For 13 years, the cassini spacecraft explored
astounding worlds ... saturn and its moons.
We discovered things we never imagined.
All of these strange, bizarre landscapes ...
geysers erupting out of moons.
Porco: We were so stunned by the images.
I mean, people were just going around in shock.
But cassini started running out of fuel.
Scientists at nasa had to decide what to do next,
and the answer was actually pretty spectacular.
Cassini goes where no spacecraft has gone before ...
a death flight revealing the deepest secrets of saturn.
out beyond jupiter lies saturn,
a planet circled by multiple moons and rings.
the capability to explore all aspects of the saturn system.
That mission was cassini.
but by September 2017, cassini was almost out of fuel.
Cassini has been orbiting saturn
and studying the saturn system for over a decade.
End the mission ... we're going to lose control soon,
so rather than let it just go derelict,
we headed into saturn.
Go out with a bang.
The cassini team goes for broke.
They program the probe to head straight for the planet.
your last bit of information, and then crash and burn?
We are in the atmosphere.
As cassini was careening toward its death,
it still had instruments that continued to work,
and as each instrument died,
there was still a set sending back data and information.
Cassini wasn't designed to plunge
through saturn's atmosphere.
No one knew how long it would last before burning up.
I remember sitting in a room
with my colleagues on cassini
and watching that radio signal,
that sharp green peak that told us cassini
was still linked to the earth.
Howett: We could monitor the atmosphere as we flew into it,
and right up until the last, it was sending back science.
I was really impressed by how long cassini lasted
in the saturn atmosphere.
I mean, go nasa engineering.
As cassini plummeted down
at 77,000 miles an hour,
it was bombarded by gas molecules
in saturn's atmosphere.
friction started tearing cassini apart
as it struggled to maintain contact.
As the antenna turned away,
we actually saw a secondary little peak,
and we thought, "okay, cassini. Hang in there. Keep fighting."
and then just that green flatline.
Just heard the signal from the spacecraft is gone,
and in the next 45 seconds, so will be the spacecraft.
Cassini's heartbeat was gone,
and we knew the mission had ended.
It just vaporized in the saturnian atmosphere,
and so it has become a part of saturn itself.
Cassini's death plunge
was the last of a series of daring dives.
Prior to ending cassini's mission
by sending it into saturn's atmosphere,
between the tops of saturn's clouds
and the innermost area of saturn's rings.
Beginning in April of 2017,
cassini ventured between saturn and its rings 22 times.
Scientists called it the grand finale.
On these dives, cassini got closest to saturn's cloud tops
than any spacecraft ever had before.
Saturn is an enormous ball of hydrogen and helium,
a gas giant.
Fundamentally, it's a really very different kind of planet
The pictures reveal a turbulent and stormy world.
We think of some storms on earth as being particularly violent.
If you've ever been in a hurricane,
it's not a fun place to be,
but the storms on saturn, the wind patterns on saturn,
can make that look like a mere breeze in comparison.
On saturn, one storms stands out.
It's location is marked by a distinct shape.
and then there's a hexagonal band up there.
No one expected that.
During its lifetime,
cassini took multiple images of the hexagon.
Whenever we posted an image of the hexagon,
the hits to the website went through the roof.
I think people thought it was so mysterious.
Bullock: When I first saw this, I was blown away.
I mean, who could imagine having something this regular,
almost geometric, on the atmosphere of a planet?
It's really just phenomenal.
In 2018, cassini data reveals this hexagonal storm
could be a towering structure hundreds of miles in height.
It's this gigantic structure.
It's many thousands of miles across,
and right in the center, right at the pole,
is this sort of permanent vortex,
a permanent hurricane.
So, it's kind of a creepy eye-like thing
staring back at us.
Each side of the hexagon is as wide as the earth.
or cloud structure on saturn?
Scientists think that saturn's spin
interacts with air currents to create this symmetrical shape.
But while the hexagon shape is stable,
the color has altered.
Over four years, it changed from mostly blue to golden brown.
The transformation is linked to saturn's seasons.
The seasons on saturn are caused by the same thing on earth.
It's the tilt of the planet,
and so as saturn is going around the sun
and it's north pole is tipping toward the sun,
you start to get more light up there.
This sunlight interacts with the atmosphere,
producing suspended particles called aerosols.
It actually looks a lot like smog.
