Wonders of the Solar System s01e04 Episode Script
The Thin Blue Line
BRIAN COX: We live on a world of wonders, a place of astonishing beauty and complexity.
We have vast oceans, incredible weather, (THUNDER RUMBLING) 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.
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.
Now, the laws of physics are simple and they're universal.
What applies here, applies everywhere else up there.
But, it's fascinating that they can manifest themselves in so many different ways across the solar system.
(NARRATING) In this programme, I'm going to look at how the universal forces of nature that created all this can also wreak devastation across the solar system.
How they can be the death of a planet and how they can keep other worlds alive.
I can just see pieces of molten rock rising up just below my chin.
These forces are so far reaching, they bridge the depths of space and transform a world long thought dead into a world of perpetual change and everlasting youth.
The intense source of heat that powers that eternal change, also drives one of the most spectacular sights in the solar system.
I've come to one of Earth's natural wonders, the Grand Canyon in Arizona.
It's a place to be humbled by nature.
Oh, this is undoubtedly one of the most beautiful places I've seen on Earth.
From sunrise to sunset, the changing light brings this immense landscape to life.
I hear stories of people coming here at sunrise with tears in their eyes at the majesty of the view, and I can see why.
It's incredible to think that this enormous valley was etched and carved by the action of running water over just a few million years.
Our knowledge of natural wonders like the Grand Canyon was once limited to our own planet.
But the Space Age has brought new worlds into view.
This is Mars, the Red Planet.
Fourth rock out from the Sun, it has a canyon so vast you could fit our own Grand Canyon into one of its side channels.
Named after the space probe that first saw it, this is the Valles Marineris.
Eight kilometres deep and over three thousand kilometres long, on Earth it would run all the way from Los Angeles to New York.
We're beginning to get a deep and quite profound understanding of the way that Mars has evolved geologically because we are now there, because we now have eyes and ears on the surface.
And there really is no substitute for actual exploration, for actually going somewhere and touching it and taking pictures of it.
Look at this picture.
This is a picture of a sunset.
These are pictures of clouds.
It's amazing to think these incredibly familiar looking photographs were taken on the surface of Mars.
This picture, or this amazing colour picture.
I could have got a camera here and just snapped any picture and it would have looked exactly like this one.
In the Grand Canyon, you can see the Colorado River running in the bottom of the valley.
So you can understand how this landscape was made.
Whereas, here on Mars, there's no sign of any water.
So Mars is a is a puzzling place.
Despite all the similarities between Mars and Earth, it's the differences between these two planets that are most telling.
Mars is now a desolate, dead wasteland.
A world where the processes that sculpted its familiar landscapes seized up long ago.
It's fascinating for me as a physicist to see how the same basic, simple laws of nature can play out in such radically different ways and produce such astonishingly varied, and beautiful, and violent, and dead worlds out there across the solar system.
(MEN TALKING OVER HELICOPTER RADIO) The Big Island of Hawaii, in the middle of the Pacific Ocean, holds the key to what happened to Mars.
This is the perfect place to witness how a planet can be kept alive by nothing more than the simple flow of heat.
You can smell the volcanic ash coming in to the helicopter.
And everywhere you look, it looks like a (LAUGHING) It's almost like an apocalyptic scene.
This volcano is Kilauea, which means "spewing".
It's been erupting almost continuously since 1983.
In fact, you can see molten rock flowing down the side of the mountain just cutting a sway through the trees.
This might look like widespread destruction, but volcanic eruptions are Earth's geological heartbeat.
Active volcanoes make our planet a vibrant, living world.
That is the most spectacular demonstration of our planet being geologically alive that I've ever seen.
A few kilometres north of Kilauea, you can see just what volcanic action can produce given enough time.
The islands of Hawaii were built entirely by volcanoes.
Today, these mountains are the largest volcanoes on our planet.
And we've seen landscapes just like this on Mars.
It's quite an experience being four kilometres high.
Makes you out of breath, you sniff a lot because your nose becomes (SNIFFLING) And makes you feel like you've had a few drinks.
Well, this is Mauna Kea, one of the five volcanoes that make up the Big Island of Hawaii.
I've known about it since I was very little because it's one of the most famous observatories on the planet.
Everywhere you look, you're surrounded by our eyes to the cosmos.
Now, though this mountain is four kilometres above the surface of the Pacific, it's actually 10 kilometres above the surface of the Pacific floor.
That makes it the highest mountain on Earth, over a kilometre higher than Mount Everest.
But it is a tiny volcano compared to the biggest volcano on the surface of Mars.
