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We live on a world of wonders.

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A place of astonishing
beauty and complexity.

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We have vast oceans

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and incredible weather.

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Giant mountains and
spectacular landscapes.

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If you think
that this is all there is,

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that our planet exists
in magnificent isolation,

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then you're wrong.

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

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that extends way
beyond the top of our atmosphere.

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I think we are living through

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the greatest age of discovery
our civilisation has ever known.

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

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photographed strange new worlds,

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stood in unfamiliar landscapes,

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tasted alien air.

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Amongst all these wonders
sits our Earth -

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

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And all that separates us

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

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our atmosphere.

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

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that we have the air
that we breathe,

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the water that we drink

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and the landscape that surrounds us.

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Atmospheres define all the
planets in the solar system.

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

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

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..one atmosphere has created
the most unexpected wonder -

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a moon that looks a lot like home.

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I've come to Cape Town
in South Africa

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to do something that
I have always wanted to do,

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but never thought
I would get the chance.

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I'm about to fly incredibly high,

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to the very edge
of the Earth's atmosphere.

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

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the thin blue line,

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the fragile strip of gas
that surrounds our whole planet.

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

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This is an English
Electric Lightning,

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the most beautiful
fighter aircraft ever built.

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

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The Lightning
is no longer in service,

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but this piece of magnificently
overpowered engineering

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is going to take me
18 kilometres, straight up.

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

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

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"Ceiling: Classified."

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

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

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

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

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To get there,
I'm going to experience

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what made the Lightning famous -

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a vertical take-off.

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Whoo-hoo!

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'It takes just seconds to
reach nine kilometres up,

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

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'called the troposphere.

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

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

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The only people above me
are on the space station.

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So beautiful.

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I'm now at 60,000 feet.

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'18 kilometres up.

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'And the highest I can go.'

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

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

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our atmosphere.

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That really is the thin blue
line that protects us.

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So...fragile and so tenuous.

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Just a tiny sliver of blue.

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Amazing.

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Between 55 and 60,000 feet,
inverted,

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the curvature of the Earth there.

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5G, vertical ascent.

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That is just a ride!

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It is remarkable to see that.

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You can see

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the...thinness and fragility,

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

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It really is astonishing.

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

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

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

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Its movements can be traced
in the gentlest breeze.

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And the most devastating hurricane.

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The oxygen and water
the atmosphere holds

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plays a fundamental role
in the ongoing survival

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of millions of different species
living on the planet.

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

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that created our unique atmosphere

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are the same laws that created many
diverse and different atmospheres

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across the solar system.

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When perfectly balanced,
a world as familiar

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and beautiful as the Earth
can evolve beneath the clouds.

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

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There are planets in our solar
system that have been transformed

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

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And just as atmospheres
can choke a planet to death,

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they are also powerful enough
to shape their surfaces.

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And there are worlds out there
which are all atmosphere.

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Giant balls of churning gas,
where storms three times

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

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All atmospheres in the
solar system are unique,

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but the ingredients and forces
that shape them are universal.

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

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a fundamental force of nature -

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gravity.

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

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

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

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I can just lift it up.

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Incredibly weak,

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

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to hold an atmosphere to the planet.

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

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

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

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Now, we don't really
notice the presence

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

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But there's a lot of it.

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

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That's the equivalent
of a weight of one kilogram

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pressing down on every square
centimetre of our bodies.

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

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that's ten tons of weight
pressing down.

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

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that's not how air pressure works.

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

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This is a glass full of water,

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

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

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

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

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Cool.

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Where did you get this water from?

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CREW LAUGH

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Life on the surface
of this planet survives,

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

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scuttling around
on the ocean floor.

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But our atmosphere does more
than allow us to breathe.

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It protects us

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

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..our sun.

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If you ask yourself the question,
"Why is Earth

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"the temperature that it is?"

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Then, the obvious answer
might seem to be,

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

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But actually,
things aren't quite that simple.

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

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

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

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

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much more than in somewhere
like Manchester, for example.

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

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

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

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

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

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somewhere over there, near the sun,

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where the temperature shift,
from day to night,

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

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Roughly 58 million kilometres from
the sun

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is the smallest planet
in the solar system...

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..Mercury.

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This tortured piece of rock

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suffers the biggest temperature
swings of all the planets,

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from 450 degrees Celsius in the day,

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to minus 180 degrees at night.

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And all because Mercury
has been stripped naked.

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It has virtually
no atmosphere at all.

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Like all the rocky inner planets
of the solar system,

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Mercury had an atmosphere
when it was formed,

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but it lost it very quickly.

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Here on Earth,

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at sea level, then...

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

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there are 10 billion
billion molecules of gas.

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

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that's 10 million million
times less.

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

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It's the only way they can

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

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

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

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

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and the consequences for the planet
were absolutely devastating.

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Atmospheres may be just
a thin strip of molecules,

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

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Without them, a planet like Mercury

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is at the mercy
of our violent solar system.

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

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

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a space rock,
weighing about ten tons,

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entered the atmosphere
right over here and actually landed

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

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

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On average,
that happens about once a month.

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

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

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

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including that one, in this garage.

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These are the actual CCTV
images captured around the city.

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They show the meteorite,

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

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

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Scientists used these
remarkable images

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to triangulate the impact site
of the meteorite.

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

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

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They are led by Dr Alan Hildebrand.

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How much energy

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does a rock like this have, then?

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

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

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say, 400 tons of TNT to explode.

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I mean, it's really quite dramatic.

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

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

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

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

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

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So, what are we looking for?

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What does a piece of
that asteroid look like?

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They... Going through the atmosphere,

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

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

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

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But... I could probably manage that.

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

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We've got one right here.

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I'll pick that up.

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Astonishing.

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It's just been
completely rounded off.

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Yeah, the heat melted
the surface of the rock.

