Wonders of the Solar System s01e03 Episode Script
Dead or Alive
BRIAN COX: We live on a world of wonders, (WIND HOWLING) A place of astonishing beauty and complexity.
We have vast oceans and incredible weather, giant mountains and spectacular landscapes.
If you think that this is all there is, that our planet exists in magnificent isolation, then you're wrong.
We're part of a much wider ecosystem that extends way beyond the top of our atmosphere.
(RUMBLING) I think we're living through the greatest age of discovery our civilisation has ever known.
We've voyaged to the farthest reaches of the solar system, photographed strange new worlds, stood in unfamiliar landscapes, tasted alien air.
(EXPLODING) COX: Amongst all these wonders sits our Earth, an oasis of calm amidst the violence of the solar system.
And all that separates us from what's out there is a thin, flimsy envelope of gas, our atmosphere.
And it's thanks to this thin blue line that we have the air that we breathe, the water that we drink, and the landscape that surrounds us.
Atmospheres define all the planets in the solar system.
They have the power to create dynamic worlds that are alien and chaotic.
But, remarkably, in the frozen wastes of the solar system, one atmosphere has created the most unexpected wonder.
A moon that looks a lot like home.
I've come to Cape Town in South Africa to do something that I have always wanted to do, but I never thought I'd get a chance.
(ENGINE WHIRRING) I'm about fly incredibly high to the very edge of the Earth's atmosphere.
From here, I'm hoping to see something that only a handful of people have ever seen, the thin blue line, the fragile strip of gas that surrounds our whole planet.
And this is what's going to take me there.
So, this is an English Electric Lightning, the most beautiful fighter aircraft ever built.
This is when England built the best aircraft in the world.
(AIR WHOOSHING) The Lightning is no longer in service, but this piece of magnificently over-powered engineering is going to take me 18 kilometres straight up.
Actually, I read somewhere that when you read about the altitude of the Lightning, it says, "Altitude, estimated 60,000 feet, "ceiling, classified.
" So, don't know how high these can go.
I've heard rumours they can go to 80,000 feet, (CHUCKLING) which is amazing.
My journey will take me beyond almost all the molecules of gas that make up our atmosphere.
- Lf you feel you're gonna get sick - Yeah.
- Use a bag, okay? - Right.
COX: Hopefully not.
(ENGINE WHIRRING) To get there, I'm going to experience what made the Lightning famous, a vertical takeoff.
(BRIAN WHOOPING) (BRIAN LAUGHING) It takes just seconds to reach nine kilometres up, but I'm still in the thickest layer of the atmosphere called the troposphere.
But the further I climb, the thinner the atmosphere becomes.
OVER RADIO: Up at 58,000 feet, 90% of the atmosphere is below me.
The only people above me are on the Space Station.
(SIGHING) So beautiful! (NARRATING) I'm now at 60,000 feet, 18 kilometres up, and the highest I can go.
Above me the sky is a deep, dark blue.
And that is what I've come to see, our atmosphere.
OVER RADIO: That really is the thin blue line that protects us.
So fragile, so tenuous.
Just a tiny sliver of blue.
Amazing.
(GRUNTING) Between 55 and 60,000 feet inverted, the curvature of the Earth there, 5g vertical ascent, that is just a ride.
It is remarkable to see that.
(STAMMERING) You can see the thinness and fragility.
You see the atmosphere going from light blue to dark blue to black.
(CHUCKLING) It really is astonishing.
(NARRATING) The thin blue line makes the Earth the wonderfully diverse place it is.
It acts as soothing blanket that traps the warmth of the Sun, yet protects us from the harshness of its radiation.
Its movements can be traced in the gentlest breeze and the most devastating hurricane.
(THUNDER RUMBLING) The oxygen and water the atmosphere holds plays a fundamental role in the ongoing survival of millions of different species living on the planet.
In this film, I want to explain how the laws of physics that created our unique atmosphere are the same laws that created many diverse and different atmospheres across the solar system.
When perfectly balanced, a world as familiar and beautiful as the Earth can evolve beneath the clouds.
But the slightest changes can lead to alien and violent worlds.
There are planets in our solar system that have been transformed into hellish worlds by nothing more than the gases in their atmosphere.
And just as atmospheres can choke a planet to death, they're also powerful enough to shape their surfaces.
And there are worlds out there which are all atmosphere.
Giant balls of churning gas where storms three times the size of the Earth have raged for hundreds of years.
All atmospheres in the solar system are unique, but the ingredients and forces that shape them are universal.
At the heart of each is the glue which holds the solar system together, a fundamental force of nature, gravity.
Gravity is by far the weakest known force in the universe.
You can see that because it's really easy for me to pick a rock up off the ground, even though there's a whole planet, planet Earth, pulling the rock down.
And I can just lift it up.
Incredibly weak.
But incredibly important because it's the only force there is to hold an atmosphere to the surface of a planet.
The more massive the planet, the greater its gravitational force.
Earth has enough mass to keep a tight grip of the gas molecules that make up our atmosphere.
It holds them against the surface and allows us to breathe.
Now, we don't really notice the presence of our atmosphere, I suppose because we live in it all the time.
But there's a lot of it.
There's five million billion tons of air surrounding the Earth.
That's the equivalent of a weight of one kilogram pressing down on every square centimetre of our bodies.
Or put it in another way, if I'm about a metre square, that's 10 tons of weight pressing down.
Now, I say "pressing down" but that's not entirely right.
That's not how air pressure works.
It presses in every direction at once.
I can demonstrate that.
This is a glass full of water.
So, if I put a piece of paper on there, turn it upside down.
Now, if I'm right, then the air pressure is pushing in every direction on this glass of water.
The air pressure is pushing up as well as down.
And it has no problem in holding the water in the glass.
Where'd you get this water from? (CREW LAUGHING) Life on the surface of this planet survives surrounded by this enormous mass of gas.
We're like lobsters scuttling around on the ocean floor.
But our atmosphere does more than allow us to breathe.
It protects us from the most powerful force in the solar system, our Sun.
COX: If you asked yourself the question, why is the Earth the temperature that it is? Then the obvious answer might seem to be, "Well, because it's 150 million kilometres away from the Sun.
" But actually things aren't quite that simple.
This is the Namib desert in Namibia in southwestern Africa.
And as the Sun sinks below the horizon, the temperature change from day to night can be as much as 30 degrees Celsius.
That's an immense amount in just a few hours, much more than in somewhere like Manchester, for example.
The reason is that this is also one of the driest places on the planet, and so there's very little water vapour in the atmosphere.
That means the atmosphere is not a very good insulator, and so when the Sun disappears, the heat just disappears quickly into space.
Now, there's a planet in the solar system, somewhere over there near the Sun, where the temperature shift from day to night is not a mere 30 degree Celsius, but an immense amount bigger.
Roughly 58 million kilometres from the Sun is the smallest planet in the solar system, Mercury.
This tortured piece of rock suffers the biggest temperature swings of all the planets, from 450 degrees Celsius in the day to minus 180 degrees at night.
And all because Mercury has been stripped naked, it has virtually no atmosphere at all.
COX: Like all the rocky inner planets of the solar system, Mercury had an atmosphere when it was formed.
But it lost it very quickly.
