All language subtitles for The Planets 2of5 The Two Sisters 1080p_Subtitles01.ENG

Lift off of Messenger on NASA'S
mission to Mercury.

MUSIC: The Void
by Muse

# They'll say no-one can see us

# That we're estranged and all alone

# They believe nothing can reach us

# And pull us out of

# The boundless gloom

# They're wrong

# They're wrong. #

Our planetary neighbour, Mars,
is a cold, barren rock.

Its rusted surface
covered in parched sand.

But, beneath the dust,

the planet bears the scars
of a former life.

Billions of years ago,
Mars was just like Earth.

A world with a thick atmosphere

that supported oceans of water.

But, today, that world is gone.

Mars lies dead,

while the Earth thrives.

Why the two planets had such
different fates is a mystery

that we've only just begun
to answer.

You see that pale red point
of light in the sky,

just there?

That's Mars.

Through a small telescope,
it appears almost Earth-like.

Our sister world -
polar ice caps and dark surface

markings that 19th-century
astronomers thought were vegetation,

even canals bringing
meltwater down from the poles

to arid equatorial cities.

Across the depths of space,

the inhabitants watched us "with
envious eyes", wrote HG Wells.

We now know that there are no
eyes looking back at us.

Mars is a frozen, arid desert world.

But a fleet of spacecraft
have revealed

that it hasn't always been that way.

Mariner 4 was successfully
launched on time

for its historic 228-day
journey to Mars.

Picture information started
to come in on July 15th, 1965.

A revelation comparable to
Galileo's first view

of the moon through a telescope.

During its brief flyby,

Mariner 4 gave us our first
close-up glimpses of Mars.

When Mariner 9 was placed
into an orbit around Mars,

it saw a planet blanketed
by a gigantic dust storm.

In nearly a year of operation,

they transmit more than 7,000
photographs.

From orbit, Mariner 9 photographed
80% of the Martian surface.

First of all, there are two eyes,
not only in colour but also

in stereo, and in the infrared
part of the spectrum.

It has a sense of touch,
it has a sense of hearing,

but by far the most important
feature of the lander is its brain.

The Viking programme took
us down to the ground

for the first time...

Touchdown, we have touched down.

..and revealed Mars...

Perfect set-down.

..like never before.

There is the first piece
of information coming in.

Oh! Oh!

The data gathered over the last 50
years has allowed us to create

detailed maps of the Martian
surface...

..and begin to piece
together its past.

Maps of Mars are like storybooks.

You can read the history
of the planet

written across its surface, and the
reason for that is that there's

virtually no erosion,
hasn't been for billions of years,

so the scars of events that
happened even four billion years ago

can still be seen.

This is a type of map
called an elevation map.

The colours correspond to difference
in heights on the surface,

so blue means low

and red and whites are high.

Now, this region here, which is much
higher on average than the rest

of Mars, is called Tharsis
and it's covered in volcanoes,

including the largest volcano
in the solar system, Olympus Mons.

At the other side of Tharsis
is the great Valles Marineris,

the Mariner Valley,
and it is a canyon that dwarfs

anything we see on Earth.

On the opposite side of the planet
is an impact basin called Hellas.

The height difference from
the crater rim

to the crater floor is 9km.

That means you could fit Everest
in the middle of there

and look down on its summit.

And the region surrounding the
basin reveals Mars' former life.

The Hellas basin is punched into the
oldest-surviving terrain on Mars.

It's called Noachis Terra

or The Land Of Noah.

And that's a wonderfully evocative
name because its surface is sculpted

by flowing water.

All across the earliest Martian
surface, we've glimpsed traces

of what appear to have
been lakes and rivers.

And so a new generation
of spacecraft has been sent to Mars,

to investigate the existence
of water...

..and what happened to the planet
for it all to disappear.

Led by the most audacious Mars
mission ever attempted...

INDISTINCT RADIO COMMUNICATION

We have two-way Doppler and orbit
around the planet Mars.