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

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6,000 degrees C would do it.

239
00:20:09,040 --> 00:20:13,160
So, this little rock
has had an amazing history.

240
00:20:13,160 --> 00:20:17,320
I mean, it approached Earth
as part of this bigger fragment,

241
00:20:17,320 --> 00:20:22,600
at about, what,
18, 19, 20 kilometres per second.

242
00:20:22,600 --> 00:20:25,960
It hit the Earth's atmosphere.

243
00:20:25,960 --> 00:20:31,200
About 85 kilometres up,
it began to feel the effects

244
00:20:31,200 --> 00:20:33,040
of the Earth's atmosphere.

245
00:20:33,040 --> 00:20:37,640
It began to squash the air in front
of it, creating a pressure wave,

246
00:20:37,640 --> 00:20:41,840
essentially, which, in turn,
causes this thing to heat up.

247
00:20:41,840 --> 00:20:46,360
And it would have heated up to
something like the temperature
of the surface of the sun.

248
00:20:46,360 --> 00:20:51,160
It would have been 5 or 6,000
degrees Celsius as it plummeted
through the atmosphere,

249
00:20:51,160 --> 00:20:56,480
lit up the sky over here
and then, quite literally,

250
00:20:56,480 --> 00:21:02,480
exploded in a series of explosions
and peppered these fields
with lumps of rock this big.

251
00:21:02,480 --> 00:21:07,240
Can you imagine
standing here on that night
and having this, these things -

252
00:21:07,240 --> 00:21:10,160
and this is heavy, right -

253
00:21:10,160 --> 00:21:11,720
raining down from the sky?

254
00:21:11,720 --> 00:21:13,960
It must have been quite incredible.

255
00:21:16,360 --> 00:21:19,040
If the meteorite
had hit the ground intact,

256
00:21:19,040 --> 00:21:24,880
the explosion would have been been
equivalent to 400 tons of TNT

257
00:21:24,880 --> 00:21:26,720
and left a crater 20 metres wide.

258
00:21:29,920 --> 00:21:35,000
The Earth was spared this
colossal impact by nothing more

259
00:21:35,000 --> 00:21:38,040
than the tenuous strip of gases
that surrounds us.

260
00:21:38,040 --> 00:21:42,120
But not all planets have
this protective blanket.

261
00:21:47,320 --> 00:21:50,040
When a meteorite hits naked Mercury,

262
00:21:50,040 --> 00:21:55,280
there is no atmosphere to
break it up or slow it down.

263
00:21:57,480 --> 00:22:02,040
It strikes the ground at full speed
and completely intact.

264
00:22:03,600 --> 00:22:06,800
For the last 4.6 billion years,

265
00:22:06,800 --> 00:22:12,880
Mercury has been bombarded with
countless asteroids and comets.

266
00:22:14,200 --> 00:22:21,280
The whole history
of the planet's violent past
is laid out on its surface,

267
00:22:21,280 --> 00:22:26,800
a world pitted with
hundreds of thousands of craters.

268
00:22:27,760 --> 00:22:32,640
Craters inside craters,
inside craters.

269
00:22:55,080 --> 00:22:57,400
Mercury was damned from the start.

270
00:22:57,400 --> 00:23:00,560
It's simply
too small and too hot

271
00:23:00,560 --> 00:23:04,400
to have retained any
meaningful traces of atmosphere.

272
00:23:04,400 --> 00:23:06,280
We, on the other hand,

273
00:23:06,280 --> 00:23:09,720
are big enough and cold enough

274
00:23:09,720 --> 00:23:12,840
to have retained
this envelope of gases.

275
00:23:16,360 --> 00:23:18,160
That, in turn, allows

276
00:23:18,160 --> 00:23:21,400
living things, like me, to evolve

277
00:23:21,400 --> 00:23:25,920
and to use that atmosphere,
to breathe and to live.

278
00:23:25,920 --> 00:23:29,480
But there are places out there
in the solar system

279
00:23:29,480 --> 00:23:33,520
whose atmospheres have
the same ingredients as our own,

280
00:23:33,520 --> 00:23:37,920
but when the formula
is even slightly remixed,

281
00:23:37,920 --> 00:23:41,600
it leads to worlds
that couldn't be more different.

282
00:23:52,440 --> 00:23:55,800
Roughly 108 million kilometres
from the sun

283
00:23:55,800 --> 00:24:00,680
sits the brightest planet
in the solar system, Venus.

284
00:24:01,600 --> 00:24:04,200
This footage shows the
luminescent world appear

285
00:24:04,200 --> 00:24:07,120
from behind our cratered moon.

286
00:24:10,560 --> 00:24:13,760
Venus and Earth
share many similarities.

287
00:24:13,760 --> 00:24:16,800
We sit next to each other in space,

288
00:24:16,800 --> 00:24:19,800
we were formed from
the same material

289
00:24:19,800 --> 00:24:25,280
and we're roughly the same size and
share a similar mass and gravity.

290
00:24:26,720 --> 00:24:29,520
But that's where
any similarities end.

291
00:24:29,520 --> 00:24:32,600
Venus is a tortured world,

292
00:24:32,600 --> 00:24:38,440
where thick clouds of sulphuric
acid are driven along by high winds

293
00:24:38,440 --> 00:24:42,000
and temperatures are hot enough
to melt lead.

294
00:24:42,000 --> 00:24:48,080
All because this planet's atmosphere
created a runaway greenhouse effect.

295
00:24:50,840 --> 00:24:53,840
The "greenhouse effect"
has become a well-known phrase.

296
00:24:53,840 --> 00:24:57,320
You know, it's synonymous
with global warming.

297
00:24:57,320 --> 00:24:59,400
But what is it?

298
00:24:59,400 --> 00:25:05,760
Well, a planet, like the Earth,
absorbs energy from the sun
as visible light.

