Wonders of the Solar System (2010–…): Season 1, Episode 3 - The Thin Blue Line - full transcript

We live on a world of wonders.

A place of astonishing
beauty and complexity.

We have vast oceans

and incredible weather.

Giant mountains and
spectacular landscapes.

If you think
that this is all there is,

that our planet exists
in magnificent isolation,

then you're wrong.

We're part of
a much wider eco-system,

that extends way
beyond the top of our atmosphere.

I think we are living through



the greatest age of discovery
our civilisation has ever known.

We've voyaged to the farthest
reaches of the solar system,

photographed strange new worlds,

stood in unfamiliar landscapes,

tasted alien air.

Amongst all these wonders
sits our Earth -

an oasis of calm amidst the
violence of the solar system.

And all that separates us

from what's out there
is a thin, flimsy envelope of gas -

our atmosphere.

And it's thanks to this
"thin blue line"

that we have the air
that we breathe,

the water that we drink

and the landscape that surrounds us.



Atmospheres define all the
planets in the solar system.

They have the power
to create dynamic worlds
that are alien and chaotic.

But, remarkably, in the
frozen wastes of the solar system...

..one atmosphere has created
the most unexpected wonder -

a moon that looks a lot like home.

I've come to Cape Town
in South Africa

to do something that
I have always wanted to do,

but never thought
I would get the chance.

I'm about to fly incredibly high,

to the very edge
of the Earth's atmosphere.

From here, I am hoping to see
something that only a handful
of people have ever seen -

the thin blue line,

the fragile strip of gas
that surrounds our whole planet.

And this
is what's going to take me there.

This is an English
Electric Lightning,

the most beautiful
fighter aircraft ever built.

This is when England built
the best aircraft in the world.

The Lightning
is no longer in service,

but this piece of magnificently
overpowered engineering

is going to take me
18 kilometres, straight up.

Actually, I read somewhere that
when you read about the altitude

of the Lightning, it says
"Altitude: Estimated, 60,000 feet.

"Ceiling: Classified."

So I don't know
how high these can go.

I have heard rumours they can
go to 80,000 feet, which is amazing.

My journey will take me beyond
almost all the molecules of gas
that make up our atmosphere.

If you feel you're going to
get sick... Yeah? ..use a bag, OK?
Right. Hopefully not.

To get there,
I'm going to experience

what made the Lightning famous -

a vertical take-off.

Whoo-hoo!

'It takes just seconds to
reach nine kilometres up,

'but I'm still in the thickest
layer of the atmosphere,

'called the troposphere.

'But the further I climb,
the thinner the atmosphere becomes.'

Up at 58,000 feet.
90% of the atmosphere is below me.

The only people above me
are on the space station.

So beautiful.

I'm now at 60,000 feet.

'18 kilometres up.

'And the highest I can go.'

Above me, the sky is a deep,
dark blue.

'And that is what
I've come to see - '

our atmosphere.

That really is the thin blue
line that protects us.

So...fragile and so tenuous.

Just a tiny sliver of blue.

Amazing.

Between 55 and 60,000 feet,
inverted,

the curvature of the Earth there.

5G, vertical ascent.

That is just a ride!

It is remarkable to see that.

You can see

the...thinness and fragility,

you can see the atmosphere
going from light blue,
to dark blue, to black.

It really is astonishing.

The thin blue line makes the Earth
the wonderfully diverse place it is.

It acts as a soothing blanket,
that traps the warmth of the sun...

..yet protects us from the
harshness of its radiation.

Its movements can be traced
in the gentlest breeze.

And the most devastating hurricane.

The oxygen and water
the atmosphere holds

plays a fundamental role
in the ongoing survival

of millions of different species
living on the planet.

In this film, I want to explain
how the laws of physics

that created our unique atmosphere

are the same laws that created many
diverse and different atmospheres

across the solar system.

When perfectly balanced,
a world as familiar

and beautiful as the Earth
can evolve beneath the clouds.

But the slightest changes can
lead to alien and violent worlds.

There are planets in our solar
system that have been transformed

into hellish worlds, by nothing more
than the gases in their atmosphere.

And just as atmospheres
can choke a planet to death,

they are also powerful enough
to shape their surfaces.

And there are worlds out there
which are all atmosphere.

Giant balls of churning gas,
where storms three times

the size of the Earth
have raged for hundreds of years.

