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

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MUSIC: The Void
by Muse

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# They'll say no-one can see us

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# That we're estranged and all alone

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# They believe nothing can reach us

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# And pull us out of

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# The boundless gloom

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# They're wrong

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# They're wrong. #

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

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..beyond the gas giant Jupiter...

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

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..lies Saturn...

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..a planet made unique, thanks to
a nearly 300,000km-wide ring

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of frozen water.

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Here, trillions of pieces of ice
have been sculpted by gravitational

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

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Here on Earth, water takes
many forms - oceans and clouds

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and rivers, but it's
in its crystalline form, ice,

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that it's at its most beautiful.

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Here, ice just adorns the landscape,
but in the outer solar system,

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where Saturn lives,
ice is so abundant that, well,

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it's become a building material.

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

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the ice rings of Saturn.

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

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over a gravitational field,
so we could see it, but how could

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something so intricate be sculpted
out of something so simple?

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Well, as is always
the case in science,

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the true beauty lies in the story.

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Saturn is a planet that has
undergone some of the most

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radical transformations
in the solar system.

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A planet that has
seen destruction...

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..and creation.

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A planet that harbours
hidden worlds...

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..where, just perhaps,

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there may exist a second home

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for life.

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Imagine a place with
no surface to stand.

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Just an endless atmosphere.

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Compared to Earth,
Saturn is so alien

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that it's hard to imagine
how it could have grown

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from the same ingredients.

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But go far enough back in time

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and Saturn would appear to be
surprisingly familiar.

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

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..tumbling chaotically
through space.

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Just like Earth, Mars,
Venus and Mercury,

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they began to merge
and clump together and grow.

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But unlike the planets of the
inner solar system, Saturn was

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forming on the other side of a
boundary we call the Snow Line...

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

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water exists only in its frozen,
solid state.

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

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providing huge amounts of extra
planetary building material.

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Under the force of gravity,
this abundant ice and rock collided

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and combined...

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

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The details of how the planets
formed in the solar system is

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not fully understood. It's
cutting-edge scientific research.

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We're talking about events that
happened over 4.5 billion years ago,

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

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perhaps ten or even 20 times the
mass of the Earth, and it's almost

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possible, in your dreams,
to imagine standing on its surface.

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But because of its immense mass
and its place

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in the solar system, Saturn didn't
remain a rocky, icy world for long.

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

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In the region in which the young
world was forming, there was

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

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And Saturn began to gain
an atmosphere...

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

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

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you appreciate
just how thin it really is.

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And it's a similar
story for the other

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worlds of the inner solar system.

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Despite being relatively thin,
these atmospheres can exert

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powerful forces on the planetary
surfaces beneath.

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On Earth, you can see the effects
of an atmosphere with nothing

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more than a sealed container.

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

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

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

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

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We've filled it with air from
the top of the mountain, which is

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about 3,000 metres.

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

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than it is down there at sea level.

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So, as we descend, the atmospheric
pressure outside is increasing

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and the pressure in
here is just what it

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was at the top of the mountain,
and so you see visually that

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force in action
and that is squashing the bottle.

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Physics in action!

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It's brilliant, isn't it?

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

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..if an atmosphere gets big enough,
it can transform an entire planet.

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

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

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And now it turned to another
building material -

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

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This gas would have been
too light for the smaller

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worlds of the inner solar system
to hold on to.

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But Saturn's great mass created
gravitational forces powerful

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enough to draw it in.

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Trillions upon
trillions of tonnes of hydrogen

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and helium gas began to
envelope the planet.

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And as this new atmosphere grew,

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it transformed the surface below.

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Enormous pressures, created by the
weight of this gas, heated rock

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and ice so much, they began to glow.

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As Saturn matured, the pressure
at the surface rose

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to ten million times the
atmospheric pressure on Earth.

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Under those kind of pressures,

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matter behaves in extremely
strange ways.

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The very idea of a surface becomes
meaningless, and Saturn was

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transformed from an icy,

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rocky world into a completely
different class of planet -

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a gas giant.

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Saturn was now so big,

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it could contain
nearly 5,000 Earth-size worlds.

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But this vast, wild protoplanet
was totally unlike the Saturn

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we see today.

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To discover how that world emerged,

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we had to send spacecraft
to meet it close up.

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We have ignition and we have liftoff
of the Titan Centaur carrying

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the first of two Voyager spacecraft
to extend Man's senses

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farther into the solar system
than ever before.

