All language subtitles for How The Universe Works S04E08 The First Oceans_ENG_Subtitles01

*

Life on Earth depends
on seas, rivers and rain.

But is our blue planet unique?

Or did the universe create countless
other wet worlds just like it?

Unlock the secrets
of Earth's first oceans

and we'll unlock the secrets
of alien life.

The Earth is the only planet we know
of that has oceans of liquid water

covering its surface.

And it's the only planet
we know of that has life.

If you look at every
living organism on Earth,

you can see that each one
has a fraction of water

that makes up the system.

We're basically bags of water

that allow chemicals to move around
and do things that we call life.

On Earth, liquid water
and life go hand in hand.

But how lucky are we to have
a watery oasis to call home?

We have a lot of water.

Was it made here
when the Earth was made?

Or was it brought here later
by something from space?

Is it a fluke to have a water world
like this, or is it inevitable?

For decades, scientists have been
trying to establish the origins

of Earth's water and they've
come to a surprising conclusion -

our planet shouldn't be wet at all.

The place where the Earth
is right now seems very dry.

So if the Earth formed as a dry rock
around a hot young star,

then how did this water get here?

Every possibility has problems,
and we want to know the answer.

Tracing the exact source
of Earth's water

is surprisingly complex.

The journey starts
over 4.6 billion years ago,

during the formation
of our solar system.

A vast cloud of gas and dust
hangs in space...

..teeming with vast quantities
of hydrogen and oxygen.

Oxygen is one of the most abundant
atoms in the universe.

Hydrogen is the most abundant
atom in the universe.

You're gonna get a lot of
whatever it is they form.

Over millions of years,
these highly reactive atoms

bind together to form H2O, water.

Water is a fairly simple molecule.

It's made of two hydrogens
and one oxygen.

This newly formed water sticks
to dust grains inside the gas cloud,

and freezes to form crystals of ice.

Eventually, the icy dust cloud
becomes so dense

that it starts to collapse
under its own gravity.

It's the start of a process that
will create our entire solar system.

There's enough water here
to fill the Earth's oceans

three million times over.

When we see stars that
are forming right now...

and we study hundreds,
thousands of them,

we see discs of material beginning
to orbit around the young stars.

Gas, dust, and there's
certainly quite a bit of water

in that material.

Gravity pulls more and more material
into the centre of the cloud,

raising the pressure and temperature.

Eventually, the extreme forces
spark nuclear fusion...

..and a protostar, our infant sun...

..bursts into life.

It's bad news for the water
surrounding the newly born star.

The environment of a star when it
forms is incredibly hot and violent.

Any water that was
existing in that region,

because water's a volatile material,
would be destroyed.

Water cannot exist near a star
early on during its formation.

Astronomers believe
the early sun may have sucked up

much of the dust and water
surrounding it.

And then blasted this debris
far out into space

in superheated jets of steam.

In the Earth's most volcanic places,

a similar process blasts hot water
high into the air.

Deep underground,
the water is superheated

and there's nowhere for it to go
when it turns into steam,

and it's driven outward
in these giant plumes.

Protostars also have
lots of water around them.

And the magnetic fields around a
protostar create, basically, tunnels

that the water can escape from and
it's blown out along these tunnels

in giant jets that spread water out
all throughout the galaxy.

In space, superheated water escapes

through the magnetic weak spots
at the poles of protostars.

Pockets of water inside
these vast, steamy jets

eventually solidify in the cold
of space to form ice pellets.

And these speed away
from the protostar,

80 times faster
than machinegun bullets.

Is this really what happened
in our young solar system?

It would have been so awesome
to be there to see that.

But the lucky thing is that our
galaxy continues to form stars,

so we can study protostars
all over the galaxy

and look at things that are a lot
like what the sun experienced.

In 2011, astronomers witness
the formation of a star

just like our own sun.

Their telescopes reveal
a central ball of gas

dragging in matter from
the clouds surrounding it...

..before blasting out water at a rate
equal to ten million Amazon Rivers.

