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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.