..to land a one-tonne rover
on the Martian surface.

Its final descent has become known
as the "seven minutes of terror".

Curiosity touched down in
Gale crater,

a 150-kilometre-wide impact basin,

thought to have been home
to an ancient lake.

The rover is a $2.5 billion
mobile chemistry lab...

..designed to take samples
of the Martian surface

and analyse its composition.

As it explored the crater,
Curiosity saw pebbles polished

and rounded by running water

in what had once been rivers
and streams.

Then, 61 days after landing,
Curiosity identified the perfect

spot to begin its primary mission.

In a sandy area of the crater
called the Rocknest,

the rover took its first
scoops of Martian soil.

Chemical analysis of the fine,
dusty sand revealed

something quite unexpected.

Even though the surface of Mars
appears completely dry,

2% of the soil is still
made up of water.

Curiosity had found evidence
of just how wet a planet

ancient Mars had been.

Mars was a water world.

Rains fell,

rivers ran,

and, in the northern hemisphere,

water collected in a vast sea

that covered a fifth
of the Martian surface.

The Red Planet was once blue.

All the evidence suggests
that there were large bodies

of standing water on Mars
around 4 billion years ago,

and the atmospheric pressure
was at least that of Earth today,

perhaps even higher.

Temperatures were around 25 degrees,
so I could have sat on Mars

all those years ago,
admittedly with a mask to breathe,

because there was very little
oxygen, but I could have sat there

and looked out over a view
like that.

So, you don't have to imagine
what Mars was like in the past.

You can experience it.

It was pretty much like this.

But, within a billion years,

all Mars' lakes and seas
had disappeared.

In our solar system,
only one blue planet survives...

..Mars' sister, Earth.

70% of our planet's surface
is covered by ocean.

Under the waves,
a million species thrive.

While on land, the rains support
Earth's delicate ecosystems...

..providing a home for
an abundance of life.

But it hasn't always been this way.

The early Earth was unrecognisable
from the planet we know today.

Its atmosphere thick
with carbon dioxide.

And its oceans acidic.

Four billions years ago,
Earth was a troubled, toxic world...

..while Mars was flourishing.

But both planets were
about to be engulfed

by a cataclysm from space.

To understand what happened,

we have to look beyond
our own world.

You can't read the deep history
of the Earth by looking

at its surface because our planet
is a geologically active world.

The surface is constantly
being reshaped by volcanic activity,

weathering, and the actions of
the oceans, but we have a companion,

the moon, which has been inactive
for many billions of years,

and so the history of events
that happened in this region

of the solar system is written
all over its surface.

The most distinctive feature
of the moon's surface

are its craters - it is literally
covered in a record of impacts

from space, and that allows us
to estimate the relative ages

of different parts of the moon.

Quite simply, if there
are more craters,

then that piece
of the moon must be older.

There's been more time
for the impacts to build up.

But we can do better
than just measure the relative ages

because we have rocks,
the moon rocks brought back

by the Apollo astronauts.

We can estimate the ages of rocks
very precisely by looking

at the rates of decay of radioactive
elements inside them.

They're like little stopwatches
that start ticking the moment

the rocks are formed, in this case
by the impacts from space.

So, the moon rocks allow us
to tie the number of craters

in a particular region of the moon

to an absolute age measured
by the rocks.

And this doesn't just allow us to
date impacts on the lunar surface.

It means that craters can be used

to read the histories of worlds
across the solar system.

Including Mars.

When we gathered all the data,
we discovered something surprising.

There was a peak in the crater
formation rate, about 3.8

to 3.9 billion years ago,

which signified a period of intense
violence in the solar system,

and that is called the
Late Heavy Bombardment.

Countless asteroids fragmented
in Mars' atmosphere,

raining havoc across the planet.

It's estimated that
53 tonnes of rock

fell on every square
metre of Mars.

Over a third of the planet's
surface was obliterated...

..and Mars was pushed
to the brink of death.

