Professor Brian Cox explores how the search for aliens in the solar system has followed the search for water. He looks at Jupiter's moon Europa - a dazzling ball of ice.
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We live on a world of wonders.
A place of astonishing beauty and complexity.
There are vast oceans and incredible weather.
and stunning landscapes.
I'm a physicist, and I'm fascinated by the way that the universal laws of nature
that made all this, also created such diverse and different worlds
out there in the solar system.
I think we're living through the greatest age of discovery our civilisation has known.
We've voyaged to the farthest reaches of the solar system.
We've photographed strange new worlds,
stood in unfamiliar landscapes, tasted alien air.
But the one thing we haven't found on those worlds
is the thing that makes our planet unique.
But is that really true?
Is the Earth the only place in the solar system that could support life?
In this film we will search the solar system for worlds that harbour the conditions to support life.
What we find on these worlds may help us answer the question, are we alone in the universe?
That's not only one of the great fundamental questions for science,
but one of the great unanswered questions in human history.
Floating in the Sea of Cortez off the cost of Mexico is the research vessel Atlantis,
the mother ship for the exploration of one of the most alien worlds we know.
But it's an alien world on our planet.
The Atlantis is the launch vessel for Alvin,
one of the world's most rugged submarines.
Built like a spacecraft,
it's designed to explore the deepest depths of the ocean.
And I'm lucky enough to have hitched a ride down to the sea floor,
two kilometres beneath the surface.
That has got to be the closest thing to going into space that you can do.
And, given that I'm not going to go into space any time soon, I think it's the next best thing.
See you in eight hours.
Roger, Alvin. Your checks are good. Permission to dive.
Roger. Alvin diving.
The parallels to spaceflight are obvious.
As the tiny capsule descends, we are leaving the familiar world
of the surface of our planet, and entering a strange, hostile world.
If anything goes wrong, we will be completely on our own.
Beeping is never good.
'Fortunately, Alvin is one of only a handful of submarines
'that can withstand the colossal pressure of the deep ocean.'
At the Earth's surface, we're used to one atmosphere of pressure.
As we descend, the pressure increases by another atmosphere every ten metres.
And it soon adds up.
We're approaching a kilometre deep. The pressure outside there is now
100 atmospheres, that's higher than the atmospheric pressure on the surface of Venus.
Without knowing, if you were asked a question, could life exist down here, 100 atmospheres,
cold, dark no sign of sunlight at all,
it's pitch black there, you would say no.
Well, I would say no.
But the depths of the ocean are not lifeless.
Illuminated by Alvin's lights,
we find oases of life in the deserts of the ocean floor.
So we have landed, after about an hour of descent.
We've just stopped in the most incredible place.
Look at those.
We've landed on top of the tube worms.
This underwater city is one of the most bizarre environments on our planet.
It's built around a hydrothermal vent, a volcanic opening in the Earth's crust
that pumps out clouds of sulphurous chemicals and water heated to nearly 300 Celsius.
And somehow, life has found a way to thrive in these most extreme conditions.
This is a genuinely remarkable place.
There are mats,
carpets of yellow bacteria.
Look at that. It's not only just bacterial blobs,
there is real complex organisms.
Alien. I want to say that word, alien environment.
It really is alien to us.
For me, the fascinating thing about finding life down here
is that the conditions on the
deep ocean floor are more similar in many ways to the conditions on
worlds hundreds of millions of kilometres away out there
in the solar system than they are to the conditions just two kilometres from my head on the Earth's surface.
It's incredibly dark, there is no sunlight,
there's a brutal mixture of hot and cold water,
and just rock and minerals.
So, if life can not only survive but even flourish in these conditions,
then you've got to feel that it's much more likely that life can
also survive and flourish out there in the solar system.
Ever since the invention of the telescope 400 years ago,
we have looked to our neighbouring worlds for signs of life.
As technology has improved, we've been able to search the planets in more and more detail,
and we have found nothing.
But that doesn't mean the rest of the solar system is dead,
because we're only beginning to scratch the surface of what's out there.