It's believed a similar process
ejected much of the water

in our embryonic solar system.

As the sun matures, the jets dry up

and a new threat to
the remaining water emerges,

a hot stream of charged particles
known as solar wind.

As the sun heats up, the ice
nearby is turning into water.

And as the sun heats up more,
it's turning into water vapour.

And then as the sun turns on
its solar wind and becomes bright,

it starts to blow that water out.

The solar wind blows
in a supersonic stream of plasma

from the sun's outer layers.

It strikes the surrounding cloud,

blasting away most of
the gas and water vapour.

What's left behind is just dust
with traces of water clinging to it.

Further out from the sun,

the solar wind has less impact
and it's also much colder.

The result is a boundary of water ice
half a billion miles from the sun,

known as the snow line.

When astronomers talk about
the snow line what they mean is

how far away from the young sun
was water able to condense.

Where did it get
cool enough for water

to finally condense into droplets,
get onto objects, become ice?

Any closer than that
and you're just gas.

The solar system's
first and biggest planet

is born at the snow line.

Where the ice is thickest,
clumps form.

And then, attracted to
each other by gravity, join up.

A colossal snowball builds.

It draws in all the matter around it,

eventually creating
the gas-giant planet Jupiter.

Where the snow line is thinner,

a similar process forms
the other gassy planets...

..Saturn, Uranus and Neptune.

On the inner, dry side of
the snow line, dust clumps together,

forming a family of small,
rocky planets, including...

..Earth.

Many astronomers believe our planet
is fashioned from little more

than arid rocks with microscopic
droplets of water sticking to them.

But even this precious reserve
of water is about to be threatened

by the most violent event
in our planet's history.

*

*

Our solar system,
4.6 billion years ago.

Earth is just one of many large,
rocky balls forming around the sun.

And our young planet's gravity
continues to pull in chunks of debris

from the surrounding dust cloud.

These rocks hitting the Earth
hold tiny amounts of water,

remnants of a time before
the sun sparked into life.

Planetary scientist Dan Durda
believes this water

had little chance of survival
on the newborn Earth,

thanks to the heating effects of
multiple high-speed asteroid impacts.

Building a planet
is a very violent process.

We can demonstrate pretty easily
here with the high-speed impact

of a bullet.

(GUNSHOT)

OK. So that's a single impact.

(GUNSHOT)

When the bullet punches
into the target,

some of its kinetic energy
is converted into heat.

You can see this sudden hot burst
using a thermal imaging camera.

In the case of a real impact,
a large asteroid impact,

the energy is a lot greater.
You're actually melting rock.

And four-and-a-half
billion years ago,

impacts like that were
happening once a month.

(GUNSHOTS)

Let's go see what we got up there.

A sub-machine-gun demonstrates
how this heat would have built up

after multiple asteroid impacts.

The cumulative effect
of all these impacts is

to heat the surface of the Earth to
near magma, lava-like temperatures.

The combined energy of the impacts
boils the surface of the young Earth.

There's a lot of impact and it's
a very high-temperature activity,

so any water that would be present,
it would be hard to hold onto it.

Because of the heat and the energy,
the water probably escaped.

After 60 million years,
the planet-building stops

and Earth's surface
cools enough to form a crust,

potentially trapping any
remaining water inside it.

But not for long.

The crowded early solar system
is home to more planets

than exist today,

and one, known as Theia,
hurtles towards Earth.

Smashing into Earth,

Theia gouges out a huge chunk
of our planet's crust.

The rocky fragments create
a colossal ring of debris

that will eventually coalesce
to form the Moon.

Reeling from the impact,
Earth reverts to a ball of lava,

and the heat drives off
yet more water.

The collision with Theia leaves
the crust of the Earth bone-dry.

So where does the water
that we see today come from?

There are only two possibilities.

In order for the water to survive,
it either has to be embedded

deeply enough in rocks that
it isn't melted and evaporated,

or it has to come to Earth
after it forms.

Was our planet originally
formed from much wetter rock

than the scientists had believed?

