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Liftoff of Messenger on
NASA's mission to Mercury.

Beyond the warm worlds
of the inner solar system...

...beyond the gas giant Jupiter...

...in the freezing regions
far beyond the Sun...

...lies Saturn...

...a planet made unique

thanks to a nearly 300,000-kilometre-wide
ring of frozen water.

Here, trillions of pieces of ice
have been sculpted

by gravitational forces

into some of the solar system's
most stunning vistas.

Here, on Earth, water takes many forms -

oceans and clouds and rivers -

but it's in its crystalline form - ice -
that it's at its most beautiful.

Now, here, ice just adorns the landscape,

but in the outer solar system,
where Saturn lives,

ice is so abundant that, well,
it's become a building material.

Now, there's no more beautiful sight
in the solar system

than the ice rings of Saturn.

It's almost as if a god has taken
snowflakes and sprinkled them

over a gravitational field
so we could see it.

But how could something so intricate

be sculpted out of something so simple?

Well, as is always the case in science,
the true beauty lies in the story.

Saturn is a planet that has undergone
some of the most radical transformations

in the solar system...

...a planet that has seen destruction...

...and creation...

...a planet that harbours hidden worlds...

...where, just perhaps,

there may exist a second home for life.

Imagine a place with no surface
to stand...

...just an endless atmosphere.

Compared to Earth, Saturn is so alien
that it's hard to imagine

how it could have grown
from the same ingredients.

But go far enough back in time,

and Saturn would appear to be
surprisingly familiar.

Saturn began life
as tiny worlds of rock and ice...

...tumbling chaotically through space.

Just like Earth, Mars, Venus and Mercury,

they began to merge
and clump together and grow.

But unlike the planets
of the inner solar system,

Saturn was forming on the other side
of a boundary we call the snow line.

A place where the Sun is so distant,
and it's so cold

water exists
only in its frozen solid state.

Out here, trillions of particles of ice
behave just like rock,

providing huge amounts
of extra planetary building material.

Under the force of gravity,

this abundant ice and rock collided
and combined...

...helping the young Saturn
to grow into a giant.

The details of how the planets formed
in the solar system

is not fully understood.

It's cutting-edge scientific research.

I mean, we're talking about events
that happened

over 4.5 billion years ago.

But it seems likely that Saturn was once
a world of rock and ice,

perhaps 10 or even 20 times
the mass of the Earth.

It's almost possible, in your dreams,

to imagine standing on its surface.

But because of its immense mass
and its place in the solar system,

Saturn didn't remain
a rocky, icy world for long.

As it grew older, Saturn became
a radically different kind of planet.

In the region in which the young world
was forming,

there was more than just ice and rock -
there were large amounts of gas.

And Saturn began to gain an atmosphere...

...but on a scale unlike anything
we experience here on Earth.

It's not until you see
the Earth's atmosphere from space

that you appreciate
just how thin it really is.

And it's a similar story

for the other worlds
of the inner solar system.

Despite being relatively thin,

these atmospheres can exert
powerful forces

on the planetary surfaces beneath.

On Earth, you can see the effects
of an atmosphere

with nothing more than a sealed container.

It's easy to forget
that the atmosphere has a weight.

There's a column of air stretching up
about 100 kilometres above my head.

That's a lot of molecules,
and it exerts a pressure.

You really see it if you look at what
happens to this bottle.

We filled it with air
from the top of the mountain,

which is about 3,000 metres.

Now, up there, the atmosphere
is about 25% less dense

than it is down there, at sea level.

So, as we descend,

the atmospheric pressure outside
is increasing,

and the pressure in here

is just what it was
at the top of the mountain,

and so you see, visually,
that force in action,

and that is squashing the bottle.

Physics in action.

It's brilliant, innit?

While just a few hundred metres
of atmosphere can crush a bottle...

...if an atmosphere gets big enough,

it can transform an entire planet.

Within just a few million years
of its birth...

...Saturn had grown as large as it could
from rock and ice alone.