Here on Earth, at sea level, then Well, in a volume about the size of this pebble, there are 10 billion billion molecules of gas.
On Mercury, in the same volume, there'd be around 100,000.
That's 10 million million times less.
Now, planets hang on to their atmosphere by the force of gravity, it's the only way they can stop that thin blue line of gas disappearing off into space.
So, the bigger the planet, the more massive the planet, the stronger the gravitational pull, and the easier it is for the planet to keep hold of its atmosphere.
So, Mercury was just too small and too hot to hang on to its atmosphere.
And the consequences for the planet were absolutely devastating.
Atmospheres may be just a thin strip of molecules but they are a planet's first line of defence.
Without them, a planet like Mercury is at the mercy of our violent solar system.
Well, this is Saskatchewan in Western Canada, and it is a cold place to be in November.
About a year ago in November, 2008, a piece of asteroid, a space rock weighing about 10 tonnes, entered the atmosphere right over here and actually landed about 30 kilometres that way at a place called Buzzard Coulee.
Now, it's not unusual for rocks that big to hit the Earth.
On average, that happens about once a month.
But what was unusual about this one was that it was over quite a densely populated area.
So, tens of thousands, if not hundreds of thousands of people saw it and heard it.
But most spectacularly, it was captured by a lot of CCTV cameras including that one in this garage.
(STATIC BUZZING) These are the actual CCTVimages captured around the city.
They show the meteorite as it streaked across the sky at 20 kilometres per second.
The fireball was brighter than the Moon and turned the night sky blue.
Scientists used these remarkable images to triangulate the impact site of the meteorite.
They traced it to a field just outside the city of Lloydminster.
A team of meteorite hunters have been searching the debris left by the enormous explosion.
They're led by Dr Alan Hildebrand.
How much energy does a rock like this have, then? You know, what is it, a 10-ton rock travelling at 50 times the speed of sound? You know, it would be like if you stacked up, say, 400 tons of TNT to explode them.
I mean, it's really quite dramatic.
Four hundred tonnes that just dissipates away in the Earth's atmosphere? Yes.
Atmosphere is slowing it down, of course, causing it to break up.
In just five seconds, it's almost all over.
And of course, you know, it's that extreme friction makes the light show, 10% of the energy goes in light.
And it's like a billion-watt bulb shining high in the sky.
COX: So, what are we looking for? What does a piece of that asteroid look like? They Going through the atmosphere the surfaces got melted, and so you end up with a dark crust on them.
So essentially, you're looking for an oddly-sculpted, dark rock.
(LAUGHING) Yeah.
Well, in all fairness, you gotta be able to tell it from the, you know, the cow patties and so on, but, you know COX: (LAUGHING) I can probably manage that, actually.
ALAN: Once you get your eye in, you'll have no trouble.
ALAN: We got one right here.
COX: Pick that up.
Astonishing! It's just been completely ALAN: Yeah.
COX: Rounded off.
ALAN: Yeah, the heat melted the surface of the rock.
I mean, how hot does something have to be to do that! COX: Yeah.
6,000 degrees C would do it.
(SIZZLING) So this little rock has had an amazing history.
I mean, it approached the Earth as part of this bigger fragment at about, what, 18, 19, 20 kilometres per second.
It hit the Earth's atmosphere.
About 85 kilometres up, it began to feel the effects of the Earth's atmosphere.
It began to squash the air in front of it and create a pressure wave, essentially, which, in turn, causes this thing to heat up.
And it would have heated up to something like the temperature of the surface of the Sun.
It would have been 5, 6,000 degrees Celsius as it plummeted through the atmosphere.
Lit up the sky over here, and then, quite literally, exploded in a series of explosions and peppered these fields with lumps of rock this big.
Can you imagine standing here on that night and having this, these things? (CHUCKLING) And this is heavy, right? Raining down from the sky.
It must have been quite incredible.
(NARRATING) If the meteorite had hit the ground intact, the explosion would have been equivalent to 400 tons of TN and left a crater 20 metres wide.
The Earth was spared this colossal impact by nothing more than the tenuous strip of gases that surrounds us.
But not all planets have this protective blanket.
When a meteorite hits naked Mercury, there is no atmosphere to break it up or slow it down.
It strikes the ground at full speed and completely intact.
For the last 4.
6 billion years, Mercury has been bombarded with countless asteroids and comets.
The whole history of the planet's violent past is laid out on its surface, a world pitted with hundreds of thousands of craters.
Craters inside craters inside craters.
Mercury was damned from the start.
It's simply too small and too hot to have retained any meaningful traces of atmosphere.
We, on the other hand, are big enough and cold enough to have retained this envelope of gases.
That, in turn, allows living things like me to evolve and to use that atmosphere to breathe and to live.
But there are places out there in the solar system whose atmospheres have the same ingredients as our own.
But when the formula is even slightly remixed, it leads to worlds that couldn't be more different.
Roughly 108 million kilometres from the Sun sits the brightest planet in the solar system, Venus.
This footage shows the luminescent world appear from behind our cratered moon.
Venus and Earth share many similarities.
We sit next to each other in space.
We were formed from the same material and we're roughly the same size and share a similar mass and gravity.
But that's where any similarities end.
Venus is a tortured world, where thick clouds of sulphuric acid are driven along by high-speed winds.
And temperatures are hot enough to melt lead.
All because this planet's atmosphere created a runaway greenhouse effect.
(WIND HOWLING) Now, the greenhouse effect has become a well-known phrase.
You know, it's synonymous with global warming.
But what is it? Well, a planet, like the Earth, absorbs energy from the Sun as visible light.
Now, atmospheres don't absorb much visible light, as you can see, because you can see the Sun.
The ground absorbs the visible light, heats up, and then reradiates it.
But it reradiates it as infrared radiation, heat radiation, if you want.
And atmospheric gases, particularly carbon dioxide, are very good at absorbing in the infrared.
And so they trap the heat and the planet heats up.
On Earth, greenhouse gases are essential to our survival.
Without them, our planet would be 30 degrees colder, too cold to support life as we know it.
But Venus' atmosphere was flooded with greenhouse gases.
The nearby Sun slowly boiled away its oceans, pumping water vapour into the atmosphere.
And carbon dioxide from thousands of erupting volcanoes added to the stifling mix.
Venus grew hotter and hotter.
The planet was slowly choked to death.
Venus is a planet with an atmosphere in overdrive.
But Earth's other rocky neighbour tells quite a different story.
(LAUGHING) Get it! Go on! These are the dunes in the Namib Desert.
It's an absolutely spectacular place.
This place is not the hottest or the driest desert in the world, but these dunes are some of the oldest sand dunes in the world.
And the reason we're here in the Namib Desert is that this is a great analogue for the surface of Mars.
This is what the surface of Mars looks like.
And these dunes are barchan dunes.
These crescent-shaped dunes are the same as the sand dunes on Mars.
So, if you want to get a feel for what it'd be like on the surface of Mars, and you want to know what driving a 4x4 around Namib would be like, then this is the place to come.
Incredibly, there is a vehicle driving across the surface of the red planet today, a space rover named Opportunity.
The rovers and spacecraft that circle the planet have sent back images which reveal Mars in exquisite detail.
Mars has vast dunes, enormous volcanoes and giant ice sheets.
It has canyons and river valleys.