This is Olympus Mons, named after Mount Olympus, the mythical home of the Greek gods.
This vast outpouring of lava stretches over 550 kilometres across.
But it's the height of this volcano that is breathtaking.
It soars 25 kilometres into the Martian sky, nearly three times the full height of Mauna Kea.
But Olympus Mons isn't just the tallest volcano in the solar system, it's the highest mountain we know.
There are striking similarities between the volcanic landscapes here on Hawaii and the giant volcanoes found on Mars.
These similarities can be traced back billions of years to the fiery birth of the solar system.
All this heat you can see driving this spectacular volcanic activity is a relic, a hangover of the Earth's formation.
Now, all the rocky planets, Earth, Mars, Venus, Mercury, were formed in the same way.
They came from a collapsing dust cloud over four and a half billion years ago.
(EXPLOSION) With the ignition of the Sun, our solar system was born.
Little by little, the rocky bodies grew, falling together under their own gravity.
This process not only generated immense amounts of heat, it also delivered radioactive material to the cores.
These two ancient sources of heat power Earth's volcanoes to this day.
But the volcanoes on Mars are little more than a petrified memory of a distant past.
When we look down on the surface of Mars, we see no evidence of this kind of volcanic activity.
As far as we can tell, Mars is a dead world.
Mars must have had similar inner heat to Earth to build its volcanoes.
Yet something obviously happened to stop the Red Planet's geological heartbeat.
Now, as everyone who's left a hot cup of tea sat on the kitchen table knows, hot things lose heat to their cooler surroundings.
And what's true for cups of tea, in physics, is also true for planets.
This is hot, and that up there, space, is cold.
And so, planets lose heat to space.
And Mars is a much smaller planet than the Earth.
It's about, well, half the diameter, it's an eighth of the volume, so there's much less heat trapped in there to begin with.
Now, planets lose heat to space through their surfaces.
And smaller things have a larger surface area in relation to their volume than big things.
So that means that Mars will lose its heat to space much quicker than the Earth does.
When the interior of Mars grew cold, the mighty volcanoes lost their lifeblood.
The Red Planet's geological heart died and, millions of years ago, the surface of Mars ground to a halt.
The fate of a whole planet was destined by the simplest of laws of physics.
Since the dawn of human history, we've been able to gaze up into the night sky, but we're lucky because we're the first generation that's been able to build machines to actually go to those planets and moons.
And we found that they're more beautiful, more violent, more magnificent and fascinating than we could've possibly imagined.
The more worlds we study, the more we realise that our solar system is a cosmic laboratory.
Even the slightest differences in size or position can create a world radically different from its neighbours.
So, here on Earth, we have one beautiful manifestation of how those simple laws of nature can play out, how they can build a planet.
In Mars, we have another example.
What happens when you take a planet that's smaller than Earth and move it further away from the Sun? Well, it loses its heat more quickly and it becomes geologically inactive.
But what would happen if we took a planet just like the Earth and moved it a little closer to the Sun? Well, we know of such a planet.
It's the brightest point of light in our night sky.
So similar in size to our own world, this planet has been called Earth's twin.
This is Venus.
Orbiting closer to the Sun, Venus was named for its shining beauty.
But our planetary twin hides its true identity beneath a thick blanket of cloud.
Over the last 4.
5 billion years, Venus has turned into an unimaginably oppressive world.
The atmosphere is so dense that the pressure is crushing.
It's 90 times atmospheric pressure here on Earth.
Now, Venus takes 243 days to rotate once on its axis, that means its day is longer than its year.
Venus has the hottest average surface temperature, other than Sun's, anywhere in the solar system.
470 Celsius.
I've come to India to a place called the Deccan Traps.
Hidden in this lush green landscape are tantalising clues to understanding how immense heat caused Venus to choke to death.
You know, when you look this landscape today, it's incredibly peaceful and beautiful, rolling green hills.
But, I think it's astonishing to think that everything you see down there is lava.
This whole landscape, half a million square kilometres of it, was created in an ongoing volcanic eruption that lasted for a million years.
If you take away the green foliage, the underlying landscape of lava is actually very similar to what we see on Venus.
Using radar to peer down through the clouds, the surface of Venus was finally revealed.
It is covered with floods of solid lava, just like we see in India, but on a scale many thousands of times larger.
We've also counted over 50,000 volcanoes, the most on any planet in the solar system.
Venus is a similar size to Earth, so it may still have a hot geological heart powering its volcanoes.
But, as yet, we haven't witnessed any eruptions.
The ancient floods of lava we see on Venus and here in India were created in much the same way.
For both planets, this was volcanic activity in overdrive.
