Earth Story 8 8 A World Apart BBC Documentary


Uploaded by NaytonScience on 05.10.2012

Transcript:
MANNING: This lush sub-tropical island off the coast of Africa
emerged from the Atlantic two million years ago.
Today, La Palma rises over two kilometres out of the sea.
At its summit, the rock suddenly falls away.
The middle of La Palma is a huge rocky chasm,
shaped like a horseshoe.
This whole vast natural amphitheatre
was first investigated by the great German geologist
Leopold von Buch in the 1820s.
He came here and worked out that this had once been a giant active volcano.
Half a million years ago, there was a colossal cataclysm here.
The whole centre of the volcano, 200 cubic kilometres of rock,
just collapsed into the sea.
This created a huge tidal wave that raced across the oceans,
leaving a trail of devastation halfway around the globe.
But events in La Palma are just one example of the vast forces
that are periodically unleashed by our planet.
And these are an inevitable part of the way it works.
In the course of making this series,
I've discovered just how dynamic our planet is.
The Earth's surface is in constant motion,
driven by its internal store of heat.
As continents collide, they create great mountain ranges.
As mountains rise and fall, they influence our climate.
And the climate shapes the life which inhabits the planet.
It's all this change which makes the Earth what it is.
With all this happening around and beneath us,
we are bound to find the Earth a special place.
But just how special is it as a planet?
And what's even more interesting, why is it the way that it is?
To begin to understand what sets Earth apart,
scientists need to find out how it was formed.
Clustered on La Palma's volcanic summit
are some of Europe's most powerful telescopes.
Here, astronomers are beginning to throw light on Earth's origins
by looking out, deep into space.
They now believe that our entire solar system
formed from the debris of dead stars,
as Mark Kidger explained to me.
I think it's a beautiful idea that the solar system
condenses out of star dust, the death of a star, as you put it.
But is that a unique event or are there many such systems?
Good heavens, no. The Hubble Space Telescope
has been searching for other solar systems that are in process of forming,
and at the last count,
it had found something around 140 of them.
KIDGER: <i>Solar systems, clouds of dust and gas</i> <i>in distinct phases of forming into solar systems.</i>
MANNING: Astronomers now think that solar systems form
when those clouds of stellar debris collapse.
You've got clouds of dust and gas in space,
but those clouds don't collapse without something to help them.
And what is that process?
Well, one idea which is particularly interesting
is the idea that a large star explodes as a supernova
and the shock wave from that supernova slams into the cloud
and that starts the cloud contracting and then gravity takes over from there.
MANNING: Here, far out in space,
scientists are observing new solar systems condensing from star dust,
just like ours did.
About four-and-a-half billion years ago,
the cloud of star dust and gas that was to become our solar system
began to spin.
As the gravity pulled the material into the centre,
it heated up.
Suddenly, the young sun ignited.
The lighter gas was swept far out into the solar system
leaving heavier, rocky material near the sun
which soon condensed to form rocky planets.
Mars, furthest from the sun, then the Earth, then Venus.
At this stage, was there anything to distinguish one from the others?
KIDGER: <i>They're very, very similar.</i> <i>They've all got an iron core,</i>
<i>they've all got lighter silicate stuff floating on top,</i> <i>they seem very similar.</i>
MANNING: So, when the solar system condensed out of this gas,
what's special about the Earth in the solar system?
Well, really, not a lot.
The Earth has exactly the same components
as the sun and all the other planets.
MANNING: But from early on, there was one thing which made Earth different from Venus and Mars,
it alone had a large satellite.
Visiting the moon was the first step in unravelling the history of the inner planets.
MAN: <i>17 Houston,</i> <i>you are go for orbit, go for orbit.</i>
MANNING: Dr Harrison Schmitt is the only geologist to visit an alien world.
SCHMITT: <i>... the corner and get that content,</i>
<i>if I can get it.</i>
MANNING: While working on the moon, he looked back
and saw how much our planet now differs from the others.
HARRISON: <i>We were in a deep mountain valley,</i>
<i>deeper than the Grand Canyon of Colorado</i> <i>of the United States, 7000 feet on either side.</i>
<i>We could see mountains rising above us,</i>
<i>brilliantly illuminated by a sun</i> <i>brighter than any desert sun that you can imagine.</i>
But the only real colour that we could see,
other than the spacecraft and maybe your colleague wandering around in the distance,
was this beautiful blue and white marbled Earth
with a desert beacon here and there
that hung always over the same part of the mountain complex.
