All language subtitles for Wonders of the Solar System E04 Dead or Alive 1080p AC3+DTS_Subtitles06.ENG

We live on a world of wonders,

a place of astonishing
beauty and complexity.

We have vast oceans,
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 ecosystem
that extends way beyond

the top of our atmosphere.

I think we're living through
the greatest age of discovery
our civilisation has known.

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

We've photographed
strange new worlds,

stood in unfamiliar landscapes,
tasted alien air.

Now, the laws of physics are
simple and they're universal.

What applies here
applies everywhere else up there.

But it's fascinating that they
can manifest themselves

in so many different ways
across the solar system.

In this programme,
I'm going to look at how
the universal forces of nature

that created all this can also wreak
devastation across the solar system.

How they can be the death
of a planet...

..and how they can keep
other worlds alive.

I can just see pieces of molten rock
rising up just below my chin.

These forces are so far reaching,
they bridge the depths of space

and transform a world
long thought dead

into a world of perpetual change
and everlasting youth.

The intense source of heat that
powers that eternal change also

drives one of the most spectacular
sights in the solar system.

I've come to one of
Earth's natural wonders,

the Grand Canyon in Arizona.

It's a place to be
humbled by nature.

Well, this is undoubtedly

one of the most beautiful places
I've seen on Earth.

From sunrise to sunset,

the changing light brings
this immense landscape to life.

I hear stories of people coming here
at sunrise with tears in their eyes

at the majesty of the view,
and I can see why.

It's incredible to think
that this enormous valley

was etched and carved

by the action of running water
over just a few million years.

Our knowledge of natural
wonders like the Grand Canyon

was once limited to our own planet.

But the space age has brought
new worlds into view.

This is Mars, the Red Planet.

Fourth rock out from the sun,
it has a canyon so vast

you could fit our own Grand Canyon
into one of its side channels.

Named after the space probe
that first saw it,

this is the Valles Marineris.

8km deep
and over 3,000km long,

on Earth it would run all the way
from Los Angeles to New York.

We're beginning to get a deep
and quite profound understanding

of the way that Mars
has evolved geologically because

we are now there, because we now
have eyes and ears on the surface.

And there really is no substitute
for actual exploration, for actually

going somewhere and touching it
and taking pictures of it.

Look at this picture.

This is a picture of a sunset.

These are pictures of clouds.

It's amazing to think
these incredibly
familiar-looking photographs

were taken on the surface of Mars.

This picture,

or this amazing colour picture...

I could have got a camera here
and just snapped any picture

and it would have looked
exactly like this one.

In the Grand Canyon,
you can see the Colorado River

running in the bottom of the valley

so you can understand
how this landscape was made.

Whereas here on Mars,
there's no sign of any water.

So Mars is a puzzling place.

Despite all the similarities
between Mars and Earth,

it's the differences between these
two planets that are most telling.

Mars is now
a desolate dead wasteland,

a world where the processes that
sculpted its familiar landscapes

seized up long ago.

It's fascinating for me
as a physicist to see how

the same basic simple
laws of nature can play out

in such radically different ways,
and produce such astonishingly

varied and beautiful
and violent and dead worlds

out there, across the solar system.

The Big Island of Hawaii,
in the middle of the Pacific Ocean,

holds the key
to what happened to Mars.

This is the perfect place to witness
how a planet can be kept alive

by nothing more
than the simple flow of heat.

You can smell the volcanic ash
coming into the...

into the helicopter. And everywhere
you look, it looks like a...

It's almost like
an apocalyptic scene.

This volcano is Kilauea,
which means spewing.

It's been erupting almost
continuously since 1983.

In fact, you can see molten rock
flowing down the side

of the mountain just cutting
a swathe through the trees.

This might look like
widespread destruction,

but volcanic eruptions
are Earth's geological heartbeat.

Active volcanoes make our planet
a vibrant, living world.

That is the most
spectacular demonstration

of our planet being geologically
alive that I've ever seen.