Whilst the evidence from the
surface of the moon tells us

that the Late Heavy Bombardment
happened, it doesn't tell us why.

For that, we have to resort
to computer models of the evolution

of the solar system,
and, when we do that,

they point the finger at Neptune.

It's thought that Neptune migrated
outwards into the Kuiper belt...

..a region of icy, rocky objects

orbiting at the edge
of the solar system.

The resulting gravitational
interactions disrupted those orbits

and sent many of the objects inwards
to the inner solar system,

and that may have been the cause
of the Late Heavy Bombardment.

Earth also suffered the onslaught,

and, for tens of millions of years,

the fortunes of the two sister
worlds hung in the balance.

But, just when conditions appeared
at their least promising,

Earth's most precious
characteristic emerged.

Life.

There is good evidence that life
was present on Earth

around 3.8 billion years ago,
and discounting the - I think -

remote possibility
that life began elsewhere

in the solar system
and was transported to the Earth

on meteorites or comets,

that means that life must
have begun here.

So, somewhere on this planet
there was a transition

from geochemistry -
the chemistry of Earth,

to biochemistry -
the chemistry of life.

And whilst the precise details
of how that transition occurred

remain a mystery, it's thought
that in warm volcanic pools

or deep sea hydrothermal vents,
conditions were right

for the chemical building blocks
of life to form spontaneously.

And that means that
if similar conditions

were to be found elsewhere
in the solar system,

it might be possible
that life began there too.

Ignition, and lift off of
the Atlas V rocket with MRO.

Surveying for the deepest
insights

into the mysterious
evolution of Mars.

So, in 2005, NASA embarked
on a mission to look

for those same environments on Mars.

For more than a decade,
the Mars Reconnaissance Orbiter

has been our eyes on the
Red Planet...

..sending back more data

than all the other Mars
missions combined.

MRO has made more than
60,000 orbits,

mapping over 99%
of the planet's surface.

Its high-resolution cameras
have revealed Mars as never before,

discovering polar avalanches,

shifting sand dunes...

..and what could be seasonal flows
of sand or even liquid meltwater.

Then, in 2017, MRO turned its gaze

to one of the Red Planet's
oldest features,

the Eridania Basin.

3.8 billion years ago,
the basin was a vast sea...

..holding ten times more water

than the Great Lakes of
North America.

And it was here that MRO found
the evidence it was looking for.

400-metre-thick deposits
of minerals that, on Earth,

form in deep sea hydrothermal vents.

In the Eridania Basin,
MRO revealed that conditions on Mars

had once been ripe
for the emergence of life.

We won't know for sure whether life
began or even perhaps still exists

on Mars until we go there
and find physical evidence -

so, microbes buried deep below the
soil in oases of liquid water,

or maybe microbe fossils -
but what we do know is that,

when life began here on Earth,
3.8 billion years ago,

the conditions on Mars
were very similar.

There were seas,
there was volcanic activity,

there were even hydrothermal vent
systems on the floors of its oceans.

So, it is at least possible
that Earth is not the only world

in the solar system
where life began.

The habitable conditions
during what's known

as Mars' Noachian era persisted
for hundreds of millions of years.

But then, prospects for life on
the Red Planet changed dramatically.

Around 3.5 billion years ago,
the Noachian era drew to a close

and Mars entered a more frozen,
arid phase, known as the Hesperian.

The water that flowed freely over
the surface during the age of Noah

became locked away
in giant reservoirs of ice.

But, around the same time, Mars
became more volcanically active,

and the volcanic eruptions
and sub-surface lava flows

occasionally melted the ice,
leading to catastrophic flooding.

They must have been some of
the most spectacular sights

in the history of the solar system.

As molten rock pushed
upwards through the crust,

meltwater poured out
onto the surface.

It raged down from
the southern highlands...

..until, in a place known
as Echus Casma, it plunged

over cliffs 4km high...

..creating the largest waterfall

the solar system has ever seen.

Echus Casma would have been like
no waterfall ever seen on Earth.

