Original subtitles

*

Our galaxy, a billion years from now.

A planet shrouded in turbulent, dense clouds...

..buried deep below an alien landscape.

The surface scorched...

..the temperature sky-high...

..the pressure unbearable.

This is not a distant extraterrestrial world...

..this is Earth.

This is our future.

We know it to be true because Earth has a twin...

..Venus,

and Venus has already descended into hell.

The solar system.

Eight planets orbit a central star -

the sun.

Among them, Earth and its neighbour Venus.

Earth is an oasis for life.

Venus, the stuff of nightmares.

Venus and Earth couldn't be more different.

The Earth is this beautiful planet and there's water everywhere.

It's ice at the poles, it's water in the ocean,

it's in the atmosphere as water vapour.

But then you look at Venus,

it is the worst place imaginable.

It is so hot on the surface, crushing pressures.

It couldn't be any less supportive of life.

To me, the planet Venus is sort of a classic definition

of the word "hell".

If you were to transport to Venus

and experience the environment there,

you'd quickly want to return back to Earth.

The conditions on Venus are among the most inhospitable

in the solar system.

It's just a horrible place.

It's so hot and there's no water and the atmosphere is so thick

and it rains sulphuric acid.

It's going to be a competition between whether or not

you're gonna be cooked to death or crushed to death.

Earth and Venus may seem like very different worlds...

..but they shouldn't be.

They're roughly the same size, the same mass,

and made from the same ingredients.

At first, they were twins.

Early Venus and early Earth were very similar.

They were twins,

probably nearly identical twins at their earliest stages.

Given that Venus is so Earth-like in so many ways,

it's really odd that it is so different than the Earth,

and this makes it one of the biggest mysteries in the solar system.

Somewhere in their two histories...

..the Earth and Venus took two very different paths.

The results - two totally different worlds.

Their paths were so different,

you can hardly believe that one would've been related to the other.

But now the opposite thing's going to happen,

we're gonna catch up with our twin.

We're gonna evolve to be a lot more like Venus in the future.

In the future, the two planets' paths will converge

and they will become twins once more.

Like Venus...

..Earth will descend into hell.

The oceans will vaporise,

the land will melt.

Our hospitable blue planet will vanish,

replaced by a fiery molten world.

We are actually on the Earth at a time

when there's water and rain

and it was so easy for life to take hold.

But that's gonna change,

and take a look in at Venus and have a look at our future.

Earth will even surpass the horrors of Venus.

A billion years from now,

Earth could be an unimaginably terrible place.

But how will this happen?

And can we stop it?

The roots of our home world's destruction

are buried deep in the past of our twin planet.

It's very much true that in studying the past of Venus,

we are also studying the distant future of Earth.

Both planets shared a violent birth,

scarred by brutal planet formation,

giant cosmic impacts and rampant volcanism.

We're trying to reconstruct things

that happened in the ancient, ancient past.

It's almost like forensic planetary geology.

4.6 billion years ago,

hundreds of infant planets began to form around the new sun.

Among them, the baby Venus and Earth.

As they hurtled around the sun, collisions were inevitable.

Planet formation is like a demolition derby.

In a derby, the cars are racing around a track,

going around in circles at different speeds.

Well, it's the same thing with planets.

The material is orbiting the sun.

It's going around and they're all going at different speeds, angles and trajectories.

And sometimes - "Boom!"

In the chaos, planets collide.

The scale of the impact is unimaginable,

as two planets...

become one.

You have these large bodies

that are hitting each other at really high velocity.

It's really a very hot, uh...violent mess.

The amount of energy released in these impacts is huge.

It completely dwarfs all of the nuclear weapons on Earth combined.

And yet, somehow, on these scales, you wind up forming

gigantic objects that we call planets.

Earth and Venus became veracious planet-eaters.

But two spectacular collisions

set the twins on very different paths.

That was the moment Venus and Earth went through this divergence

to what has now become these really dramatically different worlds.

The divergence began when a Mars-sized object hit Earth.

The impact made our planet spin faster.

The spinning core generated

a powerful magnetic field around the Earth.

The field fended off the worst of the sun's radiation

and would go on to protect life on Earth.