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Reports coming back indicate those
twin large solid motors are

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functioning perfectly, producing
1.2 million pounds of thrust each.

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

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It's there!

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CHEERING

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Voyager 2 has reached Saturn
after four years in space.

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

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after a flight of a billion
and a quarter miles.

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

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revealing the planet
in exquisite resolution.

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There's just an amazing
harvest of pictures.

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

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

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For the very first time,

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

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Saturn itself is
composed of hydrogen and helium.

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And we are seeing an exotic
meteorology here.

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Voyager showed, beyond doubt,
that Saturn's upper atmosphere was

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made almost entirely of helium
and hydrogen...

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

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But this once chaotic gas
was now organised

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into intricate weather systems.

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

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A huge hexagonal
structure in the clouds,

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so big it could fit our
entire planet within it,

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nearly four times over.

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The atmosphere of Saturn was
revealing itself to be stranger

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and more dynamic than
we could ever have imagined.

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Atmospheres are some
of the most spectacular

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and complex planetary environments.

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Not many people see the Alps
like this! Seriously! It's so great!

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I was afraid that you'd just get too
sick or something. It's awesome!

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Man! It's fun, man!

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HE LAUGHS
This IS fun!

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

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But what powers them
and the weather systems within

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depends on where
you are in the solar system.

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

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

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

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

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That means you get thermals,
so energetic

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movements of air from the ground
into the upper atmosphere.

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And that's what we're
exploiting now, to glide.

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All this beautiful weather is
driven by the Sun heating the land

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and causing air to move around.

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So we're sailing around...

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Sailing around in a turbulent,

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moving ocean of air.

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But travel further
out into the solar system

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and the driving forces within
an atmosphere must change.

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

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but Saturn is a long
way from the Sun, and so

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

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And yet the storms on Saturn
are amongst the most violent

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found anywhere in the solar system.

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With sunlight almost 100 times
weaker on Saturn than it is

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here on Earth...

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

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

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

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we can now infer a great deal about
the strange world that lies beneath.

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And the energy source that helps
power this planet.

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Below the upper atmosphere,

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great clouds of water grow.

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

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This lightning transforms methane
gas into huge clouds of soot.

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Deeper still,

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the pressures grow
so great that these

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chunks of soot are likely
transformed into diamonds.

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

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eventually liquefying.

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It's not until 40,000km down

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

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Here, pressures are so intense,
gases behave like liquid metal.

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Molten helium falls
like rain through hydrogen...

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..and releases incredible
amounts of heat.

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And we think that this is
the extraordinary heat source

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that has been helping to drive
Saturn's weather.

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Within just a few hundred million
years of its birth,

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Saturn had lived a life of drama.

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Now, it remained largely
unchanged for billions of years,

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although still very different
from the planet we know today.

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But in time, this great size

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would lead to one more iconic
transformation.

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Saturn is a planet
defined by its rings.

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00:23:37,240 --> 00:23:40,560
You get a child to draw a planet
and quite

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

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00:23:43,800 --> 00:23:48,280
Close up, they are almost impossibly
delicate and intricate.

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They're made up of trillions of ice
crystals, most of them

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00:23:52,520 --> 00:23:56,400
no bigger than snowflakes,
but some of them as large as houses.

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00:23:56,400 --> 00:23:58,640
Now,
if you look through a telescope,

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they're highly reflective.

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

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But that raises
an interesting question,

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because the solar system
is full of dust.

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Any ice crystals that hang around

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for any amount of time
will get dirty,

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and so they won't reflect sunlight
back, they won't be as bright.

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

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The answer came from one
of the most audacious

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and successful missions
to the outer solar system.

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Five, four, three, two, one, and
liftoff of the Cassini spacecraft

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on a billion-mile trek to Saturn.

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We have cleared the tower. T plus
20 seconds. All systems are go.

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00:24:52,880 --> 00:24:56,600
The Cassini probe was designed
to study Saturn and its moons

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00:24:56,600 --> 00:25:00,400
and rings up close,
to shed new light on their origins.

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All systems continuing to be go.

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00:25:03,640 --> 00:25:06,920
But whereas the Voyager probes
spent only days at Saturn,

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

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Solid rocket boosters have
been jettisoned.

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Velocity 4,378 miles per hour.

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00:25:18,000 --> 00:25:20,160
..and explore the system for years.

