All language subtitles for The.Planets.2019.S01E04.Saturn.1080i.HDTV.DD2.0.H.264-CtrlHD

Lift off 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 300,000km-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!

HE LAUGHS 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,

although still very different
from the planet we know today.

But in time, this great size

would lead to one more iconic
transformation.

Saturn is a planet
defined by its rings.

You get a child to draw a planet
and quite

a lot of the time, they'll draw
something that looks like Saturn.

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 trek 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
spent only days at Saturn,

as they sped by, Cassini was built
to enter Saturnian orbit...

Solid rocket boosters have
been jettisoned.

Velocity 4,378 miles per hour.

.. 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.

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 have 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 the planet itself,

but with the worlds
tracked 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 lapetus.

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,

20km from peak to the ground below.

And it's thought that this formed
because a ring that was once

around the Moon, just like 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 400km 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 of 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 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 learned from Cassini

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

Just beyond Saturn's atmosphere,
our leading theory suggests

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 2km 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 crisscrossed by crevasses

like this one you see
in this glacier,

but on a much grander scale.

The largest,
known as the tiger stripes,

are over 100km 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 200kg of material
was being released every second.

But this material
was not lost to space.

It seemed to be feeding one
of Saturn's outside 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
48km 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 subsurface 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.

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 gases - 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 orbit 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 its had an ocean.

If it was only tens of millions or
even hundreds of millions of years,

that may 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.

Just heard the
signal from the spacecraft is gone

and that, 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 a crew drawn
by the prospects of beauty

and new knowledge across an
uncharted 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 reassurance

that we are 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 reassurance

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.

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