All atmospheres in the
solar system are unique,

but the ingredients and forces
that shape them are universal.

At the heart of each
is the glue which holds
the solar system together,

a fundamental force of nature -

gravity.

Gravity is, by far, the weakest
known force in the universe.

You can see that
because it's really easy for me
to pick a rock up off the ground,

even though there's a whole planet,
Earth, pulling the rock down.

I can just lift it up.

Incredibly weak,

but incredibly important,
because it's the only force there is

to hold an atmosphere to the planet.

The more massive the planet,
the greater its gravitational force.

Earth has enough mass to keep a
tight grip of the gas molecules
that make up our atmosphere.

It holds them against the surface
and allows us to breathe.

Now, we don't really
notice the presence

of our atmosphere, I suppose,
because we live in it, all the time.

But there's a lot of it.

There's five million billion
tons of air surrounding the Earth.

That's the equivalent
of a weight of one kilogram

pressing down on every square
centimetre of our bodies.

Or, put it another way,
if I'm about a metre square,

that's ten tons of weight
pressing down.

Now I say pressing down,
but that's not entirely right,

that's not how air pressure works.

It presses in every direction
at once. I can demonstrate that.

This is a glass full of water,

so if I put a piece of paper
on there, turn it upside down.

Now, if I'm right,
then the air pressure is pushing

in every direction on this glass
of water, the air pressure is
pushing up as well as down.

And it has no problem in
holding the water in the glass.

Cool.

Where did you get this water from?

CREW LAUGH

Life on the surface
of this planet survives,

surrounded by this enormous mass
of gas. We're like lobsters,

scuttling around
on the ocean floor.

But our atmosphere does more
than allow us to breathe.

It protects us

from the most powerful force
in the solar system...

..our sun.

If you ask yourself the question,
"Why is Earth

"the temperature that it is?"

Then, the obvious answer
might seem to be,

"Well, because it's
150m kilometres away from the sun".

But actually,
things aren't quite that simple.

This is the Namib desert in Namibia,
in south-western Africa.

And as the sun sinks below the
horizon, the temperature change,

from day to night, can be
as much as 30 degrees Celsius.

That's an immense amount
in just a few hours,

much more than in somewhere
like Manchester, for example.

The reason is that this is also one
of the driest places on the planet

and so there is very little
water vapour in the atmosphere.

That means that the atmosphere
is not a very good insulator,

so when the sun disappears, the heat
just disappears quickly into space.

Now, there's a planet
in the solar system,

somewhere over there, near the sun,

where the temperature shift,
from day to night,

is not a mere 30 degrees Celsius,
but an immense amount bigger.

Roughly 58 million kilometres from
the sun

is the smallest planet
in the solar system...

..Mercury.

This tortured piece of rock

suffers the biggest temperature
swings of all the planets,

from 450 degrees Celsius in the day,

to minus 180 degrees at night.

And all because Mercury
has been stripped naked.

It has virtually
no atmosphere at all.

Like all the rocky inner planets
of the solar system,

Mercury had an atmosphere
when it was formed,

but it lost it very quickly.

Here on Earth,

at sea level, then...

Well, in a volume
about the size of this pebble,

there are 10 billion
billion molecules of gas.

On Mercury, in the same volume,
there would be around a 100,000,

that's 10 million million
times less.

Now, planets hang on to their
atmosphere by the force of gravity.

It's the only way they can

stop that thin blue line of gas
disappearing off into space.

So, the bigger the planet,
the more massive the planet,

the stronger the gravitational pull
and the easier it is for the planet
to keep hold of its atmosphere.

So, Mercury was just too small and
too hot to hang onto its atmosphere

and the consequences for the planet
were absolutely devastating.

Atmospheres may be just
a thin strip of molecules,

but they are a planet's
first line of defence.

Without them, a planet like Mercury

is at the mercy
of our violent solar system.

This is Saskatchewan in
western Canada and it is a
cold place to be in November.

About a year ago, in November 2008,
a piece of asteroid,

a space rock,
weighing about ten tons,

entered the atmosphere
right over here and actually landed

about 30 kilometres that way,
at a place called Buzzard Coulee.

Now, it's not unusual for rocks
that big to hit the Earth.

On average,
that happens about once a month.

What was unusual about this one
was that it was over quite
a densely-populated area.