The eruptions here in India 65 million years ago affected the Earth's climate so much that they've thought to have played a major role in the mass extinction event at the end of the Cretaceous Period which wiped out over two-thirds of the species on Earth.
Now, life on Earth recovered, but Venus wasn't so lucky.
The intense volcanic activity on both planets didn't just blast out molten rock.
It also released copious amounts of gases, like carbon dioxide.
But slight differences in the way the laws of physics played out on Venus helped push our cosmic twin down a path of no return.
Venus and Earth reacted very differently to the same kind of volcanic cataclysm.
And the reason for that is something that happens so often on Earth that we take it for granted, and just moan about it.
(THUNDER RUMBLING) Rain plays a significant role in keeping our planet a pleasant place to live.
Acting as part of a global recycling system, rain keeps our atmosphere in balance, washing out the potent greenhouse gas, carbon dioxide, ready to be locked away in rocks in our oceans.
But on Venus, the laws of physics have made it impossible for rainfall to cleanse its atmosphere.
In fact, there's no liquid water at all.
Venus lost its water essentially because it's hotter than the Earth.
The temperature is just a measure of how fast things are moving around.
So on Venus, the oceans would have evaporated into the atmosphere, and that water in the atmosphere would be moving around extremely quickly simply because it's hot.
And Venus is so close to the Sun and so hot, those water molecules are moving so fast that the gravity of the planet can't continue to hold them in the atmosphere.
And so they simply escape off into space.
With no water, there is no rain on Venus.
For billions of years there has been nothing to temper the build-up of volcanic gases in its atmosphere.
Venus ended up cocooned in a thick, high-pressured dense blanket of carbon dioxide.
And that made the temperature rise, rise and rise.
Turning Venus into the hell-like world we see today.
Compared to scorched Venus and frozen Mars, our home is a very special ball of rock.
Although governed by the same universal set of rules, our planet is not too big, not too small, not too hot, not too cold.
Earth has been called the Goldilocks planet because everything is just right.
Our world is unique but it doesn't exist in splendid isolation.
It is intimately connected with its cosmic neighbours.
Earth is not the master of its own destiny, and it never has been.
The life and death of our planet is influenced by forces emanating from the very depths of space.
As we better understand our place in space, we've come to realise that our sharing of the same physical laws with the other worlds in the solar system isn't the only connection we have with them.
Because those same laws lead to a direct physical connection between the Earth and the other worlds out there that is subtle, is complicated, but can sometimes be extremely powerful.
Out in the farthest reaches of the solar system are vast worlds that have a direct impact on our very existence.
This is our Sun from 10 billion kilometres away, just another star in a sea of stars.
But as you head towards the light, you enter a realm of giants.
The furthest planet from the Sun is icy Neptune, with its thick, blue methane atmosphere.
Uranus comes next, the only planet that lies on its back.
Further in towards the Sun, and the planets get even bigger.
Saturn, with its beautiful rings of ice.
Finally, we reach the king of the giants, Jupiter.
Jupiter is the largest planet in the solar system, so big you could fit Earth inside it over 1,000 times.
It's made up of the same stuff as our Sun, hydrogen and helium, the most common elements in the universe.
In its thick churning atmosphere, gigantic storms have raged for centuries.
Now, astrologers have said for years that Jupiter influences our lives.
But we now have scientific evidence that this mighty planet does have a significant connection with our own small world.
Now, Jupiter is so different to our planet, you know, a big ball of gas half a billion kilometres away.
It's difficult to see how it could have anything to do with us all.
But despite the fact that astrology is a load of rubbish, Jupiter can, in fact, have a profound influence on our planet.
And it's through a force that, well, surrounds us and penetrates us and binds the galaxy together, gravity.
Gravity is one of the fundamental forces of nature.
It exists between all objects.
And the effects of a gravitational field extend way beyond the planet that creates it.
(WIND HOWLING) Gravity is by far the weakest force of nature.
I mean, I can pick this rock up off the ground and this entire planet, Planet Earth, will try to pull it down.
But it's the only force that has the influence across the entire solar system.
And that's because, although it's weak, it has an infinite range, it never quite goes away.
So, no matter how far you go, you feel the force of gravity of a planet, although it drops and drops and drops away.
Jupiter has the most powerful gravitational field of all the planets.
And it's the gas giant's gravity that can directly influence the orbits of asteroids and other wandering space debris.
Jupiter is so massive, I mean, by far the most massive planet in the solar system, that its gravitational field can have a profound influence on passing interplanetary stuff.
It can do three things.