299
00:25:05,760 --> 00:25:12,520
Now, atmospheres don't absorb
much visible light, as you can see,
because you can see the sun.

300
00:25:12,520 --> 00:25:18,280
The ground absorbs
the visible light, heats up 
and then re-radiates it.

301
00:25:18,280 --> 00:25:23,000
But it re-radiates it
as infrared radiation,
heat radiation, if you want.

302
00:25:23,000 --> 00:25:27,760
And atmospheric gases, particularly
carbon dioxide, are very good

303
00:25:27,760 --> 00:25:33,160
at absorbing in the infrared
and so they trap the heat
and the planet heats up.

304
00:25:37,760 --> 00:25:41,840
On Earth, greenhouse gases
are essential to our survival.

305
00:25:41,840 --> 00:25:45,840
Without them our planet
would be 30 degrees colder,

306
00:25:45,840 --> 00:25:49,600
too cold to support life
as we know it.

307
00:25:55,760 --> 00:26:01,040
But Venus's atmosphere
was flooded with greenhouse gases.

308
00:26:01,040 --> 00:26:05,280
The nearby sun slowly
boiled away its oceans,

309
00:26:05,280 --> 00:26:08,080
pumping water vapour
into the atmosphere.

310
00:26:09,240 --> 00:26:13,720
And carbon dioxide, from
thousands of erupting volcanoes,

311
00:26:13,720 --> 00:26:16,120
added to the stifling mix.

312
00:26:17,840 --> 00:26:21,440
Venus grew hotter and hotter.

313
00:26:21,440 --> 00:26:24,080
The planet was slowly
choked to death.

314
00:26:26,880 --> 00:26:31,800
Venus is a planet with an
atmosphere in overdrive,

315
00:26:31,800 --> 00:26:36,280
but Earth's other rocky neighbour
tells quite a different story.

316
00:27:00,240 --> 00:27:02,000
Get it!

317
00:27:07,000 --> 00:27:09,960
These are the dunes
in the Namib desert.

318
00:27:09,960 --> 00:27:13,440
It's an absolutely
spectacular place.

319
00:27:20,200 --> 00:27:21,760
This place is not the hottest,

320
00:27:21,760 --> 00:27:25,160
nor the driest, desert in the world,
but these dunes

321
00:27:25,160 --> 00:27:28,600
are some of the oldest sand dunes
in the world.

322
00:27:32,120 --> 00:27:34,480
And the reason we're here
in the Namib desert

323
00:27:34,480 --> 00:27:37,760
is that this is a great analogue

324
00:27:37,760 --> 00:27:39,040
for the surface of Mars.

325
00:27:39,040 --> 00:27:42,760
This is what the surface of Mars
looks like and these dunes,

326
00:27:42,760 --> 00:27:45,600
called barchan dunes,
these crescent-shaped dunes,

327
00:27:45,600 --> 00:27:49,800
are the same
as the sand dunes on Mars.

328
00:27:56,000 --> 00:28:00,040
So, if you want to get a
feel for what it would be
like on the surface of Mars,

329
00:28:00,040 --> 00:28:06,120
and you want to know what driving
a 4x4 around on it would be like,
then this is the place to come.

330
00:28:11,680 --> 00:28:16,840
Incredibly, there is a vehicle
driving across the surface
of the "red planet" today...

331
00:28:21,960 --> 00:28:25,000
..a space rover, named Opportunity.

332
00:28:30,280 --> 00:28:33,800
The rovers and spacecraft
that circle the planet

333
00:28:33,800 --> 00:28:38,040
have sent back images which
reveal Mars in exquisite detail.

334
00:28:47,880 --> 00:28:51,400
Mars has vast dunes,

335
00:28:51,400 --> 00:28:56,000
enormous volcanoes

336
00:28:56,000 --> 00:28:59,120
and giant ice sheets.

337
00:29:02,320 --> 00:29:06,120
It has canyons and river valleys.

338
00:29:07,320 --> 00:29:14,600
Mars is a dry, frozen version of our
home, covered in red dust and sand.

339
00:29:14,600 --> 00:29:20,120
And it's all due to the fact that
Mars has virtually no atmosphere.

340
00:29:27,960 --> 00:29:30,160
But there are clues

341
00:29:30,160 --> 00:29:32,920
that things weren't always this way.

342
00:29:33,880 --> 00:29:39,600
These are pictures taken from the
surface of Mars in August 2009.

343
00:29:42,000 --> 00:29:47,560
And they caused quite a bit of
excitement, because of this,

344
00:29:47,560 --> 00:29:51,800
this rock sat on the surface
of Mars in front of the rover.

345
00:29:51,800 --> 00:29:54,240
This rock is about...

346
00:29:54,240 --> 00:29:55,560
Well, here's a close-up.

347
00:29:55,560 --> 00:29:58,000
It's actually
a nickel iron meteorite

348
00:29:58,000 --> 00:30:01,800
and it's about,
what, 60 centimetres across,

349
00:30:01,800 --> 00:30:03,680
weighs half a ton.

350
00:30:03,680 --> 00:30:07,200
It came from space,

351
00:30:07,200 --> 00:30:10,680
came through the Martian atmosphere
and landed on the ground.

352
00:30:10,680 --> 00:30:15,520
But the mystery is that a meteorite
this big, if it hit Mars today,

353
00:30:15,520 --> 00:30:17,880
would disintegrate
when it hit the surface.

354
00:30:17,880 --> 00:30:20,520
It would be travelling too fast
and that's because

355
00:30:20,520 --> 00:30:24,760
Mars's atmosphere is too thin,
too diffuse to slow it down.

356
00:30:26,280 --> 00:30:28,640
But that meteorite
is very definitely there

357
00:30:28,640 --> 00:30:32,200
so how could it
have made it to the ground?

