Lift off of Messenger on Nasa's mission to Mercury.
MUSIC: The Void by Muse
# They'll say no-one can see us
# That we're estranged and all alone
# They believe nothing can reach us
# And pull us out of
# The boundless gloom
# They're wrong
# They're wrong. #
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!
CHEERING
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.
INDISTINCT RADIO COMMUNICATION
CHEERING
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 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,
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.
WAVES ROAR
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.
WAVES ROAR
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.
WATER GUSHES
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.
WATER ROARS
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.
OVER RADIO: 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.
GROUND CONTROL: 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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