Original subtitles

COMMS: Liftoff ofMessenger on NASA 's mission to Mercury.

BRIAN COX: Our planetary neighbour, Mars, is a cold, barren rock.

Its rusted surface covered in parched sand.

But beneath the dust,

the planet bares 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.

NEWS REPORT: Mariner 4 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.

BRIAN COX: During its brief flyby,

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

NEWS REPORT: 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.

BRIAN COX: 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.

BRIAN COX: The Viking programme took us down to the ground for the first time...

COMMS: Touchdown, we have touchdown.

- BRIAN COX:...and revealed Mars... - Perfect set-down.

BRIAN COX:...like never before.

And there's the first piece of information coming in. Oh! Oh!

BRIAN COX: 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.

There hasn't been for billions of years.

So the scars of events that happened even 4 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 white 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 nine kilometres.

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's 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...

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

COMMS 1: Vehicle reports entry interface.

COMMS 2: We have two-way Doppler and orbit around the planet Mars.

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

Its final decent 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.

For hundreds of millions of years...

...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, cos 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's lakes and seas had disappeared.

In our solar system, only one blue planet survives.

Mars's 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.

4 billion 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 re-shaped by volcanic activity

and 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.

And 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.

Now, we can estimate the ages of rocks very precisely

by looking at the rates of decay of radioactive elements inside them.

They are 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's 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.

COMMS: Ignition. And liftoff of the Atlas V rocket with MRO...

...surveying for the deepest insights into the mysterious evolution of Mars.

BRIAN COX: 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 microfossils.

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's 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 Chasma,

it plunged over cliffs four kilometres high...

...creating the largest waterfall the solar system has ever seen.

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

350 cubic kilometres of water flowed over it.

That's like a cube - 70 kilometres by 70 kilometres by 70 kilometres.

It all emptied into a canyon

10 kilometres wide and 100 kilometres 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 3.5 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's core,

something was causing the planet to die,

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

COMMS: T minus ten, nine,

eight, seven, six, five, four,

three, two, one...

Main engines start.

Ignition. And liftoff of the Atlas V with MAVEN,

looking for clues about the evolution of Mars through its atmosphere.

BRIAN COX: 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's 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 150 kilometres above the surface.

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

And it was at the very top of Mars's atmosphere

that MAVEN found the key to the mystery of what happened to Mars.

(STATICKY CLICKING)

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's 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 onto hers?

150 million kilometres away in that direction is the setting Sun -

a giant nuclear fusion reactor.

You could fit a 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 a million degrees.

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

a soup of electrically charged particles.

Now, 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 are being driven away from the Sun,

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

They're crossing the solar system, hitting the Earths magnetic field,

stretching it out on the dark side of the planet.

The field then snaps back like an elastic band,

accelerating all 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 4 billion years.

So, 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.

Now, convection currents cause the molten iron to rise,

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... 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's 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's sister world

were about to take a very different turn.

For the next billion years or so,

Earth was indistinguishable from the 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 continue 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's smaller size

meant that its dynamo switched off many billions of years ago.

Being smaller meant Mars's 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

lead 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's 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's 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 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.

BRIAN COX: NASA Curiosity launched on 26th 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.

BRIAN COX: Curiosity reached the top of the Martian atmosphere

travelling at 20,000 kilometres per hour.

Curiosity is a big rover - it weighs a metric tonne - and so landing that

required every trick in the book of how we've learned to land on Mars

with previous missions.

BRIAN COX: To land safely, the rover had to be slowed

to less than four kilometres per hour.

ASHWIN VASAVADA: 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's 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.

(BEEPING)

BRIAN COX: Seven years and $2.5 billion in the making,

Curiosity finally touched down

at 6:32 Universal Time on 6th 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.

BRIAN COX: Since it landed,

Curiosity has been exploring Gale Crater for more than six years.

JENNIFER EIGENBRODE: 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 allow us

to peck 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.

ASHWIN VASAVADA: 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.

BRIAN COX: And so, thanks to Curiosity's discoveries,

the latest wave of spacecraft might finally answer the question,

has there ever been life on Mars?