.

NARRATOR: But many questions remain unanswered.

PETER GAO: How do these icy worlds even form?

How did the ice get there?

Is there life there?

NARRATOR: And of all the worlds

in our solar system,

ours hits the icy jackpot.

The ice we have here on Earth is special.

And if it didn't have its special properties,

life on Earth would not exist

and you wouldn't be here watching me.

NARRATOR: Why is our cosmic neighborhood

home to so many different and exotic ices?

"Solar System: Icy Worlds."

Right now, on "NOVA."

♪

♪

NARRATOR: Out in the dark expanse

of our solar system,

far beyond Earth and Mars...

...we enter the realm of the gas giants.

Where average temperatures of the planets

begin to plummet.

Venture even farther away from Earth,

and we pass the ice giants.

Here, temperatures can drop even lower...

...down to - 370 degrees Fahrenheit.

These worlds are made almost entirely of ice...

...existing in exotic, seemingly impossible forms.

WONG: Thanks to planetary exploration,

we have witnessed different forms of ice

behaving in ways that we could not have expected.

NARRATOR: Scientists are just beginning to uncover

the icy secrets of our solar system.

GURNEY: We really don't know much about ice worlds

and planetary scientists

are really discovering

new things every day.

NARRATOR: Discoveries that can shed light on

the strange ways ice behaves

and how dynamic it can be,

not only in our solar system,

but on other worlds in our galaxy, too.

[ice crackling]

NARRATOR: Over three billion miles from Earth,

the space probe New Horizons

is traveling far beyond the rocky planets.

And after more than nine years,

arrives at its destination.

As it passes Pluto,

...the probe gives us a glimpse

of a world made of weird ice.

DURFEY: Flyby missions

are do or die.

If you don't get those images,

if you don't get that data,

you've flown by

and you're gone now.

VERBISCER: We only get one shot because traveling at

31,000 miles an hour,

it's not like you can do a U-turn.

NARRATOR: The $780 million high-risk mission pays dividends,

delivering a mother lode of data.

[camera shutter clicking]

New Horizons surprises scientists

with incredible images of Pluto's surface,

revealing features they never expected to see.

GAO: I think when

people first saw images of Pluto,

they were just blown away.

We're seeing incredible diversity in

what the surface looks like.

VERBISCER: I walked into the geology room

and looked at an image on one of the computer screens,

and I wondered why someone was looking at

a picture of Antarctica.

And it dawned on me

[chuckling]: gradually, oh, my goodness,

that was not Antarctica, that was actually

a very close picture of Pluto.

WONG: There were tons of mysteries that we needed to unravel.

There were lots of features

that puzzled us. How could that possibly be,

so far away in the solar system?

NARRATOR: The amount of data collected

allows scientists to map Pluto's surface.

Pluto's most prominent feature

is a great icy plain around 600 miles across.

And, despite the extreme cold,

there are signs of movement in the ice;

lines etched into the surface

as if they are churning.

And then, something even more unexpected.

Glaciers.

Flowing rivers of ice

on a world so cold...

...it seems nothing should be moving.

Pluto is a wintery puzzle,

with icy landscapes as dramatic as anything seen on Earth.

How is all this dynamism possible

on a world so far from the sun?

♪

Pluto, a place right on the edge of the solar system,

looks remarkably similar to

some frozen landscapes on Earth,

like this glacier in Alaska.

LEIGHAN FALLEY: Glacier Two for the Root Canal, acknowledge.

[man speaks indistinctly over radio]

FALLEY: Okay, floor above two.

Hang on.

Never a dull moment around here.

Well, isn't this amazing?

- Incredible views. - Yes.

NARRATOR: This landscape might look frozen solid,

but it is one of the most

geologically active areas on Earth.

SINGER: In the time that you've been flying here,

have you seen a lot of changes in the glacier?

Absolutely. Are you kidding me?

[chuckling]: Yes. I've seen immense changes.