Firstly, it can capture the stuff, literally, hoover it up.
Secondly, it can deflect the stuff as such that it throws it out of the solar system.
But thirdly, and most importantly for us, it can deflect stuff onto a direct collision course with our planet.
Influenced by Jupiter's gravity, today, right now, we're highly vulnerable from asteroid impacts.
But we do have sentinels standing guard.
On top of the mountain of Haleakala on the Hawaiian honeymoon island of Maui, I've come to see Professor Nick Kaiser, who's searching our solar system for potentially hostile space debris.
The prime task is to try and find killer asteroids, things that are out there in the solar system that might hit the Earth.
There's an air of Hollywood about it, isn't there, in some sense? Well, that's right.
I would say a lot more resources have been spent on making movies about killer asteroids than actually finding them.
Anything that's a kilometre in size, if it hit the Earth it would be devastating, right? It would probably kill nearly everyone on the planet.
COX: Each night, using a revolutionary billion-pixel digital sensor, the team scans a vast swathe of the sky.
NICK: Mind your head.
They're looking for any unidentified objects that might be heading our way.
So, any one of these points of light could, in fact, be an asteroid.
That's right.
And it's very likely, in fact it's almost certain, that there are asteroids in that image.
Right, the problem is how do you figure out which ones they are.
Yeah.
The camera captures several images of the same patch of sky taken minutes apart.
The team can then see if anything has moved relative to the background of stars.
What we've done here is taken two images and subtracted them, and you see the stars have nearly all gone away, right? But there's a couple of interesting things left.
If you look over here, you see a dark thing and a white thing.
So that's something which was there in the first image - and there in the second image.
- Yeah.
NICK: And there's another one here.
So, even on this tiny patch of sky we've already detected two objects.
What that means is, when we do that kind of analysis on a whole field of view, we'll detect hundreds of objects.
COX: The beauty of our night sky belies the potential danger it holds in store.
Over 2,000 objects have been identified that pass close to the Earth, with something like 400 that could be on a future collision course.
And all of these menacing lumps of rock, at some point, come under Jupiter's gravitational influence.
Now, if you ever needed a demonstration of how congested the space is near the Earth, just look at this movie of the near-Earth objects.
So here's Mercury, Venus, the Earth, Mars, and out here is Jupiter.
Here's the asteroid belt.
But look at the congestion in there.
Every one of those points of light is an asteroid that we know of.
Just look at the Earth swimming through them.
So, when you look up into the nice, clear night sky and you want to be reassured that we're all nice and safe, (CHUCKLING) just remember this movie.
Our planet is on a deadly journey.
Earth is trapped in a cosmic game of dodge ball as it orbits the Sun.
A game where the gravitational stranglehold of Jupiter regularly throws asteroids our way.
Jupiter's gravitational influence on passing space debris has made our planet a world under constant bombardment.
One of the most famous meteorite impact sites is the Barringer Crater in Arizona.
COX: Fifty thousand years ago, a 300,000 tonne, 50 metre in diameter lump of iron and nickel entered the Earth's atmosphere and made this crater.
And it should remind us that our environment doesn't just stop at the top of our atmosphere.
Our environment stretches out into the solar system to the very edges of the solar system to wherever this rock came from.
In the grand scheme of things, the asteroid that struck here was relatively small and innocuous.
But there are much larger impact craters hidden in Earth's landscapes that have far more devastating tales to tell.
Two hundred years ago, white settlers crossed the ancient Appalachian Mountains here in America, to seek new land out west.
Little did they know what they were walking into.
Well, this place to me feels like the very definition of small town America.
It's on the border between Virginia, Kentucky and Tennessee.
And it's the kind of town where you feel that nothing much has changed for the last 100 years.
But this place was the site, way back in history, of a violent cosmic intervention.
This is Middlesboro, Kentucky.
It's a town built inside a meteorite impact crater.
The asteroid that struck here would have been huge, around half a kilometre across, hitting the Earth well over 200 million years ago.
Yeah, I find it fascinating that when you look out at this view you don't really see an obvious crater, and, indeed, it wasn't until the 1960s that anybody had any idea that there was a colossal impact from space just over there, centred right on the 18th hole of the golf course.
(THUNDER RUMBLING) We now know where the giant asteroid that struck here could have come from.
Located between Jupiter and Mars is a vast reservoir of rocky debris that forms the asteroid belt.
And it's this ancient rubble that Jupiter, our neighbourhood giant, can nudge towards the Earth.
Well, here's my model of the solar system.
There's the Sun in the middle.
Then the Earth, Mars, Jupiter.