<i>In an absolutely stark, black background.</i>
<i>It was something</i> <i>that one cannot really describe adequately.</i>
<i>It's something that everyone</i> <i>should have a chance to see for themselves.</i>
MANNING: Something must have happened to make Earth so different from its neighbours.
The moon enabled scientists to investigate the solar system's distant past.
The moon has given us a window into the early history of the Earth
that we never really expected to have, I don't think.
We hoped it would be that way, but when we actually explored it,
we found we had this beautiful window into that early history.
Pitted and dusty in some respects, but still one that we could now
understand better what kind of Earth we had
three billion years ago and later.
<i>The crust as it formed began to record</i>
<i>an extraordinarily violent period</i> <i>of solar system history</i>
<i>in which the debris left over</i> <i>from the formation of the solar system</i>
<i>was impacting the moon</i> <i>and the Earth at an incredible rate.</i>
MANNING: The surface provided scientists with a sort of clock.
The more meteorite impacts, the older a surface was.
This was true not just on the moon, but on all the inner planets.
Meteorite craters were to prove crucial
in working out the history of Earth's planetary neighbours.
How Mars became a cold, dead world.
How Venus, beneath a permanent cloud cover,
developed a surface temperature of molten lead.
And only on Earth are the conditions right for life.
How is it then that the three rocky planets that began so similarly
have ended up along such different paths?
It's a bit like the Goldilocks story.
You remember Goldilocks first tasted one porridge and it was too cold,
that's like Mars.
She then tasted another, that was too hot, that's like Venus.
Then she tasted the baby bear's porridge and that was just right.
And that's what Earth is now.
The puzzle is how did Earth come to be just right?
MANNING: As scientists continue to explore the solar system,
answers to the Goldilocks problem are beginning to emerge
from studies of our planetary neighbours, Mars and Venus.
Mike Carr is an expert on Mars.
He noticed that some geological features on the surface of that planet
look surprisingly Earth-like.
And that lead him to conclude that Mars
was not always as cold and dry as it is today.
We do have evidence that Mars and the Earth
started off relatively similarly,
two rocky planets.
That's right. When we look back at early Mars,
it appears that early Mars was warm and wet
and then it changed later.
CARR: <i>The evidence for that is that</i> <i>when we look at the oldest terrains on Mars,</i>
<i>what we see are little river valleys</i> <i>all over the place.</i>
<i>And they appear to have formed</i> <i>by slow erosion of running water.</i>
Well, to have water at the surface, you had to have warm, warm conditions.
How do we know that was early in Mars' history?
Well, it's earliest Mars because...
Very early in Mars' history, there was what we call heavy bombardment,
when the rate of impact of meteorites was very high,
and so all these old terrains are very heavily cratered.
<i>And then on the crater rims in-between the craters</i> <i>we see all these river valleys.</i>
MANNING: Four billion years ago, Mars was not unlike Earth.
The sky would have been full of clouds of water vapour.
From the clouds, rain fell.
Streams and rivers formed, eroding valleys.
But what kept the young Mars so warm and wet?
Clues to that puzzle can also be found on the surface of Mars today.
CARR: <i>The volcanoes on Mars are huge,</i> <i>absolutely huge.</i>
The crust is stable, the volcano just keeps growing and growing and growing
to these enormous sizes.
Well, Olympus Mons, which is the highest volcano,
it is 84,000 feet high.
<i>It's 550 kilometres across.</i> <i>It really is a huge volcano.</i>
MANNING: Olympus Mons is almost three times the height of Mount Everest
and half the size of France.
It is the highest volcano in the solar system.
On Earth, volcanoes produce a lot of gases,
including carbon dioxide,
which get added to the atmosphere.
And carbon dioxide has a decisive effect
on the planet's surface temperature.
It's a so-called greenhouse gas,
acting like a thermal blanket, keeping a planet warm.
So four billion years ago, the huge Martian volcanoes
must have been pumping out enough carbon dioxide
to keep the planet's surface warm and wet.
But Mars today is dry,
with no sign of running water.
Something dramatic must have happened.
The explanation may lie in some mysterious features on Mars.
Mike Carr estimates these were created
500 million years after the Martian river valleys formed.