So tens of thousands,
if not hundreds of thousands,
of people saw it and heard it.

But most spectacularly, it was
captured by a lot of CCTV cameras,

including that one, in this garage.

These are the actual CCTV
images captured around the city.

They show the meteorite,

as it streaked across the sky
at 20 kilometres per second.

The fireball was brighter than the
moon and turned the night sky blue.

Scientists used these
remarkable images

to triangulate the impact site
of the meteorite.

They traced it to a field, just
outside the city of Lloydminster.

A team of meteorite hunters have
been searching the debris
left by the enormous explosion.

They are led by Dr Alan Hildebrand.

How much energy

does a rock like this have, then?

You know, what is it,
a ten-ton rock travelling
at 50 times the speed of sound?

You know, it would be
like if you'd stocked up,

say, 400 tons of TNT to explode.

I mean, it's really quite dramatic.

400 tons that just dissipates away
in the Earth's atmosphere? Yes.

Atmosphere slowing it down,
of course, causing it to break up.

In just five seconds,
it's almost all over and, of course,

you know it's an extreme friction,
makes the light show.

10% of the energy goes in light
and it's like a billion-watt bulb
shining high in the sky.

So, what are we looking for?

What does a piece of
that asteroid look like?

They... Going through the atmosphere,

the surface has got melted, so you
end up with a dark crust on them.

So, essentially, you're looking
for an oddly-sculpted dark rock.

Yeah. Well, in all fairness, you've
got to be able to tell it from,
you know the cow patties and so on.

But... I could probably manage that.

Once you get your eye in,
you'll have no trouble.

We've got one right here.

I'll pick that up.

Astonishing.

It's just been
completely rounded off.

Yeah, the heat melted
the surface of the rock.

I mean, how hot does something
have to be to do that? Yeah.

6,000 degrees C would do it.

So, this little rock
has had an amazing history.

I mean, it approached Earth
as part of this bigger fragment,

at about, what,
18, 19, 20 kilometres per second.

It hit the Earth's atmosphere.

About 85 kilometres up,
it began to feel the effects

of the Earth's atmosphere.

It began to squash the air in front
of it, creating a pressure wave,

essentially, which, in turn,
causes this thing to heat up.

And it would have heated up to
something like the temperature
of the surface of the sun.

It would have been 5 or 6,000
degrees Celsius as it plummeted
through the atmosphere,

lit up the sky over here
and then, quite literally,

exploded in a series of explosions
and peppered these fields
with lumps of rock this big.

Can you imagine
standing here on that night
and having this, these things -

and this is heavy, right -

raining down from the sky?

It must have been quite incredible.

If the meteorite
had hit the ground intact,

the explosion would have been been
equivalent to 400 tons of TNT

and left a crater 20 metres wide.

The Earth was spared this
colossal impact by nothing more

than the tenuous strip of gases
that surrounds us.

But not all planets have
this protective blanket.

When a meteorite hits naked Mercury,

there is no atmosphere to
break it up or slow it down.

It strikes the ground at full speed
and completely intact.

For the last 4.6 billion years,

Mercury has been bombarded with
countless asteroids and comets.

The whole history
of the planet's violent past
is laid out on its surface,

a world pitted with
hundreds of thousands of craters.

Craters inside craters,
inside craters.

Mercury was damned from the start.

It's simply
too small and too hot

to have retained any
meaningful traces of atmosphere.

We, on the other hand,

are big enough and cold enough

to have retained
this envelope of gases.

That, in turn, allows

living things, like me, to evolve

and to use that atmosphere,
to breathe and to live.

But there are places out there
in the solar system

whose atmospheres have
the same ingredients as our own,

but when the formula
is even slightly remixed,

it leads to worlds
that couldn't be more different.

Roughly 108 million kilometres
from the sun

sits the brightest planet
in the solar system, Venus.

This footage shows the
luminescent world appear

from behind our cratered moon.

Venus and Earth
share many similarities.

We sit next to each other in space,

we were formed from
the same material

and we're roughly the same size and
share a similar mass and gravity.

But that's where
any similarities end.

Venus is a tortured world,

where thick clouds of sulphuric
acid are driven along by high winds

and temperatures are hot enough
to melt lead.

All because this planet's atmosphere
created a runaway greenhouse effect.

The "greenhouse effect"
has become a well-known phrase.

You know, it's synonymous
with global warming.