Mars is a dry, frozen version of our home, covered in red dust and sand.
And it's all due to the fact that Mars has virtually no atmosphere.
But there are clues that things weren't always this way.
COX: These are pictures taken from the surface of Mars in August, 2009.
And they caused quite a bit of excitement because of this.
This rock sat on the surface of Mars in front of the rover.
This rock is about Well, here's a close-up.
It's actually a nickel-iron meteorite.
And it's about 60 centimetres across, weighs half a ton.
It came from space, came through the Martian atmosphere and landed on the ground.
Well, the mystery is that a meteorite this big, if it hit Mars today, would disintegrate when it hit the surface, it would be travelling too fast.
And that's because Mars' atmosphere is too thin to diffuse, to slow it down.
But that meteorite is very definitely there.
So, how could it have made it to the ground? Well, it must be that in the past when this meteorite hit Mars, Mars' atmosphere was significantly denser, dense enough to slow this piece of rock down enough that it could land on the surface intact.
But why did Mars lose its thick atmosphere and become the barren planet we see today? Well, there are so many ways for planets to lose their atmospheres that it feels like a miracle that we've still got ours.
But, in the case of Mars, it's thought that one of the dominant mechanisms was in the interaction with the solar wind.
The solar wind is a stream of superheated, electrically-charged particles that constantly stream away from the Sun at over a million kilometres per hour.
This wave of smashed atoms has the power to strip a planet of its atmosphere.
On Earth, we're protected from this onslaught by an invisible shield that completely surrounds our planet, known as the magnetosphere.
The magnetosphere is created deep within the Earth's molten iron core.
As the core spins, it generates a powerful magnetic field which shoots out of the pole and cocoons the whole planet.
This magnetic shield is strong enough to deflect most of the solar wind that comes our way.
Now, we know that in some point in the past Mars would also have had a molten core and did have a magnetic field.
But because Mars is a smaller planet than the Earth, it lost its heat more quickly and the core solidified, electric currents could no longer flow, and its field vanished.
And that was a major factor in the solar wind being allowed to blast the planet and strip away its atmosphere.
With no atmosphere to insulate it, this once Earth-like world transformed into the frozen desert we see today, a shadow of its former self.
Now, though Mars has lost most of its atmosphere, those few molecules that remain still have the power to sculpt and transform the surface.
And that power, that transformative effect, is present on every planet in the solar system that has an atmosphere.
You can see it transforming the surface of the Namibian Desert today, as we speak.
It is, of course, the force of nature that we call weather.
(THUNDER RUMBLING) MAN: Hey, we gotta go.
Wow! Weather is a feature of every planet with an atmosphere.
Our world is transformed as this huge mass of air moves across its surface.
But as we look out into the solar system, we see it only takes the slightest atmosphere to produce extraordinary weather.
Every few years, Mars all but disappears under a maelstrom of dust.
(WIND HOWLING) Global dust storms are so huge, they dwarf Olympus Mons, a volcano three times bigger than Everest.
But to experience the most extreme and violent weather in the solar system, we need to travel to Jupiter.
This banded gas giant is over 140,000 kilometres in diameter.
Its atmosphere isn't a thin blue line, it's many thousands of kilometres thick, and in a constant state of seething motion.
The whole surface boils with gigantic storms.
Yet this most alien world shares a feature with our own planet.
(THUNDER RUMBLING) Jupiter crackles to the sound of electrical storms.
The bolts of lightning are thousands of times brighter than lightning here on Earth.
(RUMBLING CONTINUES) The physics of storms on Jupiter is, of course, the same as the physics of storms on Earth.
The warm, moist air deep in the atmosphere starts to rise, and, as it rises, it cools, and the moisture condenses out to form clouds.
Now, that rising air leaves a gap beneath it, a low pressure area, and some more warm moist air is sucked in and that fuels the rise of the storm.
Now, on Earth, those storm systems are driven by the power of the Sun.
But therein lies the mystery, because the storm systems on Jupiter are far more powerful.
And yet, Jupiter is five times further away from the Sun than the Earth is, which means it receives 25 times less solar energy.
So what mechanism could it be that powers those intensely violent storms on Jupiter? The secret to Jupiter's storm-tossed atmosphere lies hidden deep within the gas giant.
On Earth, we have clear boundaries between the gaseous sky, the liquid oceans and the solid ground.
But on Jupiter, there are no such boundaries.
It's a gas giant, it's made of the two lightest and most abundant elements in the universe, hydrogen and helium.
But as you go deep into Jupiter's atmosphere, something very strange and interesting happens to those gases.
Jupiter's atmosphere is so thick and its gravitational pull so strong, that 20,000 kilometres beneath the cloud tops, the pressure is two million times greater than the surface pressure here on Earth.
Under such immense pressure, the hydrogen gas in the atmosphere is transformed into a strange, metallic liquid.
As the gases are squeezed, a vast amount of energy is released, enough energy to fuel some of the biggest storms in the solar system.
The biggest of them all is the Great Red Spot.
This giant anticyclone has raged for hundreds of years, and is large enough to swallow the Earth three times over.
The Great Red Spot is an awesome sight, but this giant isn't one of my wonders.
My wonder is a much smaller world, a moon that orbits the gas giant Saturn, one and a half billion kilometres from Earth.
What we have found on this small world is simply astonishing.
If you've thought of our Moon as the archetypal moon of the solar system, if you like, then, well, you might think that all the other moons out there, the hundreds of them, would be dead, uninteresting worlds.
I mean, not uninteresting places to visit.
I mean, that, is in my view, the greatest thing that humans have ever achieved, landing on the surface of the Moon.
But it's a dead and lifeless place.
But as we've begun to visit those worlds, as we've flown spacecraft to within hundreds of miles of their surfaces, we've found that the moons in the outer solar system are an astonishingly interesting, varied and fascinating bunch of worlds.
This is Jupiter's moon, Europa.
This is Jupiter's moon, Io, the most volcanic object in the solar system.
But of all of the worlds out there, this one, Saturn's moon, Titan, is unique because of that.
That is an atmosphere.
And what an atmosphere it is, it's 1,000 kilometres deep, it's four times denser than the atmosphere of the Earth.
But imagine that, a moon around a distant planet in the icy distant reaches of the solar system with an atmosphere denser and thicker than our own.
Titan has the most Earth-like atmosphere in the entire solar system, a thick, blue line, rich in nitrogen and containing methane.
At first sight, a world this small shouldn't be able to hold on to such a dense atmosphere.
Except Titan lies in one of the coldest regions of the solar system and that makes all the difference.
Temperature for gases, like the gases in our atmosphere, is really a measure of how fast the molecules of the gas are moving around.
And I can demonstrate that with this thing, (LAUGHING) which is a Chinese lantern.
You see, if I light this fuel, then what's gonna happen is that the gas inside is gonna heat up.
And as you heat up a gas, what that basically means is that you speed all the molecules up.
As the molecules of air heat up and move faster, the air pressure inside the lantern begins to increase.
That means that molecules are forced out, making the air inside less dense than the air outside, and the lantern gets lighter.
And eventually, the lantern is so light that it will just float away into the atmosphere of our planet.
Hot gases have more energy to escape a planet's gravitational pull than cold gases.
Now, Titan is a much smaller body than the Earth, it has much weaker gravitational pull.