Here were cliffs hundreds of metres high,
suggesting water erosion,
but no sign of rivers.
They reminded him of a place called Dry Falls
in America's northwest.
Mike Carr's satellite photographs of Mars
show what the two sites have in common.
CARR: First, there's evidence of deep erosion such as cut this valley here
and we see that here.
MANNING: Yes, these are big...
- These are big cliffs. - Big, big cliffs.
- Yes. - We also see scour marks on the ground here
and you can trace it all the way through here.
And just above the Dry Falls over here,
you can see these same scour marks that just go northward from Dry Falls.
So, all the things that we see here
on this map, we also see here in this location.
MANNING: The explanation for Dry Falls
should provide some clues to what happened on Mars.
Geologists have worked out that Dry Falls were created
when the last Ice Age ended, 12,000 years ago.
As the climate gently recovered,
a vast lake of melt water, as big as today's Great Lakes,
formed behind a thick dam of ice.
When the dam finally broke,
the lake emptied, creating a cataclysmic flood.
The flow of the water was greater than all the rivers of the world combined.
Enormous boulders and lumps of ice scoured the land.
Their force was so great at Dry Falls
that the erosion formed cliffs like Niagara Falls.
This was briefly a giant waterfall.
Could something similar have happened on Mars?
Mike Carr's Mars images also show evidence of a vast flood,
but no sign of giant lakes to hold the water.
Now, where did the water come from?
Well, that's something of a puzzle.
If you follow this large valley here
it ends up in a large depression full of rubble.
And the same... We see all these depressions here, full of rubble,
out of which come very large channels.
- MANNING: There's nothing flowing into them. - That's right, there's nothing flowing into them.
And so what must have happened is the water must have come out of the ground.
MANNING: Somehow the water must have been trapped,
causing the pressure to build up.
Well, one way of keeping the water under high pressure
is by having a cap
and the frozen ground is that cap.
So the temperatures on Mars when these large floods formed
were probably very cold.
So you had a frozen layer
at the surface, half a kilometre to a kilometre thick.
- Permafrost. - Permafrost. Permafrost.
And that prevented water leaking out onto the surface.
And it became kind of unstable.
Be rather like an oil well, in other words.
Very much like an oil well. Of course, if you drill into an oil well,
the oil comes gushing out.
Here, a meteorite drills through that permafrost,
the water comes gushing out in enormous volumes.
MANNING: So, by three billion years ago,
Mars had already entered a permanent Ice Age.
Its water was locked up below the surface and was only occasionally released
when a giant meteorite punched through the outer layer of permafrost.
Something must have turned Mars from a warm, wet planet
into a frozen world.
There's one place on Earth which suggests how this happened.
Here, at Mono Lake, in the desert of central California,
Mike Rampino believes there are clues
to what happened to the blanket of greenhouse gas which kept Mars warm.
Why is Mars too cold, Venus too warm and the Earth just right for life?
It turns out that the key to that question
could be seen in these rocks here at Mono Lake.
These rocks are solid rock
but they contain a gas, carbon dioxide.
If we drop a small piece of this rock in acid, it will bubble.
The bubbles coming out of the rock are carbon dioxide.
MANNING: The carbon dioxide in the rocks here has been drawn out of the Earth's atmosphere.
What's happening here at Mono Lake
is very similar to what happened on Mars three and half billion years ago.
These rocks are here
because rainwater picks up carbon dioxide from the atmosphere,
forming a weak acid,
which begins to dissolve away the granite rocks of the mountains,
washing the mineral material and the carbon dioxide into the lake waters.
MANNING: As carbon dioxide is washed from the atmosphere into the lake,
conditions are set up for a chemical reaction
that traps it into newly formed rocks.
RAMPINO: When that material gets there, it's concentrated
by the evaporation of the water in this very dry environment.
Eventually, the material in Mono Lake
produces the deposits
that form those strange rock formations we were looking at
- and trap the carbon dioxide. - MANNING: Right.
MANNING: Mike Rampino showed me springs in the lake
where you can actually see this process in action.
Goodness.
You can see the spring water bubbling up
- through the lake bottom. - Yeah.
And that's where you get the deposition
of the carbonate trapping rock that forms around the lake.
And out there, is that a lump which is actually forming?
Yes, that's where the deposition of this limestone rock is taking place,
which is the material that traps the carbon dioxide from the atmosphere.