But what is it?

Well, a planet, like the Earth,
absorbs energy from the sun
as visible light.

Now, atmospheres don't absorb
much visible light, as you can see,
because you can see the sun.

The ground absorbs
the visible light, heats up
and then re-radiates it.

But it re-radiates it
as infrared radiation,
heat radiation, if you want.

And atmospheric gases, particularly
carbon dioxide, are very good

at absorbing in the infrared
and so they trap the heat
and the planet heats up.

On Earth, greenhouse gases
are essential to our survival.

Without them our planet
would be 30 degrees colder,

too cold to support life
as we know it.

But Venus's atmosphere
was flooded with greenhouse gases.

The nearby sun slowly
boiled away its oceans,

pumping water vapour
into the atmosphere.

And carbon dioxide, from
thousands of erupting volcanoes,

added to the stifling mix.

Venus grew hotter and hotter.

The planet was slowly
choked to death.

Venus is a planet with an
atmosphere in overdrive,

but Earth's other rocky neighbour
tells quite a different story.

Get it!

These are the dunes
in the Namib desert.

It's an absolutely
spectacular place.

This place is not the hottest,

nor the driest, desert in the world,
but these dunes

are some of the oldest sand dunes
in the world.

And the reason we're here
in the Namib desert

is that this is a great analogue

for the surface of Mars.

This is what the surface of Mars
looks like and these dunes,

called barchan dunes,
these crescent-shaped dunes,

are the same
as the sand dunes on Mars.

So, if you want to get a
feel for what it would be
like on the surface of Mars,

and you want to know what driving
a 4x4 around on it would be like,
then this is the place to come.

Incredibly, there is a vehicle
driving across the surface
of the "red planet" today...

..a space rover, named Opportunity.

The rovers and spacecraft
that circle the planet

have sent back images which
reveal Mars in exquisite detail.

Mars has vast dunes,

enormous volcanoes

and giant ice sheets.

It has canyons and river valleys.

Mars is a dry, frozen version of our
home, covered in red dust and sand.

And it's all due to the fact that
Mars has virtually no atmosphere.

But there are clues

that things weren't always this way.

These are pictures taken from the
surface of Mars in August 2009.

And they caused quite a bit of
excitement, because of this,

this rock sat on the surface
of Mars in front of the rover.

This rock is about...

Well, here's a close-up.

It's actually
a nickel iron meteorite

and it's about,
what, 60 centimetres across,

weighs half a ton.

It came from space,

came through the Martian atmosphere
and landed on the ground.

But the mystery is that a meteorite
this big, if it hit Mars today,

would disintegrate
when it hit the surface.

It would be travelling too fast
and that's because

Mars's atmosphere is too thin,
too diffuse to slow it down.

But that meteorite
is very definitely there

so how could it
have made it to the ground?

Well, it must be that, in the past,
when this meteorite hit Mars,

Mars' atmosphere
was significantly denser,

dense enough to slow
this piece of rock down enough

that it could land
on the surface intact.

But why did Mars lose its thick
atmosphere and become the
barren planet we see today?

There are so many ways for planets
to lose their atmospheres

that it feels like a miracle
that we've still got ours.

But with Mars,
it's thought that one of

the dominant mechanisms was
interaction with solar winds.

The solar wind
is a stream of super-heated,
electrically-charged particles

that constantly stream
away from the sun at over
one million kilometres per hour.

This wave of smashed atoms
has the power to strip
a planet of its atmosphere.

On Earth, we're protected from this
onslaught by an invisible shield

that completely surrounds our
planet, known as the magnetosphere.

The magnetosphere is created deep
within the Earth's molten iron core.

As the core spins, it generates
a powerful magnetic field

which shoots out of the pole
and cocoons the whole planet.

This magnetic shield is
strong enough to deflect most of
the solar wind that comes our way.

Now, we know that at some
point in the past, Mars

would also have had a molten core
and did have a magnetic field.

But because Mars is a
smaller planet than the Earth,

it lost its heat more quickly
and the core solidified.

Electric currents could no
longer flow and its field vanished.

And that was a major factor
in the solar wind being allowed to

blast the planet and
strip away its atmosphere.

With no atmosphere to insulate it,
this once Earth-like world

transformed into the
frozen desert we see today.

A shadow of its former self.

Although Mars has lost
most of its atmosphere,

those few molecules that remain
still have the power to sculpt
and transform the surface.