A few kilometres north of Kilauea,

you can see just what
volcanic action can produce,

given enough time.

The islands of Hawaii were built
entirely by volcanoes.

Today, these mountains are the
largest volcanoes on our planet,

and we've seen landscapes
just like this, on Mars.

It's quite an experience
being 4km high.

It makes you out of breath
and you sniff a lot
because your nose becomes...

And makes you feel like
you've had a few drinks.

This is Mauna Kea,

one of the five volcanoes that
make up the Big Island of Hawaii.

I've known about it
since I was very little

because it's one of the most famous
observatories on the planet.

Everywhere you look, surrounded by
our eyes to the cosmos.

Although this mountain is 4km
above the surface of the Pacific,

it's actually 10km above
the surface of the Pacific floor.

That makes it
the highest mountain on Earth,

over a kilometre higher
than Mount Everest.

But it is a tiny volcano
compared to the biggest volcano
on the surface of Mars.

This is Olympus Mons,

named after Mount Olympus,
the mythical home of the Greek gods.

This vast outpouring of lava
stretches over 550km across,

but it's the height of this volcano
that is breathtaking.

It soars 25km into
the Martian sky,

nearly three times
the full height of Mauna Kea.

But Olympus Mons isn't just the
tallest volcano in the solar system,

it's the highest mountain we know.

There are striking similarities
between the volcanic landscapes

here on Hawaii and the giant
volcanoes found on Mars.

These similarities can be traced
back billions of years

to the fiery birth
of the solar system.

All this heat you can see driving
this spectacular volcanic activity

is a relic,
a hangover of the Earth's formation.

Now, all the rocky planets -
Earth, Mars, Venus, Mercury -
were formed in the same way.

They came from a collapsing dust
cloud over 4.5 billion years ago.

With the ignition of the sun,
our solar system was born.

Little by little,
the rocky bodies grew,

falling together
under their own gravity.

This process not only generated
immense amounts of heat,

it also delivered radioactive
material to the cores.

These two ancient sources of heat
power Earth's volcanoes to this day,

but the volcanoes on Mars are
little more than a petrified memory
of a distant past.

When we look down
on the surface of Mars,

we see no evidence of this kind
of volcanic activity.

As far as we can tell,
Mars is a dead world.

Mars must have had similar
inner heat to Earth
to build its volcanoes.

Yet something obviously happened

to stop the Red Planet's
geological heartbeat.

Now, as everyone who's left
a hot cup of tea sat on
the kitchen table knows,

hot things lose heat
to their cooler surroundings,

and what's true for cups of tea,
in physics is also true for planets.

This is hot and that up there -
space - is cold.

So planets lose heat to space.

Mars is a much smaller planet
than the Earth.

It's about half the diameter,
it's an eighth of the volume,

so there was much less heat
trapped in there to begin with.

Now, planets lose heat to space
through their surfaces,

and smaller things have
a larger surface area

in relation to their volume
than big things.

So that means that Mars
will lose its heat to space
much quicker than the Earth does.

When the interior of Mars grew cold,

the mighty volcanoes
lost their lifeblood.

The Red Planet's
geological heart died,

and millions of years ago, the
surface of Mars ground to a halt.

The fate of a whole planet
was destined

by the simplest of laws of physics.

Since the dawn of human history,
we've been able to gaze up
into the night sky,

but we're lucky because we're the
first generation that's been able

to build machines to actually
go to those planets and moons.

And we've found that they're
more beautiful, more violent,

more magnificent and fascinating

than we could have
possibly imagined.

The more worlds we study,
the more we realise

that our solar system
is a cosmic laboratory.

Even the slightest differences
in size or position

can create a world radically
different from its neighbours.

So here on Earth, we have one
beautiful manifestation of how those

simple laws of nature can play out,
how they can build a planet.

In Mars we have another example.

What happens when you take a planet
that's smaller than Earth

and move it
further away from the sun?