It turns things more orange,
so over time the hexagon went from blue to orange.
The color change happened
during one of saturn's northern hemisphere summers,
Maybe the haze never formed in the eye
because the eye was shielded from the sun,
and the sun is responsible for creating the brownish haze
that we see on saturn.
Another reason is maybe the actual vortex
is sucking the haze down.
Maybe there's something like the eye of a hurricane.
There's haze that forms over it,
but it gets sucked down into the eye.
But the storms on saturn
aren't the only extraordinary thing about the weather.
When cassini dives through the rings, it discovers rain,
rain falling onto the planet from space.
April 2017, cassini embarked on its grand finale,
following a daring new path.
Spilker: We decided to dive in between the rings of the planet,
to go to a place no spacecraft had ever flown before
and make a unique set of measurements.
It was uncharted territory.
They didn't know exactly what they were going to find.
There could be stuff there that could have destroyed
the cassini spacecraft.
Instead, cassini encountered
something totally unexpected ... rain.
On earth it rains quite a bit.
We're getting that rain from rain clouds
which are basically just a few miles up.
On saturn it also rains, but it turns out it's raining
onto the top of the upper atmosphere,
and that rain is coming from space.
In 2018, cassini data revealed the colossal weight
of the downpour.
Icy-grained rain hits saturn
at a rate of several tons per second.
Durda: It's completely unlike anything we've ever seen.
Suddenly we've discovered rain at saturn,
but there aren't any rain clouds.
Where was it coming from?
The answer is saturn's rings.
The first thing you think of
when you hear the word saturn is the rings.
They're the most dramatic and unique aspect
of that planet.
from afar, saturn's rings look like this one whole structure,
but when you look up close it's actually a bunch of ice crystals
and ice rocks that make saturn's rings.
saturn's rings are well above the atmosphere of saturn.
They're way out in space.
And under normal circumstances those particles of ice
making up the rings would just orbit saturn forever,
but things are a little bit weird.
Something is making these orbiting ice particles
fall inward as a kind of cosmic hail.
walsh: Material is dripping inwards from the rings
and falling into the clouds of saturn.
It's like a rain with no rain cloud.
A cosmic rain trickling in and falling down.
Cassini discovers the rain
is a mix of different kinds of ice particles,
but doesn't reveal why they actually rain down.
Then the researchers realized the ice grains
were statically charged.
Ultraviolet light from the sun, for example,
can blow off an electron,
and that gives these particles a charge,
just like rubbing a balloon on your hair
makes it stick to a wall because of the static charge.
Well, if you have particles that are like that,
they can be affected by magnetism,
and saturn has a very strong magnetic field.
Earth's magnetic field springs
from it's spinning, molten-iron core.
Although saturn probably has a rocky center,
it's mostly a giant ball of hydrogen and helium,
but deep in its interior, scientists think something
much more exotic is going on.
We don't have enough data
to know exactly what's going on in saturn's interior,
but we do know the broad strokes.
Within saturn's interior,
extreme pressures and temperatures force hydrogen
to stop acting like a gas,
turning it into spinning liquid-metallic hydrogen.
Durda: You've got this band of electrons
that can just wander freely through that fluid,
so in that way, liquid hydrogen under extreme pressure
can act like a metal.
The magnetic field generated
by the spinning metallic-hydrogen outer core
pulls the ice particles from the rings.
These charged ice particles
Cassini had revealed several tons of material
is raining down on saturn every second,
but how much stuff is actually in the rings?
Again, cassini provides the answer.
In the final days of cassini,
we actually flew in between the planet saturn and the rings,
and the gravity data was able to separate out
how much mass is coming from the planet
and how much is coming from the rings,
and the surprise was the rings
are actually not very massive at all.
Even though they cover an area
as big as the moon's orbit around earth,
saturn's rings are 100,000 times less massive
than our own small planet.
They're lighter than we thought.
There's not as much material there.
The mass of the rings is a valuable clue about their age.
A more massive ring can hold itself together
for much longer than a less massive ring,
so if there's not a lot of stuff there, it must be younger.
So, how long have saturn's rings been in place,
and what is keeping them there?
It's something that I don't even think I could have imagined
if I tried.
As cassini orbited saturn,
it revealed incredible insights into the planet's rings.
are unlike anything that I could ever imagine.