358
00:30:32,200 --> 00:30:36,600
Well, it must be that, in the past,
when this meteorite hit Mars,

359
00:30:36,600 --> 00:30:39,880
Mars' atmosphere
was significantly denser,

360
00:30:39,880 --> 00:30:43,760
dense enough to slow
this piece of rock down enough

361
00:30:43,760 --> 00:30:47,360
that it could land
on the surface intact.

362
00:30:51,040 --> 00:30:57,440
But why did Mars lose its thick
atmosphere and become the
barren planet we see today?

363
00:30:59,960 --> 00:31:05,480
There are so many ways for planets
to lose their atmospheres

364
00:31:05,480 --> 00:31:08,320
that it feels like a miracle
that we've still got ours.

365
00:31:08,320 --> 00:31:10,360
But with Mars,
it's thought that one of

366
00:31:10,360 --> 00:31:13,560
the dominant mechanisms was
interaction with solar winds.

367
00:31:18,840 --> 00:31:24,480
The solar wind
is a stream of super-heated,
electrically-charged particles

368
00:31:24,480 --> 00:31:30,320
that constantly stream
away from the sun at over
one million kilometres per hour.

369
00:31:34,320 --> 00:31:40,000
This wave of smashed atoms
has the power to strip
a planet of its atmosphere.

370
00:31:47,040 --> 00:31:51,640
On Earth, we're protected from this
onslaught by an invisible shield

371
00:31:51,640 --> 00:31:57,800
that completely surrounds our
planet, known as the magnetosphere.

372
00:32:02,600 --> 00:32:08,520
The magnetosphere is created deep
within the Earth's molten iron core.

373
00:32:08,520 --> 00:32:12,880
As the core spins, it generates
a powerful magnetic field

374
00:32:12,880 --> 00:32:17,400
which shoots out of the pole
and cocoons the whole planet.

375
00:32:18,920 --> 00:32:25,240
This magnetic shield is
strong enough to deflect most of
the solar wind that comes our way.

376
00:32:27,520 --> 00:32:31,040
Now, we know that at some
point in the past, Mars

377
00:32:31,040 --> 00:32:35,360
would also have had a molten core
and did have a magnetic field.

378
00:32:35,360 --> 00:32:38,480
But because Mars is a
smaller planet than the Earth,

379
00:32:38,480 --> 00:32:42,680
it lost its heat more quickly
and the core solidified.

380
00:32:42,680 --> 00:32:46,800
Electric currents could no
longer flow and its field vanished.

381
00:32:46,800 --> 00:32:52,040
And that was a major factor
in the solar wind being allowed to

382
00:32:52,040 --> 00:32:55,880
blast the planet and
strip away its atmosphere.

383
00:33:04,000 --> 00:33:09,440
With no atmosphere to insulate it,
this once Earth-like world

384
00:33:09,440 --> 00:33:13,240
transformed into the
frozen desert we see today.

385
00:33:15,800 --> 00:33:18,120
A shadow of its former self.

386
00:33:26,760 --> 00:33:30,000
Although Mars has lost
most of its atmosphere,

387
00:33:30,000 --> 00:33:36,920
those few molecules that remain
still have the power to sculpt
and transform the surface.

388
00:33:36,920 --> 00:33:41,400
And that power,
that transformative effect,

389
00:33:41,400 --> 00:33:45,480
is present on every planet in the
solar system that has an atmosphere.

390
00:33:47,000 --> 00:33:53,600
You can see it transforming the
surface of the Namibian desert
today as we speak.

391
00:33:53,600 --> 00:34:00,760
It is, of course, the force
of nature that we call weather.

392
00:34:06,640 --> 00:34:11,120
We've got to go. Wow!

393
00:34:11,120 --> 00:34:14,800
Weather is a feature of every
planet with an atmosphere.

394
00:34:17,480 --> 00:34:23,160
Our world is transformed
as this huge mass of air
moves across its surface.

395
00:34:25,720 --> 00:34:28,520
But as we look out
into the solar system,

396
00:34:28,520 --> 00:34:34,520
we see it only takes
the slightest atmosphere to
produce extraordinary weather.

397
00:34:37,680 --> 00:34:43,840
Every few years,
Mars all but disappears
under a maelstrom of dust.

398
00:34:47,160 --> 00:34:52,240
Global dust storms are so huge
they dwarf Olympus Mons,

399
00:34:52,240 --> 00:34:55,640
a volcano three times bigger
than Everest.

400
00:34:59,640 --> 00:35:04,200
But to experience
the most extreme and violent
weather in the solar system,

401
00:35:04,200 --> 00:35:06,880
we need to travel to Jupiter.

402
00:35:06,880 --> 00:35:13,000
This banded gas giant is over
140,000 kilometres in diameter.

403
00:35:14,720 --> 00:35:20,160
Its atmosphere isn't
a thin blue line, it's many
thousand of kilometres thick

404
00:35:20,160 --> 00:35:24,520
and in a constant state
of seething motion.

405
00:35:27,000 --> 00:35:31,480
The whole surface boils
with gigantic storms.

406
00:35:34,320 --> 00:35:38,760
Yet, this most alien world shares
a feature with our own planet.

407
00:35:40,880 --> 00:35:43,280
RUMBLING

408
00:35:43,280 --> 00:35:48,160
Jupiter crackles to the sound
of electrical storms.

409
00:35:48,160 --> 00:35:53,520
The bolts of lightning are
thousands of times brighter
than lightning here on Earth.

410
00:36:11,240 --> 00:36:16,880
The physics of storms on Jupiter is,
of course, the same as the physics
of storms on Earth.

411
00:36:16,880 --> 00:36:23,640
The warm moist air deep
in the atmosphere starts to rise,
and as it rises it cools.

412
00:36:23,640 --> 00:36:26,640
And the moisture
condenses out to form clouds.