SINGER: Some aspects of what we're looking at

that really reminded me of Pluto,

there's the kind of flow lines

in the glacier

indicating the direction of flow.

Oh, Pluto has flow lines?

Yes, they are moving downhill,

similar to these glaciers.

FALLEY: We're coming in for landing.

NARRATOR: Understanding these glacial features

can help scientists understand

how Pluto's glaciers flow.

[plane powering down]

FALLEY: Welcome to the glacier, you guys.

SINGER: Wow.[Falley laughs]

- So beautiful. - FALLEY:Oh...

FALLEY: One of the greatest ranges on the continent.

[laughs]

Is the glacier under here?

Yes, we're probably

on top of about 700 to 1,500 feet

of glacier ice underneath us.

CARLY HOWETT: If you ever experience visiting or seeing a glacier,

you might think that they're completely stationary.

But actually, if you were to

stand there for long enough,

you would see that they move.

HAKEEM OLUSEYI: Glaciers form on Earth

when snow does not melt

season after season, and it packs down,

and the weight of that snow compresses it into ice.

And as more and more builds up

and it gets heavier and heavier,

it begins to flow.

NARRATOR: These huge glacial ice features in Alaska

have formed in this exact way.

SINGER: This is an incredible landscape.

It really reminds me of what it would be like to

stand on Pluto.

And it's 15 degrees Fahrenheit,

and we're in this totally frozen landscape,

and on Pluto, it's even colder.

It's -390 degrees Fahrenheit.

How can these glaciers be flowing

when it's so cold on the surface of Pluto?

♪

NARRATOR: Water ice is frozen as solid as rock here,

so it doesn't flow in the same way as glaciers on Earth.

And yet, these glaciers look like they are flowing.

We even see two glaciers that appear to flow into each other,

combining to form a mega glacier

that flows onto the ice plains below.

GURNEY: So, at Pluto, we see these glaciers

that are flowing, but it's way too cold

for there to be flowing water,

so what are they actually made of?

So, it might be surprising to hear

that the definition of ice

really depends on who you're talking to.

If you're talking to people

on Earth doing their everyday things,

then ice is just the solid form of water.

But if you're talking to a planetary scientist,

then ice is a way to describe a group of materials.

It's water, but it's also ammonia and methane,

things that are made up of a lot of carbon,

hydrogen, oxygen, nitrogen mixed in.

NARRATOR: On Earth, these are usually found

as a liquid or a gas,

but in the solar system,

they can be frozen into ices.

OLUSEYI: From afar, we can tell what the composition

of Pluto's surface is.

But once we got there, what we didn't expect

is the configuration of those materials.

♪

NARRATOR: A clue to the composition of the glaciers

is Pluto's surface temperature of -390 degrees Fahrenheit.

That's pretty close to the melting point of nitrogen,

at around - 350 degrees Fahrenheit.

Similarly, on Earth,

the melting point of water ice is 32 degrees Fahrenheit,

not far from the average surface temperature of 59 degrees.

HOWETT: For a glacier to move, it has to be made of an ice

that's not too far away from its melting temperature,

'cause at the bottom, what you need is-is

the ice to melt just a little bit

so that the glacier is able to flow,

and on Earth, water does that--

the water ice isn't that far from its melting temperature--

but on Pluto, it's so cold, that's not the case

for water ice, but it is the case for nitrogen ice.

Take a breath in.

[inhales]

The air that is in your lungs

includes nitrogen,

and it's a gas for us,

it's the air that we breathe.

On Pluto, it's so cold that that's solid.

[plane engine whirring]

NARRATOR: And it's this solid nitrogen

that creates Pluto's flowing glaciers

that are so similar to Earth's water glaciers.

WONG: The same physical forces are responsible

for carving the landscapes that we see here on Earth

and on Pluto.

It's just using different building blocks.

GAO: I think Pluto has definitely taught us

that the solar system is a giant laboratory

where different elements can be put together

in all kinds of interesting ways.