At around the same time,

Venus took a head-on hit from another infant planet...

..but this impact had an unusual side effect.

Venus is actually rotating in the wrong direction.

How could that be?

Well, what if it got hit really hard by some object?

That could do it.

An object so huge

that it didn't only stop Venus in its tracks,

it actually made the planet spin backwards.

If you think about how much energy

and what size you need to change a planet's spin,

that is an incredibly large hit.

But the backspin was slow...

..243 times slower than Earth's.

Without a fast spin,

Venus's core couldn't generate a strong magnetic field.

It had no protection from the deadly stream of particles

blasted from the sun.

Venus does not have a strong magnetic field,

and so it has suffered the full brunt of this wind

blasted out from the sun.

This is where the story of the two planets split radically.

Venus was destined to roast under a violent, suffocating atmosphere...

..while the Earth would give birth to oceans, life and intelligence.

But ultimately, these twin's fates will be the same.

Venus's present is Earth's future...

..pure hell.

*

*

Venus is a vision of hell,

and one day, we'll meet our twin's fate.

It turns out that what Venus went through in its distant past

is what Earth is going to go through in its distant future.

The clues to the fate of our blue planet

are locked inside the super-heated wastelands of Venus.

Something happened to Venus long in its past

to make it a completely different planet

with a completely different personality than the Earth as we know it today.

(BEEPING) Two, three, four.

The first probe was sent to our sister planet in 1967.

We've been sending them ever since.

What they found was incredible...

..rocks that look like granite.

What makes that interesting is that to make granite,

you need water.

That means that there must have been abundant water

for it to have formed in the first place.

Venus once harboured oceans.

Dr Lewis Dartnell thinks you can get a glimpse

of what it was like here...

in Iceland.

There's the possibility that maybe in the early solar system

there were not one but two planets with oceans,

two water-worlds - Earth and Venus.

And if Venus did once have oceans,

maybe they would've looked a lot like this here.

With a raw volcanic landscape descending down into the ocean

with the waves lapping against the coastline.

And maybe a overcast, kind of misty, hazy atmosphere,

not unlike what we are seeing here today.

If there were oceans, there might even have been life.

If Venus did have an early ocean, maybe something a bit like this,

around four billion years ago, when life was getting started on Earth,

then maybe life got started on Venus, as well.

Maybe some kind of microbial life got started in the Venusian oceans.

Just like Earth,

Venus could have been a habitable, wet planet...

..but Venus couldn't hold onto its water.

All of that water is gone.

It's just gone. Where did it go?

Something happened, either catastrophically or over time

to basically dry out this twin of the Earth.

The culprit was the sun.

Since its birth it's grown stronger.

Our sun, when we look out at it during the day,

seems the same today as it was yesterday,

but that's on a human timescale.

On cosmic timescale, the sun has been getting hotter and hotter.

Every billion years,

the sun gets 10% hotter...

..slowly turning up the temperature on Venus.

Not only that,

Venus formed 42 million kilometres nearer the sun than the Earth.

As it turns out, that distance to the sun was critical.

It's just an unfortunate circumstance

of being in the wrong place at the wrong time.

Earth is far enough from the sun to hold onto its water...

..but Venus is slightly too close.

The intensity of its sunlight got sort of just a little bit too much.

It passed this threshold where Venus couldn't hold its water

on the surface any more.

As temperatures rose, the oceans started to evaporate.

All of that water in the oceans,

all of those millions of cubic miles of water

would become water vapour.

Basically, steam clouds covering the entire planet,

hiding the surface from the outside.

Water vapour traps the sun's heat.

As the clouds start to engulf Venus, the temperature rises.

Which, in turn, causes more water to evaporate,

and more clouds to form,

which raises the temperature even higher.

But the process can't go on forever

because the clouds of water vapour in the atmosphere start to disappear,

ripped away by the solar wind.

Venus does not have a strong magnetic field,

and so the full brunt of the solar wind

has been slamming into Venus for billions of years.

Over time, if a water molecule

was in the upper part of Venus's atmosphere,

light from the sun could break it apart into oxygen and hydrogen.

And then the solar wind could blow that stuff away.