350 cubic kilometres
of water flowed over it.

That's like a cube 70km by 70km
by 70km.

It all entered into a canyon
10km wide

and 100km long,
and that happened in a few weeks.

Once the flood subsided,
the water disappeared...

..leaving the evidence of the falls
etched into the face of the planet.

We don't know precisely why
the climate of Mars changed

from warm and wet
to cold and arid.

We're talking about events
that happened

three and a half billion years ago
on a planet hundreds of millions

of kilometres away,
so it is a hard problem.

But we do strongly suspect
that changes happening

on the planet's surface were driven

at least in part by changes
in the planet's interior.

Deep within Mars' core,

something was causing
the planet to die...

..and the evidence can be found
in Mars' atmosphere.

T-minus ten, nine,
eight, seven, six,

five, four, three,

two, one.

Main engine start,
ignition, and lift-off

of the Atlas V with MAVEN,

looking for clues about
the evolution of Mars

through its atmosphere.

In September 2014,
NASA'S MAVEN probe made its final

approach to the Red Planet.

Its mission - to understand
what drove the planet's

dramatic climate change.

MAVEN is equipped with an array
of instruments designed to measure

the behaviour of the atoms
and molecules in Mars' atmosphere.

The spacecraft circles Mars
in an elliptical orbit...

..allowing it to measure the
full profile

of the planet's upper atmosphere.

At its lowest point,

it's just 150km above the surface.

At its highest, a little
over 6,000 kilometres.

And it was at the very top of
Mars' atmosphere that MAVEN found

the key to the mystery
of what happened to Mars.

Detailed measurements
revealed gas is being lost

from the Martian atmosphere,

escaping to space

at a rate of about two kilograms
every second.

Over time, it's thought this gradual
stripping away of Mars' atmosphere

has slowly thinned the insulating
layer surrounding the planet...

..causing surface
temperatures to plummet.

But what was it that caused Mars
to lose its atmosphere

while Earth clung on to hers?

150 million kilometres away in that
direction is the setting sun,

a giant nuclear fusion reactor.

You can fit one million
Earths inside it.

Now, the surface temperature

is only around 6,000 degrees
Celsius,

but the sun's atmosphere,
known as its corona,

is at one million degrees.

And that means it's in the form
of what's known as a plasma, a soup

of electrically charged particles.

Some of those particles are moving
around so fast that they can escape,

and they stream away in what's
known as the solar wind.

They reach the Earth travelling at
a few hundred kilometres per second.

And, if we weren't protected, they
would strip away our atmosphere.

And when the sun dips
below the horizon...

..there are times when that
protective force field is revealed.

Just look at that!

I mean, there is the aurora.

It's the laws of nature,
all of them, written across the sky.

Electrically-charged particles
have been driven away from the sun,

ultimately from nuclear fusion
reactions in the core of a star.

They're crossing the solar system,
hitting the Earth's magnetic field,

stretching it out
on the dark side of the planet.

The field then snaps back
like an elastic band,

accelerating all of those charged
particles up and down

the field lines to the poles,
which is here in the skies

over Iceland, and they hit nitrogen

and oxygen molecules
in the atmosphere.

And you're seeing quantum
mechanics - they're exciting the

molecules so that they emit light
in characteristic colours.

And, if you think about it,
this is the only time

that we really see
the Earth's magnetic field.

It's one of the reasons why life
on Earth

has been able to
persist for four billion years.

In a sense, that's
the reason that you exist.

It's Earth's magnetic field
that protects our atmosphere

from the ravages of the solar wind,

and that protective shield
has its origins deep

in the planet's interior.

Thousands of kilometres
down below my feet,

actually below your feet now,
is the Earth's outer core,

which is a seething
mass of molten iron.

Convection currents cause
the molten iron to rise,

and then the Earth's
rotation causes it to spiral around.

Now, a spiralling, circling flow

of an electrically conducting
liquid is a dynamo.