And if it were in the same region of the solar system as we are, then it would not be able to hold on to its atmosphere.
But it's a lot further away from the Sun than we are.
And so, that means that it's colder, its atmospheric molecules are moving around much more slowly than ours.
That means that its weak gravity is enough to hold on to that thick, dense atmosphere.
Titan's thick atmosphere was an unexpected discovery, but it took an audacious mission to reveal the world that lies beneath the blanket of clouds.
(ROCKET RUMBLING) MAN OVER RADIO: We have lift-off of the Cassini spacecraft on a seven-year trek to Saturn.
COX: In 1997, Cassini began its journey to Titan.
It carried with it the Huygens probe, a lander designed to set down on this frozen moon.
On Christmas Day, 2004, Huygens was released from Cassini and it began the bumpy ride through one of the most intriguing atmospheres in the solar system.
And then, for the first time, the thick clouds parted and the surface of Titan was revealed.
These are the actual images taken by Huygens as it slowly parachuted to the surface.
The world it revealed was more familiar than we could have possibly imagined.
One of the first people to see these incredible images was a man who helped design the probe, Ralph Lorenz.
RALPH: It was amazing 'cause we just had no idea what to expect.
We didn't know whether it would be cratered like the Moon, or just a sort of flat expanse of sand or whatever.
And then these first pictures came back and it was just astonishingly familiar.
Did that picture, or those initial series of pictures, um, I suppose it looked somewhat like this, didn't it? It was - It could be have been there, actually.
- It was like this, but It could have been right here.
COX: I do see that.
I mean, I can sit here, look at that, and that's what that picture looks like, actually.
I could take it with a camera.
The camera on the probe was about the height of your knee.
So, yeah, the view the Huygens probe had is just like this.
COX: Rounded stones dot the landscape.
They're smooth and look like they've been eroded by tumbling water, similar to stones found on riverbeds here on Earth.
It sounds to me like this was one of the easiest pictures to interpret in the history of space exploration.
You know, the way you tell it, it's just, that's a riverbed (CHUCKLING) with these stones.
I mean, is it that simple? Because you can be mislead easily, can't you? The devil's always in the details.
But I think very few people disputed the interpretation of a river channel.
I mean, it was just such a familiar thing to so many people on Earth, there really wasn't much doubt.
COX: It was an extraordinary discovery, evidence of flowing rivers had never been found before on a moon.
But it wasn't the only surprise Titan held in store.
COX: This is the Matanuska Glacier in Alaska.
It really is one of the most astonishing places I've ever seen.
And this whole landscape is testament to the erosive power of this stuff, this mixture of ice and rock as it rolls down this valley over hundreds of thousands of years, and creates this astonishing landscape.
But the reason it can do that is because of the delicate balance of the Earth's atmosphere.
You see, our planet is just at the right temperature and pressure to allow water to exist as solid, as liquid, and as gas, as vapour in the clouds.
And so, the Sun can heat up the oceans and it can move the water over the top of the mountains, it can fall as rain, turn to ice, become a glacier, and then sweep down the valley to sculpt this astonishing landscape.
Just as our atmosphere allows all this to exist, the atmosphere of Titan is the perfect temperature and pressure to allow something to exist that has never been seen before on a world beyond Earth.
This is a picture taken of the South Pole of Titan by Cassini in June, 2005.
And it's subsequently become one of the most important and fascinating pictures in the history of space exploration.
The interesting thing is this black blob here.
Now, this fascinated the Cassini scientists.
But the explanation as to what that is had to wait just over a year, till July, 2006, when this picture was taken.
And it's a radar image, this time of the North Pole of Titan.
And you see again these huge, black areas.
But black, in this case, means that the radar waves that bounced onto them didn't come back, so they're completely black.
And there's only one really good explanation for that.
That is that they're incredibly flat surfaces.
In fact, they're surfaces of liquid.
So, this picture, combined with this picture, means that this is the first observation of a liquid, a lake, on the surface of a body other than the Earth in the solar system.
These lakes, of course, cannot be lakes of liquid water because the surface temperature on Titan is minus 180 degrees Celsius.
And at those temperatures water is frozen as hard as steel.
So, if these are not lakes of water, then what are they? This is Lake Eyak in Alaska, just on Prince William Sound.
Now, I've come here to collect a molecule, or a substance, that's very abundant on Titan.
In fact, it's abundant throughout the solar system.
But, here on Earth, it exists as a gas, and it bubbles up from the floor of this lake.
Now, the floor of Lake Eyak is covered in rotting vegetation, you know, dead leaves and bits of trees, twigs.
And that's being broken down by bacteria, which produce the gas that bubbles up from the floor of the lake.
That gas is methane.
And we've been collecting it all night underneath this upturned boat, so that I can take a sample of it in this bag.
(AIR WHOOSHING) Now, on Earth, methane is very unstable.
If you give it a little kick in the presence of oxygen, then you get what chemists' call an exothermic reaction.
Methane plus oxygen goes to water, plus carbon dioxide and some energy.
The Earth's temperature and atmospheric pressure means methane can only exist as a highly flammable gas.
But Titan's atmospheric pressure and temperature is perfect to allow methane to exist as a solid, a gas, and, most importantly, a liquid.
So the images Cassini captured were gigantic lakes of liquid methane.
The first ever liquid discovered pooling on the surface of another world in the solar system.
This is Kraken Mare.
At over 400,000 square kilometres, it's the biggest body of liquid on Titan.
It's almost five times the size of Lake Superior, North America's greatest lake.
COX: On Titan, methane plays exactly the same role that water does here on Earth.
So, where we have clouds of water, Titan has clouds of methane, with methane rain.
Whereas we have lakes and oceans of water, Titan has lakes of liquid methane.
And whereas here on Earth, the Sun warms the water in the lakes and oceans, and fills our atmosphere with water vapour, on Titan, the Sun lifts the methane from the lakes and saturates the atmosphere with methane.
So, whereas on Earth, we have a hydrological cycle, on Titan there's a methanological cycle.
And rain would be an absolutely magical sight on Titan.
Because the atmosphere is so dense and the gravity of the moon is so weak, the drops of methane rain would grow to over a centimetre in size, and they would fall to the ground as slowly as snowflakes fall onto the surface of our own planet.
Thousands and thousands of gallons of liquid methane must have slowly rained down onto the surface, making rivers and streams swell and burst.
Deep gullies were cut into the frozen water landscape.
Which looks so familiar because it is familiar.
It's this.
You know, the atmosphere of Titan shapes the surface in exactly the same way that the atmosphere here on Earth shapes the surface of our planet.
Titan is like a primordial Earth caught in a deep freeze.
It's almost like looking back in time over four billion years and observing our planet before life began and began to modify our atmosphere to change it into the oxygen-rich atmosphere that we see today.
In many ways, Titan looks so familiar, it's a place with rivers, and lakes, and clouds, and rain.
It's a place with water, albeit frozen as hard as steel.
And a place of methane, albeit so cold that methane is now a liquid and flows and shapes the landscape just like water does here on Earth.
For me, the most important thing about Titan, is we now have two Earth-like worlds in our solar system.
One, in its warm region ninety three million miles away from the Sun.
And the other, in deep freeze a billion miles away from our star, in orbit around another planet.