♪

NARRATOR: When New Horizons leaves Pluto behind,

it turns back

and takes one last photograph,

revealing Pluto's breathtaking atmosphere,

rich in the nitrogen that condenses on the surface

to create its glaciers.

VERBISCER: What we've seen

in the outer solar system from the New Horizons spacecraft

at Pluto and beyond has been revolutionary,

showing us diversity in ice that we don't have here on Earth.

NARRATOR: Pluto offers a tantalizing clue

that very distant and cold worlds

may be as active and dynamic as Earth.

♪

Closer to the sun,

as we enter the realm of the ice giants,

ice appears in even more bizarre forms

behaving in unpredictable ways.

It is found not only on the surface

but also deep inside planets.

Neptune, made up of around 80% frozen icy material,

has giant ice storms raging across it.

Another billion miles closer to the sun...

...is Neptune's icy twin.

♪

Uranus is an enormous planet of icy storms.

GURNEY: When I first saw images of Uranus,

I thought they were absolutely beautiful.

Blue, almost ocean-colored.

NARRATOR: It has a feature

that has puzzled scientists for decades:

its vast, shimmering auroras.

STANLEY: The aurora on Uranus are mysterious

because flashes of light suddenly appear

all over the planetary surface,

and you don't know why they're happening there.

NARRATOR: Their location is puzzling because, on Earth,

auroras are confined to the poles.

STANLEY: Aurora on Earth happen

when high energy particles from the Sun

gets funneled along magnetic field lines

on the Earth towards the polar regions

and there they run into atmospheric particles,

and that causes the atmospheric particles

to shine light.

WONG: Earth's magnetic field is created

by the churning motion of the liquid iron

in Earth's outer core.

Because iron is electrically conducting,

when it moves, it naturally generates a magnetic field.

NARRATOR: Magnetic fields create auroras,

but on Uranus, this poses a mystery.

GURNEY: We know Uranus doesn't have

this flowing metallic core that Earth has,

but it still has, uh, a magnetic field.

So, how is that magnetic field being generated?

NARRATOR: A clue could lie

in Uranus's icy composition.

[thunder rumbles]

GURNEY: If you sent a spacecraft to Uranus,

you would go through the atmosphere

and, uh, you would see, uh, all of this, uh, gas,

uh, because it's a gas giant.

OLUSEYI: It's a turbulent, high-speed wind,

gaseous environment,

and descend inward,

you go from gases to liquids.

GURNEY: It would just get gradually more dense, um,

as the pressure increases.

And then, when you get sufficiently deep enough,

now you enter the ice world in the interior of Uranus.

♪

[thunder crackling]

GAO: The pressures and temperatures

in the interior of Uranus are extreme.

We're looking at pressures three million times that

of the surface pressure of Earth,

and temperatures reaching 9,000 Fahrenheit.

NARRATOR: Surprisingly, even at these extremely high

pressures and temperatures, water can become ice,

but not in the form we're familiar with on Earth.

GURNEY: Deep in the planet of Uranus,

you get this extremely dense, uh,

black, hot ice called "superionic ice."

NARRATOR: At extreme pressures and temperatures

hotter than the sun's surface,

water is crushed into a solid form.

Superionic ice sounds like it comes from

a science fiction novel.

It's really strange, and you just wouldn't expect

that it would exist in real life.

NARRATOR: This strange ice is unlike anything

we find naturally on Earth.

BERDIS: Here on Earth,

we see all different types of ice.

We see sleet,

we see snow, we see ice cubes.

But if we zoom in under a microscope,

we see that they are all

this very similar

hexagon pattern.

This is water ice as we know it on the Earth.

It's called "ice Ih,"

and the H stands for this hexagonal shape

that you can see in the structure.

Here, the red balls are the oxygen

and the white is the hydrogen; H2O,

two hydrogens for every oxygen.

STANLEY: There are around

20 different types of water ice

found in the solar system.

The other types that we don't find here on Earth, uh, can form

when the temperature's really, really high

and the pressure's really high.