A dynamo generates a magnetic field
and the Earth's field rises up,

not just to the surface here,
but out into space,

forming our protective shield.

And that is what you see there.

And just like Earth,

ancient Mars was also shielded
from the sun.

Aurora once danced above
its poles...

..keeping guard over the Martian
atmosphere and seas below.

But between 3.5 and 4 billion
years ago,

Mars' dynamo switched off.

The aurora surrounding the poles
slowly faded away

as the magnetic field diminished...

..allowing the atmosphere
to be stripped away

by the solar wind.

Without protection,
seas evaporated, the surface froze,

and Mars was transformed.

At the same time, the fortunes
of Mars' sister world

were about to take
a very different turn.

For the next billion years or so,
Earth was indistinguishable

from those landscapes of early Mars
-

barren continents surrounded
by ocean.

But in Earth's oceans, life was
beginning to transform the planet.

Primitive algae started
to neutralise the ocean's acidity

and replace the dense red fog
of Earth's methane-rich

atmosphere with oxygen.

Around 600 million years ago,
that oxygen-rich atmosphere allowed

complex life to evolve
in the oceans, colonise the land,

and ultimately produce this
almost-infinitely rich living world

today, of which we are a part.

While Mars died, Earth flourished.

To understand why the two sisters
had such different destinies,

you have to go right back

to the time the planets
were forming.

When Mars and Earth were born,

the solar system was a chaotic
vortex of gas and rock.

Material clumped together and grew,
only to be smashed apart.

Over time, some of the objects
became large enough to survive

at least the smaller impacts,
and continued to grow,

including the embryonic
planets Earth and Mars.

But there was one crucial difference
between the young planets.

Mars formed in a region
of the solar system

with considerably
less rocky material.

And that had a profound impact
on the planet's growth.

Mars is a significantly smaller
world - it's about half the diameter

of the Earth, and that makes
all the difference.

Although the details are not
yet fully understood,

it seems clear that Mars' smaller
size meant that its dynamo switched

off many billions of years ago.

Being smaller meant Mars' core
cooled more quickly than Earth's.

And this is certainly part
of the reason why Mars

lost its magnetic field.

Even though the planet is further
away from the sun than we are,

that meant that the solar wind
stripped away its atmosphere

and Mars died.

So, even though Earth and Mars
are so similar in so many ways,

the difference in position
and size in the solar system

led to very different fates.

Long ago, two sister
worlds were born.

In childhood, Mars was warm
and wet...

..whilst the Earth was
inhospitable and toxic.

Both young planets survived
the violence

of the Late Heavy Bombardment,

emerging as mature worlds,

primed with all the ingredients
for life.

But deep inside, the smaller
of the two was dying.

Mars' seas dried up.

And as the planet's interior cooled,
one by one her fires went out.

Olympus Mons, the largest volcano
in the solar system,

last erupted around
25 million years ago.

As the lava turned to stone,

Mars was frozen in time.

And so, today, her surface lies
rusted and gathering dust.

But that might not be
the end of Mars' story.

Because the next generation of
spacecraft are already on their way.

NASA Orion - currently
in advanced testing.

ESA ExoMars -

a fleet of spacecraft designed
to search for signs of life.

And the most ambitious private
space mission ever conceived.

A launch vehicle developed to take
humans to the surface of Mars.

Mars is, in a sense, a failed world,

a faded ember etched with the
memories of a more enticing past,

but there may have been,
and may still be, life on Mars.

And the discovery of a second
genesis in our solar system

would have profound philosophical,
scientific and cultural consequences

because it would mean
there is a sense of inevitability

about the origin of life,

and that would mean that the
universe

is most likely teeming with life -
that we are not alone.

But equally importantly, I think,
is the role that a planet

with a history like Mars
could play in our future.

Mars is rich in resources,
it has vast reservoirs of frozen

water below the surface,
and minerals -

iron, nitrogen, carbon, oxygen -
all the things

you need to support a civilisation.

And that's why I think
that, in my lifetime,

there will be Martians,
but the Martians will be us.