And that must greatly increase the probability that there are other Earth-like planets in orbit around the hundreds of billions of stars out there in the universe.
We have vast oceans and incredible weather, giant mountains and spectacular landscapes.
If you think that this is all there is, that our planet exists in magnificent isolation, then you're wrong.
We're part of a much wider ecosystem that extends way beyond the top of our atmosphere.
(RUMBLING) I think we're living through the greatest age of discovery our civilisation has ever known.
We've voyaged to the farthest reaches of the solar system, photographed strange new worlds, stood in unfamiliar landscapes, tasted alien air.
(EXPLODING) COX: Amongst all these wonders sits our Earth, an oasis of calm amidst the violence of the solar system.
And all that separates us from what's out there is a thin, flimsy envelope of gas, our atmosphere.
And it's thanks to this thin blue line that we have the air that we breathe, the water that we drink, and the landscape that surrounds us.
Atmospheres define all the planets in the solar system.
They have the power to create dynamic worlds that are alien and chaotic.
But, remarkably, in the frozen wastes of the solar system, one atmosphere has created the most unexpected wonder.
A moon that looks a lot like home.
I've come to Cape Town in South Africa to do something that I have always wanted to do, but I never thought I'd get a chance.
(ENGINE WHIRRING) I'm about fly incredibly high to the very edge of the Earth's atmosphere.
From here, I'm hoping to see something that only a handful of people have ever seen, the thin blue line, the fragile strip of gas that surrounds our whole planet.
And this is what's going to take me there.
So, this is an English Electric Lightning, the most beautiful fighter aircraft ever built.
This is when England built the best aircraft in the world.
(AIR WHOOSHING) The Lightning is no longer in service, but this piece of magnificently over-powered engineering is going to take me 18 kilometres straight up.
Actually, I read somewhere that when you read about the altitude of the Lightning, it says, "Altitude, estimated 60,000 feet, "ceiling, classified.
" So, don't know how high these can go.
I've heard rumours they can go to 80,000 feet, (CHUCKLING) which is amazing.
My journey will take me beyond almost all the molecules of gas that make up our atmosphere.
- Lf you feel you're gonna get sick - Yeah.
- Use a bag, okay? - Right.
COX: Hopefully not.
(ENGINE WHIRRING) To get there, I'm going to experience what made the Lightning famous, a vertical takeoff.
(BRIAN WHOOPING) (BRIAN LAUGHING) It takes just seconds to reach nine kilometres up, but I'm still in the thickest layer of the atmosphere called the troposphere.
But the further I climb, the thinner the atmosphere becomes.
OVER RADIO: Up at 58,000 feet, 90% of the atmosphere is below me.
The only people above me are on the Space Station.
(SIGHING) So beautiful! (NARRATING) I'm now at 60,000 feet, 18 kilometres up, and the highest I can go.
Above me the sky is a deep, dark blue.
And that is what I've come to see, our atmosphere.
OVER RADIO: That really is the thin blue line that protects us.
So fragile, so tenuous.
Just a tiny sliver of blue.
Amazing.
(GRUNTING) Between 55 and 60,000 feet inverted, the curvature of the Earth there, 5g vertical ascent, that is just a ride.
It is remarkable to see that.
(STAMMERING) You can see the thinness and fragility.
You see the atmosphere going from light blue to dark blue to black.
(CHUCKLING) It really is astonishing.
(NARRATING) The thin blue line makes the Earth the wonderfully diverse place it is.
It acts as soothing blanket that traps the warmth of the Sun, yet protects us from the harshness of its radiation.
Its movements can be traced in the gentlest breeze and the most devastating hurricane.
(THUNDER RUMBLING) The oxygen and water the atmosphere holds plays a fundamental role in the ongoing survival of millions of different species living on the planet.
In this film, I want to explain how the laws of physics that created our unique atmosphere are the same laws that created many diverse and different atmospheres across the solar system.
When perfectly balanced, a world as familiar and beautiful as the Earth can evolve beneath the clouds.
But the slightest changes can lead to alien and violent worlds.
There are planets in our solar system that have been transformed into hellish worlds by nothing more than the gases in their atmosphere.
And just as atmospheres can choke a planet to death, they're also powerful enough to shape their surfaces.
And there are worlds out there which are all atmosphere.
Giant balls of churning gas where storms three times the size of the Earth have raged for hundreds of years.
All atmospheres in the solar system are unique, but the ingredients and forces that shape them are universal.
At the heart of each is the glue which holds the solar system together, a fundamental force of nature, gravity.
Gravity is by far the weakest known force in the universe.
You can see that because it's really easy for me to pick a rock up off the ground, even though there's a whole planet, planet Earth, pulling the rock down.
And I can just lift it up.
Incredibly weak.
But incredibly important because it's the only force there is to hold an atmosphere to the surface of a planet.
The more massive the planet, the greater its gravitational force.
Earth has enough mass to keep a tight grip of the gas molecules that make up our atmosphere.
It holds them against the surface and allows us to breathe.
Now, we don't really notice the presence of our atmosphere, I suppose because we live in it all the time.
But there's a lot of it.
There's five million billion tons of air surrounding the Earth.
That's the equivalent of a weight of one kilogram pressing down on every square centimetre of our bodies.
Or put it in another way, if I'm about a metre square, that's 10 tons of weight pressing down.
Now, I say "pressing down" but that's not entirely right.
That's not how air pressure works.
It presses in every direction at once.
I can demonstrate that.
This is a glass full of water.
So, if I put a piece of paper on there, turn it upside down.
Now, if I'm right, then the air pressure is pushing in every direction on this glass of water.
The air pressure is pushing up as well as down.
And it has no problem in holding the water in the glass.
Where'd you get this water from? (CREW LAUGHING) Life on the surface of this planet survives surrounded by this enormous mass of gas.
We're like lobsters scuttling around on the ocean floor.
But our atmosphere does more than allow us to breathe.
It protects us from the most powerful force in the solar system, our Sun.
COX: If you asked yourself the question, why is the Earth the temperature that it is? Then the obvious answer might seem to be, "Well, because it's 150 million kilometres away from the Sun.
" But actually things aren't quite that simple.
This is the Namib desert in Namibia in southwestern Africa.
And as the Sun sinks below the horizon, the temperature change from day to night can be as much as 30 degrees Celsius.
That's an immense amount in just a few hours, much more than in somewhere like Manchester, for example.
The reason is that this is also one of the driest places on the planet, and so there's very little water vapour in the atmosphere.
That means the atmosphere is not a very good insulator, and so when the Sun disappears, the heat just disappears quickly into space.
Now, there's a planet in the solar system, somewhere over there near the Sun, where the temperature shift from day to night is not a mere 30 degree Celsius, but an immense amount bigger.
Roughly 58 million kilometres from the Sun is the smallest planet in the solar system, Mercury.
This tortured piece of rock suffers the biggest temperature swings of all the planets, from 450 degrees Celsius in the day to minus 180 degrees at night.
And all because Mercury has been stripped naked, it has virtually no atmosphere at all.
COX: Like all the rocky inner planets of the solar system, Mercury had an atmosphere when it was formed.
But it lost it very quickly.
Here on Earth, at sea level, then Well, in a volume about the size of this pebble, there are 10 billion billion molecules of gas.