And now it turned
to another building material -

the hydrogen and helium gas left over
from the formation of the Sun.

This gas would have been too light

for the smaller worlds
of the inner solar system to hold on to.

But Saturn's great mass
created gravitational forces

powerful enough to draw it in.

Trillions upon trillions of tonnes
of hydrogen and helium gas

began to envelop the planet.

And as this new atmosphere grew,
it transformed the surface below.

Enormous pressures,
created by the weight of this gas,

heated rock and ice so much...

...they began to glow.

As Saturn matured,
the pressure at the surface

rose to ten million times
the atmospheric pressure on Earth.

At those kind of pressures,
matter behaves in extremely strange ways.

The very idea of a surface
becomes meaningless,

and Saturn was transformed
from an icy, rocky world

into a completely different class
of planet - a gas giant.

Saturn was now so big it could contain
nearly 5,000 Earth-sized worlds.

But this vast, wild proto-planet

was totally unlike
the Saturn we see today.

To discover how that world emerged,

we had to send spacecraft
to meet it close up.

We have ignition,
and we have liftoff of the Titan-Centaur,

carrying the first
of two Voyager spacecraft

to extend man's senses farther into the
solar system than ever before.

Reports coming back indicate
those twin large solid motors

are functioning perfectly, producing
1.2 million tonnes of thrust each.

The guidance data coming back shows
that we're right on the money.

It's there!

Voyager 2 has reached Saturn
after four years in space.

It got there just a bit early,
2.7 seconds early, to be exact,

after a flight of 1.25 billion miles.

The Voyager probes gave us
our first detailed look at Saturn,

revealing the planet
in exquisite resolution.

There's just an amazing harvest
of pictures.

You know, there's tens of thousands
of photographs of Saturn

and its rings and its moons
taken by the Voyager 2 spacecraft.

For the very first time,

we were able to study
Saturn's vast atmosphere in detail.

Saturn itself is composed
of hydrogen and helium.

And we are seeing
an exotic meteorology here.

Voyager showed beyond doubt
that Saturn's upper atmosphere

was made almost entirely
of helium and hydrogen...

...the very same gasses so abundant
in the early solar system.

But this once-chaotic gas was now
organised into intricate weather systems.

And one above all others
took the Voyager team by surprise.

A huge hexagonal structure
in the clouds,

so big it could fit our entire planet
within it nearly four times over.

The atmosphere of Saturn
was revealing itself to be stranger

and more dynamic
than we could ever have imagined.

Atmospheres
are some of the most spectacular

and complex planetary environments.

Not many people see the Alps
like this. Seriously.

It's just so great!

I was afraid that you'd just get,
you know, too sick, or something.

It's awesome! Man!

It's fun, man.

This is fun!

The atmosphere of every planet
takes on its own unique character.

But what powers them
and the weather systems within

depends on where you are
in the solar system.

Most of the weather on the Earth
is driven by the heat of the Sun.

It heats the ground up, down there,

and that in turn heats the air up
close to the ground, makes it expand.

That means that the air becomes less dense
and the hot air rises.

That means you get thermals,

so energetic movements of air
from the ground into the upper atmosphere,

and that's what we're
exploiting now... to glide.

So all this beautiful weather
is driven by the Sun heating the land

and causing air to move around.

So we're sailing around.

Sailing around in a turbulent,
moving ocean of air.

But travel further out
into the solar system,

and the driving forces
within an atmosphere must change.

Now, there's been weather on Saturn
for over 4 billion years.

But Saturn is a long way from the Sun,

and so those weather systems are not
primarily driven by the Sun's heat.

And yet, the storms on Saturn
are amongst the most violent

found anywhere in the solar system.

With sunlight almost 100 times weaker
on Saturn than it is here on Earth...

...it means something else
is helping drive its weather...

...and the huge complex forms that
arise in its atmosphere.

By studying the cloud tops of Saturn
and its great storm systems...

...we can now infer a great deal

about the strange world
that lies beneath...

...and the energy source
that helps power this planet.

Below the upper atmosphere,

great clouds of water grow.