NARRATOR: And it's these conditions

deep inside Uranus that, in theory,

could allow this mysterious superionic ice to exist.

GURNEY: Superionic ice is still very new.

It took us a long time to figure out, uh, that it even existed.

NARRATOR: But in 2018,

scientists announced they'd made superionic ice.

MILLOT: We did the experiment,

and then we look at all the evidence,

and we have to say, "Well, it's true."

It's crazy, it's weird, it's strange,

but it's there.

At super-high temperatures and pressures,

ice forms a very different structure.

What you're looking at here are oxygen atoms, shown in red,

now connected with one another,

and the hydrogen atoms, in white, have broken free.

♪

NARRATOR: This unique property

may solve the mystery of Uranus's auroras.

OLUSEYI: Inside of superionic ice,

what's happened is that

the hydrogen atoms have broken free, and now they're free

to flow around within the material,

and they are the conductors of electricity.

WONG: It's like something out of science fiction where

the hydrogen atoms are flowing like water,

conducting electricity, and potentially responsible

for the wacky magnetic fields that we see on Uranus.

NARRATOR: Precisely the kind of magnetic fields

that could help generate Uranus's auroras.

But there's still much to learn.

GURNEY: We don't fully know what's going on inside Uranus.

We're only just beginning looking at these icy worlds.

We're really at the tip of the iceberg. [chuckles]

NARRATOR: And since these ice giants, Uranus and Neptune,

hold vast amounts of water,

this bizarre hot black superionic ice

may turn out to be the most common form of water

in our solar system.

As we get closer to the Sun,

ice begins to look more familiar.

Almost a billion miles from Uranus

lies Saturn.

Adorned with its famous icy rings.

Saturn's rings are truly the jewel of the solar system.

The first time anyone sees them through a telescope,

you can't believe your eyes.

NARRATOR: But we never realized the true beauty of them

until NASA's Cassini spacecraft went to visit.

The photographs and data sent back

were spectacular.

OLUSEYI: Saturn's rings

turned out to be made up primarily of water ice

that range in sizes from like

smaller than a millimeter to larger than an automobile.

NARRATOR: Icy mini-moons sweep through the rings.

Creating what look like grooves in a record.

HOWETT: Cassini gave us that view.

They're absolutely astounding.

To see the ripples in them, the gaps in them,

just really, really lovely.

NARRATOR: The rings are joined in their orbits

by over 140 moons.

And Cassini saw something intriguing

on one of the moons farthest from Saturn.

Iapetus is one of the oddest looking moons

in the solar system.

It resembles a walnut

with a mountain ridge around its middle.

But that's not its strangest feature.

In 2007,

Cassini sent back pictures of Iapetus.

One side icy white,

while the other looks as if it's been painted black.

Iapetus is one of the weirdest looking moons

in the solar system.

It looks like it has two halves.

One really bright side and one really dark side.

We would expect the moons

in the Saturn system

to have formed with roughly the same brightnesses.

The darkest materials on Iapetus

are 20 times darker than the brightest materials.

How can such a bizarre yin-yang moon exist?

NARRATOR: A clue can be found in the temperature differences

between the white and black halves of the moon.

VERBISCER: The darker side is a warmer region.

The temperatures are hotter on the darker side

than on the bright side.

NARRATOR: The dark side's daytime temperature

can be up to 54 degrees Fahrenheit hotter

than the bright side.

This temperature difference

between the black and white surfaces

creates a peculiar feedback loop.

[helicopter rotor blades whirring]

Which also happens on Earth

where scientists are studying its effects.

WONG: So right behind me, we've got this dark glacial till

right next to bright white snow

created through the erosion

of the majestic mountains around me.

NARRATOR: These dark spots are not solid rock,

but are dust and dirt covering the ice.

And there's a huge, stark contrast

between the brightness of the snow

and the darkness of the glacial till.

NARRATOR: This dark glacial till

has what scientists call a low albedo.