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00:25:27,040 --> 00:25:30,560
Many times more massive than
Voyager, Cassini had to take

241
00:25:30,560 --> 00:25:33,520
a different route to enter
orbit around Saturn.

242
00:25:37,200 --> 00:25:40,400
To conserve fuel,
it flew by multiple planets,

243
00:25:40,400 --> 00:25:43,640
slingshotting past them
to pick up speed

244
00:25:43,640 --> 00:25:47,120
and change direction over
the course of seven years.

245
00:25:53,920 --> 00:25:56,560
As Cassini finally
approached its target,

246
00:25:56,560 --> 00:26:00,680
it was travelling at almost
100,000 kilometres per hour

247
00:26:00,680 --> 00:26:04,200
and had to slow itself
with exquisite precision.

248
00:26:19,000 --> 00:26:21,680
INDISTINCT RADIO COMMUNICATION

249
00:26:23,600 --> 00:26:26,320
CHEERING

250
00:26:38,280 --> 00:26:41,760
After its 3.5 billion-kilometre
journey,

251
00:26:41,760 --> 00:26:45,240
Cassini was now able to see
Saturn's rings like never before.

252
00:26:57,840 --> 00:27:02,320
What's more, during its long stay
at Saturn, Cassini would be able

253
00:27:02,320 --> 00:27:05,520
to take physical
samples for the very first time.

254
00:27:24,960 --> 00:27:28,280
By measuring the way that the dust
from the solar system falls

255
00:27:28,280 --> 00:27:32,040
onto the rings, Cassini
made a startling discovery.

256
00:27:39,040 --> 00:27:42,760
If the rings have been around Saturn
for billions of years,

257
00:27:42,760 --> 00:27:46,560
they should have been darkened
and dimmed by the dust.

258
00:27:48,400 --> 00:27:50,760
But they are pristine and bright.

259
00:27:55,320 --> 00:27:58,560
And the reason is
because they are young.

260
00:28:01,280 --> 00:28:06,200
Nearly 4.5 billion years younger
than Saturn itself.

261
00:28:15,840 --> 00:28:19,880
The mystery of the origin of the
rings is an active area of research.

262
00:28:21,520 --> 00:28:24,960
And the evidence that Cassini
delivered hints that the solution

263
00:28:24,960 --> 00:28:28,000
lies not with the planet itself,

264
00:28:28,000 --> 00:28:31,840
but with the worlds
tracked in orbit around it -

265
00:28:31,840 --> 00:28:33,680
Saturn's moons.

266
00:28:38,760 --> 00:28:41,920
Saturn has 62 large moons
and countless smaller ones -

267
00:28:41,920 --> 00:28:43,840
we're still discovering them.

268
00:28:43,840 --> 00:28:47,200
So it's like a
mini solar system in itself.

269
00:28:47,200 --> 00:28:50,360
And those moons range in size
from planetary-size objects,

270
00:28:50,360 --> 00:28:54,200
like Titan, to small
irregular-shaped lumps of rock.

271
00:28:54,200 --> 00:28:57,680
And there's so much diversity
in size and form

272
00:28:57,680 --> 00:29:01,760
and surface features, that the whole
system is like a history book.

273
00:29:01,760 --> 00:29:05,200
And if we learn to read it
then we see that Saturn

274
00:29:05,200 --> 00:29:09,240
has had a very violent
and changeable past.

275
00:29:09,240 --> 00:29:12,920
Look at this, this is Mimas, which
is often called the Death Star moon

276
00:29:12,920 --> 00:29:14,720
for obvious reasons.

277
00:29:14,720 --> 00:29:20,200
It has this huge impact crater,
which tells you that once, long ago,

278
00:29:20,200 --> 00:29:23,040
there was a very violent impact

279
00:29:23,040 --> 00:29:26,080
that almost - but not quite -
destroyed the Moon.

280
00:29:27,560 --> 00:29:30,000
And this is Iapetus.

281
00:29:30,000 --> 00:29:32,960
It's a very large moon,
almost planetary-size.

282
00:29:32,960 --> 00:29:35,720
It's about 1,500 kilometres across.

283
00:29:35,720 --> 00:29:39,480
And half of it is white,
and half of it is dark.

284
00:29:42,120 --> 00:29:45,120
Iapetus also has another
distinctive feature

285
00:29:45,120 --> 00:29:48,440
which is this ridge all the way
around the equator, and you can see

286
00:29:48,440 --> 00:29:50,680
the scale of it
from this photograph.

287
00:29:50,680 --> 00:29:52,520
And this is clearly visible.

