NOVA

(engine igniting)

NARRATOR

A milestone in space exploration almost a billion miles from Earth. The spacecraft Cassini transforms our understanding of Saturn.

DICK FRENCH

The Cassini Mission is the most successful interplanetary mission in history.

JONATHAN LUNINE

Cassini told us the Saturn system might have life in it.

NARRATOR

Exploring mysteries of its beautiful rings, and more than 60 intriguing moons.

ATHENA COUSTENIS

More amazing, more exciting, more unbelievable than any theory you could've put together. This is our legacy.

NARRATOR

Now, the spacecraft begins its final daring mission.

EARL MAIZE

This is the capstone to one of the most phenomenal missions anybody has ever done.

NARRATOR

As Cassini's 20-year journey comes to a fiery end.

JULIE WEBSTER

The sense of an impending end is the hardest thing I've had to experience in a long time.

BRENT BUFFINGTON

It's a fitting end to an amazing mission.

NARRATOR

But how will it end? What final secrets will Cassini reveal as it makes its death dive to Saturn? Right now, on "NOVA." Major funding for "NOVA" is provided by the following. For over 80 years, we've been working on solutions to some of the world's biggest challenges, reaching beyond what's possible to support your health and help you thrive. Applying imagination and engineering, we work to protect our nation and our families so there can be moments like these. We are Draper. Do people tell you that you've got your mom's eyes or your dad's toes? 23andMe.com is a genetic service that provides personalized reports about traits, health, and ancestry, and can help you understand why you are who you are. Learn more at 23andMe.com. The David H. Koch Fund for Science. Supporting "NOVA" and promoting public understanding of science. And the Corporation for Public Broadcasting. And by PBS viewers like you. Thank you. Additional funding from The Steve Perry Foundation. Pasadena, California. Here, at the Jet Propulsion Laboratory, the mood is tense. A group of scientists-- along with their families and friends-- have gathered to share a milestone in the history of space exploration. These are the last days of a 20-year mission in the life of a 5,000-pound spacecraft named Cassini. Almost a billion miles from home, Cassini is beyond the terrestrial planets-- Mercury, Venus, Earth, and Mars-- beyond the asteroid belt and mighty Jupiter. The spacecraft is at Saturn.

ANTHONY DEL GENIO

Saturn is, I think, by anyone's estimation the most strikingly beautiful planet. If you asked anybody what planet they'd love to see close up, you'd love to see Saturn.

CALEB SCHARF

There is something special about Saturn that sets it apart even from the other giant planets in our solar system. It has this extraordinary ring system. It has 62 moons of great diversity. In some sense, Saturn, more than any of the other giant planets, resembles its own planetary system.

NARRATOR

For more than a decade, Cassini has made discovery after discovery about the planet, its magnificent rings, and perhaps the biggest surprise of all, one tiny moon with the possibility of life.

CAROLYN PORCO

I think this is our best opportunity to study an extraterrestrial habitable zone. (radio chatter)

NARRATOR

Tonight, the spacecraft begins a dangerous series of maneuvers that could result in an abrupt and premature death. I'm starting to get nervous. Yeah, I'm a little sweaty, too. We got 30 minutes to go. Anxiety is in the air, as the team waits to hear back from their beloved spacecraft.

MICHELE DOUGHERTY

I am really excited, but I am nervous, because neither the spacecraft nor the instruments were designed to do this.

NARRATOR

Throughout this daring mission, the Cassini team has faced risk after risk.

MARCELLO FULCHIGNONI

You cannot be afraid. It's an adventure.

NARRATOR

This is Cassini's final mission. Its grand finale. We are going through an area no one has gone through before. Cassini must pass through a gap between the planet and its innermost ring.

BUFFINGTON

So we think this 2,000-kilometer gap is in fact a gap, and it's clear, right? But we're not positive.

JAMES GREEN

Whether it's large dust or small dust, any of those that you hit at 76,000 miles per hour... (snaps fingers) Can kill your spacecraft. It only takes one. It only takes one larger ring particle to hit the spacecraft that could stop you cold in your tracks.

NARRATOR

But the risk is worth it, because if all goes well for the next five months, Cassini will peer inside the gas giant as never before,

to answer a puzzling question

what is Saturn's interior made of? What lies beneath its mysterious atmosphere?

LUCIANO IESS

We don't know what we are going to find, but our expectations are high. We think we will be able to determine whether it has a core and how big this core is.