Now imagine this going on on a planetary scale,
in the early history of the solar system, on the Earth or on Mars.
Bodies of water like Mono Lake, maybe even larger
where this process of formation of limestone is taking place,
trapping carbon dioxide from the atmosphere,
and taking the carbon dioxide out of the atmosphere permanently, in some cases.
MANNING: When Mars was young, the carbon dioxide being locked up in its rocks
would have been replaced by gas erupting from the huge volcanoes.
But it's clear that eventually this volcanic activity ceased.
One can look at the volcanoes on Mars from orbit
and see impact craters on the surface of those volcanoes.
Ancient impact craters.
It shows that that surface has been sitting there for a billion years
without any new flows of lava covering it up.
MANNING: Mars is much smaller than Earth
and its internal store of heat powering the volcanoes
was quickly reduced.
Eventually the volcanic activity stopped
and all of the carbon dioxide in the atmosphere, or almost all of it,
was removed from the Martian atmosphere by this process.
MANNING: Because its greenhouse atmosphere was soon locked up in its rocks,
Mars quickly froze.
And it remains frozen to this day.
MANNING: What about Venus, its surface a mystery beneath the layer of clouds?
In the 1980s, the Russians pulled off an amazing feat of space exploration.
They actually managed to land a probe on the scorching surface of Venus.
Before it burnt up, it sent back this one extraordinary image,
looking just like volcanic lava on Earth.
In the 1990s, NASA's satellite Magellan
penetrated the clouds with radar and mapped the entire surface.
The images were clearly of volcanoes.
Suddenly, Venus began to appear like parts of Earth, only hotter.
Ellen Stofan was a chief scientist on the Magellan mission.
The satellite gave her important information about the types of volcanism on Venus.
Almost everywhere on the surface of Venus,
you see some sort of volcanic landform.
There was huge lava flows, all kinds of small volcanoes, large volcanoes.
So in that sense, this is very similar to Venus.
But certain aspects of the volcanism on Venus are quite different.
MANNING: The cinder cones so common here on La Palma appear to be rare on Venus.
For example, here we have what are called cinders.
Now, this has come off what's called an effusive eruption,
where you have little clumps of magma or liquid rock
that are thrown up in the air, propelled by gas.
<i>Those red blobs, they cool as they come</i> <i>down and they form these little pieces of lava.</i>
MANNING: <i>So it's a real spurting type of eruption?</i> STOFAN: <i>Exactly.</i>
STOFAN: So these would be the glowing hot pieces and as they fall, they cool
and turn into this feature which is a cinder cone.
Now, we think we're going to see less of this on Venus
because of that high, high atmospheric pressure. It just sort of suppresses that ability
of volcanoes to throw things up in the air.
What's going to be more typical are just lava flows coming out on the surface.
More typical to this sort of eruption where you're just getting a lava flow coming out.
MANNING: It'd be much smoother on Venus, would it?
Very smooth, and in fact there are some lava flows that we've seen in the Magellan-data Venus
that are as smooth as parking lots, they're just amazingly smooth,
which is something you really don't see here on Earth.
MANNING: Venus' surface is lava flattened by an incredibly high pressure.
The pressure comes from an atmosphere crammed full of carbon dioxide.
What does this mean?
Clues come from the earliest history of Venus.
It's the same size as Earth,
so it wouldn't cool down as quickly as the smaller Mars did.
STOFAN: At about three-and-a-half billion years ago,
at the end of this early bombardment that had taken place,
Venus and the Earth are not that different. Their atmospheres are very similar at this point.
But there is a significant difference.
Venus is closer to the sun and it's warmer.
It's actually warmer than it is right now on Earth.
<i>Water, instead of being a liquid on the surface,</i>
<i>is right at the point</i> <i>where it's wanting to be a gas.</i>
<i>It's wanting to form water vapour</i> <i>in the atmosphere.</i>
As that happens, in the meantime, you've got volcanoes erupting.
As a volcano erupts, it not only makes nice lava flows,
it's also pumping carbon dioxide, silicon dioxide. It's putting a lot of gas into the atmosphere.
<i>Water vapour and carbon dioxide are both</i> <i>a type of gas that if they're in the atmosphere,</i>
<i>they start to keep the heat from the sun in.</i>
They're not letting the heat escape. So, the surface starts warming up
and warming up and warming up.