288
00:29:52,520 --> 00:29:55,960
It's one of the highest mountain
ranges in the solar system,

289
00:29:55,960 --> 00:29:59,640
20km from peak to the ground below.

290
00:30:05,800 --> 00:30:11,000
And it's thought that this formed
because a ring that was once

291
00:30:11,000 --> 00:30:15,760
around the Moon, just like Saturn's
rings, collapsed onto its surface.

292
00:30:23,440 --> 00:30:28,880
As Cassini continued its journey,
it revealed in detail multiple moons

293
00:30:28,880 --> 00:30:31,120
made almost entirely of ice.

294
00:30:33,680 --> 00:30:37,080
And many of them had taken
on extraordinary forms

295
00:30:37,080 --> 00:30:39,440
in and around the rings.

296
00:31:01,160 --> 00:31:05,320
As Cassini analysed the ice moons
in ever greater detail,

297
00:31:05,320 --> 00:31:09,280
it became apparent that many of them
were made of exactly

298
00:31:09,280 --> 00:31:12,320
the same material
as the rings themselves.

299
00:31:15,440 --> 00:31:18,960
The rings and moons
were profoundly linked.

300
00:31:23,160 --> 00:31:25,800
But for all the
moons that Cassini saw,

301
00:31:25,800 --> 00:31:29,600
it now seemed likely
that one was missing.

302
00:31:49,480 --> 00:31:51,720
When dinosaurs roamed the Earth,

303
00:31:51,720 --> 00:31:56,640
we now suspect Saturn had a moon
in orbit that no longer exists.

304
00:32:00,880 --> 00:32:04,080
A moon perhaps 400km across

305
00:32:04,080 --> 00:32:06,760
and formed almost entirely of ice.

306
00:32:10,760 --> 00:32:12,720
But this world was doomed.

307
00:32:15,920 --> 00:32:20,200
It found itself orbiting too close
to resist the immense forces

308
00:32:20,200 --> 00:32:22,200
of Saturn's gravity.

309
00:32:29,520 --> 00:32:32,040
WAVES ROAR

310
00:32:34,760 --> 00:32:38,680
Gravity is the sculptor
of the Saturnian system

311
00:32:38,680 --> 00:32:42,120
but also the instigator
of change within it.

312
00:32:42,120 --> 00:32:46,600
We tend to think of gravity as a
force that pulls things together,

313
00:32:46,600 --> 00:32:50,240
but it can also act
to rip things apart.

314
00:32:50,240 --> 00:32:53,720
Think of the tides here on Earth,
they're caused by the difference

315
00:32:53,720 --> 00:32:57,560
in gravitational pull of the Moon
from one side of the Earth

316
00:32:57,560 --> 00:32:59,320
to the other.

317
00:32:59,320 --> 00:33:03,040
And that difference, which can be
quite subtle, is also powerful,

318
00:33:03,040 --> 00:33:05,080
it can move entire oceans.

319
00:33:05,080 --> 00:33:10,000
It's called tidal gravity, but
the Earth also has a tidal effect

320
00:33:10,000 --> 00:33:13,400
on the Moon, and because
the Earth is an entire planet

321
00:33:13,400 --> 00:33:16,440
that effect is much more powerful.

322
00:33:20,920 --> 00:33:24,560
The pull of the Earth is enough
to deform the Moon's surface.

323
00:33:26,400 --> 00:33:30,640
The effect was particularly strong
4.5 billion years ago

324
00:33:30,640 --> 00:33:33,800
when the Moon was
nearly 17 times closer.

325
00:33:36,160 --> 00:33:39,920
Back then, the pull from the Earth
caused a tide of solid rock

326
00:33:39,920 --> 00:33:41,920
to rise and fall.

327
00:33:46,720 --> 00:33:50,360
If the Moon had been any nearer,
it would have crossed what we call

328
00:33:50,360 --> 00:33:55,480
the Roche limit, a place where tidal
gravitational forces are so strong,

329
00:33:55,480 --> 00:33:58,200
moons can get ripped apart.

330
00:34:01,480 --> 00:34:03,600
WAVES ROAR

331
00:34:07,640 --> 00:34:11,920
The Romans named the planet Saturn
after their god of time and harvest.

332
00:34:11,920 --> 00:34:15,760
And in one of the more gruesome
tales from classical mythology,

333
00:34:15,760 --> 00:34:20,240
Saturn actually ate his newborn
babies in order to prevent them

334
00:34:20,240 --> 00:34:22,080
from taking his power.