LINDA SPILKER

The mysteries we want to solve with the grand finale mostly have to do with revealing Saturn from the inside out.

NARRATOR

Understanding Saturn's internal structure will shed light on how the gas giant formed. Though we think we have a picture of how giant planets like Saturn or Jupiter formed, it's rather incomplete. And there's another mystery the team hopes to unravel.

GREEN

There's still quite a debate. Are the rings young? Hundreds of millions of years? Or are they four-and-a-half billion years old?

NARRATOR

As old as the solar system. The answer could finally be revealed. That is, if Cassini passes through the gap intact.

DOUGHERTY

It's about 15 or 20 minutes from when we will get that signal back that the spacecraft is fine. We're waiting with bated breath. I tried not to sit in there and bite my nails because I'm a bit nervous right now.

NARRATOR

The signal takes about 80 minutes to travel almost a billion miles across the solar system. (radio chatter) By midnight, the room is so quiet, you can hear a pin drop. It's time for Cassini to phone home. I think of it as the "Hi, Mom" moment, where the spacecraft goes through and says, "I made it safely. I'm ready to get on with the rest of the trip." And finally, the call arrives.

MAN (on radio)

Cassini Ace, I can confirm we have a good lock on the 66360 bit rate data. (cheers and applause)

NARRATOR

Cassini has survived its first close encounter with Saturn, but this is just the beginning of its final mission. Once a week for the next five months, it will plunge between the planet and the rings again, and again, and again.

MAIZE

Seven seconds of terror every seven days as we go through that little gap.

NARRATOR

Each dive is dangerous. As the orbits progress, we meet closer and closer to Saturn's atmosphere.

DEL GENIO

But you don't want to get very close to the planet until you're prepared to lose the spacecraft. Which is why we're only doing this now at the end of the mission.

NARRATOR

Even if Cassini successfully completes all of its dives, the mission and the spacecraft are still destined to come to an end.

BUFFINGTON

It's on a path towards death. That's sealed at this point.

NARRATOR

On its last dive, Cassini will plunge into the cloud tops of Saturn's atmosphere where it will burn up, and a 20-year mission will come to an end. I feel emotional even now when I think what will happen on September 15.

MAIZE

We're extended all the way out to Saturn and we're going to miss that.

WOMAN

Oh, wow. That is so cool!

NARRATOR

Saturn has long been an object of curiosity and wonder for Earth-bound observers. I see it, I see it. Getting a glimpse of the gas giant is an unforgettable experience. Oh, that's amazing! Especially if you can see its majestic rings.

WOMAN

Oh, yeah, I can see the rings! That's crazy.

MAN

Pretty awesome. Awesome is right.

NARRATOR

Saturn is named for the powerful Roman god of agriculture and wealth. It is the second-largest planet in our solar system, with a volume more than 700 times that of Earth's. It doesn't have a surface you can walk on-- it's mostly a mixture of hydrogen and helium, and that makes it incredibly light. In fact if there were a lake large enough, Saturn could float on it. Its stormy atmosphere rages with winds that blow a thousand miles an hour, some of the fastest ever recorded in the solar system. As for its temperature, the gas giant is a frigid -288 degrees Fahrenheit. At its northern pole is a massive structure, a six-sided jet stream called the hexagon. You could take two Earths and plunk them into that space and they would not fill it up. Since 2004, the Cassini imaging team, lead by Carolyn Porco, has been taking hundreds of thousands of mind-blowing images.

PORCO

Just such a surreal-looking planet. Really, it's, it's... It wins the beauty contest in the solar system, that's for sure.

NARRATOR

Exploring Saturn for over a decade has given the Cassini team a rare opportunity-- to watch as the color of the hexagon shifts from the blue of winter to summer, when its atmosphere reacts to sunlight, and forms a golden haze. It also gave the team a front-row seat to an historic event.

INGERSOLL

There have been six giant storms since the late 19th century. Astronomers have observed them from Earth, and so, when Cassini was in orbit around Saturn, we had sort of a one-in-three chance of seeing one of these storms with all of our instruments ready and working. And we caught one.

PORCO

We got to watch it from its birth, which was December 5, 2010, all the way into late 2011. It ending up circling the entire planet, and the tail ended up meeting its head and eventually it dissipated. The scientists on Cassini are still studying this monster.

