Almost everything we know today about the beautiful giant ringed planet comes from Cassini, the NASA mission that launched in 1997 and arrived at Saturn in 2004. Since then, the spacecraft has been beaming home miraculous images and scientific data, revealing countless wonders about the planet, its rings, and 62 moons—including some that could harbor life. As the mission approaches its final days in 2017, it attempts one last set of daring maneuvers—diving between the innermost ring and the top of Saturn’s atmosphere. Aiming to skim less than 2,000 miles above the cloud tops, no spacecraft has ever gone so close to Saturn and hopes are high for incredible observations that could solve major mysteries about the planet’s core. But such a daring maneuver comes with many risks. Join NASA engineers for the tense and triumphant moments as they find out if their gambit has paid off, and discover the wonders that Cassini has revealed over the years.
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Death Dive to Saturn
PBS Airdate: September 13, 2017
NARRATOR: A milestone in space exploration, almost a billion miles from Earth, the spacecraft Cassini transforms our understanding of Saturn…
RICHARD (DICK) FRENCH (Lead Scientist, Cassini Radio Science Team): The Cassini Mission is the most successful interplanetary mission in history.
JONATHAN LUNINE (Planetary Scientist, Cornell University): 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 (Planetary Scientist, Paris Observatory): More amazing, more exciting, more unbelievable than any theory you could've put together.
CAROLYN PORCO (Lead Scientist, Cassini Imaging Team): This is our legacy.
NARRATOR: Now, the spacecraft begins its final daring mission…
EARL MAIZE (Cassini Program Manager, NASA's Jet Propulsion Laboratory): This is the capstone to one of the most phenomenal missions anybody's ever done.
NARRATOR: …as Cassini's 20-year journey comes to a fiery end.
JULIE WEBSTER (Spacecraft Operations Team Member, NASA's Jet Propulsion Laboratory): The sense of an impending end is the hardest thing I have had to experience in a long time.
BRENT BUFFINGTON (Aerospace Engineer, NASA's Jet Propulsion Laboratory): 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.
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 (Planetary Scientist, NASA's Goddard Institute): 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 (Astrobiologist, Columbia University): 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.
NARRATOR: Tonight, the spacecraft begins a dangerous series of maneuvers that could result in an abrupt and premature death.
JULIE WEBSTER: I'm starting to get nervous.
LUIS ANDRADE (Guidance and Control Engineer, NASA Jet Propulsion Laboratory): Yeah, I'm a little sweaty, too. We've got 30 minutes to go.
NARRATOR: Anxiety is in the air, as the team waits to hear back from their beloved spacecraft.
MICHELE DOUGHERTY (Physicist, Imperial College London): 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 (Planetary Scientist, Paris Observatory): You cannot be afraid. It's an adventure.
NARRATOR: This is Cassini's final mission, its grand finale.
EARL MAIZE: We are going through an area no one has gone through before.
NARRATOR: Cassini must pass through a gap between the planet and its innermost ring.
BRENT 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 (Director Planetary Science Division, NASA): Whether it's large dust or small dust, any of those that you hit at 76,000 miles per hour can kill your spacecraft.
SHAWN BROOKS (Planetary Scientist, NASA's Jet Propulsion Laboratory): 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 (Aerospace Engineer, Sapienza University of Rome): 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 (Cassini Project Scientist, NASA's Jet Propulsion Laboratory): 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.
CALEB SCHARF: Though we think we have a picture of how giant planets like Saturn or Jupiter formed, it's rather incomplete.
NARRATOR: And there's another mystery the team hopes to unravel.
JIM 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.
MICHELE 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. By midnight, the room is so quiet, you can hear a pin drop. It's time for Cassini to phone home.
DICK FRENCH: 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."
NARRATOR: And finally, the call arrives.
CASSINI ACE: I can confirm we have a good lock on the 66360 bit rate data.
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.
EARL MAIZE: Seven seconds of terror every seven days, as we go through that little gap.
NARRATOR: Each dive is dangerous.
LINDA SPILKER: As the orbits progress we move closer and closer to Saturn's atmosphere.
TONY 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.
BRENT 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.
LUCIANO IESS: I feel emotional, even now, when I think on what will happen on September 15th.
EARL MAIZE: We're extended all the way out to Saturn, and we're going to miss that.