Over billions of years,

this torrent of subatomic particles blasted out from the sun

has stripped the water out of the atmosphere of Venus and has desiccated it.

Venus Express was the most recent mission to the planet.

Surfing the atmosphere, it found hydrogen and oxygen

streaming from the top,

confirming that water was leaking into space.

We see sort of a tail of these atoms being stripped off

and blown away from the planet itself.

And really the only way that could happen

is if there was a little bit of water left

that is still being destroyed, picked up and swept away.

Wisps of water are all that remain of Venus's oceans.

It's a terrifying vision of our own future.

So if Venus were, in the past, a lot more Earth-like,

then that tells us that having a habitable world

is something that is actually very precious,

and maybe is transient.

It's not something that lasts forever.

By losing the clouds that trap the heat,

you might think that Venus's temperature would have simmered down.

But in fact, it was about to rise to astonishing new heights.

It would become so hot that metal snow

would fall on its mountains.

Clues to how this happened can be found on Earth.

In Hawaii, planetary geologist Jani Radebaugh

studies the islands' volcanoes.

These volcanoes are a perfect model for early Venus.

What we're seeing out here are lava flows

come all the way down from Pu'u O'o vent

which is about 15 miles away.

Oh, there's hot... you can see the hot stuff.

You can see hot... Quick, quick, quick, quick. Right there.

It's really beautiful.

Hawaii's volcanic lava fields look like Venus in miniature.

Both produce the same kind of runny lava,

building flat shield-like volcanoes.

The big difference is there are only five active volcanoes on Hawaii...

..whereas Venus is covered in them.

One thing that really jumps out all around the planet

is uh...the number and variety of volcanoes.

I mean, Venus could almost be nicknamed Volcano World.

Venus has tens of thousands of volcanoes all over the planet.

It's not the erupting lava that turns up the heat,

it's what comes out with it.

If we think back to the histories of Venus,

I think we must have seen a landscape very similar to this one.

We have massive amounts of lava flowing out of the surface,

dumping huge amounts of gases into the atmosphere,

carbon dioxide, tons of gas into the atmosphere every single day.

It would have been amazing to see.

On the surface, the source of the gases is revealed.

If we look behind us we can see

volcanic gases gushing out of steam vents.

We've got carbon dioxide being delivered to the atmosphere.

It's exactly like what has happened on Venus.

Carbon dioxide has been delivered out of volcanoes

over and over and over again throughout its history.

So that now we have just a tremendously thick,

dense atmosphere.

The result of all of these volcanic gases pouring out of volcanoes,

major greenhouse gases, is that they've been absorbing heat

for the billions of years of the history of Venus.

The temperature's been gradually creeping up,

until today, the surface of Venus is 900 degrees.

It's like if you go into your kitchen and set your oven to broil,

wait a couple of minutes and stick your head in it,

and even that's not quite hot enough.

It's a crazy, horrible, hellish spot.

It's hard to imagine such extreme temperatures...

..but probes orbiting the planet reveal just how incredibly hot it is.

Scientists studying the images

noticed something strange on the planet's mountains.

It looks like, on the mountains,

that there's apparently snow-like structures.

But this is not like any snow found on Earth.

So if you look at the white-peaked mountains of Venus,

you would think that it was snow.

But it's actually metals that have rained down

and deposited on the top of those mountains.

Metals like bismuth and lead melt...

..then they evaporate into the atmosphere.

As they rise, they cool,

until they finally fall like snow on the mountain tops.

I'm not sure even the imagination of science fiction authors

would've come up with something as weird as Venus.

I mean, just think about that,

you have possibly metal frost on the top of mountains?

I mean, how weird is that?

(CHUCKLES) It's pretty insane.

Raining metals,

where would you ever think about that existing?

On Venus.

In the future, metallic snow is forecast for Earth, too,

and our scorching mountain caps will glitter like Venus's.

But that's not all.

Venus tells us that things will get even worse...

..our atmosphere will grow heavy enough to crush cars.

*

*

Earth and Venus were born twins...

..but they took different paths.

Earth slowly evolved into a habitable world.

Venus was covered in thick volcanic gases,

trapping the sun's heat.

Temperatures rose to over 450 degrees Celsius.

But Venus wasn't just blighted by high temperatures.