Lightning 10,000 times more powerful
than on Earth illuminates the sky.

This lightning transforms methane gas
into huge clouds of soot.

Deeper still, the pressures grow so great

that these chunks of soot
are likely transformed into diamonds.

But even these diamonds will succumb
to the pressures of Saturn...

...eventually liquefying.

It's not until 40,000 kilometres down

that the major source of Saturn's energy
is revealed.

Here, pressures are so intense

gasses behave like liquid metal.

Molten helium falls like rain
through hydrogen...

...and releases incredible
amounts of heat.

And we think that
this is the extraordinary heat source

that has been helping to drive
Saturn's weather.

Within just a few hundred million years
of its birth,

Saturn had lived a life of drama.

Now it remained largely unchanged
for billions of years,

although still very different
from the planet we know today.

But in time, it's great size would lead

to one more iconic transformation.

Saturn is a planet defined by its rings.

If you get a child to draw a planet,
and quite a lot of the time

they'll draw something
that looks like Saturn.

Now, close up, they are almost impossibly
delicate and intricate.

They're made up of trillions
of ice crystals,

most of them no bigger than snowflakes,

but some of them as large as houses.

Now, if you look through a telescope,
they're highly reflective.

They reflect sunlight back
just as powerfully as the planet itself.

But that raises an interesting question

because the solar system is full of dust.

Any ice crystals that hang around
for any amount of time

will get dirty,
and so they won't reflect sunlight back,

they won't be as bright.

So the question is, why, then,
are Saturn's rings so reflective?

The answer came
from one of the most audacious

and successful missions
to the outer solar system.

Five, four, three, two,

one... and liftoff
of the Cassini spacecraft

on a billion-mile trip to Saturn.

We have cleared the tower.

T plus 20 seconds,
all systems are go.

The Cassini probe was designed
to study Saturn and its moons

and rings up close,
to shed new light on their origins.

All systems continuing to be go.

But whereas the Voyager probes

had spent only days at Saturn
as they sped by,

Cassini was built
to enter Saturnian orbit...

Solid rocket boosters
have been jettisoned.

Velocity 4,378mph.

...and explore the system for years.

Many times more massive than Voyager,

Cassini had to take a different route
to enter orbit around Saturn.

To conserve fuel,
it flew by multiple planets,

slingshotting past them to pick up speed

and change direction
over the course of seven years.

As Cassini finally approached its target,

it was travelling at almost
100,000 kilometres per hour

and had to slow itself with exquisite
precision.

Flight to telecom.

Go ahead.

The Doppler has flattened out.

After its 3.5-billion-kilometre journey,

Cassini was now able to see
Saturn's rings like never before.

What's more,
during its long stay at Saturn,

Cassini would be able to take
physical samples for the very first time.

By measuring the way

that the dust from the solar system
falls onto the rings,

Cassini made a startling discovery.

If the rings had been around Saturn
for billions of years,

they should have been darkened
and dimmed by the dust.

But they are pristine and bright.

And the reason is
because they are young.

Nearly 4.5 billion years younger
than Saturn itself.

The mystery of the origin of the rings
is an active area of research.

And the evidence that Cassini delivered

hints that the solution lies
not with planet itself

but with the worlds trapped in orbit
around it - Saturn's moons.

Saturn has 62 large moons
and countless smaller ones.

We're still discovering them.

So it's like a mini solar system
in itself.

And those moons range in size
from planetary-size objects, like Titan,

to small, irregular-shaped lumps of rock.

And there's so much diversity in size
and form and surface features

that the whole system
is like a history book,

and if we learn to read it,

then we see that Saturn has had
a very violent and changeable past.

Look at this. This is Mimas,

which is often called the Death Star moon
for obvious reasons.

It has this huge impact crater,

which tells you that once, long ago,

there was a very violent impact

that almost, but not quite,
destroyed the moon.

And this is Iapetus.

It's a very large moon,

almost planetary size.

It's about 1,500 kilometres across.

And half of it is white
and half of it is dark.