Or were the oceans delivered
to the Earth much later...

..from somewhere else?

Initially, delivery seems
the most likely possibility.

Far out beyond the orbit of Neptune,

lies a vast band of icy material
called the Kuiper Belt.

It's made up of
the leftover building-blocks

of the gas-giant planets.

Occasionally, chunks of this
icy debris, known as comets,

tumble into the inner solar system.

Did such marauding comets
bring water to the early Earth?

Comets are basically
big, dirty snowballs.

They're giant balls of ice that
have rock, pebbles, gravel, dust,

embedded in them.

We think that comets are
about 50% made of ice.

So if you looked at
everything in the solar system,

trying to find a source
for water on Earth

there's seemingly an obvious answer,
and you'd look at comets.

Passing comets present some
of the most spectacular sights

in the night sky.

As they approach the sun, the solar
wind blasts water from the surface

of these dusty snowballs,

generating a bright tail that can
stretch for millions of kilometres.

Today, comets are relatively rare
visitors to the inner solar system,

but four billion years ago,
they were common

and Earth was in the firing line.

It's completely reasonable to expect

that icy bodies from
the outer solar system

came inward and hit the Earth.

How much of a contribution
is the question?

Rocks dating to soon
after the Moon formed

prove the early Earth
had vast oceans.

But just how many comet impacts would
it have taken to fill these seas?

The answer is staggering.

Comets come in different sizes,

and if you take
the run-of-the-mill average comet,

it might take 20 or 30 million
comets to make the Earth's oceans.

If millions of comets did
bring water to the early Earth,

they must have done it
in a very short period

and just after the Moon formed.

Astronomers scour
the solar system for evidence

of this rapid-fire icy attack.

And they find it in the
most unlikely of places -

our seemingly waterless moon.

Hey, I've got a picture. Yeah?

Pick up that little rock. Atta boy.

In the 1970s,
Apollo astronauts collect rocks

from the Moon's largest craters
to determine when they formed.

They brought them back to our
laboratories and we could date them.

When were those rocks actually made?

Planetary geologists had
assumed these craters had been

blasted out around
the time the Moon formed.

They were in for a big surprise.

We found that many of
the big impact basins on the Moon

were formed not in the earliest days
of the accretion of the Moon,

but several hundred
million years later

During the period we call
the Late Heavy Bombardment.

Late because it happened
several hundred million years

after the Earth and Moon had formed.

The Late Heavy Bombardment
begins when the gas giants align.

Their combined gravity disrupts
a vast belt of asteroids

lying close to Mars.

Sending a shower of rocks
towards Earth,

the Moon and the inner planets.

Then Neptune swings outwards,

smashing into the comets
of the Kuiper Belt

and sending many of them
hurdling inward, too.

All hell breaks loose.

99% of the Kuiper Belt
and the asteroid belt disappear,

lots of bodies get
thrown every which way.

We look at the Moon,
we see that it is scarred,

it is covered with craters.

The Earth didn't somehow magically
escape that same bombardment.

For every crater
you see on the Moon,

the Earth is a bigger target
out there in space,

there were probably 20 or 30
craters formed on the Earth.

We don't necessarily
see them everywhere today

because it's a lively planet
with geologic processes

that erase those craters.

The Earth is pummelled
by the Late Heavy Bombardment.

But how many of these impacts were
delivered by water-rich comets?

We don't know.

And that's one of the forefront
science questions is,

could the comets have come in to
deliver ocean water at that time?

If comets made our oceans,

they should have left
a unique chemical signature behind,

because not all water is the same.

(THUNDER CRACKS)

On Earth, for every 10,000 drops
of ordinary water,

there exists three drops
of semi-heavy water,

a rare molecule made from
deuterium instead of hydrogen.

Deuterium is a normal
hydrogen nucleus,

except instead of just
being one proton by itself,

it's a proton and a neutron
connected together.

The extra neutron in
deuterium adds weight,

and that's why water made
from these atoms is called heavy.

Semi-heavy water forms more easily
in cold conditions.