413
00:36:26,640 --> 00:36:31,800
Now, that rising air leaves a gap
beneath it, a low pressure area,

414
00:36:31,800 --> 00:36:36,640
and so more warm, moist
air is sucked in and that
fuels the rise of the storm.

415
00:36:36,640 --> 00:36:41,680
Now, on Earth, those storm systems
are driven by the power of the sun.

416
00:36:41,680 --> 00:36:46,600
But therein lies a mystery because
the storm systems on Jupiter are
far more powerful

417
00:36:46,600 --> 00:36:51,600
and yet Jupiter is five times
further away from the sun
than the Earth is,

418
00:36:51,600 --> 00:36:56,600
which means it receives
25 times less solar energy.

419
00:36:56,600 --> 00:37:01,680
So, what mechanism could it be that
powers those intensely violent
storms on Jupiter?

420
00:37:07,480 --> 00:37:13,640
The secret to Jupiter's storm-tossed
atmosphere lies hidden deep
within the gas giant.

421
00:37:22,440 --> 00:37:27,600
On Earth, we have clear
boundaries between the gaseous sky,

422
00:37:27,600 --> 00:37:31,360
the liquid oceans
and the solid ground.

423
00:37:31,360 --> 00:37:34,360
But on Jupiter, there
are no such boundaries.

424
00:37:39,080 --> 00:37:44,200
It's a gas giant, made of the two
lightest and most abundant elements
in the universe,

425
00:37:44,200 --> 00:37:46,200
hydrogen and helium.

426
00:37:46,200 --> 00:37:49,960
But as you go deep
into Jupiter's atmosphere,

427
00:37:49,960 --> 00:37:54,400
something very strange and
interesting happens to those gases.

428
00:37:58,000 --> 00:38:02,640
Jupiter's atmosphere is so thick and
its gravitational pull so strong

429
00:38:02,640 --> 00:38:06,520
that 20,000 kilometres
beneath the cloud tops,

430
00:38:06,520 --> 00:38:12,160
the pressure is 2,000,000
times greater than the surface
pressure here on Earth.

431
00:38:14,400 --> 00:38:18,120
Under such immense pressure,
the hydrogen gas in the atmosphere

432
00:38:18,120 --> 00:38:22,400
is transformed
into a strange metallic liquid.

433
00:38:24,400 --> 00:38:29,480
As the gases are squeezed,
a vast amount of energy is released,

434
00:38:29,480 --> 00:38:34,200
enough energy to fuel some of the
biggest storms in the solar system.

435
00:38:37,840 --> 00:38:41,600
The biggest of them all
is the Great Red Spot.

436
00:38:45,920 --> 00:38:50,800
This giant anti-cyclone
has raged for hundreds of years

437
00:38:50,800 --> 00:38:54,960
and is large enough to swallow
the Earth three times over.

438
00:38:57,240 --> 00:39:01,560
The Great Red Spot is an
awesome sight.

439
00:39:01,560 --> 00:39:06,120
But this giant
isn't one of my wonders.

440
00:39:06,120 --> 00:39:09,280
My wonder is a much smaller world.

441
00:39:11,160 --> 00:39:17,320
A moon that orbits the gas giant
Saturn, 1.5 billion kilometres
from Earth.

442
00:39:19,240 --> 00:39:25,200
What we have found on this
small world is simply astonishing.

443
00:39:48,280 --> 00:39:55,800
If you thought of our moon as
the archetypal moon of the solar
system, if you like,

444
00:39:55,800 --> 00:40:01,280
then... Well, you might think that
all the other moons out there,

445
00:40:01,280 --> 00:40:06,360
hundreds of them, would be dead,
uninteresting worlds.

446
00:40:06,360 --> 00:40:09,040
I mean not uninteresting
places to visit.

447
00:40:09,040 --> 00:40:14,480
I mean that is, in my view,
the greatest thing that
humans have ever achieved,

448
00:40:14,480 --> 00:40:22,120
landing on the surface of the moon
but it's a dead and lifeless place.

449
00:40:22,120 --> 00:40:26,240
But as we've begun to visit those
worlds, as we've flown spacecraft

450
00:40:26,240 --> 00:40:33,040
to within hundreds of miles of their
surfaces, we've found that the moons
in the outer solar system

451
00:40:33,040 --> 00:40:40,800
are of an astonishingly interesting
and varied and fascinated
bunch of worlds.

452
00:40:40,800 --> 00:40:44,000
This is Jupiter's moon, Europa.

453
00:40:44,000 --> 00:40:49,720
This is Jupiter's moon, Io, the most
volcanic object in the solar system.

454
00:40:49,720 --> 00:40:56,360
But of all the worlds out there,
this one - Saturn's moon, Titan -
is unique, because of that.

455
00:40:56,360 --> 00:41:00,520
That is an atmosphere,
and what an atmosphere it is!

456
00:41:00,520 --> 00:41:07,040
It's 1,000 kilometres deep,
it's four times denser than
the atmosphere of the Earth.

457
00:41:07,040 --> 00:41:09,160
I mean imagine that,

458
00:41:09,160 --> 00:41:15,880
a moon around a distant planet
in the icy, distant reaches
of the solar system

459
00:41:15,880 --> 00:41:21,160
with an atmosphere
denser and thicker than our own.

460
00:41:28,360 --> 00:41:34,400
Titan has the most
Earth-like atmosphere in
the entire solar system,

461
00:41:34,400 --> 00:41:40,240
a thick blue line, rich in
nitrogen and containing methane.

462
00:41:41,760 --> 00:41:48,120
At first sight, a world this small
shouldn't be able to hold
onto such a dense atmosphere,

463
00:41:48,120 --> 00:41:53,280
except Titan lies in one of the
coldest regions of the solar system,

464
00:41:53,280 --> 00:41:56,320
and that makes all the difference.