It loses its heat more quickly and
it becomes geologically inactive.

But what would happen if you took
a planet just like the Earth

and moved it
a little closer to the sun?

Well, we know of such a planet.

It's the brightest point
of light in our night sky.

So similar in size to our own world,

this planet has been
called Earth's twin.

This is Venus.

Orbiting closer to the sun, Venus
was named for its shining beauty.

But our planetary twin hides
its true identity

beneath a thick blanket of cloud.

Over the last 4.5 billion years,

Venus has turned into
an unimaginably oppressive world.

The atmosphere is so dense that
the pressure is crushing.

It's 90 times atmospheric pressure
here on Earth.

Now, Venus takes 243 days
to rotate once on its axis.

That means its day
is longer than its year.

So Venus has the hottest
average surface temperature,

other than the sun's, anywhere
in the solar system - 470 Celsius.

I've come to India, to a place
called the Deccan Traps.

Hidden in this lush green landscape
are tantalising clues

to understanding how immense heat
caused Venus to choke to death.

When you look at
this landscape today,

it's incredibly peaceful and
beautiful, rolling green hills,

but I think
it's astonishing to think

that everything you see down there
is lava.

This whole landscape, half a million
square kilometres of it, was created

in an ongoing volcanic eruption
that lasted for a million years.

If you take away the green foliage,
the underlying landscape of lava

is actually very similar
to what we see on Venus.

Using radar
to peer down through the clouds,

the surface of Venus
was finally revealed.

It's covered with floods of solid
lava, just like we see in India,

but on a scale many thousands
of times larger.

We've also counted
over 50,000 volcanoes,

the most on any planet
in the solar system.

Venus is a similar size to Earth,

so it may still have
a hot geological heart

powering its volcanoes but as yet
we haven't witnessed any eruptions.

The ancient floods of lava
we see on Venus

and here in India were created
in much the same way.

For both planets, this was
volcanic activity in overdrive.

The eruptions here in
India 65 million years ago

affected the Earth's climate so much
that they're thought to have played

a major role in the mass
extinction events at the end of
the cretaceous period,

which wiped out over two-thirds
of the species on Earth.

Now, life on Earth recovered
but Venus wasn't so lucky.

The intense volcanic activity
on both planets

didn't just blast out molten rock.

It also released copious amounts
of gases, like carbon dioxide.

But slight differences in the way
the laws of physics played out

on Venus helped push our cosmic twin
down a path of no return.

Venus and Earth reacted very
differently to the same kind
of volcanic cataclysm,

and the reason for that is something
that happens so often on Earth

that we take it for granted,
and just moan about it.

Rain plays a significant role
in keeping our planet
a pleasant place to live.

Acting as part of
a global recycling system,

rain keeps our atmosphere
in balance,

washing out the potent
greenhouse gas, carbon dioxide,

ready to be locked away
in rocks in our oceans.

But on Venus, the laws of physics
have made it impossible

for rainfall
to cleanse its atmosphere.

In fact,
there's no liquid water at all.

Venus lost its water essentially
because it's hotter than the Earth.

You see, temperature is
just a measure of how fast
things are moving around.

So on Venus the oceans would have
evaporated into the atmosphere

and that water in the atmosphere
would be moving around

extremely quickly,
simply because it's hot.

And Venus is so close to the sun
and so hot,

those water molecules are moving so
fast that the gravity of the planet

can't continue to hold them
in the atmosphere,

and so they simply escape
off into space.

With no water,
there is no rain on Venus.

For billions of years,
there has been nothing

to temper the build-up
of volcanic gases in its atmosphere.

Venus ended up cocooned in
a thick, high-pressure,

dense blanket of carbon dioxide,

and that made the temperature
rise and rise and rise,

turning Venus into the
hell-like world we see today.

Compared to scorched Venus
and frozen Mars,

our home is a very special
ball of rock.

Although governed by
the same universal set of rules,

our planet is not too big,
not too small,

not too hot, not too cold.