If I was alien visiting our solar system,
I don't know what would stand out to me more,
is how old are they.
Could something like that really have existed
from the beginning of the solar system,
or is it relatively recent?
Cassini provided an answer.
The rings could be as young as only 100 million years old.
A couple of clues ... the low mass and the fact
that they're so bright and icy
that it hasn't had time to get polluted
from all the micrometeoroids and darkened
over a long time like the age of the solar system.
So, the amazing thing is
that, you know, if you were on earth
about the time of the dinosaurs,
there might have been a saturn in the sky with no rings.
So, if the rings didn't form
to form the rings 100 million years ago,
you need to find an object ...
maybe a comet or a moon gets too close to saturn.
Saturn's gravity tears it apart and forms the rings.
As the object is torn apart,
the pieces spread out around saturn to form the rings.
They keep colliding, breaking into smaller and smaller pieces.
Like pebbles on a beach, subsequent jostling
and self collisions between each other
will take the sharp edges off of them
creating rounded particles.
From a distance, saturn's rings appear
One of our results from cassini
as it took its final plunge into saturn
was as we flew past those rings,
we noticed that the rings were actually not a uniform density.
That's something that nobody had seen.
I mean, you had to get really close to see that,
and it wasn't expected.
I'm a ring scientist,
and I just love seeing that detail
and trying to figure out
why do saturn's rings look the way they do.
Durda: There are very intricate structures,
knife-edge little ringlets
and, like, almost like the grooves in a record.
It begs the question, of course,
The clue is hidden within the rings.
There's not five rings. There's not 500 rings.
There's thousands of rings.
There's potentially millions of tiny, little ringlets
with small gaps between them, and sometimes large gaps,
and cassini saw that there are moons embedded inside the rings.
These moons and moonlets seem to be shaping the rings.
When I think about the rings of saturn,
I almost hear symphonies playing in my head.
It's all about this wonderful structure and these harmonies,
Saturn's moons stir the rings particles
with their gravitational pull, creating waves.
spilker: There's a special place where the resonance exists.
Imagine where the ring particles are going around twice
for each single time the moon goes around.
It's like pushing someone on a swing.
If you push them at just the right rate
they go higher and higher,
and these places are where the waves generated.
So, there's this ballet, this dance,
between the ring and the moons.
It is one of the most elegant things I've ever seen.
But the rings aren't just being shaped
into waves by the moons.
They're being held in place by them.
Sutter: Through gravitational interactions,
these moons might be shaping the rings, shepherding them,
keeping their flock in a nice, tight orbit around the planet.
In 2017, cassini reveals
there are more than one or two moons
shepherding the rings.
A whole team of moons holds saturn's outermost visible ring,
the "a" ring, in place.
One of the really cool things that cassini discovered
during its death dive
was that there are seven moons of saturn
that are all working together
to keep that ring system in configuration,
so it's like the magnificent seven
holding this thing together.
Of the seven magnificent moons, the biggest is mimas.
It's one-eighth the size of our moon.
the smallest, pan, is only 20 miles across.
forming the ring system that we see today.
Cassini has truly opened our eyes to the wonder
of saturn's rings and many moons.
Saturn has a lot of moons ... I mean, a lot of moons,
and they're all really interesting and different.
Coming up with the exact number is a little difficult
'cause it almost changes every year as we discover new ones.
The latest count is over 60 moons,
each with a different character,
but one has a split personality and a very dark side.
May 2017.
Cassini was on its grand finale.
the probe snapped its last photo of a strange moon
2 million miles from saturn...
Iapetus.
Porco: Iapetus was discovered hundreds of years ago,
and right from the start it was recognized
This dark and light moon
confused italian astronomer giovanni cassini
when he first spotted it in 1671.
It's been puzzling scientists ever since.
so, we get there with cassini, and of course iapetus
was a very major target for us
because we were interested to know
what was with this crazy two-toned moon.
Cassini reveals that the answer lies even farther out
from saturn in the form of another moon.
There's one pretty big but really dark moon,
phoebe, that's outside of iapetus
and is orbiting the opposite direction around saturn.
Pheobe orbits saturn four times farther out than iapetus.
As it travels around the planet, micrometeorite impacts
toward saturn going the opposite direction of iapetus,
which is the perfect material for iapetus
to sweep up in its orbit to create one dark side.