So the edges of the solar system
have more of it

than regions closer to the sun.

The ratio of deuterium to hydrogen
is a very sensitive probe

of where water formed
in our solar system.

And therefore we can look
at the abundance of heavy water

to determine where
that water formed and how.

In 1986, scientists get
their first opportunity

to test the chemistry
of cometary water...

when Halley's Comet makes a fleeting
return to the night sky.

Astronomers look for the telltale
signature of semi-heavy water...

..but the result is not
what they're expecting.

They measured the water
for the first time

and found it was about
twice as heavy as Earth oceans.

Then in the 1990s,

comets Hyakutake and Hale-Bopp
pay a visit.

Just like Halley's,
these comets are as old as the ones

that smashed into the Earth during
the Late Heavy Bombardment.

But both Hyakutake and Hale-Bopp

also turn out to have
way more semi-heavy water

than the Earth's oceans.

People started to get worried

because the entire Earth water
budget, as measured by oceans,

could not be made of
just these comets.

In 2015, the Rosetta space probe
analyses a comet up close,

and this time,
the data is indisputable.

A semi-heavy water content

three times greater
than the water on Earth.

For the most part, the chemistry of
Earth's oceans and atmospheres...

..is actually a very poor match
for the chemistry of comets.

When you look at the flavour of
hydrogen in the water molecules

that make up the comets
that we've measured so far,

it doesn't exactly match

the flavour of the water
that we find on our planet.

It's clear in my mind

that comets could not have
brought all of Earth's water.

But if the dirty snowballs
weren't to blame, what was?

An unexpected candidate
begins to emerge.

In 2011,

the Dawn space probe flies
by the giant asteroid Vesta.

We used to think rocky objects
like Vesta were completely dry,

but the scientists see evidence
of water on the surface.

And Vesta's water turns out
to be a perfect chemical match

for Earth's oceans.

The type of water
is exactly what we think

is contributing
to the water on the Earth.

So it looks like a really solid deal

that those types of asteroids were
putting the water on the Earth.

Scientists turn their focus
from comets to asteroids.

But how could these dry-looking
rocks have provided enough water

to fill the Earth's oceans?

The explanation could lie
within the haunting remains

of a failed planet.

*

*

Today our solar system
hosts four rocky planets.

Mercury,

Venus,

Earth...

..and Mars.

But there should have been five.

Billions of years ago, planets were
forming all over our solar system.

But there was an area
in-between Mars and Jupiter

where the gravity of Jupiter
pretty much pulled apart anything

that tried to form.

The scattered remains
of that failed rocky planet

now fill this gravitational
battleground.

Its debris forms a vast band
of rubble around the sun,

called the asteroid belt.

Rocks inside the asteroid belt
range in size from grains of sand

to giant boulders
hundreds of kilometres wide.

When I first started studying
astronomy, we called them rocks,

dry rocks.

Now we understand that
there may be a lot of water,

maybe even liquid water
on some of the larger asteroids.

Our new-found understanding
of asteroid water

comes from a study of meteorites,

tiny fragments from the asteroid
belt that occasionally fall to Earth.

I've got a sample of a meteorite
called a Carbonaceous chondrite.

And it looks and feels
rather dry to the touch,

but I can tell you that
that sample actually contains

about 20%, by weight, water.

Even crushing doesn't
release the hidden moisture,

because the water is
chemically bound to the minerals

that make up the rock.

Yeah, let's see if we can get
some heat going here on our...

on our burner.

Heat allows the water molecules
to break their chemical bonds

and escape as vapour.

Look at all that water coming out.

Just this small sample of meteorite
is driving off all of this water.

So here is direct tangible evidence
of the amount of water,

the astonishing amount of water
that can be delivered to the Earth

from the impact of asteroids.

Four billion years ago,
countless asteroids

smash into Earth during
the Late Heavy Bombardment.

Each impact generates
an intense burst of heat

that releases the water
trapped inside the asteroid.

This water vapour then falls back
to the ground as rain.