NARRATOR

Over the last 13 years, the imaging team has been documenting Saturn's turbulent atmosphere. They even made the first movie of lightning on another planet. This thunderstorm is larger than the continental United States, and these lightning bolts, a thousand times stronger than the ones we have on Earth.

INGERSOLL

The weather on Earth actually after a while gets boring and then you go to some other place and you see, "My God, look at what is possible."

DEL GENIO

It puts the Earth into this larger perspective and helps us understand the universe of possible configurations that a planetary atmosphere could have. Once we can do that, then we can go back and say, "Oh, okay, I look at what I see on the Earth in a little different way than I was looking at it before."

NARRATOR

After almost 20 years in space, Cassini is on its final chapter, flying closer to the gas giant than ever before. Now, just two days after Cassini's first daring dive between the planet and its innermost ring, the team gathers at JPL to make a critical decision-- should they put their spacecraft in harm's way, again? On its first dive, just how much of a beating did Cassini take? It's William Kurth's job to figure it out. Kurth is in charge of the radio and plasma wave science instrument.

KURTH

This is the one instrument that can detect the impact of the dust that is hitting the spacecraft.

NARRATOR

He translates those hits into audio signals we can hear. First, he plays a recording made in 2016, when Cassini flew just outside one of Saturn's rings, called the F-Ring. This is the sound of dust hitting Cassini's antenna. (soft static crackling) So how does that sound compare to what he heard during Cassini's first dive between the rings and the planet? (high-pitched hiss) He hears a hiss, the sound of charged particles in the environment, but not much dust.

KURTH

I really expected to see a lot more dust than we did.

NARRATOR

In fact, the team now calls this 1,200-mile gap the Big Empty. But it doesn't mean it's danger-free.

MAIZE

If we're surprised, the show's over early, and we don't want that to happen. In my business, we call them the unknown unknowns. You don't know what to worry about, but you know there might be something out there.

NARRATOR

Far from public view, in a basement at JPL, a group of engineers is hard at work making sure Cassini's mission doesn't end before it's supposed to. We are responsible for the health and safety of the spacecraft. This is the Cassini integrated test lab, also known as the ITL.

WEBSTER

The ITL is our representation of the spacecraft on the ground.

NARRATOR

Throughout this clean room are dozens of vintage circuit boards, exact copies of the guts of the Cassini spacecraft, along with the ground-based equipment needed to run it.

WEBSTER

The equipment's 30 years old. Everybody talks about gigabits these days, we're down in kilobits.

NARRATOR

The memory and computing power here is less than in your smartphone, but it's kept Cassini running. We were built to be bulletproof.

MAN

And we are go for orbit trim maneuver 467 test.

NARRATOR

Right next door, Julie Webster's team uses this vintage hardware to test contingency maneuvers for Cassini's final dives. No red alarms and we are go for orbit trim maneuver 467.

WEBSTER

The timing of everything is highly choreographed. The accelerometer has powered on at this time. Because we are doing something almost every second on the spacecraft, but certainly every minute.

WOMAN

Wind roll turn has started.

WEBSTER

To have either an anomaly on the spacecraft or a sequence that isn't quite right, there's very little time to figure out what's wrong, fix it, clean it back up, put the sequence back on board the spacecraft. We don't have a lot of time to recover at that point. We're game on. At this time, the wind roll turn has completed.

NARRATOR

How does the team keep track of a spacecraft a billion miles away? We actually carry about a 2,400-star map and those stars are the same stars that sailors used with a sextant. Just like a navigator sailing the seas hundreds of years ago, Cassini looks at the stars around it and compares their positions to those on a star map it carries on board.

WEBSTER

It's just built into the flight software. That's how we know where we are in space.

NARRATOR

Today, we've developed sophisticated technology to expand our view of the solar system. We can even see galaxies far beyond our own. But the effort to see beyond Earth began with a handful of 17th-century astronomers

who strived to develop a revolutionary new tool

the telescope. At the Dutch National Museum for the History of Science and Medicine, one of the few early telescopes that has survived the test of time is taken out of storage.

PAUL STEENHORST

What you see actually is a black iron tube.

NARRATOR

This telescope must be handled with extreme care.

STEENHORST

The material's pretty thin, so I will carefully take out piece by piece till I have the whole tube.

NARRATOR

When fully extended, it's almost 15 feet long.

STEENHORST

The lens is mounted by a metal spring. Beautiful craftsmanship they used to make this.

VINCENT SCHEERMAN

That's perfect.