Earth has been called
the Goldilocks planet

because everything is just right.

Our world is unique but it
doesn't exist in splendid isolation.

It is intimately connected
with its cosmic neighbours.

Earth is not the master of its
own destiny and it never has been.

The life and death of our planet
is influenced by forces

emanating from
the very depths of space.

As we better understand
our place in space,

we've come to realise that our
sharing of the same physical laws

with the other worlds
in the solar system

isn't the only connection
we have with them,

because those same laws lead to
a direct physical connection

between the Earth
and the other worlds out there

that is subtle, is complicated, but
can sometimes be extremely powerful.

Out in the farthest reaches of
the solar system are vast worlds

that have a direct impact
on our very existence.

This is our sun from
ten billion kilometres away -

just another star
in a sea of stars.

But as you head towards the light,
you enter a realm of giants.

The furthest planet from the sun
is icy Neptune,

with its thick, blue
methane atmosphere.

Uranus comes next, the only planet
that lies on its back.

Further in towards the sun,
and the planets get even bigger.

Saturn,
with its beautiful rings of ice.

Finally we reach the king
of the giants - Jupiter.

Jupiter is the largest planet
in the solar system,

so big you could fit Earth
inside it over 1,000 times.

It's made up of the same stuff
as our sun - hydrogen and helium -

the most common elements
in the universe.

In its thick churning atmosphere,

gigantic storms have
raged for centuries.

Now, astrologers have said for years
that Jupiter influences our lives,

but we now have scientific evidence
that this mighty planet does have

a significant connection
with our own small world.

Jupiter is so different
to our planet -

you know, a big ball of gas,
half a billion kilometres away -

it's difficult to see how it could
have anything to do with us at all.

But despite the fact that
astrology is a load of rubbish,

Jupiter can, in fact, have a
profound influence on our planet

and it's through a force that, well,
surrounds us and penetrates us

and binds the galaxy together -
gravity.

Gravity is one of the fundamental
forces of nature.

It exists between all objects,
and the effects

of a gravitational field extend way
beyond the planet that creates it.

Gravity is by far the weakest force
of nature. But it's the only force

that has an influence
across the entire solar system,

and that's because, although
it's weak, it has an infinite range.

It never quite goes away.

So, no matter how far you go,
you feel the force of gravity

as a planet, although it drops
and drops and drops away.

Jupiter has the most powerful
gravitational field
of all the planets,

and it's the gas giant's gravity
that can directly influence

the orbits of asteroids
and other wandering space debris.

Jupiter is so massive,
by far the most massive planet
in the solar system,

that its gravitational field
can have a profound influence on
passing interplanetary stuff.

It can do three things.

Firstly, it can capture the stuff,
literally hoover it up.

Secondly,
it can deflect the stuff,

such that it throws it out
of the solar system.

But thirdly,
and most importantly for us,

it can deflect stuff onto a direct
collision course with our planet.

Influenced by Jupiter's gravity,

today, right now, we're highly
vulnerable from asteroid impacts.

But we do have sentinels
standing guard.

On top of the mountain of Heleakala
on the Hawaiian honeymoon
island of Maui,

I've come to see Professor
Nick Kaiser, who's searching

our solar system for
potentially hostile space debris.

The prime task is to try
and find killer asteroids,

things that are out there
in the solar system

that might hit the Earth.

There's an air of Hollywood about
it, isn't there, in some sense?

Well, that's right. I would say
a lot more resources have been spent

on making movies about killer
asteroids than actually finding them.

Anything that's a kilometre
in size, if it hit the Earth
it would be devastating.

It would probably kill
nearly everyone on the planet.

Each night, using a revolutionary
billion-pixel digital sensor,

the team scans
a vast swathe of the sky.

Mind your head!

They're looking for
any unidentified objects
that might be heading our way.

So any one of these points of light
could in fact be an asteroid?