And that power,
that transformative effect,

is present on every planet in the
solar system that has an atmosphere.

You can see it transforming the
surface of the Namibian desert
today as we speak.

It is, of course, the force
of nature that we call weather.

We've got to go. Wow!

Weather is a feature of every
planet with an atmosphere.

Our world is transformed
as this huge mass of air
moves across its surface.

But as we look out
into the solar system,

we see it only takes
the slightest atmosphere to
produce extraordinary weather.

Every few years,
Mars all but disappears
under a maelstrom of dust.

Global dust storms are so huge
they dwarf Olympus Mons,

a volcano three times bigger
than Everest.

But to experience
the most extreme and violent
weather in the solar system,

we need to travel to Jupiter.

This banded gas giant is over
140,000 kilometres in diameter.

Its atmosphere isn't
a thin blue line, it's many
thousand of kilometres thick

and in a constant state
of seething motion.

The whole surface boils
with gigantic storms.

Yet, this most alien world shares
a feature with our own planet.

RUMBLING

Jupiter crackles to the sound
of electrical storms.

The bolts of lightning are
thousands of times brighter
than lightning here on Earth.

The physics of storms on Jupiter is,
of course, the same as the physics
of storms on Earth.

The warm moist air deep
in the atmosphere starts to rise,
and as it rises it cools.

And the moisture
condenses out to form clouds.

Now, that rising air leaves a gap
beneath it, a low pressure area,

and so more warm, moist
air is sucked in and that
fuels the rise of the storm.

Now, on Earth, those storm systems
are driven by the power of the sun.

But therein lies a mystery because
the storm systems on Jupiter are
far more powerful

and yet Jupiter is five times
further away from the sun
than the Earth is,

which means it receives
25 times less solar energy.

So, what mechanism could it be that
powers those intensely violent
storms on Jupiter?

The secret to Jupiter's storm-tossed
atmosphere lies hidden deep
within the gas giant.

On Earth, we have clear
boundaries between the gaseous sky,

the liquid oceans
and the solid ground.

But on Jupiter, there
are no such boundaries.

It's a gas giant, made of the two
lightest and most abundant elements
in the universe,

hydrogen and helium.

But as you go deep
into Jupiter's atmosphere,

something very strange and
interesting happens to those gases.

Jupiter's atmosphere is so thick and
its gravitational pull so strong

that 20,000 kilometres
beneath the cloud tops,

the pressure is 2,000,000
times greater than the surface
pressure here on Earth.

Under such immense pressure,
the hydrogen gas in the atmosphere

is transformed
into a strange metallic liquid.

As the gases are squeezed,
a vast amount of energy is released,

enough energy to fuel some of the
biggest storms in the solar system.

The biggest of them all
is the Great Red Spot.

This giant anti-cyclone
has raged for hundreds of years

and is large enough to swallow
the Earth three times over.

The Great Red Spot is an
awesome sight.

But this giant
isn't one of my wonders.

My wonder is a much smaller world.

A moon that orbits the gas giant
Saturn, 1.5 billion kilometres
from Earth.

What we have found on this
small world is simply astonishing.

If you thought of our moon as
the archetypal moon of the solar
system, if you like,

then... Well, you might think that
all the other moons out there,

hundreds of them, would be dead,
uninteresting worlds.

I mean not uninteresting
places to visit.

I mean that is, in my view,
the greatest thing that
humans have ever achieved,

landing on the surface of the moon
but it's a dead and lifeless place.

But as we've begun to visit those
worlds, as we've flown spacecraft

to within hundreds of miles of their
surfaces, we've found that the moons
in the outer solar system

are of an astonishingly interesting
and varied and fascinated
bunch of worlds.

This is Jupiter's moon, Europa.

This is Jupiter's moon, Io, the most
volcanic object in the solar system.

But of all the worlds out there,
this one - Saturn's moon, Titan -
is unique, because of that.

That is an atmosphere,
and what an atmosphere it is!

It's 1,000 kilometres deep,
it's four times denser than
the atmosphere of the Earth.

I mean imagine that,

a moon around a distant planet
in the icy, distant reaches
of the solar system

with an atmosphere
denser and thicker than our own.

Titan has the most
Earth-like atmosphere in
the entire solar system,

a thick blue line, rich in
nitrogen and containing methane.