There are literally hundreds of other worlds.
Planets and their moons which we have barely explored.
Among them may be worlds that hold the conditions to support life.
And the best way to find out what those conditions are
is to look at the one place we know life flourishes.
Life is pretty much only chemistry.
It's just the reactions between atoms and molecules.
And so for life to exist, you only really need three things.
First of all, you need the right chemistry set.
Now, I'm made of something like 40 elements,
almost half of the known elements, which is pretty complicated.
But actually 96% of me is only made of four of them, carbon, nitrogen, oxygen and hydrogen.
Secondly, you need a power source.
You need a battery, something to make a flow of electrons
that powers the processes of life.
Now here on Earth, most life uses the power of the sun.
And thirdly, you need some kind of medium for life to play itself out in,
for processes to happen.
And here on Earth,
you don't have to look very far at all to find that medium, that solvent.
Because it's this, water.
If you want to see how important water is to life,
there's no better place to come than the Atacama desert in Chile.
The soil here is more sterile than a hospital operating theatre.
In fact, scientists have looked for the most basic form of life, bacteria,
in some parts of the Atacama, and they found absolutely nothing.
All deserts are characterised by a lack of moisture.
But the Atacama takes that to the extremes.
The Sahara is 50 times wetter than the Atacama.
There are weather stations here that have measured 1mm of rainfall in 10 years.
There are river valleys that have been dry for 120,000 years.
There are rocks that haven't seen rainfall for 20 million years.
It's this dryness that explains why nothing can survive here.
Even the most primitive form of life on Earth, the bacteria,
need water for their survival.
And there are no exceptions.
And this seemingly fundamental link between water and life
is driving the search for life out there in the solar system.
Because, wherever we find water,
that will be the best place to look for life beyond the Earth.
The Earth is the only planet that currently has liquid water on its surface.
The other planets are either too close to the sun,
like Mercury, and baked dry.
Or they are too far away.
Saturn's rings are made of water, but in the depths of space, it's frozen into lumps of solid ice.
But that doesn't mean that liquid water has never existed elsewhere in the solar system.
And if it has, we should be able to find the evidence,
because wherever water goes, it leaves its footprints.
These are the Scablands, a remote part of the North Western United States.
It's one of the most spectacular places to come to see how water
carves its signature into the landscape.
The largest flood on Earth went through this area here.
Jim Rice is an astro-geologist. He believes that understanding the events that created this landscape
can help in the search for water on other planets.
We are kind of like CSI arriving at the scene of a crime, this is the evidence left here.
-We've come to piece it together.
-I can see this is not a normal river system.
You can see, because it is so straight.
There is no meandering of a river here, it's just a big hole.
This entire landscape was created at the end of the last Ice Age.
200 miles to the east lay a huge lake, held in place by a wall of glacial ice.
When that wall ruptured, over 2,000 cubic kilometres of water swept out in a single catastrophic event.
The flood waters were at least 400 feet deep here.
But actually they were another 200 feet stacked on top of that, coming across here.
So we would be under 200 feet of water standing right here.
So am I to imagine a wave?
Yeah, a massive wave rolling, rumbling, this water would
be charged full of big chunks of ice from that ice dam.
It would be loaded with big chunks of the salt bed rock being gouged, ripped out of here.
It would be an impressive sight.
As the floodwaters tore across the landscape, they carved out this 20 mile long canyon.
And at its head, it left these giant horseshoes.
At over 400 feet high and five miles across,
this was the largest waterfall the world has ever known.
The easiest way of thinking about it is if you took every river in the world, put them in
the same location, had them flowing at the same time, these floods are 10 times larger than that.
And how long do we think it took to sculpt this landscape?
48 hours to a week.
It's instantaneous, geologically.
The Scablands reveal the characteristic signature
that water carves into the landscape.
It's a signature that can be seen from space, and not just on the Earth.
When we turn our telescopes on our next door neighbour and prime candidate for finding
alien life, the planet Mars,
we find almost identical features cut into its surface.
The Red Planet is covered in outflow channels.