465
00:42:02,400 --> 00:42:07,120
Temperature for gases like this,
the gases in our atmosphere,

466
00:42:07,120 --> 00:42:12,080
is really a measure
of how fast the molecules
of the gas are moving around,

467
00:42:12,080 --> 00:42:18,600
and I can demonstrate that with this
thing, which is a Chinese lantern.

468
00:42:20,960 --> 00:42:23,880
If I light this fuel,

469
00:42:23,880 --> 00:42:25,560
then what's going to happen...

470
00:42:27,320 --> 00:42:31,640
is that the gas inside
is going to heat up.

471
00:42:33,440 --> 00:42:37,200
And as you heat up a gas,

472
00:42:37,200 --> 00:42:41,680
what that basically means is that
you speed all the molecules up.

473
00:42:43,920 --> 00:42:48,200
As the molecules of air heat up and
move faster,

474
00:42:48,200 --> 00:42:51,640
the air pressure inside
the lantern begins to increase.

475
00:42:51,640 --> 00:42:58,640
That means that molecules are
forced out, making the air inside
less dense than the air outside,

476
00:42:58,640 --> 00:43:01,360
and the lantern gets lighter.

477
00:43:03,200 --> 00:43:07,600
And eventually the lantern
is so light...

478
00:43:09,120 --> 00:43:15,000
..that it will just float away in
the atmosphere of our planet.

479
00:43:35,680 --> 00:43:42,480
Hot gases have more energy to escape
a planet's gravitational pull
than cold gases.

480
00:43:42,480 --> 00:43:50,080
Now Titan is a much smaller
body than the Earth. It has much
weaker gravitational pull,

481
00:43:50,080 --> 00:43:53,320
and if it were in the same region
of the solar system as we are,

482
00:43:53,320 --> 00:43:56,320
then it would not be able to
hold onto its atmosphere.

483
00:43:56,320 --> 00:44:00,400
But it's a lot further away
from the sun than we are

484
00:44:00,400 --> 00:44:07,720
and so that means that it's colder,
its atmospheric molecules are moving
around much more slowly than ours.

485
00:44:07,720 --> 00:44:14,760
That means that its weak gravity
is enough to hold on to that
thick dense atmosphere.

486
00:44:17,000 --> 00:44:21,800
Titan's thick atmosphere
was an unexpected discovery,

487
00:44:21,800 --> 00:44:24,240
but it took an audacious mission

488
00:44:24,240 --> 00:44:28,880
to reveal the world that lies
beneath the blanket of clouds.

489
00:44:32,120 --> 00:44:36,720
We have lift off of the Cassini
spacecraft on a billion-mile
trek to Saturn.

490
00:44:36,720 --> 00:44:41,200
In 1997,
Cassini began its journey to Titan.

491
00:44:44,560 --> 00:44:51,920
It carried with it the Huygens
probe, a lander designed to
set down on this frozen moon.

492
00:44:57,080 --> 00:44:59,120
On Christmas Day 2004,

493
00:44:59,120 --> 00:45:04,720
Huygens was released from Cassini
and it began the bumpy ride

494
00:45:04,720 --> 00:45:08,680
through one of the most intriguing
atmospheres in the solar system.

495
00:45:11,800 --> 00:45:14,120
And then, for the first time,

496
00:45:14,120 --> 00:45:19,280
the thick clouds parted and
the surface of Titan was revealed.

497
00:45:27,720 --> 00:45:31,720
These are the actual images
taken by Huygens

498
00:45:31,720 --> 00:45:35,320
as it slowly
parachuted to the surface.

499
00:45:40,680 --> 00:45:42,240
The world it revealed

500
00:45:42,240 --> 00:45:46,360
was more familiar than we
could have possibly imagined.

501
00:45:59,600 --> 00:46:01,360
One of the first people to see

502
00:46:01,360 --> 00:46:05,240
these incredible images was a man
who helped design the probe,

503
00:46:05,240 --> 00:46:07,440
Ralph Lorenz.

504
00:46:07,440 --> 00:46:11,720
It was amazing because we
just had no idea what to expect.

505
00:46:11,720 --> 00:46:18,120
We didn't know whether it would be,
you know, cratered like the moon or
just sort of a flat expanse of sand

506
00:46:18,120 --> 00:46:22,720
and then these first pictures
came back and it was just
astonishingly familiar.

507
00:46:22,720 --> 00:46:27,120
Did that picture,
that initial series of pictures...

508
00:46:27,120 --> 00:46:30,720
I suppose it did look somewhat
like this, didn't it? It did.

509
00:46:30,720 --> 00:46:34,520
It could have been there.
It could have been right here.
I do see that.

510
00:46:34,520 --> 00:46:36,000
I could sit here,

511
00:46:36,000 --> 00:46:38,480
look at that and that's what
that picture looks like.

512
00:46:38,480 --> 00:46:40,320
I could take it with a camera.

513
00:46:40,320 --> 00:46:44,480
The camera on the probe was about
the height of your knee, so yeah,

514
00:46:44,480 --> 00:46:47,680
the view the Huygens probe had
is just like this.

515
00:46:47,680 --> 00:46:50,240
Rounded stones dot the landscape.

516
00:46:50,240 --> 00:46:56,360
They're smooth and look like they
have been eroded by tumbling water,

517
00:46:56,360 --> 00:46:59,760
similar to stones found on
river beds, here on Earth.

518
00:47:01,640 --> 00:47:05,720
It sounds to me like this was one of
the easiest pictures to interpret

519
00:47:05,720 --> 00:47:07,960
in the history of space exploration.

520
00:47:07,960 --> 00:47:14,000
You know, the way you tell it, it's
just that's a river bed with these
stones. I mean, is it that simple?

521
00:47:14,000 --> 00:47:16,160
Because you can be misled easily,
with...