At first sight, a world this small
shouldn't be able to hold
onto such a dense atmosphere,

except Titan lies in one of the
coldest regions of the solar system,

and that makes all the difference.

Temperature for gases like this,
the gases in our atmosphere,

is really a measure
of how fast the molecules
of the gas are moving around,

and I can demonstrate that with this
thing, which is a Chinese lantern.

If I light this fuel,

then what's going to happen...

is that the gas inside
is going to heat up.

And as you heat up a gas,

what that basically means is that
you speed all the molecules up.

As the molecules of air heat up and
move faster,

the air pressure inside
the lantern begins to increase.

That means that molecules are
forced out, making the air inside
less dense than the air outside,

and the lantern gets lighter.

And eventually the lantern
is so light...

..that it will just float away in
the atmosphere of our planet.

Hot gases have more energy to escape
a planet's gravitational pull
than cold gases.

Now Titan is a much smaller
body than the Earth. It has much
weaker gravitational pull,

and if it were in the same region
of the solar system as we are,

then it would not be able to
hold onto its atmosphere.

But it's a lot further away
from the sun than we are

and so that means that it's colder,
its atmospheric molecules are moving
around much more slowly than ours.

That means that its weak gravity
is enough to hold on to that
thick dense atmosphere.

Titan's thick atmosphere
was an unexpected discovery,

but it took an audacious mission

to reveal the world that lies
beneath the blanket of clouds.

We have lift off of the Cassini
spacecraft on a billion-mile
trek to Saturn.

In 1997,
Cassini began its journey to Titan.

It carried with it the Huygens
probe, a lander designed to
set down on this frozen moon.

On Christmas Day 2004,

Huygens was released from Cassini
and it began the bumpy ride

through one of the most intriguing
atmospheres in the solar system.

And then, for the first time,

the thick clouds parted and
the surface of Titan was revealed.

These are the actual images
taken by Huygens

as it slowly
parachuted to the surface.

The world it revealed

was more familiar than we
could have possibly imagined.

One of the first people to see

these incredible images was a man
who helped design the probe,

Ralph Lorenz.

It was amazing because we
just had no idea what to expect.

We didn't know whether it would be,
you know, cratered like the moon or
just sort of a flat expanse of sand

and then these first pictures
came back and it was just
astonishingly familiar.

Did that picture,
that initial series of pictures...

I suppose it did look somewhat
like this, didn't it? It did.

It could have been there.
It could have been right here.
I do see that.

I could sit here,

look at that and that's what
that picture looks like.

I could take it with a camera.

The camera on the probe was about
the height of your knee, so yeah,

the view the Huygens probe had
is just like this.

Rounded stones dot the landscape.

They're smooth and look like they
have been eroded by tumbling water,

similar to stones found on
river beds, here on Earth.

It sounds to me like this was one of
the easiest pictures to interpret

in the history of space exploration.

You know, the way you tell it, it's
just that's a river bed with these
stones. I mean, is it that simple?

Because you can be misled easily,
with...

The devil is always in the details,

but I think there were
very few people

disputed the interpretation
of a river channel.

I mean it's just such a familiar
thing to so many people on Earth,
there really wasn't much doubt.

It was an extraordinary discovery.

Evidence of flowing rivers had
never been found before on a moon.

But it wasn't the only surprise
Titan held in store.

This is the Matanuska glacier
in Alaska.

It really is one of the most
astonishing places I've ever seen.

And this whole landscape
is testament to the erosive power
of this stuff,

this mixture of ice and rock

as it rolls down this valley over
hundreds of thousands of years

and creates this
astonishing landscape.

But the reason it can do that

is because of the delicate balance
of the Earth's atmosphere.

You see, our planet is just at
the right temperature and pressure
to allow water to exist as solid,

as liquid and as gas,
as vapour in the clouds.

And so the sun can heat up the
oceans and it can move the water
over the top of the mountains.

It can fall as rain, turn to ice,
become a glacier

and then sweep down the valley to
sculpt this astonishing landscape.

Just as our atmosphere
allows all this to exist,

the atmosphere of Titan
is the perfect temperature and
pressure to allow something to exist

that has never been seen before
on a world beyond Earth.

This is a picture taken
of the south pole of Titan

by Cassini in June 2005,

and it's become
one of the most important
and fascinating pictures

in the history of
space exploration.