WONG: Which means that it absorbs most of the sunlight

that hits it, heating it up,

and that heat gets transferred to the surrounding snow,

melting it into liquid water,

causing the dark streaks that you see

coming down the side of that cliff.

And a very similar process happens on Iapetus.

NARRATOR: With so little light and heat from the sun,

it's a very slow process.

Over a billion years,

it's estimated the dark regions can lose around 66 feet of ice

compared to the white side.

As this feedback mechanism continues,

it keeps the black side black...

...and the white side white.

But a mystery remains.

Where did all the dark material come from in the first place?

VERBISCER: Did the dark material

come from inside of Iapetus

or did it come from someplace else?

NARRATOR: The smoking gun was discovered by complete accident.

VERBISCER: So, in 2005, I was observing

another moon of Saturn called Phoebe,

which orbits Saturn much further away than Iapetus does.

SINGER: Phoebe is in an unusual orbit

because it goes backwards with respect to the other moons

in its direction around Saturn.

VERBISCER: If you look at the Cassini images of Phoebe,

you see that it is a pretty heavily cratered surface.

So, it's been hit with things for billions of years,

and material has been excavated

and thrown into the space around Phoebe.

The question is, where did all that material go?

Where are all those dust particles?

So, we got the idea to use the Spitzer Space Telescope

to try and look for dust

in the region around Phoebe.

NARRATOR: The Spitzer Space Telescope used infrared light,

or heat signatures, to detect

what's normally hidden from view.

SINGER: If you were able to see

how the Spitzer telescope sees,

it would be like when you put on night vision goggles,

and all of a sudden, you're able to see in the dark.

VERBISCER: We got our images,

and they were extremely interesting.

NARRATOR: Phoebe rides within a colossal ring

about ten million miles across,

wrapped around Saturn.

VERBISCER: It is the largest ring by far in the solar system.

This ring is so big that a billion Earths

could fit inside the volume of the ring.

It makes perfect sense that this ring

should be called the Phoebe ring

because it is associated so closely with the moon Phoebe.

NARRATOR: The Phoebe ring moves around Saturn,

in the same direction as its moon Phoebe,

backwards from the other moons and rings.

Each time a passing object or rocky debris

gets too close to Phoebe...

boom.

The resulting impact

throws dark material out into space.

SINGER: Some of this dark material

in this huge ring starts to fall in towards Saturn,

and Iapetus is basically

running through those particles in its orbit.

NARRATOR: How Iapetus orbits Saturn

is key to solving the mystery

of why it has these black and white halves.

[wind howling]

WONG: So Iapetus, like our moon that orbits the Earth,

is tidally locked, which means one side of Iapetus

always faces Saturn,

the other side always faces away from Saturn.

Imagine this snowball is a freshly formed Iapetus,

completely uniform in its brightness on all sides.

Now, as it orbits around Saturn, through the Phoebe ring,

it will pick up dark dust particles

from that ring.

And because it is tidally locked to Saturn,

the same hemisphere

will keep piling on dust

every time it goes around

for millions and millions of years,

continuously darkening this side of Iapetus.

DURFEY: As soon as the front side

of Iapetus became darkened,

the ice underneath started heating up more quickly,

so that would become a gas and migrate around

to the colder, lighter side of Iapetus.

NARRATOR: The ice vaporizes from the darker, warmer side

and refreezes onto the colder, lighter side.

DURFEY: And so, this means over time,

the dark side becomes darker,

the light side becomes lighter.

NARRATOR: It's a slow process,

one dust particle at a time.

But it's the best explanation

of how this peculiar moon

got its self-sustaining, warm, dark face.

OLUSEYI: Iapetus just shows us the potential

of the interconnectedness

between the planetary bodies of our solar system.

We think they're isolated, but nothing's isolated.

NARRATOR: Iapetus is not the only icy moon affected

by the system it exists within.

Another moon closer to the sun

reveals not only a unique connection with its neighbor,

but also hints

that something potentially profound

could lurk beneath its surface.