NARRATOR

This telescope was built by Christiaan Huygens, the 17th-century Dutch astronomer. At his home in the Netherlands, Huygens and his brother Constantin designed and built some of the best telescopes of the day. They even ground their own lenses. The better the lens, and the longer the telescope, the more detail he could see. One, two... But using a long, fragile tube was anything but easy.

STEENHORST

If you have two or three supporting people, then you can look through the tube. But you have to hold it still, also.

NARRATOR

With a telescope a lot like this, Huygens set out to solve a mystery-- one that even baffled Galileo, the great astronomer who first observed Saturn. Galileo couldn't figure out what was sticking out of the planet's sides. As Huygens searched for the answer, he filled his notebooks with drawings of his observations.

CEES ANDRIESSE

Here again Saturn, another night.

NARRATOR

In 1656, at the ripe old age of 27,

Huygens finally reached the conclusion

that Saturn is surrounded by a ring. He also discovered its largest moon, Titan. One, two, three moons. And the number two is his moon. He adds the word "meus", my moon. The others were discovered by Cassini.

NARRATOR

Jean-Dominique Cassini ran the newly built Paris Observatory. There, he discovered six more moons of Saturn, and that instead of a single ring, Saturn was surrounded by several rings separated by gaps. Over time, telescopes got better, and better, and better. (engine igniting) And by the 20th century,

we accomplish the unimaginable

we take our telescopes into space. In 1979, Pioneer 11 flies by Saturn, sending back images that bring us closer to the cosmos than ever before. A year later, Voyager's state-of-the-art cameras arrive at Saturn and exceed our expectations. We thought during Voyager days, this was, like... Nothing could top this.

NARRATOR

Then comes Cassini, built with a suite of scientific instruments designed to explore Saturn, its rings, and its moons in extraordinary detail. Cassini's Battlestar Galactica, because it's just festooned with instruments. It's a flagship.

ANNOUNCER

And liftoff of the Cassini spacecraft on a billion-mile trek to Saturn.

NARRATOR

19 countries pool their talent and resources to get this massive effort off the ground.

LUCIANO IESS

Science is international by definition. I never represented my country with Cassini. Cassini is a family.

NARRATOR

Seven years later, Cassini arrives at Saturn, and the team gets ready to explore

one of the most intriguing objects in the solar system

Saturn's largest moon, Titan.

RALPH LORENZ

Before Cassini arrived, Titan was Terra Incognita and that was a big draw, of course.

NARRATOR

This "unknown land" has an atmosphere so thick that 25 years earlier, the spacecraft Voyager could not see beneath Titan's haze, but it did detect one surprising characteristic. The main constituent of Titan's atmosphere is, like on Earth, nitrogen. The similarity to Earth's atmosphere was too intriguing to ignore.

COUSTENIS

Scientists just wanted to go back. It sounds like a clich, but every space mission raises more questions.

NARRATOR

Scientists want to go back so badly that the European Space Agency takes on the challenge of building a probe to land on the surface of this alien world. Called Huygens, it fits snugly on Cassini's side. Huygens was there to teach us about Titan's atmosphere, take pictures of the surface also, and then it would either crash or sink, but it would have a disastrous end.

JEAN-PIERRE LEBRETON

We did not know anything about the surface of Titan. It was really very difficult to design for the unknown.

NARRATOR

So do their efforts pay off? As Huygens makes its way through Titan's hazy atmosphere, in a control room in Darmstadt, Germany, scientists anxiously wait to hear back from the probe.

FULCHIGNONI

And so, at the right time, we were starting to be a little bit nervous.

NARRATOR

Then they get a signal. The Huygens probe has survived its descent to Titan. Soon data comes streaming in, including the echo of Huygens' radar as it bounces off the moon's surface. (radar zip echoing) (zip abruptly stops) (laughter and applause) I think this is the best techno music we've ever heard. (laughter) During its descent, the probe measures wind speeds of 270 miles per hour, and confirms Titan's atmosphere is made of mostly nitrogen, along with methane and ethane. Finally, Huygens gives the world our first look beneath Titan's dense haze. I thought it was a joke. It looked so much like an image of the surface of Mars. My colleague says, "This is Titan." I look at it and said, "That can't be Titan. There's pebbles on it, it looks like... it looks red." He says, "It's Titan, you better get used to it." Over the next decade, Cassini continues to explore Titan, making more than 100 flybys.