The interesting thing is
this black blob, here.

Now this fascinated the Cassini
scientists but the explanation
as to what that is

had to wait just over a year
till July 2006,

when this picture was taken,

and it's a radar image, this time
of the north pole of Titan,

and you see, again,
these huge black areas.

The black in this case means
that the radar waves that
bounced onto them didn't come back

so they're completely black,
and there's only one
really good explanation for that.

That is that they are incredibly
flat surfaces.

In fact, they're surfaces of liquid

so this picture combined
with this picture

means that this is the first
observation of a liquid,

a lake on the surface of a body
other than the Earth
in the solar system.

But these lakes, of course, cannot
be lakes of liquid water because

the surface temperature on Titan
is minus 180 degrees Celsius and,
at those temperatures,

water is frozen as hard as steel.

So if these are not lakes of water,
then what are they?

This is Lake Eyak in Alaska,
just on Prince William Sound,

and I've come here to collect
a molecule or a substance that's
very abundant on Titan.

In fact, it's abundant throughout
the solar system, but here on Earth

it exists as a gas and it bubbles up
from the floor of this lake.

The floor of Lake Eyak is covered in
rotting vegetation, you know, dead
leaves and bits of trees, twigs,

and that's been broken down by
bacteria which produce the gas

that bubbles up
from the floor of the lake.

That gas is methane and we've
been collecting it all night

underneath this upturned boat

so that I can take a sample of it in
this bag.

Now, on Earth,
methane is very unstable.

If you give it...

a little kick...

in the presence of oxygen,
then you get what chemists
call an exothermic reaction.

Methane plus oxygen goes to water
plus carbon dioxide, and...

some energy.

The Earth's temperature
and atmospheric pressure

means methane can only exist
as a highly-flammable gas.

But Titan's atmospheric pressure
and temperature

is perfect to allow methane
to exist as a solid,
a gas and, most importantly,

a liquid.

So the images Cassini captured were
gigantic lakes of liquid methane...

..the first ever liquid discovered

pooling on the surface of another
world in the solar system.

This is Kraken Mare.

At over 400,000 square kilometres,

it's the biggest
body of liquid on Titan.

It's almost five times
the size of Lake Superior,

North America's greatest lake.

On Titan,
methane plays exactly the same role
that water does here on Earth.

So, where we have clouds of water,

Titan has clouds of methane
with methane rain.

Whereas we have
lakes and oceans of water,
Titan has lakes of liquid methane.

And whereas, here on Earth,
the sun warms the water
in the lakes and oceans,

and fills our atmosphere
with water vapour,

on Titan the sun lifts the methane

from the lakes and saturates
the atmosphere with methane.

So, whereas on Earth we have a
hydrological cycle, on Titan
there's a methanological cycle.

And rain would be an
absolutely magical sight on Titan.

Because the atmosphere is so dense
and the gravity of the moon
is so weak,

the drops of methane rain would
grow to over a centimetre in size

and they would fall to the ground
as slowly as snowflakes fall
onto the surface of our own planet.

Thousands and thousands
of gallons of liquid methane

must have slowly rained down
onto the surface,

making rivers and streams
swell and burst.

Deep gullies were cut
into the frozen water landscape...

Which looks so familiar
because it is familiar.

It's this. You know, the atmosphere
of Titan shapes the surface
in exactly the same way

that the atmosphere here on Earth
shapes the surface of our planet.

Titan is like a primordial Earth
caught in a deep freeze.

It's almost like looking back in
time over four billion years

and observing our planet
before life began,
and began to modify our atmosphere,

to change it into the oxygen-rich
atmosphere that we see today.

In many ways,
Titan looks so familiar.

It's a place with rivers
and lakes and clouds and rain.

It's a place with water, albeit
frozen as hard as steel,

and a place of methane, albeit so
cold that methane is now a liquid

and flows and shapes the landscape
just like water does here on Earth.

For me, the most
important thing about Titan

is we now have two Earth-like
worlds in our solar system

One in this warm region,
93 million miles away from the sun,

and the other in deep freeze,
a billion miles away from our star

in orbit around another planet,
and that must greatly increase
the probability

that there are other
Earth-like planets in orbit

around the hundreds of billions of
stars out there in the universe.

# Somewhere over the rainbow

# Skies are blue

# And the dreams
that you dare to dream

# Really do come true. #