Iapetus also has
another distinctive feature,

which is this ridge
all the way around the equator.

And you can see the scale of it
from this photograph.

And this is clearly visible.

It's one of the highest mountain ranges
in the solar system -

20 kilometres from peak
to the ground below.

And it's thought that this formed

because a ring
that was once around the moon -

just liked Saturn's rings -
collapsed onto its surface.

As Cassini continued its journey,

it revealed in detail multiple moons
made almost entirely of ice.

And many of them
had taken on extraordinary forms

in and around the rings.

As Cassini analysed the ice moons
in ever greater detail,

it became apparent that many of them

were made of exactly the same material
as the rings themselves.

The rings and moons
were profoundly linked.

But for all the moons that Cassini saw,

it now seemed likely
that one was missing.

When dinosaurs roamed the Earth,

we now suspect Saturn had a moon in orbit
that no longer exists.

A moon perhaps 400 kilometres across
and formed almost entirely of ice.

But this world was doomed.

It found itself orbiting too close

to resist the immense forces
of Saturn's gravity.

Gravity is the sculptor
of the Saturnian system,

but also the instigator
of change within it.

We tend to think of gravity
as a force that pulls things together,

but it can also act to rip things apart.

Think about the tides here on Earth.

They're caused by the difference
in gravitational pull of the Moon

from one side of the Earth to the other.

And that difference, which can be
quite subtle, is also powerful.

It can move entire oceans -
it's called tidal gravity.

But the Earth also has a tidal effect
on the Moon,

and because the Earth is an entire planet,
that effect is much more powerful.

The pull of the Earth is enough
to deform the Moon's surface.

The effect was particularly strong
4.5 billion years ago,

when the Moon was nearly 17 times closer.

Back then, the pull from the Earth
caused a tide of solid rock

to rise and fall.

If the Moon had ever been any nearer,

it would have crossed
what we call the Roche limit,

a place where tidal gravitational forces
are so strong

moons can get ripped apart.

The Romans named the planet Saturn
after their god of time and harvest,

and in one of the more gruesome tales
from classical mythology,

Saturn actually ate his newborn babies

in order to prevent them
from taking his power.

What's sort of interesting
is that what we've learnt from Cassini

is that, at least metaphorically speaking,
they weren't far wrong.

Just beyond Saturn's atmosphere,

our leading theories suggest

an icy moon must have approached close to,
or even just inside,

the planet's Roche limit.

As Saturn's immense
tidal gravitational forces acted

across the moon, it began to rupture.

Saturn began to devour its child.

Up to 15,000 trillion tonnes of ice
broke apart in orbit around Saturn.

Because of the speeds
the ice fragments were travelling,

it's likely that in just a few days

they spread out
to encircle the great giant.

Saturn's iconic ring had been born.

Today, the giant ring has evolved.

Saturn's powerful gravitational field

has helped keep
its near-perfect circular shape,

but collisions within
have caused it to flatten out.

Now this debris
forms a disc wider than Jupiter,

yet on average
just ten metres thick.

Within, moon-sized chunks of ice
orbit the structure,

clearing great voids...

...turning one ring...

...into many.

In places, moons have pulled
particles of ice upwards

to create strange peaks
over two kilometres high.

And in just the right conditions,

they cast spectacular shadows
across the rings.

This once-tiny world of rock and ice...

...that has undergone
so many great transformations...

...has today become
the solar system's jewel.

Cassini had deepened
our understanding of the origin

and evolution of Saturn.

But its mission was far from over.

Because just beyond the rings,
it discovered another treasure.

A place that may hold answers
to some of our deepest questions

about the possibility of life
in the solar system and beyond.

Over a billion kilometres
from the warmth of the Sun,

just on the outer edge of Saturn's rings,

lies the icy moon Enceladus.

Enceladus is quite a small moon.

It's only about the size of Iceland,
actually.

But that does not make it dull.

It is the most reflective object
in the solar system.

Over 90% of the light that hits it
bounces back,

and that's because its surface
is covered in pure ice.