That's right. And it's very likely,

in fact, it's almost certain that
there are asteroids in that image.

The problem is, how do you
figure out which ones they are?

Yeah.

The camera captures several
images of the same patch of sky
taken minutes apart.

The team can then see
if anything has moved,

relative to the background of stars.

What we've done here is taken
two images and subtracted them,

and you see the stars
have nearly all gone away.

There's a couple of
interesting things left.

If you look over here, you see
a dark thing and a white thing,

so that's something which was
THERE in the first image

and THERE in the second image.
And there's another one here.

So even on this tiny patch of sky,
we've already detected two objects.

What that means is, when we do
that kind of analysis

on a whole field of view,
we'll detect hundreds of objects.

The beauty of our night sky
belies the potential danger
it holds in store.

Over 2,000 objects have been
identified that pass close
to the Earth,

with something like 400 that could
be on a future collision course.

And all of these menacing lumps
of rock, at some point,

come under Jupiter's
gravitational influence.

Now, if you ever needed
a demonstration of how congested
the space is near the Earth,

just look at this movie
of the near-Earth objects,

so here's Mercury, Venus, the Earth,
Mars and out here is Jupiter.

Here's the asteroid belt, but look
at the congestion in there.

Every one of those points of light
is an asteroid that we know of.

Just look at the Earth swimming
through them, so when you look up
into the nice clear night sky and

you want to be reassured
that we're all nice and safe,

just remember this movie.

Our planet is on a deadly journey.

Earth is trapped in a cosmic game
of dodgeball as it orbits the sun,

a game where the gravitational
stranglehold of Jupiter regularly
throws asteroids our way.

Jupiter's gravitational
influence on passing space debris

has made our planet a world
under constant bombardment.

One of the most famous
meteorite impact sites
is the Barringer crater in Arizona.

50,000 years ago,

a 300,000 ton, 50 metre in diameter
lump of iron and nickel
entered the Earth's atmosphere

and made this crater,
and it should remind us

that our environment doesn't just
stop at the top of our atmosphere.

Our environment stretches out
into the solar system,

to the very edges of
the solar system,
to wherever this rock came from.

In the grand scheme of things,

the asteroid that struck here
was relatively small and innocuous.

But there are much larger impact
craters hidden in Earth's landscapes

that have far more
devastating tales to tell.

200 years ago, white settlers
crossed the ancient Appalachian
mountains here in America

to seek new land out west.

Little did they know
what they were walking into.

Well, this place to me feels
like the very definition
of small town America.

It's on the border between
Virginia, Kentucky and Tennessee.

It's the kind of town
where you feel that

nothing much has changed
for the last 100 years.

But this place was the site,

way back in history, of a violent
cosmic intervention.

This is Middlesboro, Kentucky.

It's a town built inside
a meteorite impact crater.

The asteroid that struck here
would have been huge, about
half a kilometre across,

hitting the Earth well over
200 million years ago.

I find it fascinating that when
you look out of this view, you
don't really see an obvious crater.

And indeed, it wasn't until the
1960s that anybody had any idea
that there was

a colossal impact from space

just over there, centred right on
the 18th hole of the golf course.

We now know where the giant
asteroid that struck here
could have come from.

Located between Jupiter and Mars
is a vast reservoir of rocky debris
that forms the asteroid belt,

and it's this ancient rubble that
Jupiter, our neighbourhood giant,
can nudge towards the Earth.

Well, here's my model
of the solar system.

There's the sun in the middle,

then the Earth, Mars, Jupiter,

and the asteroids sort of
scattered in between the big region

between Mars and Jupiter.

In fact they extend
over 150 million miles,

which is further than the distance
from the Earth to the sun.

This is my coffee, by the way,
which doesn't represent anything.

I'll put it over there.
But now and again,

because of collisions in the
asteroid belt, a stray asteroid

will get thrown into the position
where they keep rhythmically meeting
Jupiter over and over again.