335
00:34:22,080 --> 00:34:25,720
What's sort of interesting is,
that what we've learned from Cassini

336
00:34:25,720 --> 00:34:30,040
is that, at least metaphorically
speaking, they weren't far wrong.

337
00:34:43,400 --> 00:34:47,520
Just beyond Saturn's atmosphere,
our leading theory suggests

338
00:34:47,520 --> 00:34:50,880
an icy moon must
have approached close to

339
00:34:50,880 --> 00:34:54,560
or even just inside
the planet's Roche limit.

340
00:34:58,680 --> 00:35:01,680
As Saturn's immense tidal
gravitational forces

341
00:35:01,680 --> 00:35:05,360
acted across the Moon,
it began to rupture.

342
00:35:09,960 --> 00:35:13,240
Saturn began to devour its child.

343
00:36:02,400 --> 00:36:05,680
Up to 15,000 trillion tonnes of ice

344
00:36:05,680 --> 00:36:09,200
broke apart in orbit around Saturn.

345
00:36:21,880 --> 00:36:25,360
Because of the speeds the ice
fragments were travelling,

346
00:36:25,360 --> 00:36:28,800
it's likely that in just a few days
they spread out

347
00:36:28,800 --> 00:36:30,840
to encircle the great giant.

348
00:36:37,120 --> 00:36:40,800
Saturn's iconic ring had been born.

349
00:37:01,280 --> 00:37:03,720
Today, the giant ring has evolved.

350
00:37:06,000 --> 00:37:09,520
Saturn's powerful gravitational
field has helped keep

351
00:37:09,520 --> 00:37:11,880
its near perfect circular shape.

352
00:37:13,080 --> 00:37:16,560
But collisions within
have caused it to flatten out.

353
00:37:21,320 --> 00:37:24,880
Now this debris forms
a disc wider than Jupiter,

354
00:37:24,880 --> 00:37:28,360
yet, on average,
just ten metres thick.

355
00:37:33,800 --> 00:37:37,880
Within, moon-sized chunks
of ice orbit the structure

356
00:37:37,880 --> 00:37:40,720
clearing great voids.

357
00:37:42,400 --> 00:37:44,360
Turning one ring...

358
00:37:45,800 --> 00:37:47,600
..into many.

359
00:37:53,120 --> 00:37:57,800
In places, moons have pulled
particles of ice upwards

360
00:37:57,800 --> 00:38:01,640
to create strange peaks
over 2km high.

361
00:38:04,280 --> 00:38:06,520
And in just the right conditions,

362
00:38:06,520 --> 00:38:09,880
they cast spectacular shadows
across the rings.

363
00:38:18,080 --> 00:38:21,360
This once tiny world
of rock and ice...

364
00:38:23,800 --> 00:38:27,000
..that has undergone so many
great transformations...

365
00:38:33,600 --> 00:38:37,000
..has today become
the solar system's jewel.

366
00:38:50,800 --> 00:38:54,440
Cassini had deepened our
understanding of the origin

367
00:38:54,440 --> 00:38:56,280
and evolution of Saturn.

368
00:38:57,360 --> 00:39:00,080
But its mission was far from over.

369
00:39:00,080 --> 00:39:04,400
Because just beyond the rings
it discovered another treasure.

370
00:39:06,040 --> 00:39:10,480
A place that may hold answers
to some of our deepest questions

371
00:39:10,480 --> 00:39:14,640
about the possibility of life
in the solar system and beyond.

372
00:39:35,040 --> 00:39:39,280
Over a billion kilometres
from the warmth of the Sun,

373
00:39:39,280 --> 00:39:42,600
just on the outer edge
of Saturn's rings,

374
00:39:42,600 --> 00:39:45,040
lies the icy moon Enceladus.

375
00:39:48,280 --> 00:39:50,120
Enceladus is quite a small moon,

376
00:39:50,120 --> 00:39:53,160
it's only about the size
of Iceland actually.

377
00:39:53,160 --> 00:39:55,960
But that does not make it dull.

378
00:39:55,960 --> 00:39:59,480
It is the most reflective
object in the solar system -

379
00:39:59,480 --> 00:40:03,720
over 90% of the light
that hits it bounces back,

380
00:40:03,720 --> 00:40:07,760
and that's because its surface
is covered in pure ice.