NASA LAUNCH ANNOUNCER: T minus five, four,

three, two, one.

Ignition and liftoff

of the Atlas V with Juno on a trek to Jupiter.

NARRATOR: The Juno mission is to investigate the origin

and evolution of the gas giant Jupiter,

the largest planet in our solar system.

LYNNAE QUICK: The main goal of the Juno spacecraft

is to study Jupiter.

However,

as it orbits Jupiter,

it's taking these wonderful images

of Jupiter's largest moons.

NARRATOR: Ganymede,

the largest moon in the solar system.

Io, the most volcanically active.

And then, there's the icy moon, Europa,

the odd one out with the smoothest surface.

It's an ancient moon, billions of years old,

and yet, mysteriously,

has a young surface, with few craters.

DURFEY: We can get

an idea of how old a planetary surface is

by counting the number of impact craters.

Europa barely has any impact craters,

which means that its surface must be really young.

NARRATOR: And that's not all that makes Europa different.

It's criss-crossed by a strange network of striking red lines.

GONZALEZ: The surface of Europa

has a lot of interesting cracks and features to it,

and has lots of lines,

which look almost like a network

of blood vessels in an eye.

♪

NARRATOR: These lines are vast ice cracks,

some of which are 15 miles wide,

with a dark red floor.

OLUSEYI: What could be creating

these criss-crossing lines and patterns on Europa's surface?

♪

QUICK: The canyons on Europa are so significant

that NASA's planning a mission

to go to Europa to explore them.

NARRATOR: And in preparation,

scientists are exploring similar environments on Earth,

like these icy terrains in Alaska.

[wind whistling]

SAM HOWELL: I think many of the most important questions

in planetary science can be addressed

by going to worlds like Europa.

NARRATOR: NASA scientist Sam Howell is studying

what may be at the heart of Europa's mysterious appearance.

HOWELL: What I'm most excited about,

coming from a background

in studying plate tectonics here on Earth,

we see systems of rigid,

icy plates moving around similar

to the way Earth's plate tectonics looks on the surface.

But digging in, we expect those processes to be very different.

NARRATOR: Europa's surface features indicate

they are a result of tectonic activity.

BERDIS: This is an up-close image

of Europa's surface, and you can see

this large, dark red band

in the very center of the image.

We know that this feature is created

due to tectonic activity

because it appears that the crust

of Europa has pulled apart,

and you've had material flow up from underneath.

But if we were to take this feature

and extract it from this image,

both sides of the crust would fit back together

like a puzzle piece.

We did not expect to see tectonic activity

on a tiny moon like this, but there it is.

NARRATOR: So far, this kind

of global tectonic activity has only been observed on Earth.

OLUSEYI: Take a look at Earth,

and you'll see that South America looks like

it fits right neatly in there into Africa.

And so, we know that in the Atlantic,

the seafloor is spreading,

and that's what spread those two features apart.

NARRATOR: On Earth, the rocky crust is broken into plates.

They move as softer rock beneath flows very slowly

over geologic time.

QUICK: Europa does not have molten rock like Earth does,

but what we do believe Europa has is a subsurface ocean.

NARRATOR: A vast body of liquid water

that exists underneath its icy surface.

WONG: There is more water

in Europa's subsurface

than all of Earth's oceans, rivers and lakes combined.

NARRATOR: This massive ocean sits beneath a layer

of water ice, which could be up to 15 miles thick in places.

So what force is powerful enough

to crack Europa's extensive icy shell?

BERDIS: As Europa and all of the other large moons orbit Jupiter

and interact with it,

they are all constantly pushing

and pulling and tugging on each other,

and this process is called "tidal heating."

NARRATOR: The tidal heating varies,

depending on how close or far Europa is from Jupiter

and the other moons,

resulting in temperature changes in the ocean.

QUICK: And as the temperature of the ocean changes,

the ice shell thickens and thins and expands and contracts.

[wind whistling]

NARRATOR: The mechanics of which can be seen on Earth.