WOMAN #1 LOOKING INTO TELESCOPE: Oh, wow. That is so cool!
NARRATOR: Saturn has long been an object of curiosity and wonder for earthbound observers.
MAN #1 LOOKING INTO TELESCOPE: I see it, I see it.
NARRATOR: Getting a glimpse of the gas giant is an unforgettable experience…
MAN #2 LOOKING INTO TELESCOPE: Oh, that's amazing!
NARRATOR: …especially if you can see its majestic rings.
WOMAN #2 LOOKING INTO TELESCOPE: Oh, yeah, I can see the rings. That's crazy.
MAN #3 LOOKING INTO TELESCOPE: Pretty awesome.
MAN #4 LOOKING INTO TELESCOPE: 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 minus-288 degrees Fahrenheit.
At its northern pole, is a massive structure, a six-sided jet stream called "the hexagon."
ANDREW INGERSOLL (Planetary Scientist, California Institute of Technology): You could take two Earths and plunk them into that space, and they would not fill it up.
NARRATOR: Since 2004, the Cassini Imaging Team, led by Carolyn Porco, has been taking hundreds of thousands of mind-blowing images.
CAROLYN PORCO: It's just such a surreal-looking planet. Really, it, 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.
ANDREW 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.
CAROLYN PORCO: We got to watch it from its birth, which was December 5th, 2010, all the way into late 2011. It ended up circling the entire planet; its tail ended up meeting its head and, eventually, it dissipated. The scientists of Cassini are still studying, 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.
ANDREW 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.
TONY 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 J.P.L. 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.
WILLIAM (BILL) KURTH (Research Scientist, University of Iowa): 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.
So, how does that sound compare to what he heard during Cassini's first dive between the rings and the planet? He hears a "hiss," the sound of charged particles in the environment, but not much dust.
BILL KURTH: I really expected to see a lot more dust than we did.
NARRATOR: In fact, the team now calls this 1200-mile gap the "big empty." But it doesn't mean it's danger-free.
EARL 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 J.P.L., a group of engineers is hard at work making sure Cassini's mission doesn't end before it's supposed to.
JULIE WEBSTER: We are responsible for the health and safety of the spacecraft.
NARRATOR: This is the Cassini Integrated Test Lab, also known as the I.T.L.
JULIE WEBSTER: The I.T.L. 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.
JULIE 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.
JULIE WEBSTER: We were built to be bulletproof.
CASSINI INTEGRATED TEST LAB TEAM MEMBER 1: We are going to work on 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.
CASSINI INTEGRATED TEST LAB TEAM MEMBER 1: No red alarms, and we are go for orbit trim maneuver 467.
JULIE WEBSTER: The timing of everything is highly choreographed…
CASSINI INTEGRATED TEST LAB TEAM MEMBER 2: The accelerometer is powered on at this time.
JULIE WEBSTER: …because we are doing something almost every second on the spacecraft, but certainly every minute.
CASSINI INTEGRATED TEST LAB TEAM MEMBER 2: The wind roll turn has started.
JULIE 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, we're game on.
CASSINI INTEGRATED TEST LAB TEAM MEMBER 2: At this time, the wind roll turn has completed.
NARRATOR: How does the team keep track of a spacecraft a billion miles away?
JULIE WEBSTER: We actually carry about a 2,400-star map, and those stars are the same stars that sailors used with a sextant.
NARRATOR: 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 onboard.
JULIE 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: (Dutch National Museum for the History of Science and Medicine): What you see, actually, is a black iron tube.
NARRATOR: This telescope must be handled with extreme care.
PAUL STEENHORST: The material is pretty thin. So, I will carefully take it out piece by piece, until I have the whole tube.
NARRATOR: When fully extended, it's almost 15 feet long.
PAUL STEENHORST: The lens is mounted by a metal spring; beautiful craftsmanship they used to make this.
NARRATOR: This telescope was built by Christiaan Huygens, the 17th century Dutch astronomer. At his home in the Netherlands, Huygens and his brother, Constantijn, 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.
But using a long, fragile tube was anything but easy.
PAUL 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 (Physicist/Historian, Utrecht University): 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.
CEES ANDRIESSE: One, two, three moons; and the number two is his moon. He adds the word "mius," 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, 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.
CAROLYN PORCO: 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.