We will go to Mars
and make it our home,

and that old red world
will become our first step

beyond the cradle, and out
to the stars.

Mars really captures

our imagination,

partly because it's so close.

I think people are really interested
in Mars because it actually

is so similar to Earth.

It's close by, it's easy to travel
there with robots

and space missions, and so we've
done a lot of exploration.

And, every time you go and look,
you discover something new.

NASA Curiosity launched
on the 26th of November, 2011.

But the biggest obstacle
facing the mission team

wasn't leaving the Earth.

Mars has a unique set of challenges

compared to other places
we go with spacecraft.

Mars has an atmosphere
but it's thin, so it's not enough

to really slow you down,

but it is enough to actually burn
you up as you're trying to land.

Curiosity reached the top
of the Martian atmosphere,

travelling at 20,000km per hour.

Curiosity is a big rover.
It weighs a metric ton,

and so landing that required
every trick in the book

of how we've learned to land
on Mars with previous missions.

To land safely, the rover
had to be slowed

to less than 4km per hour.

You end up arriving
at Mars going really fast,

so you actually have to slow down,

and we do that using a heat shield,

which burns off a lot of energy
and creates a lot of heat,

so you have to absorb that somehow
and not damage the spacecraft.

Then a parachute comes out.

The biggest parachute we've ever
used in a planetary mission.

And that even doesn't slow
Curiosity down enough,

because Mars' atmosphere is quite
thin, so then rockets carry

the spacecraft and guide
the spacecraft to the surface.

There's nothing you can do
at that point to ensure its success

or prevent its crashing...

..and yet you've invested so much
in the outcome.

All I could do was sort of curl
up in a ball and wait for the

green light that Curiosity was
safely on Mars.

Seven years and $2.5 billion
in the making,

Curiosity finally touched down

at 6.32 Universal Time,
on the 6th of August, 2012.

I was sitting in the control room
watching the engineers,

who were actually monitoring the
signals coming in from Curiosity,

and so they were reading out
the data that they were getting

and they detected
the wheels touching the soil.

Then a few seconds went
by when cables had to be cut

and the rocket jet
pack had to fly away.

And, only then, they understood
that Curiosity was safe

on the ground, and the whole room
just erupted in celebration.

Since it landed, Curiosity
has been exploring Gale Crater

for more than six years.

Curiosity is a roving laboratory.

We actually collect samples
by scooping it or by drilling,

or just by sucking in some
of the atmospheric gas.

And it's that type of data
that allows us to pick apart

the story that those things hold.

In 2015, we made our first
identification of organic molecules

that we think were coming
from the Martian materials.

And that is a turning point for us.

What we found in those rocks

is what we expected
of natural organic matter.

It's what you would
expect to find on Earth.

Finding the organic matter
is the clue to searching for life.

What everybody wants to know
is whether or not Mars

once had life, and the short
answer is - we don't know.

The somewhat longer answer is -

we see all the signs of materials
that could have supported life.

We have evidence for
lots of water early on.

We see the nutrients,
we see carbon, we see oxygen,

we see nitrogen, we see phosphorus,

we see all the stuff that life needs

in order to reproduce and survive
as simple microorganisms.

For me personally,
I find it might actually

be more surprising if we never
found evidence of life on Mars.

Everything we've found suggests
that Mars was such a friendly,

supportive place for life
in its early history,

and there should be a lot of planets
like that around other stars,

and lots of life in the universe.

So, maybe we're getting to the point
where it'll be more surprising

if we never find other life.

And so, thanks to Curiosity's
discoveries, the latest wave

of spacecraft might finally
answer the question -

has there ever been life on Mars?

Next time...

..we enter the realm of the gas
giants...

..to discover how the largest
and oldest of the planets

sculpted the entire solar system.

Jupiter, the godfather.

Journey through our solar system
with this free poster produced

by the Open University, and discover
more about its planets and moons.

Order your free copy by calling...

..or go to...

..and follow the links
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