The volcanoes kept spewing gas up into the atmosphere,

forming a 250km-deep cloud of carbon dioxide around the planet.

We don't think about gases as weighing anything,

but they actually do.

Trillions and trillions of tonnes of gas pressed downwards.

There is simply so much air on Venus

that on the surface it's pushing down with a huge amount of force.

The atmospheric pressure on Venus is a monster.

Think about it this way.

All right, car. It's time for you to be crushed, baby.

If you're on Venus,

you're gonna have 155 miles of atmosphere above your head.

As a result, atmospheric pressure is 90 times that on Earth.

So on Earth, it's about 14.6lb per square inch.

On Venus, we're talking about 1,300lb per square inch.

So if you're driving your car in Venus, this is what might happen.

The crusher delivers the same force as the weight of Venus's atmosphere.

This is pretty serious stuff.

And this is why it's so hard on Venus,

you get down to the surface,

you have the crushing atmosphere to deal with.

A spaceship tasked with landing on Venus

must first survive the violent weather on the way down.

The extreme heat powers a vortex

the size of Europe over the south pole.

As the Earth heats up, we see storms that are more violent.

You can only imagine the level of storms on Venus.

The upper atmosphere of Venus is whipping around

hundreds of miles per hour.

Wind speeds reach 400km per hour.

That's faster than most hurricanes on Earth.

It must be a weatherman's paradise in a sense.

There are also huge lightning strikes.

But these bolts are not formed in water clouds like on Earth...

..they form in something much more toxic.

Venus's volcanoes release sulphur dioxide gas,

which mixes with water vapour,

creating sulphuric acid.

You have clouds that are raining sulphuric acid.

It rains sulphuric acid!

Sulphuric acid in your lungs, your eyes, your mouth, your nose -

that would be deeply uncomfortable.

The extreme pressure and heat

make Venus nearly impossible to explore.

Only one nation has ever succeeded

in getting a probe to the planet's surface.

Truly one of the engineering triumphs of the human race

was the Soviet Union's Venera programme.

The Russians sent over a dozen probes to the planet Venus

and only a few of them were able to survive

long enough to even be able to take pictures from the surface.

The Venera missions were incredible.

It's such a hostile environment on the surface for electronics,

and they were able to land on the surface and survive.

The probe that sent back these images

was crushed and burnt out in 90 minutes.

Some day in the future, there are going to be

interplanetary tour guides,

taking people to every planet in the solar system.

And you can imagine going to Saturn

and seeing the rings and Jupiter and its panoply of moons.

There are all these great tourist attractions in the solar system.

At the very bottom of that list is Venus.

That is the last place in the solar system

I would ever want to visit.

The sun's heat and volcanic gases

have transformed Venus into a vision of hell...

..the most lethal rocky planet in our solar system.

Suppose you decided that you were gonna put me

in a star-ship transporter beam

and transport me to the surface of Venus.

What would happen?

Well, the pressures there are incredibly intense,

and the rain there, as I say, it is acidic.

The atmosphere is acidic.

So I would be unaffected,

but the average man would be crushed and melted.

If you could imagine standing on Venus,

it would be a torture in the extreme.

Your feet would be burning from the molten fire below you

and your head would be hurting

from the incredible pressure pushing you down.

You probably wouldn't survive long enough

to even notice it, actually.

So why hasn't Earth followed the same path?

Our volcanoes also spew out carbon dioxide,

and we orbit around the same sun...

..but we're not being crushed and broiled.

One reason is, Earth formed farther away from the sun...

..and stayed cool enough to hold on to its oceans.

Oceans do a lot of things for us on Earth.

Because not only, obviously, are we water creatures

and we depend on the water cycle for our existence in so many ways.

But people don't realise the oceans

also help to regulate the climate of Earth.

Our oceans are full of tiny creatures that eat carbon dioxide.

Richard Zeebe from the University of Hawaii

is diving on the islands' coral reefs.

He's studying how tiny marine organisms

turn carbon dioxide into rock.

What you see here, the white stuff,

this is what we call calcium carbonate.

And on top of this, what you see, these brown layers,

this is essentially the living organism.

This is the coral itself.