And this same water remains with us
to this day, in our oceans...

..our rivers...

..and even in our coffee cups.

When we look at this fingerprint
of deuterium on the Earth's water,

it better matches meteorites
and asteroids than it does comets.

So, yes, certainly some of
the water came from comets,

but the majority of water
in your body right now,

amazingly, may have come
from the asteroid belt.

But water-bearing asteroids
may not completely solve

the mystery of Earth's first oceans.

A remarkable new geological discovery
suggests these impacts

only tell part of the story.

There's an amazing amount of water
on the surface of the Earth.

The Pacific Ocean
has an area of roughly

half the surface of the Earth.

Millions and millions of
cubic miles of water

and yet that's not where
all the water on Earth is.

There's quite a bit of it
under the surface.

In recent years, geologists
have made a stunning discovery -

a layer of heated rock lying
deep below the Earth's crust,

which holds vast quantities of water.

Seismologists stumble on the layer

while analysing the rumble
of earthquakes.

When a big earthquake strikes,
low-frequency sound waves

travel through the difference layers
of Earth's interior

before reaching the crust
on the other side of the planet.

Studies of these long-range rumbles
show some of the sound waves

slow down when they reach
a scorching layer of rock,

sitting 480km below the crust.

And there's only one thing known
to delay the passage of sound

through rock -

water.

Now, it's not like
an ocean of water.

It's water molecules bound up in
minerals and with other molecules.

But if you take all that water
and put it all together,

we think it actually would add up
to more than all the water

in all the oceans
on the Earth combined.

This vast underground reserve
of water is a genuine puzzle...

..because there's no way
comets or asteroids

could have penetrated so deeply
below the Earth's crust.

That's actually inside the Earth.

It doesn't seem that there's an
easy way to get it from the surface

down hundreds of miles
into the mantle.

So it seems far more likely
that that water that exists,

that was discovered,
came with the Earth when it formed.

Was the Earth born wet?

It's a controversial idea,
but the evidence is mounting.

Steve Mojzsis believes
this grey dust provides

the most conclusive proof to date
of the wet-birth theory.

This is a vial filled
with little zircon minerals.

These zircon minerals are amongst
the oldest known substances

that we have from our planet.

And this sample here
formed a mere 150 million years

after our planet formed,

and it's the very best record
of the earliest Earth.

Until recently, scientists believed
Earth was a scorched, dry ball

this early in its history.

But ancient zircon samples
paint a very different picture...

..because the zircon contains
the chemical signature

of the Earth's first water oceans.

The amazing find
from samples such as these

is that liquid water on our planet
is a primordial phenomenon.

These tiny zircon crystals
are compelling evidence

that the Earth was
bathed in liquid water,

millions of years before
the Late Heavy Bombardment

brought comets and asteroids
to the Earth.

Vast quantities of water
must have been in the mix

when the Earth was created.

But this simple fact

means everything we think we know
about the birth of our planet...

is wrong.

The heat of the sun evaporates water

from the surface of
the Earth's hottest places.

Our vast, parched deserts
are almost liquid-less.

Five billion years ago,
the great cosmic desert

that stretches from the young sun
to the snow line

is just as dry.

How could the wet interior
of our planet

form out of this rocky, arid dust?

We think that the materials that
were forming in the solar system

right where the Earth is today,
would have been much more dry

than the Earth actually is.

So we think that the Earth
had to get an extra contribution

of water-rich material.

Where did all this extra
cosmic water come from?

Something must have
transported it in bulk

from the wet side of the snow line.

A clue comes from observing
distant exoplanets,

being cooked alive
by their parent stars.

What we see a lot of
are Jupiter-sized planets

sitting really close to their star,
sometimes extremely close,

sometimes much closer
than Mercury is to the sun.

Initially, these star-grazing giants
were a mystery.

How did they grow so big
so far away

from the icy riches of the snow line?

The only possibility is that
these planets must have formed

far out from their parent stars

and then later, migrated in.

We know that those kind of
planets can't form there,

they're simply too big.