Straight, wide canyons, exactly like the Scablands.
And they are filled with identical geological features.
It all suggests that similar huge floods once tore across the surface of Mars.
This is a picture of here from the air.
I am sat somewhere around here.
And here are the horseshoe shapes of the dry folds which are just over there.
This is a picture taken of the surface of Mars,
and you see those typical horseshoe shapes of the folds.
Also, you see the structures upstream of the folds, these grooves cut into the landscape.
And you see that here, grooves cut into the landscape as the water
cascades down and then flows over the folds
and cuts the gigantic valleys out as it moves downstream.
So, all this adds up, I think, to an overwhelming smoking gun
that there were vast amounts of water that flowed very quickly
over the surface of Mars at some point in the past.
But although the outflow channels are proof that liquid water once flowed across Mars,
it may not point to the existence of life.
Because if the Martian landscapes were formed by the same processes that formed the Scablands on Earth,
the floods that created them may only have lasted a matter of days.
For life to get a foothold, you need more than that.
You need areas of standing water.
Lakes and rivers that persist for millions of years.
In order to look for evidence of that standing water, we've done the only thing we can,
we have sent an army of robotic explorers to the surface of the planet.
We have touch down, we have touch down.
Over the last 35 years,
we've landed six robot probes on Mars.
And one of them, Opportunity, is still rolling across the surface, investigating the Martian geology.
The Mars rovers has really captured our imaginations.
I suppose, because they genuinely are explorers in the old-fashioned sense.
They are the extension of our senses to the surface of another world.
But they have also been very important scientifically, because
you can't really get to know another planet from orbit.
You have got to get down to the surface, you've got to touch it,
you've got to dig down and examine it microscopically.
And the Rovers really have, by doing that,
made some extremely important scientific discoveries.
One of the most significant of those discoveries was made in November 2004.
The Opportunity rover was examining an impact feature called the Endurance crater,
when it detected deposits of a remarkable mineral.
This is the world's largest salt works on the Baha peninsula in Mexico.
And what they do here is pump sea water into these lagoons and let it evaporate.
What they're after is this stuff, which is sodium chloride, table salt.
But, at different stages, different salts, different minerals, crystallise out.
So all the things really that are in sea water emerge, crystallise out
at different stages of the process.
In one of the lagoons, pond number nine, the sea water is at exactly the right concentration
to precipitate out these beautiful crystals that cover the entire floor of the lagoon.
This is gypsum,
and it's exactly the same stuff that Opportunity found on the surface of Mars.
Now, what's interesting about that discovery is how you make gypsum.
You see, its chemical formula is CaSO4.
So it's calcium sulphate.
So, the only way we know of, the only way to make gypsum here on Earth, is to have calcium
and sulphate ions in the presence of liquid water.
So, large deposits of gypsum on the surface of Mars tells you
that there must have been big areas of water present for a very long time.
The discovery of gypsum has helped to build a picture
of an ancient Mars that was much warmer and wetter.
Subsequent discoveries of gypsum in networks of sand dunes
suggest that large areas of Mars were once covered in standing water.
And where there is standing water, there is the chance of life.
This area of the salt flats is, we think,
very similar to areas that have been seen on Mars.
And it certainly looks extremely inhospitable.
It's hard at first sight to see how anything could live here.
But, if you just dig
a tiny bit below the surface,
then you see that this layer of gypsum is only a few millimetres thick,
and then immediately the ground beneath it turns this greeny colour.
It's green because that is bacteria that thrive in these seemingly inhospitable conditions.
Now if these bacteria can survive here,
then there seems to be no good reason why they couldn't also have survived and even flourished on Mars
when there was water present at some point in the very distant past.
But although it may once have been more hospitable,
any liquid water has long since disappeared from the surface of Mars.
About three billion years ago, it died as a planet.
Its core froze and the volcanoes that had produced its atmosphere seized up.
The solar wind stripped away the remains of that atmosphere.
Any liquid water would have evaporated
or soaked into the soil where it froze.
It left the surface of Mars too cold, too exposed
and too dry to support life.
