Catastrophe

This city, the people in it, in fact all life on Earth, were only here by chance.

We survived, 99% of all the species that ever existed didn't.

They were wiped out in a series of global catastrophes,

disasters that brought life to the verge of extinction.

Four and a half billion years ago, the Earth collided with another planet.

The impact nearly destroyed our world, but instead it made it a home.

This is the story of our planet's difficult birth.

When I look around at all these people,

it's hard to believe quite how lucky we are because we're only all here by chance.

Evolution wasn't an orderly progression from single cells to us.

It was an imperfect, often violent process,

and it's a miracle that we're here at all.

In this series, we'll look at the theories about catastrophes that shaped our world

and very nearly stopped evolution in its tracks.

Like the time the planet was encased in ice for

25 million years, freezing the land and the oceans.

Life survived, but only just.

When huge volcanic eruptions poisoned the planet's atmosphere

and pushed all life to the edge of extinction.

At the moment, a six-mile-wide asteroid smashed into the planet,

killing 70% of life on Earth,

including the dinosaurs.

Even humans didn't escape unscathed.

We might only have been around for a short while,

but we've faced and survived supervolcanoes, ice ages, and even cosmic impacts.

These catastrophes nearly wiped out life,

but from their ashes, new species evolved.

Without them, we might not be here at all.

Tonight, we examine the first great

catastrophe, the day our planet collided with another.

The Earth's around 4.5 billion years old, so it's

tough getting a handle on such an enormous amount of time.

To put it in perspective, imagine the whole of Earth's

history compressed into the 24 hours of a single day.

Each minute on our clock represents around 3 million years.

It starts ticking at midnight and the first catastrophe was just minutes away.

Our solar system hadn't even finished forming.

Twenty infant planets circled a new star, our Sun.

One of them was Earth.

Its surface was a vision of hell, a barren,

lifeless place shrouded in toxic volcanic gases.

There was no water, no oxygen, and no moon.

But this is all about to change.

As our clock reaches 9 minutes past 12, that's 4.5 billion years ago,

the Earth faces its first and greatest disaster, an impact of biblical proportions.

Reconstructing Earth's early history is a major challenge.

It's been obliterated and obscured by

billions of years of erosion and volcanic activity.

Astronomer Bill Hartman has spent his life

studying the events of the early solar system.

The record of the very first part of the Earth's

history, all of that is wiped away on the Earth itself

because we have erosion and rain and continental drift

and continents colliding and mountains coming up and so on.

But all is not lost.

There is a record of Earth's earliest days, just not on Earth.

It turns out the best place to look is on nearby Mercury and Mars.

Their surfaces have barely changed in over 4 billion years,

providing us with a unique record of events

in the very earliest days of our solar system.

They're dotted with ancient impact craters.

You start looking at those craters and you discover that

there are not only 100-mile craters and 200-mile craters,

there's 600-mile features and some very large objects.

These craters paint a picture of an intensely violent period,

of a solar system littered with cosmic debris, where

millions of asteroids and comets smashed into the young planets.

In the thick of this all-out assault, the Earth too must have been struck.

It was a window into the early history of the Earth and it made us realise that the

Earth itself has had this tremendous history of impacts,

enormous impacts that could have really damaged the whole planet.

It got people thinking about what would be

the effects of giant impacts on the Earth.

Hartman realised that it wasn't just small asteroids

hitting the planets, there were much larger objects too.

The larger the object, the more dramatic the consequences.

So the impact process, it's a wonderful kind of paradox. On one hand, the small

impacts tend to make everything the same,

millions and millions of impacts averaged out, but the

big impacts give individual personalities to the planet.

Take our planet, tilted on its axis at 23

degrees, with a nearby orbiting satellite, the Moon.

We used to think that they'd been born

together, until Hartman proposed a radical theory.

The Earth had been hit by something the size of

another planet, creating that tilt and the Moon.

The key to our idea was that as the planets grew, you

had the finished planets but you still had leftover bodies.

If one of those crashes into the Earth, just as the Earth has finished forming,

that can blow out material from which the

Moon could form in orbit around the Earth.

Picture the scene. The newly formed Earth hurtling round the Sun, and loads of

other planets doing exactly the same thing.

Including this one, Theia, about the size of Mars.

It was orbiting the Sun at exactly the same distance as

the Earth. The two planets were on a collision course.

Theia hit the Earth at 25,000 miles per hour,

with the force of billions of megaton bombs.

The impact ripped off huge sections of the Earth's

crust. Billions of tonnes of debris blasted into space.

A ring of red hot dust and rock formed around the Earth.

Over the next hundred years, the rocks and dust slowly

clumped together into a ball one-fiftieth the size of Earth.