It's also criss-crossed by crevasses,

like this one you see on this glacier,
but on a much grander scale.

The largest, known as the tiger stripes,
are over 100 kilometres long.

But for all its surface beauty,

it's what's going on below the ice that
makes Enceladus so special.

What we found
beneath Enceladus's shell of ice

must rank as one
of the greatest discoveries

in 21st-century space exploration.

But it was a discovery
that had been hiding in plain sight.

This is a photograph taken
by Voyager 1 of Enceladus in 1980.

Now, nobody noticed anything remarkable
about it at the time,

but this image has been enhanced,

and now you can see this sort of misty
blob off one side of the moon.

Now, that is fascinating.

It means something is rising up
from inside Enceladus.

Now, when Cassini got back there
24 years later,

it saw the same thing,
but this time in much more detail.

As Cassini approached Enceladus,

the anomaly revealed itself.

Giant plumes of water vapour and ice
were erupting from its surface.

Over 200 kilograms of material
was being released every second.

But this material was not lost to space.

It seemed to be feeding
one of Saturn's outer rings,

helping to replenish it.

And these discoveries inspired
an audacious risk.

Piloted from over
a billion kilometres away,

Cassini was guided dangerously close
to the plumes.

The craft passed within just 48 kilometres
of the surface of Enceladus...

...and Cassini was able to
touch the plumes.

What we discovered,
thanks to a series of these flybys,

was breathtaking.

The ice in the plumes was actually
frozen particles of salty water.

We had found a sub-surface ocean
leaking into space.

Far, far from the Sun,

Enceladus was harbouring an ocean
of liquid water,

and it was there because of Saturn.

Enceladus orbits Saturn
in an elliptical orbit,

which means the gravitational forces
across the moon are constantly changing.

It's maintained in this orbit by the pull
of another larger moon, Dione.

These ever-shifting gravitational forces
stretch and squeeze its heart,

warming and melting its icy interior.

But Cassini's data
had one more surprise in store.

As the plumes of water ice were analysed
in ever-greater detail,

we discovered complex organic compounds

and silica particles
that could only have come

from hot hydrothermal vents.

In the frozen outer reaches
of the solar system...

...we had found a warm, watery oasis.

Cassini has given us a glimpse
beneath the ice of Enceladus,

and it is fascinating, genuinely,
in a scientific sense,

because many biologists believe

that hydrothermal vents, like those
that are almost certainly present

on the floor of Enceladus's ocean,

were the cradle of life on Earth.

See, all the ingredients are present.

There's hot water
in touch with ice and minerals.

That's a reactive cauldron of chemistry.

There are reactive gasses, methane.

Cassini found molecular hydrogen
in the plumes,

and that was one of the food sources
of primitive organisms on Earth.

So, there really is a possibility

that there is life in orbits
around Saturn today.

The prospect of life on Enceladus
is exciting,

but if it's there,

it's likely to be only the simplest
and most primitive of organisms.

And given how violent and changeable
Saturn's past has been,

this world of ice and liquid water
may only have arisen relatively recently.

Now, we don't know how long Enceladus
has been geologically active,

how long it's had an ocean.

If it's only tens of millions -
or even hundreds of millions - of years,

that may have not have been enough time
for life to get going.

But if there is life there,
then we can glimpse its future

because there are still
hydrothermal vent systems present

on Earth today, and life doesn't just
survive there, it thrives.

Often found in the deep oceans
along fault lines in the Earth's crust,

hydrothermal vents
are some of the richest

and most complex ecosystems
on Earth.

Feeding off the abundant bacteria
that breed there,

these habitats support a multitude
of strange and complex organisms.

Whilst creatures like this
will never exist on Enceladus,

billions of years from now,

it is just possible
that this distant world may become home

to its own unique forms of life.

But for Cassini, the brave new world
under the ice of Enceladus

was a bittersweet discovery.

13 years after it arrived at Saturn,

Cassini's fuel finally began to run dry.