381
00:40:07,760 --> 00:40:10,760
It's also crisscrossed by crevasses

382
00:40:10,760 --> 00:40:13,320
like this one you see
in this glacier,

383
00:40:13,320 --> 00:40:15,680
but on a much grander scale.

384
00:40:15,680 --> 00:40:18,480
The largest,
known as the tiger stripes,

385
00:40:18,480 --> 00:40:20,240
are over 100km long.

386
00:40:20,240 --> 00:40:22,840
But for all its surface beauty,

387
00:40:22,840 --> 00:40:25,480
it's what's going on below the ice

388
00:40:25,480 --> 00:40:27,920
that makes Enceladus so special.

389
00:40:33,960 --> 00:40:37,840
What we found beneath Enceladus's
shell of ice must rank

390
00:40:37,840 --> 00:40:41,760
as one of the greatest discoveries
in 21st-century space exploration.

391
00:40:43,720 --> 00:40:47,480
But it was a discovery
that had been hiding in plain sight.

392
00:40:58,160 --> 00:41:02,520
This is a photograph taken
by Voyager 1 of Enceladus in 1980.

393
00:41:02,520 --> 00:41:06,360
Now, nobody noticed anything
remarkable about it at the time

394
00:41:06,360 --> 00:41:10,520
but this image has been enhanced
and now you can see

395
00:41:10,520 --> 00:41:14,360
this sort of misty blob
off one side of the Moon.

396
00:41:14,360 --> 00:41:17,000
Now, that is fascinating.

397
00:41:17,000 --> 00:41:21,720
It means something is rising
up from inside Enceladus.

398
00:41:21,720 --> 00:41:26,720
Now, when Cassini got back there 24
years later it saw the same thing,

399
00:41:26,720 --> 00:41:29,600
but this time in much more detail.

400
00:41:33,200 --> 00:41:37,800
As Cassini approached Enceladus,
the anomaly revealed itself.

401
00:41:41,320 --> 00:41:46,280
Giant plumes of water vapour and
ice were erupting from its surface.

402
00:41:50,160 --> 00:41:55,080
Over 200kg of material
was being released every second.

403
00:42:00,040 --> 00:42:03,120
But this material
was not lost to space.

404
00:42:03,120 --> 00:42:06,600
It seemed to be feeding one
of Saturn's outside rings,

405
00:42:06,600 --> 00:42:08,800
helping to replenish it.

406
00:42:16,240 --> 00:42:20,000
And these discoveries
inspired an audacious risk.

407
00:42:22,480 --> 00:42:26,000
Piloted from over a
billion kilometres away,

408
00:42:26,000 --> 00:42:29,760
Cassini was guided
dangerously close to the plumes.

409
00:42:46,240 --> 00:42:50,640
The craft passed within just
48km of the surface

410
00:42:50,640 --> 00:42:52,240
of Enceladus.

411
00:42:55,160 --> 00:42:58,200
And Cassini was able
to touch the plumes.

412
00:43:08,720 --> 00:43:12,000
What we discovered,
thanks to a series of these flybys,

413
00:43:12,000 --> 00:43:13,920
was breathtaking.

414
00:43:15,600 --> 00:43:20,560
The ice in the plumes was actually
frozen particles of salty water.

415
00:43:22,600 --> 00:43:26,640
We had found a subsurface ocean
leaking into space.

416
00:43:31,360 --> 00:43:33,840
Far, far from the Sun,

417
00:43:33,840 --> 00:43:37,920
Enceladus was harbouring
an ocean of liquid water.

418
00:43:39,240 --> 00:43:41,720
And it was there because of Saturn.

419
00:43:48,720 --> 00:43:52,400
Enceladus orbits Saturn
in an elliptical orbit,

420
00:43:52,400 --> 00:43:55,320
which means the gravitational forces
across the Moon

421
00:43:55,320 --> 00:43:57,320
are constantly changing.

422
00:44:00,160 --> 00:44:04,000
It's maintained in this orbit by
the pull of another larger moon -

423
00:44:04,000 --> 00:44:05,440
Dione.

424
00:44:07,520 --> 00:44:11,720
These ever-shifting gravitational
forces stretch and squeeze

425
00:44:11,720 --> 00:44:15,800
its heart, warming and melting
its icy interior.

426
00:44:22,600 --> 00:44:26,360
But Cassini's data had
one more surprise in store.