MANNING: Ellen Stofan believes that as Venus got hotter,
its climate became unstable.
It goes back to the whole water question.
Water actually helps to remove carbon dioxide from the atmosphere.
On Venus again, the problem is, it's just a little too warm.
There is not that liquid water that's wearing down the rocks.
This whole chemical reaction just isn't taking place.
So there's no way to get the carbon dioxide out of the atmosphere.
The carbon dioxide is building up, building up, building up. It's not being fixed into rocks.
And that's where this runaway greenhouse comes from.
Those processes ran away, the surface heated up and heated up and heated up
- till it's 500 degrees centigrade on the surface. - Right.
And it all started at that point when Venus was just a little bit too close to the sun,
a little bit too warm, and you end up with the Venus we have now.
<i>Without the ability to bring the carbon dioxide</i> <i>out of the atmosphere, the planet ran away.</i>
<i>And that's why it's so hot there today.</i>
MANNING: The Magellan radar images were put together
to form this computerised flyover of Venus.
They show a surface locked into a temperature of nearly 500 degrees.
So the climates of both Earth's neighbours have changed dramatically since they were young
and are now at opposite extremes.
What strikes me about the Earth and makes it so special
is its stability over a long, long period of time.
For example, there's been liquid water on the surface
for over three and a half billion years.
Yet if I've learnt one thing from this series,
it's the compelling fact of how dynamic and variable our planet's been,
constantly changing and recycling.
Is it possible that this constant geological change
may even be responsible for the Earth's climatic stability?
Earth's surface is made up of a few large rocky plates,
their edges marked by chains of volcanoes, earthquakes and mountain ranges.
This moving system of rocky plates, called plate tectonics,
is fundamental to the functioning of our planet.
It was natural to assume that Venus had a similar system.
As on Earth, the inner heat powering its volcanoes
could also drive a system of moving plates.
The Magellan radar camera should have enabled Ellen Stofan's team to detect these plates.
STOFAN: When we started getting the Magellan data set of Venus and we had a global view,
we started wondering where were the geologic features located?
The reason that we cared about this is because if you look at the Earth's surface,
the Earth's surface is broken up into large plates
and most of the geologic activity is concentrated at the edges of those plates.
- So things like volcanoes and faults... - Earthquakes?
Yeah, exactly. So we said okay, if you can look for patterns on the Earth and see them so clearly,
if Venus has plates, we should be able to look for geologic features and patterns.
<i>So we mapped out all the volcanoes on Venus.</i>
<i>We mapped out the faults.</i>
<i>And you couldn't find any plates,</i> <i>you couldn't find any plate edges.</i>
MANNING: Why is that a crucial difference?
It's telling you that the Earth and Venus
again, just differ fundamentally in the way they work.
It's not only hotter on Venus, it's not only higher pressure,
it's also the whole planet is operating in a different way.
So, is it activity at the edges of these plates, movements and so on,
which are crucial for the way Earth has gone?
We think so. We think it's part of the whole system that we have here on the Earth,
that the plates and how they move is a critical part of that.
And Venus is showing us how unique the Earth really is.
MANNING: And then it began to emerge how this unique movement of rocky plates
might maintain the climatic stability of Earth.
As they move, one plate can slide beneath another.
This process, subduction, buries rock beneath the surface.
Some rock melts and erupts out of volcanoes back onto the surface.
This recycles the rocks.
The circulation of rocks is what links climate with plate tectonics.
And there are processes on the Earth, geological processes,
that take the rocks that form on the surface, that lock up the carbon dioxide,
push it down to depth inside the Earth, those rocks are heated up
and the carbon dioxide is released back through the volcanoes.
There is a cycling of the carbon dioxide in the Earth,
that takes place because the Earth is an active, geologically active planet
and has been so for the past four-and-a-half billion years.
MANNING: So on Earth, volcanoes pump carbon dioxide into the atmosphere,
as they do on Venus.
Rain dissolves the carbon dioxide, just as happened on Mars very early in its history.
And the carbon dioxide is trapped in rocks.
But on Earth, most of that rock ends up on ocean floors.
Then it's subducted down as a slab, burying the carbon.
The carbon dioxide erupts from volcanoes
and so completes a cycle which stabilises the planet.
This cycle happens only on Earth.
But why does Venus have no plate tectonics?