They must have formed farther out
and moved inward,

migrated towards their star.

And that is interesting
because that makes you wonder,

was our solar system always
the configuration it is today?

Or have our planets
moved back and forth?

Exoplanet observations
have forced astronomers

to devise a radical new theory

about the formation
of our own solar system.

Known as the Grand Tack Hypothesis,

this theory suggests Jupiter
radically altered its course.

There was a time when
the disc of dust and gas

was very thick around the young sun.

And that actually put a drag
on planets as they orbited around.

In the Grand Tack model,

Jupiter forms on the outer,
wet side of the snow line.

But, slowed down
by the matter around it,

the gas giant's orbit
spirals in closer to the sun.

There's amazing evidence
that Jupiter may have moved in

as far as the orbit of Mars.

As Jupiter moves in,
it brings with it

massive quantities of water
from beyond the snow line.

This is a chance to push material

from much further out
in the solar system,

and throw it into the region
where the Earth is forming.

A chance to add a bunch
of water-rich material

to an otherwise dry Earth.

It's kind of like a huge snowplough,

just blasting this material
and pushing it inwards.

So that while the Earth was forming,

Jupiter could have been
scattering a bunch of icy bodies

from the outer part
of the solar system
in to where the Earth was forming,

while the Earth
was still being put together.

Jupiter's inward spiral
stops after 100,000 years,

when Saturn forms.

As the gravity of these two
massive planets interact,

they change tack,
heading away from the sun.

The water Jupiter leaves behind
clumps together with dust

to form Earth
and its neighbouring rocky planets.

But how did this water
trapped inside the Earth,

turn into the first oceans?

(RUMBLING)

Volcanoes may have
played a crucial role.

Think about the very young Earth
as a blister of volcanic activity.

You see these giants clouds
of ash and dust falling down,

but in there, there also
would have been water vapour.

Water vapour that could have cooled
and condensed in the atmosphere,

built up clouds over hundreds,
or maybe even thousands of years.

Until there was a moment

when there was enough water
in the atmosphere to begin to rain.

There really was a first rain,
billions of years ago.

(THUNDER RUMBLES)

As this volcanic water rains down
on the surface of the Earth,

the first rivers and oceans develop.

This happens long before
the Late Heavy Bombardment

that brings comets
and asteroids to Earth.

Based on evidence from the rocks,
it appears that that liquid water

is indigenous, native to our planet.

Comets and asteroids
brought some water to Earth.

But if the Grand Tack Hypothesis
is correct,

then Jupiter delivered
most of the water

we see filling our oceans today.

And Earth wasn't the only planet
watered by Jupiter's foray

into the inner solar system.

Both Mars and Venus
may have once had oceans, too.

To truly appreciate how remarkable
our living blue planet is...

..we need to find out
why we remained watery,

whilst our planetary
neighbours dried up.

*

*

Water defines the sights
and sounds of our planet.

As vapour, it paints the sky
with rolling clouds.

And as a liquid, it sculpts
and shapes the Earth's surface.

Water fills every cell
of every living thing.

And seen from space,
our brilliant blue oceans are unique,

a stark contrast
to our drab planetary neighbours.

Looking at our nearest neighbours,

we see the catastrophe that
happens when you lose water.

Not only is water important
for biological life,

but the evolution of
a planet really changes

when you lose
this particular molecule.

All the inner planets were
sculpted from the same materials,

and there's good evidence
that both Venus and Mars

once had oceans, too.

Mars had a lot of water in its past.

The whole surface is covered
with these incredible rivers

and stream beds that are empty now,

but really looked just
like what we see on the Earth.

And with Venus
we think we see evidence

that it was also a water world
when it was young, too.

Although we still
haven't explored Venus

as well as we've explored Mars.

So the best evidence we have
suggests that all of these planets

started out wet
and went through a watery phase.

Clearly something is happening in
the intervening billions of years

that is erasing the water away
from these planets.

Why did our neighbours dry up?

Around four-and-a-half
billion years ago, Mercury forms.