J. HUNTER WAITE

In the upper atmosphere, there was this organic factory that was producing huge amounts of very complex organic compounds, and those molecules were then falling down through the atmosphere, ending up on the surface and forming these organic dunes.

NARRATOR

Massive dunes, as large as ones found in the Sahara Desert, next to long, river-like channels.

LORENZ

Now we've mapped the whole world, and it's, in many ways, almost as diverse as the Earth.

NARRATOR

Radar and infrared cameras paint a picture of a moon with dozens of lakes. In fact, Titan is the only celestial object in our solar system known to have liquid on its surface, except, of course, planet Earth. What would it be like to stand by a lake on Titan? How would it compare to Earth?

ALEXANDER HAYES

We're here on the shore of Cayuga Lake in upstate New York, because the view that we have here would not be too different from the view you might see from the shores of a Titan lake. Behind me, we have a beautiful blue sky that's reflecting off the surface of the lake, making it look blue. And, on Titan, that sky would be a hazy orange. And the lake would be reflecting that hazy orange sky, and upon closer inspection, we'd notice it's not made of water. It's a mixture of methane and ethane that's similar to liquid natural gas. In fact, all of Titan's lakes and seas combined is 300 times the volume of all the proven gas reserves here on Earth. Now, they create a landscape that's similar to the rocky deserts of the southwestern United States. The ground is going to be a broken-up mixture of water, ice, and organics with the texture of plastic shavings or Styrofoam beads. Sometimes, it'll rain, and when it rains on Titan, those raindrops are going to fall slower than we might expect and they're going to make large splashes... (splash echoes)...because the gravity on Titan is 1/7 that of Earth's, similar to what the astronauts felt on the moon's surface. And finally, because we have this colder, denser atmosphere, sound travels faster than it does here on Earth, and the sound of the waves will be coming to us more compressed and perhaps be perceived as slightly pitched. (voice

distorted)

My voice itself will sound a little bit more alien and complex. But I hope you're holding your breath, because while Titan's atmosphere is predominantly nitrogen, just like Earth, there is very little to no oxygen to speak of. And if you can visualize all of this, I'd like to welcome you to... the shores of a lake on Titan.

It's so Earth-like

rain, lakes, rivers, erosion, clouds. To me, there's a romance to that that I just can't escape. I'm from Scotland, which is a place that has a lot of cold rain, and I don't like rain, but I like to think that I'm privileged to live on one of only two worlds in the solar system where rain falls onto the surface and flows in rivers. And the other one is Titan. (splash echoes)

NARRATOR

But Titan is not the only moon to pique the team's interest. Saturn has 62 moons, and counting. From Prometheus, which looks like a potato, Janus, which resembles a meatball, and Mimas, which has an uncanny resemblance to the Death Star in Star Wars. Let's not forget Iapetus, the yin-yang moon, because it has one side as dark as night and the other as bright as snow. And Pan, a tiny moon shaped like a ravioli. And then there's Enceladus, which is just such a total surprise. Saturn's moon Enceladus is a tiny, bright, white ball of ice, the most reflective object in the solar system.

IESS

Enceladus is a small body. It's less than the distance between Los Angeles and San Francisco. And everybody expected to be a dead body.

NARRATOR

Little Enceladus poses a great mystery. Its north pole is pockmarked with craters, the remnants of asteroid impacts that were common billions of years ago. But its south pole is surprisingly smooth, almost craterless. The question is, why? Early in the mission, the magnetometer, an instrument that measures magnetic fields, picks up on something strange.

DOUGHERTY

It was really spectacular. It was like a plume of water vapor coming off from the south pole.

NARRATOR

Around the same time, Cassini's cameras capture a mind-boggling image. This is what we saw. We saw dozens of fine jets shooting off the south pole of Enceladus. More instruments, designed to detect the chemical composition of gases and dust, find that these jets, or plumes, are made of water ice. Enceladus spews out 1,000 tons of it every hour. Some of it comes back down like freshly fallen snow, and create the moon's smooth, white surface. It's pretty startling. We started to try to figure out, well, why is this happening? This moon is supposed to be cold and dead. In subsequent flybys, Cassini gets even closer, and the instruments reveal even more clues.

LUNINE

We have tasted this material coming out of Enceladus. It has organic molecules, carbon-bearing molecules, nitrogen-bearing molecules.

And one mineral no one ever expected to find

salt. That told us these ice grains were frozen seawater.