One side always faces the planet
while another side drives forward through the dust.
Walsh: It's plowing through a bunch of dust
than the trailing bright side by 50 degrees fahrenheit.
When you have something that's dark like the dust
on the leading edge of iapetus, that gets warmer.
It absorbs sunlight better,
and if it's warmer then things that can evaporate more easily,
like water for example, tends to blow off the surface.
The front side gets darker and warmer.
any visible ice turns to vapor
and makes its way to the colder trailing side
where it refreezes.
so, the white side gets whiter and the dark side gets darker.
So, you have this self-sustaining dark side
and bright side, and you wind up with this two-faced moon.
but cassini discovers that they're not
the weirdest thing about this moon.
The weirdest thing is that it is a walnut.
iapetus has a mountain range that exactly circles its equator
all the way around the moon,
a mountain range higher than the himalayas.
These mountains are over 12 miles high,
more than twice the altitude of earth's highest peak,
mount everest.
It's crazy. It's this huge crazy ridge
on this really strange moon.
how do you form a smooth equatorial mountain ridge
around an entire world?
A really interesting idea is that for some period of time,
just like saturn itself has this gigantic ring system,
iapetus had a ring system, as well.
Over time, iapetus' ring collapsed,
falling in a circle around the moon.
As it fell to the surface,
it built up a mountain range
right below where the ring was orbiting
so that all that material just built up
and built a mountain range
ringing the equator all the way around.
Walnut-shaped iapetus
is not the only strange moon around saturn.
Cassini discovered a moon
hiding many secrets beneath its icy surface ...
enceladus, a moon that could even harbor life.
August 2017,
cassini captured six images of enceladus,
one of the most intriguing moons in the solar system.
Enceladus is a relatively small moon of saturn
that's pretty easy to ignore,
but once you pay attention to it, hosts a lot of surprises.
We've known something was unusual about this icy world
ever since the voyager mission took photographs in 1980.
Spilker: We thought that enceladus would be frozen solid,
and yet we knew from voyager data the surface
of enceladus was bright white.
And we could see on the surface
that vast tracks of it were smooth,
at least at the resolution that we had with voyager,
and that immediately says that there's been internal activity
because that's really, on an airless moon,
Then cassini sent back pictures of enceladus backlit,
and all was revealed.
Spilker: We saw these icy jets shooting out from enceladus,
and everyone was so amazed that a moon so tiny
and assumed to be a frozen-solid ice cube could be so active.
When I first saw a picture of a geyser on enceladus,
I mean, I was floored. That's amazing.
I had no idea that was even possible.
The cassini discovery of geysers on enceladus was a game changer.
All of the sudden, here's water jetting out.
It was, like, too good to be true.
Cassini revealed the geysers are blasting out liquid water.
Spilker: As we got more data from cassini,
we found that enceladus had a wobble
basically covering the inside of that moon,
but where's the heat coming from?
When planets and moons form,
their cores are incredibly hot,
but over time they cool down.
The smaller the planet, the faster it cools.
A tiny world like enceladus over a billion miles from the sun
should have frozen solid by now.
That's what we expected.
If things are smaller, then they would be roughly dead,
and they'd be covered by craters,
but enceladus shows us that that's not the case at all.
How could a tiny moon so far from the sun
have enough warmth for liquid water?
One idea is tidal heating.
Howett: If you've ever played racquetball,
you know as you play the game the ball heats up,
and this is because as the ball hits the racket or the wall,
it's getting squished, and then it relaxes.
saturn's gravity squishes and relaxes
enceladus as it orbits the planet,
heating it like a racquetball,
but this alone wouldn't generate enough heat
to stop enceladus' water from freezing.
Something else must be going on in the core.
What if the core of enceladus is actually kind of gravelly?
Instead of it just being solid, it's actually made of rocks
and pebbles and gravel all put together.
Then what happens is as the tides are stretching
and squeezing it, those rocks are rubbing together,
and that actually generates even more energy.
2017, a computer model based on cassini data
revealed this tidal friction generates more energy
than america's biggest power station.
Water heated to 194 degrees fahrenheit rises to the surface.
It sprays through cracks in the moon's
south pole creating misty plumes.
spilker: As we flew seven times through and tasted and sampled
the gas and the particles, we found salty particles,
that the ocean was salty very much like the earth's ocean.