It takes calcium out of the seawater

and it takes carbonate out of the seawater,

combines them and makes this piece of calcium carbonate.

There's over 70 million billion tons of carbon

locked up in carbonate rocks.

This helps regulate carbon dioxide levels in the atmosphere,

keeping temperatures from rising.

If all the carbon that is being locked up in carbonate rocks

in these corals would be put into the atmosphere as CO2,

that would be certainly bad news for us.

Currently, our carbon cycle helps stabilise our climate...

..but in the future this won't be able to save us.

Forces far greater than the ones at work on Earth

will overpower our natural defences.

Like Venus, our oceans will burn off.

Temperatures will rocket as our life-giving sun

starts to kill the Earth.

Right now we live just the right distance from the sun,

where it's just the right temperature for water to exist

as a liquid.

But that's going to change, just as it did for Venus.

Venus started off, probably, in the habitable zone

and then the inner limit to the habitable zone crossed

the distance of Venus's orbit.

Well, it's gonna cross the distance of Earth's orbit, too.

There is an expiration date to the Earth,

and that's due to the sun's evolution.

Ever since its birth, the sun has been getting hotter.

That increased heat devastated Venus,

and in the future, it will destroy Earth.

In 1.1 billion years,

the sun will be 10% hotter than it is today.

The oceans will start to evaporate into thick clouds,

which will trap more of the sun's heat.

Catastrophically, very rapidly on a geological timescale,

the oceans will put so much water vapour into the atmosphere

that we will get a runaway greenhouse effect.

The clouds forming in the atmosphere will trap more and more heat,

driving temperatures even higher.

Spiralling temperatures will cause even more evaporation,

so the clouds will get thicker.

Which led to more heating, which led to more evaporation

and you can see where this is going.

It's a vicious cycle, it's a positive feedback.

All of the oceans' water will boil away, millions of cubic miles of it.

We have all of this water that will go into the atmosphere,

covering the Earth and shrouding it in, basically, steam.

Earth has had oceans for billions of years,

but it will take just 10,000 years to lose them.

All of the water in the Earth's oceans will be in the atmosphere.

We'll have an incredibly dense cloud-coverage system

where the temperatures on Earth will be approaching 1,000 degrees.

Like Venus in the past, Earth will get hotter and hotter.

But unlike Venus, which plateaued at 500 degrees Celsius...

..temperatures on Earth will keep climbing.

Venus lost its water to space, blasted away by the solar wind.

But Earth will hold on to its water,

because it's protected by our magnetic shield.

However, that is no longer a good thing.

Unlike Venus, Earth has a strong magnetic field which protects it

from the erosion of the solar wind.

That water will stay with us.

The Earth could have a thicker, hotter greenhouse atmosphere

than Venus does today, much worse.

This huge volume of water vapour will push down on the surface...

..increasing the surface pressure by 270 times.

If by some miracle, a human stepped out onto the surface...

..they would instantly be crushed by the atmosphere.

Its weight would be more than a fully armoured tank,

pushing down on their head.

It is ironic to think that the water on Earth will one day

help contribute to its demise.

After all, the water has been the source of life on Earth.

But in the far future, it'll become our enemy.

And with no oceans and their microscopic creatures

to absorb the carbon dioxide,

there's nothing to hold back the rising temperatures.

Earth is doomed.

*

*

In 1.2 billion years,

a probe visiting Earth would see an alien world...

..a scorched, barren landscape.

The pressure is crushing.

Temperatures reach 650 degrees Celsius.

Molten metals snow down on the mountain tops.

It's so hot, granite rock melts.

The surface liquefies.

At that point, the Earth will become a molten ball

very similar to what it was at the very beginning.

Earth and Venus started as twins.

Venus was destroyed by rampant global warming.

Earth will follow the same path, then overtake it.

It is inevitable that the Earth will someday not only be like Venus,

but actually put it to shame.

In 1.3 billion years, Earth could hit 2,000 degrees Celsius...

..four times hotter than Venus.

It'll be the hottest and deadliest planet in our solar system.

For Earth and its inhabitants, it's the end of the road.

Maybe we'll escape to space

and send probes back to study our former home.

A billion years from now,

Earth could be an unimaginably terrible place.