It's highly unlikely that there will be life on the surface of Mars today.
But that's not to say that life couldn't exist somewhere on the Red Planet,
maybe we're just looking in the wrong place.
There are other potential habitats for life on Mars.
Detailed pictures of the surface show the entrances to caves,
revealing the existence of a world beneath the Martian surface.
We know there may be water down there.
Satellite data shows permafrost, ice frozen in the soil.
Deep below the surface, that ice may melt to form liquid water.
It all hints at an undiscovered subterranean world
that may be a more likely place to find life.
If you were to imagine the perfect habitat for life,
then it would surely be somewhere like this.
A warm climate, lots of liquid water,
a beautiful, dense atmosphere.
You see the results everywhere, just life everywhere you look.
All the life we're familiar with thrives in pretty much the same
conditions that we do, driven by the heat and light of the sun.
But this is by no means the only life on Earth.
There's another living planet hidden beneath the surface
that exists in completely different conditions.
It raises fascinating possibilities for the caves on Mars.
This is the Cueva de Villa Luz in Tabasco, Mexico,
the Cave of the House of Light.
And it is the definition of a hostile environment to me.
Because (HE SNIFFS) it's full of hydrogen sulphide gas, hence
the gas monitor which says at the moment one part per million hydrogen sulphide, very toxic for me,
which is why I have got this gas mask in case it all gets too much.
So, it's a place where you, at first sight,
would not expect a great many life forms to survive and flourish.
Although the cave is a death-trap for us, that doesn't mean that nothing lives here.
In fact, it's teeming with life.
Look at these fish, just everywhere in the cave water. And they're
adapted to live in these conditions.
In fact, if you look at them closely,
they're quite pink.
That's thought to be because they've got lots of haemoglobin
because there's not much oxygen down here,
so they need to have an efficient way of moving oxygen around their bodies.
But the really interesting life is found in the depths of the caves,
where the concentration of poisonous gas is high enough to set off my alarm.
Down here, far from the light of the sun,
are organisms whose energy source comes from the air around them.
They use the hydrogen sulphide gas bubbling up through these springs.
The same gas that could be fatally poisonous to me
is their source of life.
These things are what I came deep underground to see.
These are snottites. And you can see why they're called that.
They're really one of the most alien life forms that I can conceive of
on the Earth
Because they metabolise hydrogen sulphide, so they metabolise this
faintly acidic and nasty gas that I'm just breathing in now.
You can almost feel it on your tongue, actually, the acidity of it.
They metabolise it, they react it with oxygen, and they produce sulphuric acid.
So their breathing process, if you like, their version of what I do,
I breathe in oxygen, react that with sugars and breathe out CO2 and get energy
these guys breathe in hydrogen sulphide and oxygen and produce sulphuric acid.
In fact, I can test it here with this.
Yes, you see, look at that.
That, well, what looks like water, that secretion of dripping off the snottites, has actually got a pH...
well, it's now about between 0.5 and 0.
That's strong acid.
That's as strong as battery acid.
It's actually highly concentrated sulphuric acid.
So, what a strange organism.
Alien in every sense of the word.
Except that it's present on, well, just below the surface, of our planet.
And the snottites are not alone.
Organisms that can extract energy from the minerals around them
are found under the ground all over the world.
In fact, this way of life is so successful that it's thought there
may be more life living beneath the Earth's surface than there is on it.
And that raises an intriguing possibility.
If life can thrive below the Earth's surface,
why couldn't organisms like snottites survive and flourish
beneath the surface of Mars?
If you think about it, living below the surface of Mars might actually
be quite a good idea, because the surface is incredibly hostile.
It's subjected to intense ultraviolet radiation from the sun.
It's a very cold place, and the atmospheric pressure doesn't
allow liquid water to exist on the surface.
But, if there is life below the surface of Mars, then obviously we have a problem.
How could you possibly detect it?
Well, actually, there is a perhaps tantalising clue that
there might be something interesting going on below the Martian surface.
These are termites, or white ants.
And they're very unusual animals because they eat wood.
This is their food.