EARL MAIZE: Cassini's Battlestar Galactica, because it's just festooned with instruments. It's a flagship.
TELEVISION NEWS ANNOUNCER: And liftoff of the Cassini spacecraft, on a billion-mile trek to Saturn.
NARRATOR: Nineteen 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 within 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 (Planetary Scientist, Johns Hopkins University Applied Physics Laboratory): Before Cassini arrived, Titan was terra incognita and that was a, 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.
FRANCOIS RAULIN (Exobiologist, Université Paris-Est Créteil): The main constituent of Titan's atmosphere is, like on Earth, nitrogen.
NARRATOR: The similarity to Earth's atmosphere was too intriguing to ignore.
ATHENA COUSTENIS: Scientists just wanted to go back. It sounds like a cliché, 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 snuggly on Cassini's side.
ATHENA COUSTENIS: 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 (Huygens Project Scientist, European Space Agency): 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.
MARCELLO 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.
WOMAN: I think it's the best techno music we've ever heard.
NARRATOR: 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.
FRANCOIS RAULIN: I thought it was a, it was a joke. It looked so much like an image of the surface of Mars.
ATHENA COUSTENIS: 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."
NARRATOR: Over the next decade, Cassini continues to explore Titan, making more than a hundred flybys.
J. HUNTER WAITE (Planetary Scientist, Southwest Research Institute): 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.
RALPH LORENZ: Now, we've mapped the whole world, and it's in many ways almost as diverse as 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 (Astronomer, Cornell University): 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 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, 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 you might expect, and they're going to make large splashes, because the gravity on Titan is 1/7th that of Earth, 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. 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.
EARL MAIZE: It's so Earth-like: rain, lakes, rivers, erosion, clouds. To me, that's, that's, there's a romance to that that I just can't escape.
RALPH LORENZ: 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. The other one is Titan.
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.
EARL MAIZE: And then there's Enceladus, which is just such a total surprise.
NARRATOR: Saturn's moon Enceladus is a tiny, bright white ball of ice, the most reflective object in the solar system.
LUCIANO IESS: Enceladus is a small body—it's less than the distance between Los Angeles and San Francisco—and everybody expected it 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.
MICHELE 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.
CAROLYN PORCO: This is what we saw. We saw dozens of fine jets shooting off the south pole of Enceladus.
NARRATOR: 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 a thousand tons of it every hour. Some of it comes back down like freshly fallen snow and creates the moon's smooth, white surface.
HUNTER WAITE: It's pretty startling. We started to try to figure out, "Why is this happening? This moon is supposed to be cold and dead."
NARRATOR: In subsequent flybys, Cassini gets even closer and the instruments reveal even more clues.
JONATHAN LUNINE: We have tasted this material coming out of Enceladus. It has organic molecules, carbon-bearing molecules, nitrogen-bearing molecules.
NARRATOR: And one mineral no one ever expected to find: salt.
JONATHAN LUNINE: 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.
CAROLYN 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 that 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?
LINDA SPILKER: It took about 10 years of Cassini data to answer that question.
NARRATOR: An instrument which measures gravity helped solve the mystery.
DICK FRENCH: From the gravitational field, we can unpeel what the interior is like.
NARRATOR: And what they find inside Enceladus is stunning. The gravity data revealed something denser than ice under the surface: water.
DICK FRENCH: We provided one important clue: that there was an underground ocean.
NARRATOR: Enceladus has an ocean about 20 miles beneath its icy crust.
LUCIANO IESS: The ocean under the surface of Enceladus is not limited to the southern polar region, but is global.
LINDA SPILKER: But there's still more. We found tiny particles of nanosilica. What's so amazing is that those nanosilica grains could only form in really hot water. All of the sudden the pieces started to fall into place, and, so, we're thinking maybe you have hydrothermal vents on the seafloor 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 seafloor where heated water flows. Although it's dark and cold, life thrives.
JONATHAN 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.
HUNTER 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.
HUNTER WAITE: H2 is a nutrient for microbial systems. So, it's like, microbial candy.
NARRATOR: Could this be food, for life?
CAROLYN 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.
LUCIANO IESS: 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.
SHAWN BROOKS: The E ring is the only known ring that is, basically, made up of samples of a moon.
JONATHAN 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 homebuilt 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.