We call it the Moon.

When Hartman suggested the idea in the 1960s, people found it hard to accept.

Scientists were thinking of everything in terms of slow geologic processes, one

grain of sand at a time, you know, wearing down mountains.

To think of something as colossal as the Moon forming as

a result of a single event was hard for people to swallow.

Left arm, left arm, very simple, and cute.

But then Hartman got the first real clue that his theory might be true.

The Apollo Project.

It's one small step for man, one giant leap for mankind.

American astronauts made six visits to the Moon.

They explored its surface, drove around its

craters, and brought back 840 pounds of Moon rock.

For the first time, scientists could find out what the Moon was actually made of.

The lunar samples had a remarkably similar

chemistry to the outermost layer of the Earth's crust.

To most researchers, it was an interesting

discovery. But to Hartman, it was vital new evidence.

So you have crustal rocks, you have rocks on the surface, and

a big impact comes in and blows all those crustal rocks away.

And that material goes into space and forms the Moon.

But many scientists were still sceptical. They just couldn't see how a massive

impact could create the Moon and the Earth as they are today.

I actually had people telling me we should exhaust every

other theory first because this was such an outlandish idea.

It was a chance meeting at a conference with astrophysicist Robin Canup that gave

Hartman the breakthrough he was looking for.

She was using computer models to study Saturn's moons.

Bill Hartman came up to me after my talk and asked me, "Have you ever thought about

applying your models about how moons form within

and near Saturn's rings to the origin of the Moon?"

And I said, "No."

So she tried it. Canup used modelling software to recreate the early solar system.

Then she plotted a planetary collision of the kind Hartman was suggesting.

So we're four and a half billion years ago at the end of the Earth's formation.

And we're in space and we're watching as a small planet, the planet on the right,

is about to hit the young Earth, represented by the larger planet on the left.

The collision takes place and as we see it hit, it hits in a glancing blow.

And you can see the impactor is completely destroyed by the collision.

Thea, the impactor, is annihilated.

Earth survives, but only just.

Now this collision is incredibly violent, so violent

that there's enough energy to completely melt the Earth.

And in fact, at the end of this impact, the Earth

is surrounded by an atmosphere of vaporised rock.

Trillions of tonnes of debris blast out into space.

Here we see part of the impacting planet sheared out into this long arm of material

that produces a disk that we're seeing almost edge-on in this view.

And it's from that disk that the Moon later coalesces.

Canup's model demonstrates that the Moon was

probably made of debris from both Thea and Earth.

It explains why those Moon rocks were so similar to rocks from the Earth's crust.

So I actually called my colleague and said, "You're not going to believe this."

But I tried this Mars-sized impactor case with

about a 45-degree impact angle and everything worked.

And he said, "You better check it again."

And so I did check it again and did many more of these simulations.

And sure enough, that type of impact is the

one that gives us the Earth-Moon system today.

Canup's work was further evidence that

Hartman's radical theory might just be right.

So it was very exciting as Robin did her models and they started to say,

"Yes, there can be Moon-forming debris left in orbit

around the Earth, and the Moon would form from that debris."

Hartman and Canup's work showed that our Moon was created by a violent cataclysmic

collision between Earth and its twin Thea.

The Earth narrowly survives complete destruction.

But the collision triggered a series of events that transformed our planet.

It became a climate hell-hole with extreme weather conditions and giant tides.

But bizarrely, those deadly conditions created

the building blocks for life itself to evolve.

On our clock, only 10 minutes have passed.

That's 30 million years in real time.

It was the single most important period in the Earth's history,

a time of incredible violence that saw the creation of a new Earth and our Moon.

The massive impact which created the Moon very nearly destroyed our planet.

But out of this catastrophe came a new beginning,

because the impact set in motion a chain of events that transformed the Earth from

a vision of hell to the blue-green oasis we call home.

Scientists set out to reconstruct those events,

starting with the moments immediately after the impact.

Ithaca, Upstate New York.

Palaeontologist Judith Nagel-Myers hunts for clues that

might reveal what happened to Earth just after it was struck.

No evidence remains on Earth from the time of the collision,

so scientists are constantly searching for ingenious ways to look back in time.

Nagel-Myers uses fossilized corals.

These ones are just 400 million years old.

But they hold a vital clue to what conditions

were like on Earth four and a half billion years ago.

I personally love about fossils that you

find them and they open up a window in time.

Just by looking at their remains and their skeletons,

you can reconstruct the environment that was

here long before humans are even on the Earth.

The coral fossils have an unusual property that allows them

to capture a day-by-day snapshot of conditions on early Earth.