There are many, many species of these, billions of individuals across the planet.
And, in the process of digesting wood, they produce the gas methane.
Because there are so many of them, they actually produce an estimated
50 million tonnes of methane
and pump it into the Earth's atmosphere every year.
And it's not just termites.
There's lots of methane naturally in our atmosphere.
It's all produced either biologically...
or by active geological processes like mud volcanoes.
And that makes it all the more surprising that methane
has been detected in the atmosphere of the supposedly dead planet Mars.
It was telescopes on Earth, using infrared spectroscopy,
that first identified methane in Mars's tenuous atmosphere.
Those first measurements appeared to show only tiny amounts.
But closer observations have revealed that the gas
is concentrated in a handful of plumes that vary with the seasons.
In the warmer summer months,
thousands of tonnes of the gas is released from vents in the surface.
Something under the surface of Mars must be producing it.
It may be coming from previously unknown geological processes.
But it could be that it's coming from a biological source.
Now no-one, I don't think, is seriously suggesting that there
are termites running around beneath the surface of Mars.
But it's not actually the termites that are particularly interesting about this story.
It's the way they digest the wood.
You see, they use symbiotic bacteria, bacteria that live in their guts, called Archaea.
And Archaea, these bacteria that can digest wood and produce methane,
are the most common organisms beneath the surface of the Earth.
The snottites are members of the Archaea,
as are many of the microorganisms found living around deep-sea hydrothermal vents.
In fact, it's Archaea that we find thriving in many of the Earth's most extreme environments.
So I think it's quite a fascinating prospect that the methane we see
in Mars's atmosphere might just be produced by organisms like Archaea,
living below the Martian surface.
But while Mars remains a tantalising possibility,
it's no longer the only place in the solar system
we think could harbour alien life.
Far out, a billion kilometres from the sun,
the solar system becomes a very different place.
The planets, like Saturn, are made of gas, not rock.
There's plenty of water out here, but it's frozen solid.
The planets are surrounded by networks of moons, carved from ice.
They're cold and desolate.
They don't seem likely places to find life.
Any places on Earth remotely similar are completely barren.
This is central Iceland.
And, at this time of year, in mid-November, it's an increasingly inhospitable place.
It's about 3 o'clock in the afternoon, it's already well below freezing.
The sun is dipping below the horizon.
And it will stay this way for another six months.
And there's pretty much no visible life here at all.
There are no trees, no grass, and just listen.
No insects, no birds.
But it's because these places are so cold and inhospitable
that they're of increasing interest to astro-biologists.
Because discoveries in these frozen places of Earth have raised new hope
of finding life among the icy worlds of the outer solar system.
And in those frozen wastes
we have found one world that is of particular interest.
It's one of Jupiter's moons.
Jupiter has a vast network of moons.
The four largest have been known
since they were discovered by Galileo in 1610.
And they're a varied bunch.
Closest to the planet is the tortured moon Io.
It's torn apart by volcanoes that carpet its surface with bright yellow sulphur.
In total contrast to the heat of Io comes its neighbour,
the ice moon Europa.
It's about the same size as our moon.
And it's the smoothest body in the solar system.
Its surface is made of an unbroken shell of ice.
Though it's etched with a network of mysterious red markings.
It exists at a chilly minus 160 Celsius.
It seems an incredibly unlikely home for life.
The photographs of Europa from space
reveal a vast, icy wilderness.
But, if you look more closely, then you start to see surface features.
And those features tell you a lot about what's going on deep beneath the ice.
Close-up, we can see deep cracks that criss-cross the surface of Europa.
At higher magnification
we see areas where the ice has been broken into icebergs
and jumbled up before refreezing.
We see the same formations in sea ice on Earth,
where the movements of the ocean have caused the ice to bend and crack.
It suggests something similar may be happening on Europa.
But it's the way the cracks are broken and fractured that provide
the compelling evidence that there is liquid water on Europa.
You see, as Europa orbits around Jupiter,
Jupiter's intense gravity stretches and squashes the moon.
And that stresses the ice and causes it to fracture and crack.