On Mercury, in the same volume, there'd be around 100,000.
That's 10 million million times less.
Now, planets hang on to their atmosphere by the force of gravity, it's the only way they can stop that thin blue line of gas disappearing off into space.
So, the bigger the planet, the more massive the planet, the stronger the gravitational pull, and the easier it is for the planet to keep hold of its atmosphere.
So, Mercury was just too small and too hot to hang on to its atmosphere.
And the consequences for the planet were absolutely devastating.
Atmospheres may be just a thin strip of molecules but they are a planet's first line of defence.
Without them, a planet like Mercury is at the mercy of our violent solar system.
Well, this is Saskatchewan in Western Canada, and it is a cold place to be in November.
About a year ago in November, 2008, a piece of asteroid, a space rock weighing about 10 tonnes, entered the atmosphere right over here and actually landed about 30 kilometres that way at a place called Buzzard Coulee.
Now, it's not unusual for rocks that big to hit the Earth.
On average, that happens about once a month.
But what was unusual about this one was that it was over quite a densely populated area.
So, tens of thousands, if not hundreds of thousands of people saw it and heard it.
But most spectacularly, it was captured by a lot of CCTV cameras including that one in this garage.
(STATIC BUZZING) These are the actual CCTVimages captured around the city.
They show the meteorite as it streaked across the sky at 20 kilometres per second.
The fireball was brighter than the Moon and turned the night sky blue.
Scientists used these remarkable images to triangulate the impact site of the meteorite.
They traced it to a field just outside the city of Lloydminster.
A team of meteorite hunters have been searching the debris left by the enormous explosion.
They're led by Dr Alan Hildebrand.
How much energy does a rock like this have, then? You know, what is it, a 10-ton rock travelling at 50 times the speed of sound? You know, it would be like if you stacked up, say, 400 tons of TNT to explode them.
I mean, it's really quite dramatic.
Four hundred tonnes that just dissipates away in the Earth's atmosphere? Yes.
Atmosphere is slowing it down, of course, causing it to break up.
In just five seconds, it's almost all over.
And of course, you know, it's that extreme friction makes the light show, 10% of the energy goes in light.
And it's like a billion-watt bulb shining high in the sky.
COX: So, what are we looking for? What does a piece of that asteroid look like? They Going through the atmosphere the surfaces got melted, and so you end up with a dark crust on them.
So essentially, you're looking for an oddly-sculpted, dark rock.
(LAUGHING) Yeah.
Well, in all fairness, you gotta be able to tell it from the, you know, the cow patties and so on, but, you know COX: (LAUGHING) I can probably manage that, actually.
ALAN: Once you get your eye in, you'll have no trouble.
ALAN: We got one right here.
COX: Pick that up.
Astonishing! It's just been completely ALAN: Yeah.
COX: Rounded off.
ALAN: Yeah, the heat melted the surface of the rock.
I mean, how hot does something have to be to do that! COX: Yeah.
6,000 degrees C would do it.
(SIZZLING) So this little rock has had an amazing history.
I mean, it approached the Earth as part of this bigger fragment at about, what, 18, 19, 20 kilometres per second.
It hit the Earth's atmosphere.
About 85 kilometres up, it began to feel the effects of the Earth's atmosphere.
It began to squash the air in front of it and create a pressure wave, essentially, which, in turn, causes this thing to heat up.
And it would have heated up to something like the temperature of the surface of the Sun.
It would have been 5, 6,000 degrees Celsius as it plummeted through the atmosphere.
Lit up the sky over here, and then, quite literally, exploded in a series of explosions and peppered these fields with lumps of rock this big.
Can you imagine standing here on that night and having this, these things? (CHUCKLING) And this is heavy, right? Raining down from the sky.
It must have been quite incredible.
(NARRATING) If the meteorite had hit the ground intact, the explosion would have been equivalent to 400 tons of TN and left a crater 20 metres wide.
The Earth was spared this colossal impact by nothing more than the tenuous strip of gases that surrounds us.
But not all planets have this protective blanket.
When a meteorite hits naked Mercury, there is no atmosphere to break it up or slow it down.
It strikes the ground at full speed and completely intact.
For the last 4.
6 billion years, Mercury has been bombarded with countless asteroids and comets.
The whole history of the planet's violent past is laid out on its surface, a world pitted with hundreds of thousands of craters.
Craters inside craters inside craters.
Mercury was damned from the start.
It's simply too small and too hot to have retained any meaningful traces of atmosphere.
We, on the other hand, are big enough and cold enough to have retained this envelope of gases.
That, in turn, allows living things like me to evolve and to use that atmosphere to breathe and to live.
But there are places out there in the solar system whose atmospheres have the same ingredients as our own.
But when the formula is even slightly remixed, it leads to worlds that couldn't be more different.
Roughly 108 million kilometres from the Sun sits the brightest planet in the solar system, Venus.
This footage shows the luminescent world appear from behind our cratered moon.
Venus and Earth share many similarities.
We sit next to each other in space.
We were formed from the same material and we're roughly the same size and share a similar mass and gravity.
But that's where any similarities end.
Venus is a tortured world, where thick clouds of sulphuric acid are driven along by high-speed winds.
And temperatures are hot enough to melt lead.
All because this planet's atmosphere created a runaway greenhouse effect.
(WIND HOWLING) Now, the greenhouse effect has become a well-known phrase.
You know, it's synonymous with global warming.
But what is it? Well, a planet, like the Earth, absorbs energy from the Sun as visible light.
Now, atmospheres don't absorb much visible light, as you can see, because you can see the Sun.
The ground absorbs the visible light, heats up, and then reradiates it.
But it reradiates it as infrared radiation, heat radiation, if you want.
And atmospheric gases, particularly carbon dioxide, are very good at absorbing in the infrared.
And so they trap the heat and the planet heats up.
On Earth, greenhouse gases are essential to our survival.
Without them, our planet would be 30 degrees colder, too cold to support life as we know it.
But Venus' atmosphere was flooded with greenhouse gases.
The nearby Sun slowly boiled away its oceans, pumping water vapour into the atmosphere.
And carbon dioxide from thousands of erupting volcanoes added to the stifling mix.
Venus grew hotter and hotter.
The planet was slowly choked to death.
Venus is a planet with an atmosphere in overdrive.
But Earth's other rocky neighbour tells quite a different story.
(LAUGHING) Get it! Go on! These are the dunes in the Namib Desert.
It's an absolutely spectacular place.
This place is not the hottest or the driest desert in the world, but these dunes are some of the oldest sand dunes in the world.
And the reason we're here in the Namib Desert is that this is a great analogue for the surface of Mars.
This is what the surface of Mars looks like.
And these dunes are barchan dunes.
These crescent-shaped dunes are the same as the sand dunes on Mars.
So, if you want to get a feel for what it'd be like on the surface of Mars, and you want to know what driving a 4x4 around Namib would be like, then this is the place to come.
Incredibly, there is a vehicle driving across the surface of the red planet today, a space rover named Opportunity.
The rovers and spacecraft that circle the planet have sent back images which reveal Mars in exquisite detail.
Mars has vast dunes, enormous volcanoes and giant ice sheets.
It has canyons and river valleys.
Mars is a dry, frozen version of our home, covered in red dust and sand.
And it's all due to the fact that Mars has virtually no atmosphere.