We found hydocarbons, methane, carbon dioxide, ammonia.
We found the key ingredients for life
coming out of the jets of enceladus ... so remarkable.
In 2018, scientists reanalyze the enceladus data
and found something even more remarkable ...
complex organic molecules.
What they found were larger organic compounds
than initially thought there.
At first, it was just dust and water and some basic organics.
Now they're seeing more complex stuff
coming up from enceladus' interior,
and that asks the question what else is down there.
On earth, we find life huddled around hot vents on the seabed.
Could the same be true in the oceans of enceladus?
Could these complex organic molecules be signs of life?
Mckay: My favorite name for them is "goo."
they're gooey things, and think of life
as a collection of gooey-like molecules.
Now, that doesn't mean that all gooey molecules are biological,
but certainly biology makes use
of these complex organic molecules.
Deep in enceladus' oceans,
heat from hydrothermal vents drives chemical reactions...
complex molecules that could serve as the precursors to life.
What's amazing is the chemistry of that ocean.
Everything needed for life is there.
The eternal question is is there life in the universe,
and enceladus is a great place to try and answer that question.
And cassini revealed secrets
of another moon of saturn with amazing chemistry,
titan.
four days before the mission ended,
cassini flew past one of saturn's
most spectacular moons, titan.
Here's why I like titan ... not just because it has a name
that means "big" and "strong," which makes you think of me,
but because it has an extensive atmosphere
that's made primarily of nitrogen, just like earth's.
Titan's always been a mystery.
What is hiding underneath that thick atmosphere?
Viewed through a telescope,
this moon seemed little more than a hazy orange ball.
Then cassini launched the huygens probe.
It traveled beneath the clouds
and sent back images of titan's surface.
I almost can't describe how thrilling it was,
the landing of the huygens probe on the surface of titan.
It was like a jules verne adventure come true.
as you're coming through the atmosphere
and the world emerges,
and it's this incredible world that looks so familiar.
Mountains and these streams flowing into this ocean.
Wow!
Sutter: You might be standing on the shores of a lake,
but this lake doesn't look like water.
Instead, it's methane.
It's much darker.
It's a frigid, bizarre world
with geologic features that look familiar,
but in a very, very alien setting.
Titan is like a home away from home.
It's just colder by about 350 degrees fahrenheit.
Radebaugh: The cassini mission revealed that titan is
really exactly like earth in terms of its landscape.
In fact, almost a quarter of the body
is covered in sand dunes exactly like what you see behind me.
But cassini reveals titan's dunes are not what they seem.
Radebaugh: The dunes on titan are made of something
completely different than sand dunes on the earth.
You know, most sand on the earth is made of quartz,
but on titan sand dunes, it turns out,
are made entirely of organics.
Titan's sand is made from tiny particles
of organic gunk called hydrocarbons.
these organic dunes contain the building blocks of life.
For scientists, this is a tantalizing hint.
Could titan harbor life?
stricker: Here on earth, there's life that exists
in so many extreme environments,
so its not impossible to think that life
It's going to be really different from life on earth.
To me, there's going to be this wonderful moment
in history when we really do have
another example of how life can be,
and if I had to place my bets on it,
I think we're going to find it in the saturn system.
But cassini's discovery of potential life
sentenced the probe to death.
Porco: We didn't want to leave it just indefinitely in orbit
you don't want it crashing into titan or enceladus.
Cassini interacted with titan one last time.
Spilker: With a gentle nudge from titan's gravity,
we call it titan's goodbye kiss.
We ended the mission with a plunge into saturn's atmosphere,
vaporizing cassini and saying goodbye to our friend.
mitchell: I hope you're all as deeply proud
of this amazing accomplishment.
I'm going to call this the end of mission.
Project manager bob mitchell.
Although cassini is gone, its legacy lives on.
Now we're sifting through
all of the data collected, still finding discoveries,
putting together the pieces of the puzzle to understand
saturn, the rings, and the moons.
Porco: Cassini is going to go down in history
as one of the most scientifically productive
interplanetary missions that humanity has ever flown.
I'm immensely proud and feel enormously privileged
to have been a part of it.
Besides the amazing science cassini returned,
just the beauty of this planet
I think sparked something inside of us.
Spilker: Whenever I look up at saturn now,
I know that cassini is there, too,
and so saturn is even a more special place.