And because Jupiter is such
a massive planet,

that means that it gets a kick,
it gets a gravitational kick.

And that changes the orbit of these
asteroids and over time, their orbit

can become, well, elongated or
elliptical rather than circular.

That means that they can get thrown

into the inner solar system
and cross the orbits

of the inner planets,
including the orbit of the Earth.

And you get a potentially
catastrophic collision.

Jupiter was once thought
to be our protector,

its enormous gravity
swallowing up dangerous asteroids.

Yet we now realise its gravitational
influence can propel some of
those asteroids in our direction.

But surprisingly, catastrophic
impacts with space debris
might not be a bad thing,

at least, in Earth's past.

Impacts from space
shaped our planet.

They made our world
what it is today.

Take life on Earth, for example.

Now, it's possible,
or probable even,

that impacts on a colossal scale
changed the climate so much

that huge swathes of life on Earth
were wiped out,

creating ecological niches into
which other species could evolve -

us, for example.

It's incredible to think
that a planet

half a billion kilometres away
could dictate the fate of our world.

Jupiter's immense gravity
bridges the depths of space

and even though its power could
one day devastate Earth,

that same gravitational field
breathes life into other corners
of the solar system.

For better or worse,

Jupiter's powerful gravitational
field has had a direct influence
on our planet.

We're part of a much wider ecosystem
that extends to the very edges of
the solar system,

and that ecosystem is bound
together by the force of gravity,

and it's gravity that has power
to bring worlds to life.

Our understanding of the solar
system began much closer to home.

Gazing down at us, it was our moon,
with its regularly changing face,

that first fired our interest
in worlds beyond our own.

When we could look further out, we
discovered the solar system was full

of moons, each invisibly connected
to their parent planets by gravity.

Our moon is a cold,
geologically dead world,
but the powerful gravitational bond

that exists between another moon
and its parent planet has done
something astonishing.

It has brought this moon to life,

making it the most violent place
in the solar system.

400 years ago, it was Galileo
who first looked up at
the night sky through a telescope.

Turning his attention to Jupiter,
he noticed that this giant planet
was not alone.

Oh, yeah!

Absolutely magnificent.

You see a disc surrounded by...

Well, I can see
three points of light.

Galileo, over several nights,
saw four points of light.

And he correctly surmised that those
are actually moons, other worlds
in orbit around Jupiter.

Jupiter's four largest moons
are named after the four lovers
of the Greek god Zeus.

Furthest out is Callisto.

Then there's huge Ganymede, the
largest moon in the solar system.

Next is icy Europa,

and finally, the small,
yellow-tinged moon nearest
to Jupiter, Io.

So the legend goes, Zeus tried
to protect his lover, Io,
by turning her into a cow

to hide her from the jealous gaze
of his wife, Hera.

But it didn't work, and Hera sent
a gadfly to torment Io.

And it was prescient in a way
to name that satellite Io,
the tormented moon.

Because we've since learned that
it is indeed an incredibly
tormented world.

For 400 years, we expected Io
to be as dead as our own moon.

But in the late 1970s, when the
first spacecraft passed by Jupiter,

we finally saw Io up close.

But it didn't make any sense,
as here was a moon with
no meteorite impact craters.

Now, it's impossible to believe
that Io could have escaped

the bombardment that we see
on our moon and practically every

other body in the solar system,
so the only explanation is that
that surface is young.

It must have been recently produced,
and that in turn means that Io,

that tiny moon of Jupiter out there,
must be a geologically active world.

The truth about Io
would blow us away.

We may not have stood on Io,
but there are places we can go here
on Earth to help unlock its secrets.

This is Ethiopia in East Africa.

We're being flown out by
military helicopter to the very hot,

very inhospitable, Afar region
in the north-east of the country.

And this is what I've come to see.

It's one of the rarest
geological phenomena on our planet,

a volcano
with a lake of molten lava.

As the sun goes down,
the lava lake comes to life.

This volcano is called Erta Ale
by the local Afar people.