But there are clues that things weren't always this way.
COX: These are pictures taken from the surface of Mars in August, 2009.
And they caused quite a bit of excitement because of this.
This rock sat on the surface of Mars in front of the rover.
This rock is about Well, here's a close-up.
It's actually a nickel-iron meteorite.
And it's about 60 centimetres across, weighs half a ton.
It came from space, came through the Martian atmosphere and landed on the ground.
Well, the mystery is that a meteorite this big, if it hit Mars today, would disintegrate when it hit the surface, it would be travelling too fast.
And that's because Mars' atmosphere is too thin to diffuse, to slow it down.
But that meteorite is very definitely there.
So, how could it have made it to the ground? Well, it must be that in the past when this meteorite hit Mars, Mars' atmosphere was significantly denser, dense enough to slow this piece of rock down enough that it could land on the surface intact.
But why did Mars lose its thick atmosphere and become the barren planet we see today? Well, there are so many ways for planets to lose their atmospheres that it feels like a miracle that we've still got ours.
But, in the case of Mars, it's thought that one of the dominant mechanisms was in the interaction with the solar wind.
The solar wind is a stream of superheated, electrically-charged particles that constantly stream away from the Sun at over a million kilometres per hour.
This wave of smashed atoms has the power to strip a planet of its atmosphere.
On Earth, we're protected from this onslaught by an invisible shield that completely surrounds our planet, known as the magnetosphere.
The magnetosphere is created deep within the Earth's molten iron core.
As the core spins, it generates a powerful magnetic field which shoots out of the pole and cocoons the whole planet.
This magnetic shield is strong enough to deflect most of the solar wind that comes our way.
Now, we know that in some point in the past Mars would also have had a molten core and did have a magnetic field.
But because Mars is a smaller planet than the Earth, it lost its heat more quickly and the core solidified, electric currents could no longer flow, and its field vanished.
And that was a major factor in the solar wind being allowed to blast the planet and strip away its atmosphere.
With no atmosphere to insulate it, this once Earth-like world transformed into the frozen desert we see today, a shadow of its former self.
Now, though Mars has lost most of its atmosphere, those few molecules that remain still have the power to sculpt and transform the surface.
And that power, that transformative effect, is present on every planet in the solar system that has an atmosphere.
You can see it transforming the surface of the Namibian Desert today, as we speak.
It is, of course, the force of nature that we call weather.
(THUNDER RUMBLING) MAN: Hey, we gotta go.
Wow! Weather is a feature of every planet with an atmosphere.
Our world is transformed as this huge mass of air moves across its surface.
But as we look out into the solar system, we see it only takes the slightest atmosphere to produce extraordinary weather.
Every few years, Mars all but disappears under a maelstrom of dust.
(WIND HOWLING) Global dust storms are so huge, they dwarf Olympus Mons, a volcano three times bigger than Everest.
But to experience the most extreme and violent weather in the solar system, we need to travel to Jupiter.
This banded gas giant is over 140,000 kilometres in diameter.
Its atmosphere isn't a thin blue line, it's many thousands of kilometres thick, and in a constant state of seething motion.
The whole surface boils with gigantic storms.
Yet this most alien world shares a feature with our own planet.
(THUNDER RUMBLING) Jupiter crackles to the sound of electrical storms.
The bolts of lightning are thousands of times brighter than lightning here on Earth.
(RUMBLING CONTINUES) The physics of storms on Jupiter is, of course, the same as the physics of storms on Earth.
The warm, moist air deep in the atmosphere starts to rise, and, as it rises, it cools, and the moisture condenses out to form clouds.
Now, that rising air leaves a gap beneath it, a low pressure area, and some more warm moist air is sucked in and that fuels the rise of the storm.
Now, on Earth, those storm systems are driven by the power of the Sun.
But therein lies the mystery, because the storm systems on Jupiter are far more powerful.
And yet, Jupiter is five times further away from the Sun than the Earth is, which means it receives 25 times less solar energy.
So what mechanism could it be that powers those intensely violent storms on Jupiter? The secret to Jupiter's storm-tossed atmosphere lies hidden deep within the gas giant.
On Earth, we have clear boundaries between the gaseous sky, the liquid oceans and the solid ground.
But on Jupiter, there are no such boundaries.
It's a gas giant, it's made of the two lightest and most abundant elements in the universe, hydrogen and helium.
But as you go deep into Jupiter's atmosphere, something very strange and interesting happens to those gases.
Jupiter's atmosphere is so thick and its gravitational pull so strong, that 20,000 kilometres beneath the cloud tops, the pressure is two million times greater than the surface pressure here on Earth.
Under such immense pressure, the hydrogen gas in the atmosphere is transformed into a strange, metallic liquid.
As the gases are squeezed, a vast amount of energy is released, enough energy to fuel some of the biggest storms in the solar system.
The biggest of them all is the Great Red Spot.
This giant anticyclone has raged for hundreds of years, and is large enough to swallow the Earth three times over.
The Great Red Spot is an awesome sight, but this giant isn't one of my wonders.
My wonder is a much smaller world, a moon that orbits the gas giant Saturn, one and a half billion kilometres from Earth.
What we have found on this small world is simply astonishing.
If you've thought of our Moon as the archetypal moon of the solar system, if you like, then, well, you might think that all the other moons out there, the hundreds of them, would be dead, uninteresting worlds.
I mean, not uninteresting places to visit.
I mean, that, is in my view, the greatest thing that humans have ever achieved, landing on the surface of the Moon.
But it's a dead and lifeless place.
But as we've begun to visit those worlds, as we've flown spacecraft to within hundreds of miles of their surfaces, we've found that the moons in the outer solar system are an astonishingly interesting, varied and fascinating bunch of worlds.
This is Jupiter's moon, Europa.
This is Jupiter's moon, Io, the most volcanic object in the solar system.
But of all of the worlds out there, this one, Saturn's moon, Titan, is unique because of that.
That is an atmosphere.
And what an atmosphere it is, it's 1,000 kilometres deep, it's four times denser than the atmosphere of the Earth.
But imagine that, a moon around a distant planet in the icy distant reaches of the solar system with an atmosphere denser and thicker than our own.
Titan has the most Earth-like atmosphere in the entire solar system, a thick, blue line, rich in nitrogen and containing methane.
At first sight, a world this small shouldn't be able to hold on to such a dense atmosphere.
Except Titan lies in one of the coldest regions of the solar system and that makes all the difference.
Temperature for gases, like the gases in our atmosphere, is really a measure of how fast the molecules of the gas are moving around.
And I can demonstrate that with this thing, (LAUGHING) which is a Chinese lantern.
You see, if I light this fuel, then what's gonna happen is that the gas inside is gonna heat up.
And as you heat up a gas, what that basically means is that you speed all the molecules up.
As the molecules of air heat up and move faster, the air pressure inside the lantern begins to increase.
That means that molecules are forced out, making the air inside less dense than the air outside, and the lantern gets lighter.
And eventually, the lantern is so light that it will just float away into the atmosphere of our planet.
Hot gases have more energy to escape a planet's gravitational pull than cold gases.
Now, Titan is a much smaller body than the Earth, it has much weaker gravitational pull.
And if it were in the same region of the solar system as we are, then it would not be able to hold on to its atmosphere.
