The Planets

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

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...

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.

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,

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,