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 pattern of 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.

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