But it's a lot further away from the Sun than we are.
And so, that means that it's colder, its atmospheric molecules are moving around much more slowly than ours.
That means that its weak gravity is enough to hold on to that thick, dense atmosphere.
Titan's thick atmosphere was an unexpected discovery, but it took an audacious mission to reveal the world that lies beneath the blanket of clouds.
(ROCKET RUMBLING) MAN OVER RADIO: We have lift-off of the Cassini spacecraft on a seven-year trek to Saturn.
COX: In 1997, Cassini began its journey to Titan.
It carried with it the Huygens probe, a lander designed to set down on this frozen moon.
On Christmas Day, 2004, Huygens was released from Cassini and it began the bumpy ride through one of the most intriguing atmospheres in the solar system.
And then, for the first time, the thick clouds parted and the surface of Titan was revealed.
These are the actual images taken by Huygens as it slowly parachuted to the surface.
The world it revealed was more familiar than we could have possibly imagined.
One of the first people to see these incredible images was a man who helped design the probe, Ralph Lorenz.
RALPH: It was amazing 'cause we just had no idea what to expect.
We didn't know whether it would be cratered like the Moon, or just a sort of flat expanse of sand or whatever.
And then these first pictures came back and it was just astonishingly familiar.
Did that picture, or those initial series of pictures, um, I suppose it looked somewhat like this, didn't it? It was - It could be have been there, actually.
- It was like this, but It could have been right here.
COX: I do see that.
I mean, I can sit here, look at that, and that's what that picture looks like, actually.
I could take it with a camera.
The camera on the probe was about the height of your knee.
So, yeah, the view the Huygens probe had is just like this.
COX: Rounded stones dot the landscape.
They're smooth and look like they've been eroded by tumbling water, similar to stones found on riverbeds here on Earth.
It sounds to me like this was one of the easiest pictures to interpret in the history of space exploration.
You know, the way you tell it, it's just, that's a riverbed (CHUCKLING) with these stones.
I mean, is it that simple? Because you can be mislead easily, can't you? The devil's always in the details.
But I think very few people disputed the interpretation of a river channel.
I mean, it was just such a familiar thing to so many people on Earth, there really wasn't much doubt.
COX: It was an extraordinary discovery, evidence of flowing rivers had never been found before on a moon.
But it wasn't the only surprise Titan held in store.
COX: This is the Matanuska Glacier in Alaska.
It really is one of the most astonishing places I've ever seen.
And this whole landscape is testament to the erosive power of this stuff, this mixture of ice and rock as it rolls down this valley over hundreds of thousands of years, and creates this astonishing landscape.
But the reason it can do that is because of the delicate balance of the Earth's atmosphere.
You see, our planet is just at the right temperature and pressure to allow water to exist as solid, as liquid, and as gas, as vapour in the clouds.
And so, the Sun can heat up the oceans and it can move the water over the top of the mountains, it can fall as rain, turn to ice, become a glacier, and then sweep down the valley to sculpt this astonishing landscape.
Just as our atmosphere allows all this to exist, the atmosphere of Titan is the perfect temperature and pressure to allow something to exist that has never been seen before on a world beyond Earth.
This is a picture taken of the South Pole of Titan by Cassini in June, 2005.
And it's subsequently become one of the most important and fascinating pictures in the history of space exploration.
The interesting thing is this black blob here.
Now, this fascinated the Cassini scientists.
But the explanation as to what that is had to wait just over a year, till July, 2006, when this picture was taken.
And it's a radar image, this time of the North Pole of Titan.
And you see again these huge, black areas.
But black, in this case, means that the radar waves that bounced onto them didn't come back, so they're completely black.
And there's only one really good explanation for that.
That is that they're incredibly flat surfaces.
In fact, they're surfaces of liquid.
So, this picture, combined with this picture, means that this is the first observation of a liquid, a lake, on the surface of a body other than the Earth in the solar system.
These lakes, of course, cannot be lakes of liquid water because the surface temperature on Titan is minus 180 degrees Celsius.
And at those temperatures water is frozen as hard as steel.
So, if these are not lakes of water, then what are they? This is Lake Eyak in Alaska, just on Prince William Sound.
Now, I've come here to collect a molecule, or a substance, that's very abundant on Titan.
In fact, it's abundant throughout the solar system.
But, here on Earth, it exists as a gas, and it bubbles up from the floor of this lake.
Now, the floor of Lake Eyak is covered in rotting vegetation, you know, dead leaves and bits of trees, twigs.
And that's being broken down by bacteria, which produce the gas that bubbles up from the floor of the lake.
That gas is methane.
And we've been collecting it all night underneath this upturned boat, so that I can take a sample of it in this bag.
(AIR WHOOSHING) Now, on Earth, methane is very unstable.
If you give it a little kick in the presence of oxygen, then you get what chemists' call an exothermic reaction.
Methane plus oxygen goes to water, plus carbon dioxide and some energy.
The Earth's temperature and atmospheric pressure means methane can only exist as a highly flammable gas.
But Titan's atmospheric pressure and temperature is perfect to allow methane to exist as a solid, a gas, and, most importantly, a liquid.
So the images Cassini captured were gigantic lakes of liquid methane.
The first ever liquid discovered pooling on the surface of another world in the solar system.
This is Kraken Mare.
At over 400,000 square kilometres, it's the biggest body of liquid on Titan.
It's almost five times the size of Lake Superior, North America's greatest lake.
COX: On Titan, methane plays exactly the same role that water does here on Earth.
So, where we have clouds of water, Titan has clouds of methane, with methane rain.
Whereas we have lakes and oceans of water, Titan has lakes of liquid methane.
And whereas here on Earth, the Sun warms the water in the lakes and oceans, and fills our atmosphere with water vapour, on Titan, the Sun lifts the methane from the lakes and saturates the atmosphere with methane.
So, whereas on Earth, we have a hydrological cycle, on Titan there's a methanological cycle.
And rain would be an absolutely magical sight on Titan.
Because the atmosphere is so dense and the gravity of the moon is so weak, the drops of methane rain would grow to over a centimetre in size, and they would fall to the ground as slowly as snowflakes fall onto the surface of our own planet.
Thousands and thousands of gallons of liquid methane must have slowly rained down onto the surface, making rivers and streams swell and burst.
Deep gullies were cut into the frozen water landscape.
Which looks so familiar because it is familiar.
It's this.
You know, the atmosphere of Titan shapes the surface in exactly the same way that the atmosphere here on Earth shapes the surface of our planet.
Titan is like a primordial Earth caught in a deep freeze.
It's almost like looking back in time over four billion years and observing our planet before life began and began to modify our atmosphere to change it into the oxygen-rich atmosphere that we see today.
In many ways, Titan looks so familiar, it's a place with rivers, and lakes, and clouds, and rain.
It's a place with water, albeit frozen as hard as steel.
And a place of methane, albeit so cold that methane is now a liquid and flows and shapes the landscape just like water does here on Earth.
For me, the most important thing about Titan, is we now have two Earth-like worlds in our solar system.
One, in its warm region ninety three million miles away from the Sun.
And the other, in deep freeze a billion miles away from our star, in orbit around another planet.
And that must greatly increase the probability that there are other Earth-like planets in orbit around the hundreds of billions of stars out there in the universe.