427
00:44:29,800 --> 00:44:33,880
As the plumes of water ice were
analysed in ever greater detail,

428
00:44:33,880 --> 00:44:37,640
we discovered complex
organic compounds

429
00:44:37,640 --> 00:44:40,760
and silica particles
that could only have come

430
00:44:40,760 --> 00:44:43,600
from hot hydrothermal vents.

431
00:44:48,720 --> 00:44:52,960
In the frozen outer reaches
of the solar system

432
00:44:52,960 --> 00:44:56,640
we had found a warm watery oasis.

433
00:44:58,880 --> 00:45:01,120
WATER GUSHES

434
00:45:06,560 --> 00:45:11,040
Cassini has given us a glimpse
beneath the ice of Enceladus,

435
00:45:11,040 --> 00:45:16,520
and it is fascinating genuinely
in a scientific sense because many

436
00:45:16,520 --> 00:45:20,920
biologists believe that hydrothermal
vents like those that are almost

437
00:45:20,920 --> 00:45:24,040
certainly present on the floor
of Enceladus's ocean

438
00:45:24,040 --> 00:45:26,680
were the cradle of life on Earth.

439
00:45:26,680 --> 00:45:28,920
All the ingredients are present -

440
00:45:28,920 --> 00:45:32,160
there's hot water in touch
with ice and minerals.

441
00:45:32,160 --> 00:45:35,400
That's a reactive cauldron
of chemistry.

442
00:45:35,400 --> 00:45:37,880
There are reactive gases - methane.

443
00:45:37,880 --> 00:45:41,520
Cassini found molecular hydrogen
in the plumes,

444
00:45:41,520 --> 00:45:46,400
and that was one of the food sources
of primitive organisms on Earth.

445
00:45:46,400 --> 00:45:50,240
So there really is a possibility
that there is life

446
00:45:50,240 --> 00:45:53,280
in orbit around Saturn today.

447
00:46:01,600 --> 00:46:05,240
The prospect of life on Enceladus
is exciting.

448
00:46:06,920 --> 00:46:10,160
But if it's there, it's likely
to be only the simplest

449
00:46:10,160 --> 00:46:12,680
and most primitive of organisms.

450
00:46:12,680 --> 00:46:14,640
WATER ROARS

451
00:46:16,400 --> 00:46:21,320
And given how violent and changeable
Saturn's past has been,

452
00:46:21,320 --> 00:46:24,360
this world of ice and liquid water
may only have arisen

453
00:46:24,360 --> 00:46:26,600
relatively recently.

454
00:46:29,760 --> 00:46:31,680
Now, we don't know how long

455
00:46:31,680 --> 00:46:36,160
Enceladus has been geologically
active, how long its had an ocean.

456
00:46:36,160 --> 00:46:40,240
If it was only tens of millions or
even hundreds of millions of years,

457
00:46:40,240 --> 00:46:44,440
that may not have been enough
time for life to get going.

458
00:46:44,440 --> 00:46:49,320
But if there is life there
then we can glimpse its future

459
00:46:49,320 --> 00:46:54,440
because there are still hydrothermal
vent systems present on Earth today

460
00:46:54,440 --> 00:46:58,680
and life doesn't just
survive there, it thrives.

461
00:47:17,320 --> 00:47:22,280
Often found in the deep oceans along
fault lines in the Earth's crust,

462
00:47:22,280 --> 00:47:25,920
hydrothermal vents are some
of the richest and most complex

463
00:47:25,920 --> 00:47:28,160
ecosystems on Earth.

464
00:47:29,920 --> 00:47:33,360
Feeding off the abundant bacteria
that breed there,

465
00:47:33,360 --> 00:47:38,120
these habitats support a multitude
of strange and complex organisms.

466
00:47:49,920 --> 00:47:54,000
Whilst creatures like this will
never exist on Enceladus,

467
00:47:54,000 --> 00:47:58,960
billions of years from now,
it is just possible that this

468
00:47:58,960 --> 00:48:03,200
distant world may become home
to its own unique forms of life.

469
00:48:14,280 --> 00:48:18,600
But for Cassini, the brave new world
under the ice of Enceladus

470
00:48:18,600 --> 00:48:21,160
was a bittersweet discovery.

471
00:48:35,040 --> 00:48:38,640
13 years after it arrived at Saturn,

472
00:48:38,640 --> 00:48:42,080
Cassini's fuel
finally began to run dry.

473
00:48:44,040 --> 00:48:48,440
NASA couldn't risk letting
this probe crash land on Enceladus

474
00:48:48,440 --> 00:48:52,240
and contaminate
a potential habitat for life.