NASA couldn't risk letting this probe
crash-land on Enceladus

and contaminate
a potential habitat for life.

Systems, ACS One. We've just had
transition to high-rate mode.

And so the decision was made
to send Cassini on one final journey.

A journey that would take it
into the atmosphere of Saturn itself.

A journey
from which it would never return.

And we are in the atmosphere.

Radio signal still holding -
30 seconds.

We just heard the signal
from the spacecraft is gone,

and within the next 45 seconds,

so will be the spacecraft.

The planet that had borne witness

to some of the greatest dramas
in the history of the solar system

was now consuming the craft
that had told its extraordinary story.

Cassini and Saturn had now become
inseparable companions.

We journeyed out to Saturn to experience
and explore this great beauty.

But in the end, we were rewarded
with insights closer to home.

Cassini and her team of scientists
and engineers

should be counted
amongst our greatest of explorers.

The mission had all the ingredients -
a vessel and her crew

drawn by the prospect of beauty
and new knowledge

across an un-charted ocean
to an alien world,

and as is so often the case
in exploration,

the real treasure was found in the
unexpected shadows,

in this case, in the shadow of the rings,

where we found this tiny world, Enceladus,

that may harbour life and provide us
with re-assurance that we're not alone

in the universe.

See? The beauty is in the story.

Would it matter, really,
if we found life on Enceladus?

It would only be microbes after all.

Yes, it would matter.

We are at a time in our history, I think,

where we need re-assurance,
or perhaps a reminder,

that there is beauty and knowledge
and perhaps even life and meaning

beyond our shores.

We will, I'm sure,
go back to Saturn and its moons,

explore deeper, stay longer,

and maybe one day even visit ourselves.

We don't know what we'll discover,

but we can be sure that
the story has only just begun.

Cassini has been one
of the most successful probes ever flown.

But constructing a machine
that could survive 13 years

in the outer solar system was a huge
challenge for everyone involved.

T plus 20 seconds.

Even when they were proposing Cassini,
I thought,

"No, no, this is never going to happen!"

It was, like, maybe too ambitious,
or something.

It's difficult to put a number on how many
people were involved in Cassini.

Um, tens of thousands, I would say.

This is a very, very complex effort.

There's over ten miles of wiring,

and you have to check out every wire.

So it took us two-and-a-half years
to put the thing together.

All of us spent a lot of effort
to get to that moment,

so there's a lot of emotion
attached to it.

And then you see it launch,
and you know that it's on a journey

to a very, very distant place.

And so you feel like you're going, too.

When Cassini first began to
send close-up pictures back to Earth...

...it was a special moment
for the whole team.

I was sitting in what they call
the blue room,

being interviewed while they were beaming
for the first time,

and then one of these pictures came on,
and I just went, "Wow, look at that!"

That was really the
jumping-out-of-the-chair moment for me.

And it was just the first day.

What we were seeing
was the highest resolution

that had ever been seen.

I saw phenomena that I'd only seen
in kind of computer simulations before,

and now I was seeing it for the first time

and realising all this was real.

And we were seeing
details in the rings

that were shocking. Shocking.

We just lacked the imagination
that it would require

to predict what it would look like.

For me, when that great backlit picture
came out,

that's when it hit, for me.

"Oh, my God, we've done something
really unique and wonderful."

It was like, "This is going to be...
This is going to be huge."

Over Cassini's 13 years at Saturn,

it explored
the planet's incredible storms...

...the formation of its rings...

...and discovered the oceans of Enceladus.

But Cassini's greatest insights
may be yet to come.

There's such a wealth of information
in the data.

I think we've just skimmed
the top of the data.

I think people will continue
to make discoveries from Cassini data

for another 30 to 40 years.

The data Cassini collected
in its final moments

is already changing
our understanding of the planet.

It has revealed
the rings are disappearing.

As much as 10,000 kilograms of material
is falling into the planet each second.

It now seems likely that
just 100 million years from now,

Saturn's great rings will be no more.

Cassini has shown us
that we have been uniquely fortunate

to behold this great beauty.