It means "smoking mountain".

For many, this place
is a vision of hell.

Yet it holds the key
to understanding Io, a world
over half a billion kilometres away.

This hot, tortured landscape of lava
will be our home for the next
three days.

Temperatures here reach 50 Celsius
in the shade and we're camping
right on top of the volcano.

But we're in good hands,
as Io specialist Dr Ashley Davies
is part of the team.

It is absolutely spectacular,
isn't it?

Extraordinarily beautiful.
You feel like you're looking
into the core of the planet.

It's a window into the interior
of the Earth, so magma is rising up

from some kilometres down,
circulating through the surface
and then sinking back down again.

It is very difficult to breathe,
it's very acidic and very bitter.

The magma has gases in it,

and as it comes up to the surface,
just like when you pour

out a bottle of Coca-Cola,
the gases come out.

So what we have here is
sulphur dioxide, hydrogen sulphide,

water vapour, carbon dioxide,
coming out of the magma

before the magma then cools and sinks
down, and that's what we're breathing
now, it's incredibly unpleasant.

This volcanic phenomenon
is a product of immense amounts

of primordial heat escaping
from inside our planet.

Yet we have seen something similar
in the far reaches of
the solar system.

Io is the same size as our moon
and should be a cold, dead world.

Yet our first glimpses of Io
revealed it as seething with heat,
alive with volcanic activity.

Just one of the many lava lakes
on Io releases more heat than all
Earth's volcanoes put together.

If we were to stand
on the surface of Io now,

what would be the similarities
and what would be the differences?

The lake would probably appear
very, very similar to this,
except for the scale.

The lava lakes on Io
are vastly larger.

The biggest one, we think,
is 180km in diameter.

180km? So that would stretch way...
obviously way beyond
the horizon on Earth!

Yeah, it's almost beyond description,

to see something that size and it's
just this huge pool of molten lava.

Io is the most volcanic place
in the solar system,

endlessly pumping out heat
into the cold vacuum of space.

But what's really interesting
is that it's so small that it
shouldn't be volcanic at all.

It was one of the greatest surprises
of planetary science

when these massive volcanoes
were discovered on Io.

Mighty planets like Mars -
has big volcanoes.

They're not erupting anymore, they
haven't erupted for a long time.

Venus, lots of volcanoes there,
but they haven't been erupting
in a long time, probably.

And here we have Io, which is
just insanely volcanic.

It's just pumping out vast amounts
of energy in a zone, in a part of

the solar system where it was
thought that everything was dead.

Everything we now know about Io
comes from looking at it
from a distance,

but measuring the heat pumping out
of this lava lake

will give us a better idea of the
true scale of the heat loss on Io.

Far from being a benign,
bubbling cauldron,

this volcano has the power
to kick off at a moment's notice.

I can just see pieces of molten rock
rising up just below my chin,

and with it a cloud of heat,
absolutely overpowering heat.

There must be a hell...
a hell of an eruption going on.

Seeing active volcanism like this
on such a small moon like Io

changed our view of the workings
of the solar system.

A world like Io, having such
a powerful internal heat source,
cries out for an explanation.

You see, Io is far too small a world
to have retained any internal heat

to have the same heat source
that powers the Earth's volcanoes.

So something else must be driving
that powerful volcanism on Io.

New images sent back from recent
space probes confirm that Io is
a surprising and bizarre world.

Being so far from the sun,
Io's surface is mostly very cold.

It is covered in frozen sulphur,
which gives it its yellow colour.

Yet Io is pockmarked
by cauldrons of molten lava.

You know, I think it is remarkable,
and fortunate in a sense,

that you can come to a place
like this on our planet and just

get the tiniest sense
of what it must be like

to stand on the edge of one of
those magnificent lava lakes on Io.

When we first saw hot
volcanic action on Io,

it was quite a surprise
for most planetary scientists.

But by considering
simple laws of physics,
it didn't surprise everyone.