QUICK: As water freezes,

it takes up more space and expands into ice.

It's very similar to if we were to take a bottle of water

and place it into the freezer.

First, it would expand,

and then we'd notice a large bit of cracking.

This is what is happening on Europa.

NARRATOR: Scaled up to the size of a moon,

these ice expansion cracks create massive canyons

across Europa.

And as these canyons form,

a key ingredient from the ocean below surges up

through the cracks,

giving Europa's markings their distinctive red.

BERDIS: So the red marks that we see on Europa's surface--

these cracks and lines--

are salts

that have come up from below the surface.

♪

NARRATOR: Suggesting a tantalizing possibility.

This crisscrossing system of salty canyons means

we might find life in the ocean

underneath Europa's icy shell.

SINGER: Life as we know it needs three basic things:

water,

a source of energy, and also, essential elements.

NARRATOR: Europa has an ocean,

and a source of energy from tidal movements.

But it's missing the final key ingredient:

some essential elements.

However, with a little help from its neighboring moon Io,

these elements can be found on Europa.

SINGER: Io

is an extremely volcanically-active place.

And these volcanoes emit

sulphur and other sulphur products,

and these make it all the way onto Europa.

And sulphur is one of those essential elements that life

on Earth uses.

♪

BERDIS: All of the sulphur that's coming from Io

and landing on Europa's surface

is interacting with the salts and water

that's coming up from underneath Europa's icy crust.

NARRATOR: Chemical reactions between the sulphur from Io

and the salts on Europa create essential molecules

needed for life.

And this allows for this perfect breeding ground

for potential life to form and evolve.

NARRATOR: Europa is such a strong candidate for life

that NASA is already preparing to explore the moon.

The goal is to study Europa's surface

in more detail

so that a future mission can one day land

and explore beneath its icy crust.

And scientists like Sam are already testing ideas

for studying Europa's watery world.

HOWELL: What we're doing is we're deploying a robotic probe

into the ice and under it

to explore what this lake environment is like.

We might have the opportunity

in my own lifetime

to answer a question that's intrinsic to all of us:

are we alone in our universe?

Are we alone in our solar system?

And one of the most exciting aspects of exploring Europa

is the potential of someday knowing

whether or not there's life in our own solar system.

NARRATOR: Europa is one of the last ice moons

that can exist this close to the sun.

Worlds existing any closer to the warmth of the sun,

like the rocky worlds of Mars, Earth, and our moon,

have relatively little ice.

Because they lie inside what is known as the ice line.

DURFEY: You can't see the ice line,

but you can see its effects.

When small bodies that are

primarily composed of ice

cross inside of the ice line,

the ice within them begins to vaporize.

NARRATOR: Sometimes, icy rocks are ejected

from the frigid outer regions of the solar system

and pulled towards the sun.

These are known as comets.

HOWETT: Comets are basically dirty snowballs,

mostly water ice with a little bit of dust,

and as they come into the inner solar system,

they get heated up by the sun.

NARRATOR: As comets cross this invisible ice line,

their ice starts turning into gas.

HOWETT: And that's what forms these long tails

that we can see when you look at a comet,

and those tails stream away from the sun

as the comet moves in towards it.

NARRATOR: Inside the ice line, ice is rare,

but there are places where it can hang on.

DURFEY: Mars is within the ice line of the solar system,

but it still has ice in it and on it.

NARRATOR: It's the end of winter,

and it's been dark at Mars's south pole

for over 150 Martian days straight.

Mars holds onto much of its ice at its poles.

The ice sheet here is not like any on Earth.

Because as winter turns to spring,

the surface becomes covered in strange features.

And with the launch of the Mars Reconnaissance Orbiter

in 2005, we began to see them

like never before.

OLUSEYI: The Mars Reconnaissance Orbiter

is a satellite of Mars that takes

photograph after photograph

of Mars's surface,

and so it's able to see these things come and go

and change with the seasons.

NARRATOR: The images show dark fans on the surface

meaning something must have risen into the air above them.