NARRATOR

Frozen seawater coming out of enormous fractures in the south pole, and these fractures are radiating heat.

PORCO

This is the south pole. There's more energy coming from these fractures than anywhere else on Enceladus. It would be as if on Earth, we found there was more heat coming from Antarctica than there was from the Equator.

NARRATOR

Plumes of frozen seawater gushing out of massive fractures. So, could there be liquid water beneath the frozen moon's surface? It took about ten years of Cassini data to answer that question. An instrument which measures gravity helped solve the mystery. From the gravitational field, we can unpeel what the interior is like. And what they find inside Enceladus is stunning. The gravity data revealed

something denser than ice under the surface

water.

FRENCH

We provided one important clue that there was an underground ocean.

NARRATOR

Enceladus has an ocean about 20 miles beneath its icy crust. The ocean under the surface of Enceladus is not limited to the southern polar region, but is global.

SPILKER

But there's still more. We found tiny particles of nano-silica. What's so amazing is that those nano-silica grains could only form in really hot water. All of a sudden, the pieces started to fall into place, and so we're thinking, maybe you have hydrothermal vents on the sea floor of Enceladus.

NARRATOR

Not only is there liquid water, but it's hot. On Earth, at the bottom of the ocean, there are hydrothermal vents, openings in the sea floor where heated water flows. Although it's dark and cold, life thrives.

LUNINE

Biochemists have suggested that it's in environments like this where life might've got going billions of years ago.

NARRATOR

To learn more, Cassini flies even closer to the plumes, and makes another startling discovery.

WAITE

On the last flyby that we operated, we were able to detect native H2 in the plume.

NARRATOR

H2 is a form of hydrogen gas found around Earth's hydrothermal vents. H2 is a nutrient for microbial systems. So it's, like, microbial candy. Could this be food for life?

PORCO

It doesn't get any better than this, to go to Saturn and come away having discovered what we think might be the best place in the solar system to go to search for life. Nobody can say that there is life on Enceladus, but it's a possibility.

NARRATOR

Not only do these plumes from Enceladus offer tantalizing clues of life beyond Earth, but their icy particles also create one of Saturn's most majestic features, its outermost, or E-Ring.

BROOKS

The E-Ring is the only known ring that is basically made up of samples of a moon.

LUNINE

This E-Ring is not like the classical rings of Saturn. It's not sharp and dramatic. You can't see it through a home-built telescope. But it's there.

NARRATOR

While all the gas giants of the outer solar system have rings, they're faint. Except, of course, for Saturn's. Saturn's rings are composed of billions of particles made mostly of water ice. The rings are mostly ice, and then there's a veneer of contaminant schmutz. We don't really know what it is. Ring particles come in a wide variety of shapes and sizes. From the size of a grain of sand to the size of a house.

JEFF CUZZI

Saturn's rings are the biggest and most massive in the solar system.

NARRATOR

Extending about 90,000 miles into space.

CUZZI

On the other hand, if you were to look at them on edge, they are a flat disc of particles only tens of meters thick.

NARRATOR

Within this shallow band of ice and dust, Cassini discovered a dynamic world.

PORCO

Just look at these shadows, they're just wild.

NARRATOR

Long shadows cast by the sun reveal particles reaching miles above the ring plane. We get to see lots of places just really densely packed, where the particles are protruding two miles above the ring plain. I mean, it's astonishing. I've often loved imagining that I'm in a shuttle craft, flying low over the rings. And from a low elevation above the rings, the rings would look to you like they extended to infinity, because they're really very big. So, flying low over the rings, it's only 30 feet thick, and you're passing all these particles, and then eventually coming upon a wall of rubble two miles high. I mean, come on... That's like science fiction. Glorious, just absolutely glorious.

CUZZI

They do look calm and beautiful and tranquil from afar, but they're really a kind of an explosion of events that are going on. The rings are fluid.

NARRATOR

Ring particles run into each other, forming mysterious waves hundreds of miles long.

MAIZE

To see all these little waves, these little bow waves going through, just a phenomenal picture.

NARRATOR

Cassini's cameras reveal what's causing

these strange formations

Saturn's collection of tiny moons.

CUZZI

We have grades of moons. We have little things we call ring moons, and we have smaller things we call moonlets, embedded within the rings and sprinkled around very close to the rings. In addition, there's a whole class of objects we call propeller objects, swarms... Hundreds or thousands or more of these things.