Just weeks before Voyager
arrived at Jupiter,
three scientists made a prediction,

and it was one of those predictions
that, when you see it,
is almost obvious.

It was using physics that had
been known for hundreds of years,
but nobody had thought of it.

They predicted that Io should have
an intense internal heat source
because of its unique position

in the solar system,
very close to a giant planet
and surrounded by other large moons.

Io sits about the same distance
from Jupiter as our own moon
does from Earth,

but don't forget that orbiting
outside Io are its sister moons,
Europa and Ganymede.

Io is under the influence not just
of the massive gravitational pull

of Jupiter, but also the additional
pull of its neighbouring moons.

It's this gravitational tug of war
that conspires to breathe life
into Io.

Now, Io has a very interesting
relationship with Europa
and Ganymede,

because for every four orbits
that Io makes around the planet,

Europa goes around
almost exactly twice

and Ganymede goes around just once.

Periodically, they line up together,
bang, bang, bang, and Io gets

a powerful gravitational kick
on a very regular basis.

And that has the effect of moving Io
out of the nice circular orbit

into an elliptical
or an eccentric orbit.

Io comes close to Jupiter
and then far away from Jupiter,
and then close to Jupiter again.

And because Jupiter's gravity
is so big, that has the effect
of stretching and squashing Io.

Now, imagine it was a squash ball.
If you stretch and squash
and stretch and squash,

then it gets hot by friction, and
the same thing happens to this moon.

The power of the gravitational
interaction between Jupiter
and Io is extraordinary.

It contorts the shape of this
tiny moon, moving rock as if it
were nothing more than water.

Now, this crater is about, what,
30 metres from the base

that you can see down there
up to the edge of the rim.

Now, Io, when it orbits around
Jupiter every 1.8 days,

flexes by something like 100 metres.

That's three times
the height of that crater.

Remember, Io's surface
is pretty much like this,

solid rock, so imagine
how much energy that takes,

and all that energy comes from
Jupiter's gravitational field,

and that is the energy
that powers the volcanoes.

Io is a world beyond
our imagination.

Its unique gravitational connections

provide a seemingly
inexhaustible supply of heat.

As well as its huge lava lakes,
the heat also powers

the largest volcanic eruptions
in the solar system.

Molten rock and gas
blasts out from the frigid surface.

The gas expands,
shattering lava into a giant
fountain of fine particles.

With weak gravity
and a sparse atmosphere,

Io's volcanic plumes can reach
500 km above the moon's surface.

This incredible phenomenon,
volcanism, comes from the simplest
of laws of physics,

the law that says that heat
contained in a planet

must eventually find a way to escape
into the coldness of space.

But what a spectacular way for
the laws of physics to play out.

In the most unexpected of places,

in the coldest reaches
of the solar system,

the laws of physics created
a fiery world of wonder,

and Io is not alone.

Many of the hundreds of moons
in the solar system

are not dead, barren
and uninteresting worlds,

but active, often violent and
always beautiful worlds of wonder.

Io is fascinating.

It doesn't derive its energy
from an internal heart source
in the same way that the Earth does.

It extracts energy from its orbit
around its giant parent planet,

Jupiter, and for all those reasons,
Io is a wonder of the solar system.

Our exploration of the planets
and moons orbiting our star

has given us valuable insights
into the nature of our own world.

Our view of the Earth's place in
space has been turned on its head.

Out there are many truly violent
and hostile worlds,

but they're driven by the same laws
that shape and control
our own world.

And so, I suppose,

it's in many ways a miracle
that we exist at all.

Our solar system is like
a cosmic laboratory.

Until we went there,
we had no idea of what
the laws of nature could produce.

I think one of the most important
lessons that our exploration

of the solar system has taught us
is that the laws of nature

can create vastly different worlds
with the tiniest of changes.

We now see how the life and death
of planets and moons

is governed by interconnections
which span the solar system,

and we wouldn't be here
if it wasn't for those connections.

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