GONZALEZ: We see these features,

and they will go in the direction

depending on the wind patternof Mars.

So we know that material

is being pushed out from underneath.

NARRATOR: This phenomenon seen on Mars

depends on the type of ice that falls from the Martian skies.

During winter at the South Pole,

more and more snow falls.

But with so little water in Mars's atmosphere,

this isn't the type of snow we have on Earth.

This is carbon dioxide snow.

GAO: Every winter, three to four trillion tons

of carbon dioxide snow falls on Mars.

That's so much snow that the atmospheric pressure

at the surface of Mars decreases by about a third.

NARRATOR: The air is literally freezing to the ground,

forming carbon dioxide ice caps at the poles.

But in spring, as the warmth of the sun reaches

the poles again, something incredible happens.

DURFEY: As the sun comes over the horizon,

the carbon dioxide ice is heated.

This carbon dioxide ice can turn to gas,

and when it does, it forms these geysers

that also bring other material from the subsurface

to the surface.

GONZALEZ: And depending on which way the wind

is blowing, it'll form these nice looking fan features.

NARRATOR: Because of Mars's combination

of atmospheric pressure and temperature...

...the carbon dioxide turns from ice to gas

without passing through the liquid phase.

WONG: On Mars, its atmosphere

actually falls onto the surface

and then vaporizes back into the atmosphere

on an annual basis.

That's absolutely wild

because it's so different from what's happening here on Earth.

NARRATOR: These conditions on Mars

have enabled carbon dioxide snow

to create unique features on its surface.

But the conditions found on its nearest neighbor,

close enough to see from Mars with the naked eye,

have allowed for something extraordinary to form.

And just like Mars,

Earth, too, has ice caps at its poles.

But the similarities end there.

HOWETT: Earth's ice is really unique

because we operate in a very small temperature range

and a very small pressure range on the surface.

The ice that we experience is a very small subset

of the total ice that's in our universe.

NARRATOR: Earth's temperature and pressure ranges

allow water to exist in all three states:

solid, liquid, and gas.

Earth is the only place known to have floating icebergs

and bodies of water not completely frozen over.

And frozen water under Earth's conditions

has an important property.

We might take this for granted,

but the ice that we have here on Earth

floats because its density

is lower than the water that it is floating in.

And so, the molecules of water ice,

as it freezes, expands out slightly

and moves away from each other

to lower the overall density of the ice,

and this allows it to float on top of the water.

NARRATOR: This property of water ice has enormous consequences

for life on Earth,

the only place life is known to exist to date.

BERDIS: The ice on Earth provides a form of protection

to any life below the surface

that might need protection from anything that could harm it.

NARRATOR: Ice's ability to float

means that, during Earth's colder periods,

bodies of water didn't freeze from the bottom up,

instead remaining a liquid

where life may have held on.

[birds and insects chirping]

For around four billion years,

liquid water on Earth

has allowed an unbroken chain

of life to evolve.

Resulting in oceans.

Home to countless species.

And rivers teeming with life.

All made possible by the unique interplay

between liquid water and floating ice on Earth.

OLUSEYI: The next time you find yourself

making a beverage and you put ice in it,

just take a moment and reflect

on the amazingness of the ice

that made your life possible.

NARRATOR: Across our solar system,

scientists have found worlds

shaped by ice,

made of a diversity of substances

behaving in unbelievable ways.

♪

DURFEY: There's a massive variety of ice worlds out there.

Each one has something we haven't seen before.

NARRATOR: We are in a golden age of discovery,

but our knowledge of these ice worlds is far from complete.

There is so much left to learn about ice worlds.

Our understanding is only skin deep.

We want to explore these worlds because

it really tells us where we fit into the universe.

Why are we here? How did we first get here?

NARRATOR: We may not have all the answers,

but we now know that ice plays a critical role

in our story.

WONG: Without the kind of ice that we have here on Earth,

we might not even be here to be discussing ice

and all of its wondrous forms.

♪