Right now, today, Venus is the evil twin of Earth,

but in the distant future, Earth could be the evil twin of Venus.

Earth's surface temperature will be far hotter

and its surface pressure far greater than Venus's.

At these kind of temperatures where

rock on the surface of the Earth is molten,

it's hard to imagine any place there could be life.

But there is hope.

Astrobiologist Lewis Dartnell thinks that some forms of life could survive

such terrible conditions.

We're here on top of a volcanic outcrop

in Iceland with this howling gale whistling past our ears.

The stench of hydrogen sulphide,

of sulphurous fumes, filling our nostrils.

And this is about as close as we can get on Earth

to high up in the Venusian atmosphere.

About 30-35 miles above the surface of the planet Venus,

the air pressure is about the same as on Earth's surface,

the temperature is pretty similar as well.

But the cloud droplets are full of concentrated sulphuric acid,

many, many times more concentrated than battery acid.

It's a hostile, horrible environment.

But bizarrely enough,

there's good reasons to think that there may be life,

Venusian life, high up in the clouds there.

A kind of high-altitude aerial biosphere.

If life can live up there, then perhaps it might survive

high up in the clouds of future Earth.

You can easily imagine these microorganisms evaporating

in water particles and being transported to the upper atmosphere.

Even though it turned into this toxic greenhouse planet,

life could potentially still survive in that upper atmosphere.

But what about us?

We couldn't survive the high temperatures

or pressures at the surface...

..and a life in the clouds doesn't seem likely.

What's our future?

If there was this life on Venus,

clearly they weren't advanced enough to stop

the changes in their atmosphere that led to Venus's current state.

The question is, are we?

Perhaps we'll leave our planet and find a new home.

As the sun gets brighter, Earth will go the way of Venus,

but then the world's further out will get warmer and warmer.

It'll be a progression, outward, of habitability.

First, the polar ice-caps on Mars will melt into oceans.

Then even further out, the sun's heat will unlock the water

on Jupiter's moon Europa...

..transforming it into a water-world.

If we do this right,

we could maybe sort of hop from world to world

and, like, move farther out from the sun as the sun warms, sort of.

Because these are gonna be slow transitions, so there'd be, like,

plenty of time to kind of engineer the right environments

and sort of hop from world to world, farther from the sun.

So that might be one way that, you know, if there is intelligent life on a solar system

where this is happening, it might just be a matter of sort of skipping

from world to world and following the habitable zones as they...

as they move outward in your solar system.

But there is another solution...

..stay and move the Earth further from the sun.

Moving the Earth is at least imaginable because, in fact,

as objects exchange gravitational energy

they move in or out in the solar system.

It has happened to our planet,

so I could imagine engineering things where we

directed large asteroids and comets close to the Earth but not to hit it.

The gravity from these large objects would slowly alter our orbit.

Gravitational energy would be exchanged

and the Earth could slowly move out.

Each gravitational jolt would only move Earth a short distance.

But do it thousands or millions of times

and we could push the Earth away from the sun...

..in theory, at least.

Over a billion-year period, it's possible to imagine.

It would require incredible technology

and incredible coordination.

The technology is possible,

whether humanity as a species could ever coordinate it

is something I'm a little more sceptical about.

It sounds like science fiction,

but if we don't do something the Earth will die

and we'll become just like Venus.

Earth and Venus were probably born together as identical twins,

but then their paths diverged.

But now lifetime is gonna send that cycle all the way back

and they'll die as identical twins again.

In the grand scheme of things, they'll just be two charred twins

spinning to oblivion in a backwater of the universe.

On the cosmic scale, life is short.

When you look at how Earth evolved and how Venus evolved,

you can see the difference, even though it's two almost twin planets,

how life and habitability could change over time.

So habitability isn't always a permanent thing.

On human scales, the universe seems the same every single day.

But of course, that's because

human life and human civilisation is but a brief instant in cosmic time.

On cosmic scales, the universe evolves and changes,

and that makes the history of the universe remarkable.

Our tale of two planets converges in the end,

a cautionary tale about forces beyond our control.

Perhaps a billion years of learning from Venus

will ultimately save us from the same terrible fate.

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