The critical pointer was that it had no oceans, where rocks on Earth get buried by subduction.
Subduction is really the key to the whole process of plate tectonics.
It turns out that if you calculate sort of all the forces of what's going on,
the pull of the slab as it sinks
really helps the whole process of plate tectonics go as a system.
MANNING: And for subduction to keep working, it needs a lubricant.
STOFAN: And there is one thing that we know on Earth, is we have plenty of water.
The water lowers the melting temperature of basalt,
makes it easier for the slab to subduct.
Really does, as you say, it lubricates the whole process.
MANNING: So, water, it seems, is a vital part of plate tectonics on Earth.
Ever since our planet looked like this, three-and-a-half billion years ago,
the carbon cycle, driven by plate tectonics and by water, has maintained a temperate climate.
But as I discovered, keeping this astonishing system going over billions of years
has required another surprising factor.
To find out more, I went to Paris.
I've come to the Paris Observatory,
which for centuries has been a centre of research on the Earth's motion through the heavens,
motion determined by the force of gravity.
Here, Jacques Laskar uses a gyroscope
to demonstrate the way gravity affects the Earth as it spins on its axis.
I've always been delighted by gyroscopes.
The gyroscope, it can help us to understand the motion of the axis of the Earth.
You see, if I put the gyroscope here, without it rotating, it will just fall down
due to the gravity.
But if I make it rotate now,
if I just take it like that and if I put it back,
it is rotating now.
Instead of falling down, it will
- just slowly... - Slowly rotate.
Slowly rotate like that, then the... So, its axis will slowly rotate.
In the case of the Earth, it's roughly the same thing.
The axis of the Earth is tilted
and the moon and the sun are attracting this part
to make it go back in the upright position.
And it will make the axis of the Earth slowly rotate in a period of 26,000 years.
So, the sun and the moon are both trying to pull the Earth upright
and the Earth is trying to stay on its angle of rotation.
And the result is that the axis is itself, turning.
- Yes, it's turning, but it's a very slow motion. - Very slow.
MANNING: This slow rotation is controlled mainly by the combined gravity of sun and moon.
But the planets play a part, too, and that upsets Earth's stability.
In Jacques Laskar's computer are the orbits of the planets over millions of years.
These shifting orbits produce a fluctuating gravitational pull on the Earth.
They make the Earth's own tilt shift.
So the red circle drawn here by its axis changes in size.
Although small, these shifts have a major influence on climate.
As the Earth wobbles, vast ice sheets wax and wane over much of the globe.
But without the moon at the Earth's side,
it seems these fluctuations could cause catastrophe, worse than any Ice Age.
MANNING: Now, here's the moon. You can take it away...
That's something which is difficult to do in reality,
- but on the computer simulation, it is quite easy. - Yes, on the computer...
So, you see, I put the moon out.
(MANNING CHUCKLING)
And the immediate effect, you see immediately
we are getting to much higher tilt, you see how it's changing.
MANNING: With the moon's gravity removed,
the Earth comes much more under the influence of the other planets.
They cause chaos in the tilt of its axis
and the red circle changes, apparently unpredictably.
In reality, the moon being so close and large
keeps the Earth's tilt within narrow limits.
If the moon did go away, Earth could tip to vertical.
The result would be no seasons.
And it could go to any degree of tilt, even almost horizontal.
At this angle, one hemisphere would stay light for months
the other dark for months,
as the Earth moved round its annual orbit of the sun.
Effectively, a year would become a day, with colossal effects on climate.
And events like this have happened to planets without big moons.
Mars wobbles all the time and Venus appears to have flipped upside down.
No one can be sure, but Jacques Laskar's calculations suggest,
fascinatingly,
that the moon's steadying influence has enabled the Earth's climate to remain stable.
But there is a final surprising twist to the complex story of our climate.
Something with the power to overwhelm the intricate system
regulating the planet's temperature.
Astronomers have discovered that the sun itself is not completely stable.
It's gradually heating up.
Today, it burns 25% more brightly than it did when the solar system was young.
This colossal change should've had an absolutely catastrophic effect on the Earth,
and yet it appears that our world has hardly been affected.
The evidence for this comes from Greenland,
where geologists have found the oldest rocks on the Earth's surface.
As I discovered in the very first programme of this series,
these rocks paint a vivid picture of the planet nearly four billion years ago.