SHAWN BROOKS: 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.
NARRATOR: From the size of a grain of sand to the size of a house…
JEFFREY CUZZI (Planetary Scientist, NASA's Ames Research Center): Saturn's rings are the biggest and most massive in the solar system.
NARRATOR: ….extending about 90,000 miles into space.
JEFFREY CUZZI: On the other hand, if you were to look at them on edge, they are a flat disk of particles only tens of meters thick.
NARRATOR: Within this shallow band of ice and dust, Cassini discovered a dynamic world.
CAROLYN 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.
CAROLYN PORCO: 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, it's astonishing. I've often loved imagining that I'm in a shuttlecraft 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!
JEFFREY CUZZI: They do look calm and beautiful and tranquil from far, 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.
EARL 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.
JEFFREY 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 that we call propeller objects, swarms, hundreds or thousands or more of these things.
EARL 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.
MATTHEW TISCARENO (Planetary Scientist, SETI Institute): 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.
CAROLYN 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 look 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.
JIM GREEN: Four-and-a-half-billion years ago, this is what we believe happened. We have a very large cloud of material, gas but also dust, 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.
LINDA 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 disk 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.
NARRATOR: But there's a catch. These propellers can't grow into full-fledged planets.
JEFFREY CUZZI: They're kind of frustrated, in that 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.
CAROLYN 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's, 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.
JEFFREY CUZZI: 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.
SHAWN BROOKS: 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.
LINDA 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.
HUNTER WAITE: Every week we get a new dataset and we go, "Oh!"
REBECCA PERRYMAN (Operations, Cassini Ion and Neutral Mass Spectrometer): Everything is speeding up; the spacecraft's speeding up, the data is speeding up and the discovery…
HUNTER WAITE: There's never a dull moment now.
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.
MICHELE 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.
MICHELE 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.
JONATHAN 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 Michelle Dougherty can't find that tilt on Saturn.
MICHELE DOUGHERTY: I'm only going to 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.
MICHELE 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.
TONY 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.
MICHELE 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, "Oh, do you understand your data?" I said "Nah." And they said, "That's great isn't it?" And I said, "Yeah."
DICK FRENCH: I think scientists are disappointed when they are 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.
DICK 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.
SHAWN BROOKS: What will really seal the deal is to find out from Cassini just how massive the rings are.
NARRATOR: 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.
LINDA SPILKER: So far, we have hints that maybe the rings are not as massive as we thought.
JEFFREY CUZZI: My hunch has been young, and my hunch is still young.
NARRATOR: On September 15th, 2017, after 20 years, the final hour of the mission has arrived.
SHAWN 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.
EARL MAIZE: We just heard 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.
EARL MAIZE: This has been an incredible mission; incredible spacecraft; and you're all an incredible team.
LINDA 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.
JONATHAN LUNINE: Cassini told us that 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.
JULIE WEBSTER: It was perfect. It was truly perfect. It did everything we asked it to do.
NARRATOR: Including sending back to earth a final gift: these images of Saturn's atmosphere, closer than ever before.
EARL 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 time to say goodbye.
WRITTEN, PRODUCED, AND DIRECTED BY Terri Randall EDITED BY Jedd Ehrmann CAMERA David Arabia Yoann Le Gruiec
Thomas van Krugten
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A NOVA Production by Terri Randall Productions, Inc. for WGBH Boston
© 2017 WGBH Educational Foundation
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This program was produced by WGBH, which is solely responsible for its content.
Original funding for this program was provided by Draper, 23andMe, the David H. Koch Fund for Science, The Neil & Anna Rasmussen Foundation and the Corporation for Public Broadcasting.
IMAGE: Image credit (Cassini animation) Courtesy NASA
- Cees Andriesse, Shawn Brooks, Brent Buffington, Athena Coustenis, Jeffrey Cuzzi, Anthony Del Genio, Michele Dougherty, Richard French, Marcello Fulchignoni, James Green, Alexander Hayes, J. Hunter Waite, Luciano Iess, Andrew Ingersoll, Jean Pierre LeBreton, Ralph Lorenz, Johnathan Lunine, Earl Maize, Rebecca Perryman, Francois Raulin, Caleb Scharf, Vincent Scheerman, Linda Spilker, Paul Steenhorst, Matthew Tiscareno, Julie Webster