522
00:47:16,160 --> 00:47:18,440
The devil is always in the details,

523
00:47:18,440 --> 00:47:21,000
but I think there were
very few people

524
00:47:21,000 --> 00:47:24,720
disputed the interpretation
of a river channel.

525
00:47:24,720 --> 00:47:29,840
I mean it's just such a familiar
thing to so many people on Earth,
there really wasn't much doubt.

526
00:47:32,040 --> 00:47:35,360
It was an extraordinary discovery.

527
00:47:35,360 --> 00:47:40,000
Evidence of flowing rivers had
never been found before on a moon.

528
00:47:41,600 --> 00:47:46,480
But it wasn't the only surprise
Titan held in store.

529
00:48:05,040 --> 00:48:08,720
This is the Matanuska glacier
in Alaska.

530
00:48:10,720 --> 00:48:15,880
It really is one of the most
astonishing places I've ever seen.

531
00:48:18,240 --> 00:48:25,640
And this whole landscape
is testament to the erosive power
of this stuff,

532
00:48:25,640 --> 00:48:30,000
this mixture of ice and rock

533
00:48:30,000 --> 00:48:34,560
as it rolls down this valley over
hundreds of thousands of years

534
00:48:34,560 --> 00:48:37,640
and creates this
astonishing landscape.

535
00:48:46,840 --> 00:48:48,200
But the reason it can do that

536
00:48:48,200 --> 00:48:52,680
is because of the delicate balance
of the Earth's atmosphere.

537
00:48:52,680 --> 00:49:00,280
You see, our planet is just at
the right temperature and pressure
to allow water to exist as solid,

538
00:49:00,280 --> 00:49:05,480
as liquid and as gas,
as vapour in the clouds.

539
00:49:05,480 --> 00:49:12,600
And so the sun can heat up the
oceans and it can move the water
over the top of the mountains.

540
00:49:12,600 --> 00:49:17,680
It can fall as rain, turn to ice,
become a glacier

541
00:49:17,680 --> 00:49:23,320
and then sweep down the valley to
sculpt this astonishing landscape.

542
00:49:34,400 --> 00:49:39,160
Just as our atmosphere
allows all this to exist,

543
00:49:39,160 --> 00:49:45,720
the atmosphere of Titan
is the perfect temperature and
pressure to allow something to exist

544
00:49:45,720 --> 00:49:50,200
that has never been seen before
on a world beyond Earth.

545
00:49:56,280 --> 00:50:00,280
This is a picture taken
of the south pole of Titan

546
00:50:00,280 --> 00:50:02,320
by Cassini in June 2005,

547
00:50:02,320 --> 00:50:08,720
and it's become
one of the most important
and fascinating pictures

548
00:50:08,720 --> 00:50:12,360
in the history of
space exploration.

549
00:50:12,360 --> 00:50:15,520
The interesting thing is
this black blob, here.

550
00:50:17,120 --> 00:50:22,760
Now this fascinated the Cassini
scientists but the explanation
as to what that is

551
00:50:22,760 --> 00:50:26,720
had to wait just over a year
till July 2006,

552
00:50:26,720 --> 00:50:28,920
when this picture was taken,

553
00:50:28,920 --> 00:50:33,240
and it's a radar image, this time
of the north pole of Titan,

554
00:50:33,240 --> 00:50:37,960
and you see, again,
these huge black areas.

555
00:50:37,960 --> 00:50:44,280
The black in this case means
that the radar waves that
bounced onto them didn't come back

556
00:50:44,280 --> 00:50:49,400
so they're completely black,
and there's only one
really good explanation for that.

557
00:50:49,400 --> 00:50:54,320
That is that they are incredibly
flat surfaces.

558
00:50:54,320 --> 00:50:58,120
In fact, they're surfaces of liquid

559
00:50:58,120 --> 00:51:02,320
so this picture combined
with this picture

560
00:51:02,320 --> 00:51:08,480
means that this is the first
observation of a liquid,

561
00:51:08,480 --> 00:51:14,280
a lake on the surface of a body
other than the Earth
in the solar system.

562
00:51:18,960 --> 00:51:23,880
But these lakes, of course, cannot
be lakes of liquid water because

563
00:51:23,880 --> 00:51:28,840
the surface temperature on Titan
is minus 180 degrees Celsius and,
at those temperatures,

564
00:51:28,840 --> 00:51:34,440
water is frozen as hard as steel.

565
00:51:34,440 --> 00:51:39,240
So if these are not lakes of water,
then what are they?

566
00:52:03,040 --> 00:52:08,960
This is Lake Eyak in Alaska,
just on Prince William Sound,

567
00:52:08,960 --> 00:52:15,560
and I've come here to collect
a molecule or a substance that's
very abundant on Titan.

568
00:52:15,560 --> 00:52:19,800
In fact, it's abundant throughout
the solar system, but here on Earth

569
00:52:19,800 --> 00:52:24,520
it exists as a gas and it bubbles up
from the floor of this lake.

570
00:52:37,240 --> 00:52:44,760
The floor of Lake Eyak is covered in
rotting vegetation, you know, dead
leaves and bits of trees, twigs,

571
00:52:44,760 --> 00:52:47,960
and that's been broken down by
bacteria which produce the gas

572
00:52:47,960 --> 00:52:50,720
that bubbles up
from the floor of the lake.

573
00:52:50,720 --> 00:52:55,240
That gas is methane and we've
been collecting it all night

574
00:52:55,240 --> 00:52:57,760
underneath this upturned boat

575
00:52:57,760 --> 00:53:01,200
so that I can take a sample of it in
this bag.

576
00:53:03,920 --> 00:53:09,120
Now, on Earth,
methane is very unstable.

577
00:53:09,120 --> 00:53:10,960
If you give it...

578
00:53:10,960 --> 00:53:13,840
a little kick...

579
00:53:13,840 --> 00:53:18,800
in the presence of oxygen,
then you get what chemists
call an exothermic reaction.