475
00:48:52,240 --> 00:48:53,880
OVER RADIO: Systems, ACS One.

476
00:48:53,880 --> 00:48:56,120
We've just had transition
to high rate mode.

477
00:48:59,160 --> 00:49:03,640
And so the decision was made to send
Cassini on one final journey.

478
00:49:07,080 --> 00:49:08,920
A journey that would take it

479
00:49:08,920 --> 00:49:11,440
into the atmosphere
of Saturn itself.

480
00:49:12,960 --> 00:49:16,360
A journey from which
it would never return.

481
00:49:21,760 --> 00:49:25,160
GROUND CONTROL: And we
are in the atmosphere.

482
00:49:25,160 --> 00:49:27,200
Radio signal still holding.

483
00:49:27,200 --> 00:49:28,600
30 seconds.

484
00:49:49,520 --> 00:49:53,200
Just heard the
signal from the spacecraft is gone

485
00:49:53,200 --> 00:49:57,320
and that, within the next 45
seconds, so will be the spacecraft.

486
00:50:01,080 --> 00:50:04,440
The planet that had borne witness
to some of the greatest dramas

487
00:50:04,440 --> 00:50:08,600
in the history of the solar system
was now consuming the craft

488
00:50:08,600 --> 00:50:11,600
that had told its
extraordinary story.

489
00:50:24,280 --> 00:50:28,960
Cassini and Saturn had now become
inseparable companions.

490
00:50:52,000 --> 00:50:55,480
We journeyed out to Saturn
to experience and explore

491
00:50:55,480 --> 00:50:57,160
this great beauty.

492
00:51:03,480 --> 00:51:07,960
But in the end we were rewarded
with insights closer to home.

493
00:51:16,760 --> 00:51:19,520
Cassini and her team of scientists
and engineers

494
00:51:19,520 --> 00:51:22,520
should be counted amongst
our greatest of explorers.

495
00:51:22,520 --> 00:51:24,520
The mission had all
the ingredients -

496
00:51:24,520 --> 00:51:28,040
a vessel and a crew drawn
by the prospects of beauty

497
00:51:28,040 --> 00:51:32,760
and new knowledge across an
uncharted ocean to an alien world.

498
00:51:32,760 --> 00:51:36,000
And, as is so often the case
in exploration,

499
00:51:36,000 --> 00:51:39,480
the real treasure was found
in the unexpected shadows.

500
00:51:39,480 --> 00:51:42,240
In this case,
in the shadow of the rings,

501
00:51:42,240 --> 00:51:45,520
where we found this tiny world,
Enceladus,

502
00:51:45,520 --> 00:51:49,480
that may harbour life
and provide us with reassurance

503
00:51:49,480 --> 00:51:52,400
that we are not alone
in the universe.

504
00:51:52,400 --> 00:51:55,720
See? The beauty is in the story.

505
00:51:55,720 --> 00:51:59,120
Would it matter really
if we found life on Enceladus?

506
00:51:59,120 --> 00:52:01,520
It would only be microbes after all.

507
00:52:01,520 --> 00:52:03,720
Yes, it would matter.

508
00:52:03,720 --> 00:52:08,480
We are at a time in our history,
I think, where we need reassurance

509
00:52:08,480 --> 00:52:13,520
or perhaps a reminder that
there is beauty and knowledge

510
00:52:13,520 --> 00:52:17,960
and perhaps even life and meaning
beyond our shores.

511
00:52:29,000 --> 00:52:33,440
We will, I'm sure, go back
to Saturn and its moons,

512
00:52:33,440 --> 00:52:36,080
explore deeper, stay longer,

513
00:52:36,080 --> 00:52:39,440
and maybe one day
even visit ourselves.

514
00:52:42,920 --> 00:52:45,240
We don't know what we'll discover

515
00:52:45,240 --> 00:52:49,720
but we can be sure that the story
has only just begun.

516
00:53:07,800 --> 00:53:11,240
Journey through our solar system
with this free poster

517
00:53:11,240 --> 00:53:12,880
produced by the Open University,

518
00:53:12,880 --> 00:53:16,120
and discover more about its planets
and moons.

519
00:53:16,120 --> 00:53:24,040
Order your free copy by calling...

520
00:53:24,040 --> 00:53:29,040
..or go to...

521
00:53:29,040 --> 00:53:33,040
..and follow the links to
the Open University.