MAIZE

The rings are full of little propellers, just festooned with them.

NARRATOR

A young researcher on the imaging team, Matt Tiscareno, is the first to notice them.

TISCARENO

What I saw was two horizontal dashes oriented in the same direction as each other, about 100 meters across, about the size of a football field.

PORCO

Someone might have dismissed that as cosmic ray hits or imperfections in the camera or something, but he noticed something that was very common among all these streaks. They all looked like little propellers.

NARRATOR

At the center of each streak, a tiny moon tugs and tosses ring particles about. They clump together along the edges of what looks like a distinctive shape. These little two-bladed propellers orbiting Saturn's rings can, some scientists suggest, shed light on a mysterious process

that took place billions of years ago

the formation of our solar system.

GREEN

Four-and-a-half billion years ago, this is what we believed happened. We have a very large cloud of material, gas, but also dust. (explosion) A blast wave-- we actually believe it's this nearby supernova-- rippled through this gas cloud. Things end up in a disk and then they start to form.

NARRATOR

As they orbit around the young sun, gas and dust stick together. Pebbles turn into boulders, boulders into mountain-size comets. Comets turn into protoplanets. Protoplanets become planets. Over hundreds of millions of years, the planets take shape. The best place in our solar system to observe a process like this in action is in Saturn's rings.

SPILKER

So, watching the propellers, other clumping we've seen in the rings, perhaps we can better understand how the planets might have formed from a disc of gas and dust to form the larger objects. The propellers are kind of like the seeds that came together from which the planets formed.

But there's a catch

these propellers can't grow into full-fledged planets. They're kind of frustrated in, they can't form, they can't go on to complete the process because they're too close to Saturn.

NARRATOR

The power of Saturn's gravity pulls them apart.

PORCO

It actually prevents things from coalescing. It's like the formation of baby planets, but it's an arrested state of development. They don't get very big. So we are seeing here an initial process. It went to completion in the solar system billions of years ago. But we get to see it in Saturn's rings, and that's what's so special about it. That was the golden opportunity we had in going back to Saturn's rings with Cassini.

NARRATOR

In the last 13 years, Cassini has made startling discoveries about Saturn's extraordinary rings, but there's a fundamental question

the team has yet to answer

how old are the rings? Did they come together alongside the gas giant 4.5 billion years ago, when the solar system formed? Or are they much younger? There are two schools of thought. We're thinking that Saturn's rings may not be as old as the solar system. They may be a recent addition, maybe 400 or 500 million years ago. My inclination is to believe that they are probably older rather than younger.

NARRATOR

As Cassini completes one daring dive after another, the team is hoping to resolve this question once and for all. In June 2017, team members from around the world gather at the European Space Agency in Noordwijk, Holland.

SPILKER

Well, we've just had our seventh plunge in between the rings and the planet, and there are all kinds of surprises for Cassini. Every week, we get a new dataset and we go, "Oh."

REBECCA PERRYMAN

Everything is speeding up. The spacecraft is speeding up; the data is speeding up. And the discoveries. There's never a dull moment.

NARRATOR

Especially when it comes to probing Saturn's interior. A key instrument to do this is the magnetometer, which measures the magnetic field of a planet.

DOUGHERTY

What that magnetic field does, if you can measure it outside, it allows you to understand what's going on in the inside.

NARRATOR

So far, what the magnetometer is showing is downright baffling.

DOUGHERTY

We've got an internal planetary field that's been generated, but it's really weird. It's different from any other planetary fields we've seen.

NARRATOR

On Earth, our magnetic field is produced deep inside the planet by the motion of churning liquid metal surrounding a hard inner core. It works a lot like a large generator or dynamo.

LUNINE

According to a theory called planetary dynamos, you get permanent magnetic fields when you have electrically conducting fluids inside planets that are circulating in these twisting motions that will then generate the magnetic field.

NARRATOR

These twisting motions in the center of the Earth create a magnetic field on a tilt from Earth's axis. But Michele Dougherty can't find that tilt on Saturn.

DOUGHERTY

I'm only gonna tell you this. I haven't told anyone else on the Cassini project yet. There are five members of my team who know this. But we think the rotation axis of the planet and the magnetic axis of the planet, at this stage, as far as we can tell, are lying on top of each other.

NARRATOR

The problem is, this doesn't fit planetary dynamo theory, the current idea of how magnetic fields work.