But the position of those cracks is not quite where you would expect it to be.
And the explanation for that is that the icy surface of Europa
has shifted, it's moved relative to the rocky core.
And the only way that could happen is if there's a layer, or an ocean of liquid water,
surrounding the rocky core that allows the outer ice surface to slip around.
Measurements of Europa's magnetic field have confirmed that its icy shell
is sitting on top of a salty ocean that may be a staggering 100km deep.
That would mean that there is more than twice as much life-giving
liquid water on this tiny moon than there is on planet Earth.
But it's not just the discovery of the hidden ocean
that makes us believe that Europa may be the most likely home to alien life.
And that's why I've come to this spectacular ice cave in the Vatnajokull glacier.
You see, the laws of nature are universal.
That may not only apply to laws of physics, but also to the laws of biology as well.
And if that's the case,
then what we find in these ice caves of Iceland may tell us something
about what we could expect to find below the frozen surface of Europa.
It's hard to describe this place.
It's absolutely magnificent.
Visually, the quality of the ice, it's just completely
transparent and clear.
You can see straight through it.
The cave tunnels into the heart of the glacier,
where the ice has been frozen for a thousand years.
It's what astro-biologists find in this ice
that makes us think that Europa could be teeming with life.
NASA scientist Richard Hoover
has spent his career looking for life in unlikely places.
Well, that went very well.
-So, will any organisms that you find in that ice be living in a sense that I would understand it?
They're actually alive now, and metabolising?
For a long time it was thought that ice microorganisms
were present only in a state of what is called deep anabiosis.
Suspended animation. It's now becoming quite clear that that isn't necessarily
the case for all the microorganisms, there may be others that are actually actively living in the ice.
So in this glacier, the whole place, this whole cave
may be populated by living things, not frozen things?
Things existing, living, cell dividing, reproducing, all the things you do?
All of this.
It's this prospect of finding things living in solid ice
that has had the greatest impact
on our ideas of where life could survive in the solar system.
OK, we're at lowest magnification.
So, that is 100,000 millionths of a metre?
Yes. We have bacteria.
So, these are organisms that have been trapped in that glacier for thousands of years?
Yes, look at this.
Beautiful. You're seeing life in ice.
We now know that some microorganisms
are capable of actually causing the ice to melt,
because they generate, essentially, anti-freeze proteins.
They change the temperature at which ice goes from a solid state to a liquid state.
And they could have been forming little tiny pockets,
maybe only a few microns in diameter,
but if he can make a two or three micron diameter ball of liquid water,
and he has the ability to move,
then that bacterium is now not in a glacier, but he's in an ocean.
What are the implications of these discoveries?
The fact that you've got living bacteria inside ice on Earth, what are the implications for Europa?
You can clearly have bacteria like this in the frozen ice near the surface crust.
And the thing that is most exciting to me,
is that surface crust of Europa has a wide variety of colours
that are highly suggestive of microbial life.
And so there is a very, very strong possibility
that the ice of Europa may contain viable, living microorganisms.
It's a controversial idea, but it is a dizzying thought
that the mysterious red stains on the surface of Europa
could be the visible signs of alien life.
The discovery of the huge ocean of liquid water
under the surface of this tiny moon, combined with the potential for life in ice,
and the intriguing red markings that criss-cross its surface,
have made Europa the most fascinating and important alien world we know.
A true wonder of the solar system,
because it's our best hope of finding extraterrestrial life.
That question, are we alone in the universe?
Is this the only planet amongst the billions of planets in our galaxy,
amongst the billions of galaxies in the universe, that harbours life?
Is, I think, one of the most important questions,
perhaps THE most important question that we can ask.
Think about what it would mean for us
if the answer was that there was no other life in the solar system,
in our galaxy, perhaps even in the universe.
How valuable would that make planet Earth?
How valuable would that make us?
But then imagine that the answer is that, on every moon of every planet
where the conditions are right, then life survives and flourishes.
That makes us part of a wider cosmic community,
if the universe is teeming with life.