580
00:53:18,800 --> 00:53:24,480
Methane plus oxygen goes to water
plus carbon dioxide, and...

581
00:53:24,480 --> 00:53:26,000
some energy.

582
00:53:28,040 --> 00:53:31,160
The Earth's temperature
and atmospheric pressure

583
00:53:31,160 --> 00:53:35,200
means methane can only exist
as a highly-flammable gas.

584
00:53:39,160 --> 00:53:42,680
But Titan's atmospheric pressure
and temperature

585
00:53:42,680 --> 00:53:48,840
is perfect to allow methane
to exist as a solid,
a gas and, most importantly,

586
00:53:48,840 --> 00:53:50,840
a liquid.

587
00:53:54,280 --> 00:54:00,280
So the images Cassini captured were
gigantic lakes of liquid methane...

588
00:54:02,960 --> 00:54:05,840
..the first ever liquid discovered

589
00:54:05,840 --> 00:54:10,840
pooling on the surface of another
world in the solar system.

590
00:54:10,840 --> 00:54:13,840
This is Kraken Mare.

591
00:54:13,840 --> 00:54:17,160
At over 400,000 square kilometres,

592
00:54:17,160 --> 00:54:21,160
it's the biggest
body of liquid on Titan.

593
00:54:22,960 --> 00:54:26,920
It's almost five times
the size of Lake Superior,

594
00:54:26,920 --> 00:54:29,600
North America's greatest lake.

595
00:54:45,320 --> 00:54:52,920
On Titan,
methane plays exactly the same role
that water does here on Earth.

596
00:54:55,440 --> 00:54:59,720
So, where we have clouds of water,

597
00:54:59,720 --> 00:55:04,400
Titan has clouds of methane
with methane rain.

598
00:55:04,400 --> 00:55:12,040
Whereas we have
lakes and oceans of water,
Titan has lakes of liquid methane.

599
00:55:12,040 --> 00:55:18,240
And whereas, here on Earth,
the sun warms the water
in the lakes and oceans,

600
00:55:18,240 --> 00:55:21,440
and fills our atmosphere
with water vapour,

601
00:55:21,440 --> 00:55:24,040
on Titan the sun lifts the methane

602
00:55:24,040 --> 00:55:29,000
from the lakes and saturates
the atmosphere with methane.

603
00:55:29,000 --> 00:55:35,880
So, whereas on Earth we have a
hydrological cycle, on Titan
there's a methanological cycle.

604
00:55:38,040 --> 00:55:42,400
And rain would be an
absolutely magical sight on Titan.

605
00:55:42,400 --> 00:55:47,680
Because the atmosphere is so dense
and the gravity of the moon
is so weak,

606
00:55:47,680 --> 00:55:52,920
the drops of methane rain would
grow to over a centimetre in size

607
00:55:52,920 --> 00:56:00,320
and they would fall to the ground
as slowly as snowflakes fall
onto the surface of our own planet.

608
00:56:02,280 --> 00:56:07,720
Thousands and thousands
of gallons of liquid methane

609
00:56:07,720 --> 00:56:11,320
must have slowly rained down
onto the surface,

610
00:56:11,320 --> 00:56:16,000
making rivers and streams
swell and burst.

611
00:56:16,000 --> 00:56:20,480
Deep gullies were cut
into the frozen water landscape...

612
00:56:20,480 --> 00:56:23,960
Which looks so familiar
because it is familiar.

613
00:56:23,960 --> 00:56:31,960
It's this. You know, the atmosphere
of Titan shapes the surface
in exactly the same way

614
00:56:31,960 --> 00:56:37,240
that the atmosphere here on Earth
shapes the surface of our planet.

615
00:56:48,360 --> 00:56:54,480
Titan is like a primordial Earth
caught in a deep freeze.

616
00:56:54,480 --> 00:56:58,880
It's almost like looking back in
time over four billion years

617
00:56:58,880 --> 00:57:04,960
and observing our planet
before life began,
and began to modify our atmosphere,

618
00:57:04,960 --> 00:57:09,680
to change it into the oxygen-rich
atmosphere that we see today.

619
00:57:13,600 --> 00:57:18,480
In many ways,
Titan looks so familiar.

620
00:57:18,480 --> 00:57:24,280
It's a place with rivers
and lakes and clouds and rain.

621
00:57:24,280 --> 00:57:28,800
It's a place with water, albeit
frozen as hard as steel,

622
00:57:28,800 --> 00:57:35,960
and a place of methane, albeit so
cold that methane is now a liquid

623
00:57:35,960 --> 00:57:41,560
and flows and shapes the landscape
just like water does here on Earth.

624
00:57:45,920 --> 00:57:50,040
For me, the most
important thing about Titan

625
00:57:50,040 --> 00:57:54,120
is we now have two Earth-like
worlds in our solar system

626
00:57:57,520 --> 00:58:03,520
One in this warm region,
93 million miles away from the sun,

627
00:58:03,520 --> 00:58:08,720
and the other in deep freeze,
a billion miles away from our star

628
00:58:08,720 --> 00:58:14,760
in orbit around another planet,
and that must greatly increase
the probability

629
00:58:14,760 --> 00:58:17,960
that there are other
Earth-like planets in orbit

630
00:58:17,960 --> 00:58:24,600
around the hundreds of billions of
stars out there in the universe.

631
00:58:32,120 --> 00:58:38,840
# Somewhere over the rainbow

632
00:58:38,840 --> 00:58:45,800
# Skies are blue

633
00:58:45,800 --> 00:58:53,800
# And the dreams
that you dare to dream

634
00:58:53,800 --> 00:58:56,920
# Really do come true. #

635
00:58:56,920 --> 00:58:59,160
E-mail subtitling@bbc.co.uk

636
00:59:00,160 --> 00:59:10,160
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