Among them is this deposit of rounded pebbles in a muddy matrix,
the remains of an ancient beach or shoreline.
This is unequivocal proof that then, as now, there was liquid water on the Earth.
So, despite the steady increase in the sun's activity,
Earth's temperature has changed little in four billion years.
Something which controls the planetary climate must be reacting to the changing sun,
so keeping the world cool.
But what?
The answer may lie here, in one of the most famous of all landmarks,
the white cliffs that look out over the English Channel.
As Rory Mortimore explained to me,
these rocks have enormous significance for global climate.
What sort of depth of rock is there beneath our feet here?
It's got to be at least 200 metres.
MANNING: But it's not only the height of the cliffs which matters.
It's what they're made of. Chalk.
There's something special about chalk. It's not made by a purely geological process.
It's made by living things.
MORTIMER: It forms by a rain of plankton onto the seabed.
They die in cycles and they come through in blooms in the oceans.
So the chalk is made up of millions and millions of tiny coccoliths.
They are so small that we cannot see them with the naked eye.
And even with an ordinary light microscope, they're almost impossible to see.
MANNING: As these tiny organisms grow,
they use carbon dioxide from the atmosphere to build their shells.
When they die and sink to the sea floor,
these shells can become compressed to form chalk and other limestones.
Let's look at this sample of chalk which we have here.
Now, this weighs about six kilograms.
And locked into it, we have about 1,300 litres of carbon dioxide.
And that is an enormous amount of carbon dioxide locked into the rocks.
MANNING: Today, almost all the carbon locked up in the rocks
is found in deposits like limestone, chalk and coal, made from living things.
In another words, on Earth today,
it's life that pulls carbon dioxide from the atmosphere,
locks it up and keeps our planet cool.
So, at some point in Earth history, possibly billions of years ago,
life must've taken over a key role in the carbon cycle
which keeps the Earth's temperature stable.
And of course, living things react to changes in sunlight.
Perhaps as the energy from the sun increased, life flourished,
drawing ever more carbon dioxide from the atmosphere.
That gradually lessened the greenhouse effect and so kept the Earth from overheating.
If life has played this crucial role in controlling the climate,
we can perhaps begin to answer the question I started with.
What makes our planet special
is above all the unique partnership between the Earth and living things.
That partnership can be seen in miniature here in La Palma.
At its heart lies the planet's interior energy,
which drives plate tectonics and the Earth's volcanism.
This fantastic area here,
which to me looks more like a giant rubbish heap of burnt plastic,
is a graphic example of the Earth's volcanic activity.
This was molten magma which flowed out of the volcanoes
on June 24, 1949.
It flowed down here and went down directly into the sea.
Throughout Earth history, geological upheavals like these
have been one of the driving forces of evolution.
Living things today are the descendents of those that survived the many cataclysms
unleashed by the planet.
Like the pine trees on La Palma's volcanic slopes,
whose insulating bark enables them to survive the fires triggered by volcanic eruptions.
It's a remarkable thought that the planet's activity shapes life.
But to me, even more astonishing is that through the carbon cycle,
life plays a major role in maintaining Earth's activity.
Whole hillsides of bananas on La Palma
are also part of the planet's carbon cycle,
which began here with gases released in the last volcanic eruption.
Some of the carbon dioxide that came out of this volcano 27 years ago,
is now fixed in these bananas and if I eat one, some of it will come out in my breath.
Rainwater may dissolve it and it will end up in the ocean
and get deposited in rocks deep in the ocean.
Thence it'll move along on the plates and get subducted,
and perhaps 100 million years from now,
it will be erupted again from a volcano to complete this remarkable cycle.
For millions of years, this partnership has controlled
the level of carbon dioxide in our atmosphere,
keeping the Earth's climate in the narrow zone where water remains liquid,
neither freezing as on Mars nor boiling away as on Venus.
In turn, liquid water has lubricated the motion of the plates
and of course without it, life would be impossible.
So every part of this astonishing system is essential.
In <i>Earth Story,</i> I set out to explore the links between the Earth and life.
Those links have turned out to be far more profound than I ever imagined.
For me, this television series has been a scientific journey of discovery.
I've learnt to look at the planet in a totally different way.
As an intricate system in which changes to one part affect all the others.
And as living beings, we're part of that system.
And our own story is intimately tied up with the story of the Earth,
our ancient, extraordinary living home.