If knowing the answer to the question is so profoundly important,
then surely striving to find the answer should be of overwhelming importance.
I believe it's the most important question you can possibly ask.
Because we have a chance of answering it.
What we've learned from the extreme places on Earth
is that, if there is life out there in the solar system, it will almost certainly be simple.
Single-celled organisms like bacteria eking out an existence in the most hostile of environments.
One thing seems certain.
The only place in the solar system where there is complex life,
life that can build a civilisation,
is here on planet Earth.
But how did that happen? What is it that makes our world so special?
Because, after all, everything in the solar system shares the same genesis.
It was all created out of nothing more than a spinning cloud of gas and dust 4.5 billion years ago.
Solid worlds condensed out of the swirling mists.
But those worlds were radically different.
Around the solar system, there are worlds that erupt with volcanoes of sulphur.
And others with geysers of ice.
There are worlds with rich atmospheres and swirling storms.
And there are moons carved from ice
that hide huge oceans of liquid water.
But there's only one world where the laws of physics have conspired
to combine all these features in one place.
On Earth, the temperature and atmospheric pressure are just right
to allow oceans of liquid water to exist on the surface of the planet.
And it's big enough to have retained its molten core
that not only powers geysers and volcanoes,
but also produces our magnetic field
that fends off the solar wind and protects our thick, nurturing atmosphere.
It's the combination of all those wonders in one place
that allowed life to begin and to get a foothold here on Earth.
But, to allow that life to evolve into such complex creatures as ourselves
requires one more ingredient.
And that's time. Deep time.
The kind of time over which mountains rise and fall, and planets are formed and stars live and die.
And it's perhaps that that makes the earth so rare and so precious in the cosmos.
Because it's been stable enough for long enough for life to evolve
into such magnificent complexity.
The life we have on Earth today is the result of millions of years of stability.
And the pinnacle of that is us, humankind.
A species that has developed to the point where we can bend
and shape and change the world around us.
We have even left our own planet behind
to begin exploring our cosmic surroundings.
You could take the view that our exploration of the universe
has made us somehow insignificant.
One tiny planet around one star amongst hundreds of billions.
But I don't take that view.
Because we've discovered that it takes the rarest combination of chance, and the laws of nature,
to produce a planet that can support a civilisation.
That most magnificent structure
that allows us to explore and understand the universe.
And that's why, for me, our civilisation is THE wonder of the solar system.
MUSIC: "Calling Occupants Of Interplanetary Craft" by the Carpenters
# Calling occupants of interplanetary craft
# Calling occupants...
And if you were to be looking at the Earth from outside the solar system,
that much would be obvious.
# Calling occupants of interplanetary craft...
We have written the evidence of our existence onto the surface of our planet.
Our civilisation has become a beacon that identifies our planet as home to life.
# We'd like to make a contact with you
# Calling occupants of interplanetary, anti-adversary craft
# We are your friends
# We are your friends...#
If you'd like to know more about the solar system,
go to bbc.co.uk/science.
Subtitles by Red Bee Media Ltd
Professor Brian Cox visits some of the most stunning locations on earth to describe how the laws of nature have carved natural wonders across the solar system.
Brian descends to the bottom of the Pacific in a submarine to witness the extraordinary life forms that survive in the cold, black waters. All life on Earth needs water so the search for aliens in the solar system has followed the search for water.
Soaring above the dramatic Scablands of the United States, Brian discovers how the same landscape has been found on Mars. And it was all carved out in a geological heartbeat by a monumental flood.
Armed with a gas mask, Brian enters a cave in Mexico where bacteria breathe toxic gas and leak concentrated acid. Yet relatives of these creatures could be surviving in newly discovered caves on Mars.
But Brian's sixth wonder isn't a planet at all. Jupiter's moon Europa is a dazzling ball of ice etched with strange cracks. The patterns in the ice reveal that, far below, there is an ocean with more potentially life-giving water than all the oceans on Earth.
Of all the wonders of the solar system forged by the laws of nature, there is one that stands out. In the final episode of this series, Brian reveals the greatest wonder of them all.