DOUGHERTY

If we're seeing what I think we're seeing, the internal field is generated in a different way than people think or the field is dying.

NARRATOR

So what does that tell us about what's inside Saturn? Though the planet is made mostly of hydrogen and helium, scientists believe that Saturn, like Earth, has a solid core.

DEL GENIO

So we think Saturn has a small core at the very center. But maybe the more interesting part of Saturn is that it would be surrounded by a metallic hydrogen envelope.

NARRATOR

That metallic hydrogen is likely the source of Saturn's magnetic field. Here on Earth, the magnetic field protects us from the sun's solar wind, which could erode our atmosphere, making life on Earth impossible. Without a magnetic field, Earth could look a lot like Mars, which was once a warmer and wetter place. But its magnetic field weakened, leaving it a barren desert. Cassini's data raises the possibility for Michele that Saturn's magnetic field might also be dying.

DOUGHERTY

If this actually happened at Saturn, the gas surface would be stripped away. And I don't know what will happen as a result of that. It's crazy. But that's why you do science. You know, I said to someone. Someone said to me, "Ooh, do you understand your data?" I said, "Nah." And they said, "That's great, isn't it?" I said, "Yeah." I think scientists are disappointed whenhey're always right. This is a field where you can take delight in being baffled. My favorite time is when we find out that we're profoundly wrong.

NARRATOR

And one thing they could be wrong about is how Saturn formed from the disk of the early solar system. They've assumed that, like other planets, Saturn came together through a clumping of dust and debris, then attracting hydrogen and helium gas around it. Now they're hoping to discover in the billions of bytes of data yet to be analyzed whether this idea is right or wrong.

FRENCH

I think it's too early for us to say whether it's changing our view of planet formation. But what it's saying is we have to confront the data directly and see what the observations are telling us.

NARRATOR

And the observations might finally be telling them about the age of Saturn's rings. Crucial clues may come from an instrument that measures mass. What will really seal the deal is to find out from Cassini just how massive the rings are. The older the rings are, the more debris they may have gathered up, becoming more massive over time. Less massive rings point to a younger age.

SPILKER

So far, we have hints that maybe the rings are not as massive as we thought.

CUZZI

My hunch has been young, and my hunch is still young.

NARRATOR

On September 15, 2017, after 20 years, the final hour of the mission has arrived.

BROOKS

We are bringing the mission to a close, because, basically, we are out of gas.

NARRATOR

With its fuel tanks almost empty, Cassini must be destroyed to reduce the risk of contaminating Saturn's moons with earthly microbes. And so, in the early hours before dawn, the team gathers to witness Cassini's final moments together. The probe is traveling more than 70,000 miles an hour as it enters Saturn's upper atmosphere. Thrusters position the spacecraft so its antenna can send data back to Earth for as long as possible. As it dives, pushing against the atmosphere, the temperature rises. Cassini can no longer maintain stability, and communication with Earth comes to an end.

MAIZE

The signal from the spacecraft is gone and, in the next 45 seconds, so will be the spacecraft.

NARRATOR

All alone, nearly a billion miles away, Cassini begins to break apart, as temperatures reach those on the surface of the sun. The propellant tanks explode. The instruments atomize in the intense heat. What remains sinks into the cloud tops, and Cassini becomes part of the distant world it explored.

MAIZE

This has been an incredible mission, an incredible spacecraft. And you're all an incredible team. (applause)

SPILKER

I've worked on Cassini for almost 30 years, and that's the time it takes for Saturn to go around the sun. I have a mixture of feelings. A feeling of sadness-- this group of people I've worked together with for decades will now be scattering and going their separate ways. At the same time, a tremendous sense of pride. We've rewritten the textbooks on Saturn.

LUNINE

Cassini told us the Saturn system might have life in it, that it has moons that are every bit as interesting as the Earth and Mars, the Saturn ring system may be relatively young. And so it's told us such an incredible amount about this one planetary system within our solar system.

WEBSTER

It was perfect. It was truly perfect. It-it did everything we asked it to do.

Including sending back to Earth a final gift

these images of Saturn's atmosphere, closer than ever before.

MAIZE

It's just been an amazing machine. We've used every bit of it, and it's been phenomenal. So, I... You know, it's-it's time to say good-bye. This "NOVA" program is available on DVD. To order, visit shopPBS.org or call 1-800-PLAY-PBS. "NOVA" is also available for download on iTunes.