The closest to the sun of all
the planets in our solar system,

it's also the smallest.

Tiny Mercury barely
outsizes our moon.

And when it comes to holding
onto surface water,

size matters.

If it's a small planet,

it's actually going
to lose water to space

because it doesn't have
the gravity to hold onto it.

Next in line from the sun
sits Venus.

Earth-sized Venus holds onto
its water, at least for a while.

Four billion years ago,
Venus and Earth looked like twins.

Both have oceans of liquid water

and both are cloaked
in thick atmospheres.

But Venus takes up residence
closer to the sun

and grows hotter.

Its oceans evaporate,

pumping the atmosphere
full of water vapour,

a powerful greenhouse gas.

And Venus heats up even more.

Venus got itself into
a terrible vicious cycle.

Water got baked out of rocks.

The minerals themselves were
baked to such high temperatures,

they released their water vapour.

There was no way for
the water to condense.

It was too hot
so there were no rains

and so the water
moved higher and higher

into the atmosphere over time,
where it got blown away.

And now Venus
is this hellish landscape,

cooked under a heavy atmosphere.

Of all the rocky planets,

Mars sits furthest from the sun.

Billions of years ago,
an ocean a mile deep

covered half its northern hemisphere.

But it wasn't heat
or a lack of gravity

that caused Mars
to lose all its liquid water.

Mars's oceans were blasted away...

by radiation.

Mars does not have a magnetic field
and you need a magnetic field

to protect yourself
from the solar wind,

these sub-atomic particles
blasting away from the sun.

Earth has a magnetic field,

generated by
its spinning molten-iron core.

This field protects our atmosphere
from the solar wind.

Mars, however, is smaller
than Earth, and its core cooled,

shutting down its magnetic field

and exposing its atmosphere
to the savagery of solar radiation.

Mars's water didn't stand a chance.

Incoming radiation split apart
the hydrogen from the oxygen.

So the hydrogen's very light

and just went to space
and it was gone.

So then we were left on Mars
with a lot of oxygen.

This is why we hypothesise
Mars is a red planet

because it's very rusty,
and that's because

all the oxygen that used to be
in the water is now in the rocks.

Mars's oceans evaporated,
leaving behind traces of ice

and staining its landscape
a vivid red.

Four rocky planets
created at the same time

from the same building materials...

..but only one got lucky.

Water is fundamental
to life on Earth.

It's the perfect solvent
for organic molecules

to let the machinery of life
do what it does.

You and I could not
survive without water.

Water enables the geology,
it enables the climate

and it enables the biology of Earth.

So I think that that marks it
as a pretty special substance.

It took 14 billion years

and a great deal of luck
for our watery Earth to form,

and then to stay watery
long enough for life to evolve.

And the more we learn about water,

the more we'll discover
just how many other worlds

in the universe got lucky, too.

And as we discover more
and more exoplanets,

and more and more planetary systems,

we're gonna get
a better lay of the land.

And have a better view
of whether or not our system

is something that could be common,

or whether something like
the Earth is actually rare.

It's going to require tomorrow's
technology to get a better view.

For now, our telescopes
don't have the power

to see exoplanets clearly
enough to identify water.

These places are still so far away

that we're not gonna be able
to resolve pictures of oceans

and continents
and little clouds whipping around.

But chemically, we could detect
the signs, not only of water vapour,

but organic molecules.

Scientists hope
the next generation of telescopes

will detect water
on Earth-sized exoplanets

by analysing
the chemical signatures of light

passing through the atmospheres
of these distant worlds.

And when that happens,

there's a good chance
we'll discover a Milky Way,

packed full of watery worlds.

And we've already discovered planets

where, in principle,
liquid water could exist.

We don't yet know for certain.
We will find out.

I think in the next few decades,
we will know.

We will be able to identify
a planet where we can say,

"Yes, there's liquid water
on the surface of that planet."

That will be
when the universe changes

and we really grow up and realise
we have brothers and sisters

in the Milky Way galaxy.

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