The Planets

Lift off of Messenger on Nasa's mission to Mercury.

The Void by Muse

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 300,000km-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.

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 had 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.

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 ten or even 20 times the mass of the Earth and 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 100km 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've 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, isn't it?

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 envelope 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.

Under 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-size worlds.

But this vast, wild protoplanet 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 pounds 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 a billion and a quarter 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 gases 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 so great!

I was afraid that you'd just get 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 Sun.

It heats the ground up down there and that, in turn,

heats the air up close to the ground, makes it expand,

and 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.

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 four 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 40,000km down

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

Here, pressures are so intense, gases 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,