And then the asteroids sort of scattered in between, a big region, actually, between Mars and Jupiter.
In fact they extend over 150 million miles, which is further than the distance from the Earth to the Sun.
Now This is my coffee by the way, which doesn't represent anything.
I'll put it over there.
Now and again, because of collisions in the asteroid belt, a stray asteroid will get thrown into the position where they keep rhythmically meeting Jupiter over and over again.
And because Jupiter's such a massive planet, that means that it gets a kick, it gets a gravitational kick.
And that changes the orbit of these asteroids.
And, over time, their orbit can become, well, elongated or elliptical rather than circular.
That means that they can get thrown in to the inner solar system and cross the orbits of the inner planets, including the orbit of the Earth.
And you get a potentially catastrophic collision.
Jupiter was once thought to be our protector, its enormous gravity swallowing up dangerous asteroids.
Yet we now realise its gravitational influence can propel some of those asteroids in our direction.
But, surprisingly, catastrophic impacts with space debris might not be a bad thing, at least in Earth's past.
Impacts from space shaped our planet, they made our world what it is today.
Take life on Earth, for example.
Now, it's possible or probable even that impacts on a colossal scale changed the climate so much that huge swathes of life on Earth were wiped out, creating ecological niches into which other species could evolve.
Us, for example.
It's incredible to think that a planet half a billion kilometres away could dictate the fate of our world.
Jupiter's immense gravity bridges the depths of space.
And even though its power could one day devastate Earth, that same gravitational field breathes life into other corners of the solar system.
For better or worse, Jupiter's powerful gravitational field has had a direct influence on our planet.
Now, we're part of a much wider ecosystem that extends to the very edges of the solar system.
And that ecosystem is bound together by the force of gravity.
And it's gravity that has power to bring worlds to life.
Our understanding of the solar system began much closer to home.
Gazing down at us, it was our moon, with its regularly changing face, that first fired our interest in worlds beyond our own.
When we could look further out, we discovered the solar system was full of moons, each invisibly connected to their parent planet by gravity.
Our Moon is a cold, geologically dead world.
But the powerful gravitational bond that exists between another moon and its parent planet has done something astonishing.
It has brought this moon to life, making it the most violent place in the solar system.
Four hundred years ago, it was Galileo who first looked up at the night sky through a telescope.
Turning his attention to Jupiter, he noticed that this giant planet was not alone.
Oh, yeah.
That's absolutely magnificent.
You see a disc surrounded by Well, I can see three points of light.
Galileo, over several nights, saw four points of light.
And he correctly surmised that those are actually moons, other worlds in orbit around Jupiter.
Jupiter's four largest moons are named after the four lovers of the Greek god Zeus.
Furthest out, is Callisto.
Then there's huge Ganymede, the largest moon in the solar system.
Next is icy Europa, and, finally, the small yellow-tinged moon nearest to Jupiter, Io.
So the legend goes, Zeus tried to protect his lover Io by turning her into a cow to hide her from the gaze of, the jealous gaze of his wife, Hera, but it didn't work, and Hera sent a gadfly to torment Io.
And it was prescient in a way to name that satellite Io the tormented moon, because we've since learned that it is indeed an incredibly tormented world.
For 400 years, we expected Io to be as dead as our own moon.
But in the late 1970s, when the first spacecraft passed by Jupiter, we finally saw Io up close.
But it didn't make any sense, as here was a moon with no meteorite impact craters.
Now, it is impossible to believe that Io could've escaped the bombardment that we see on our moon and practically every other body in the solar system.
So, the only explanation is, is that surface is young, it must have been recently produced.
And that, in turn, means that Io, that tiny moon of Jupiter out there, must be a geologically active world.
The truth about Io would blow us away.
We may not have stood on Io, but there are places we can go here on Earth to help unlock its secrets.
This is Ethiopia in east Africa.
We're being flown out by military helicopter to the very hot, very inhospitable Afar region in the northeast of the country.
And this is what I've come to see.
It's one of the rarest geological phenomena on our planet.
A volcano with a lake of molten lava.
As the Sun goes down, the lava lake comes to life.
This volcano is called Erta Ale by the local Afar people.
It means "smoking mountain".
For many, this place is a vision of hell.
Yet it holds the key to understanding Io, a world over half a billion kilometres away.
This hot, tortured landscape of lava will be our home for the next three days.
Temperatures here reach 50 Celsius in the shade, and we're camping right on top of the volcano.
But we're in good hands, as Io specialist Dr Ashley Davies is part of the team.
It is absolutely spectacular.
DAVIES: It is extraordinarily beautiful.
COX: You feel like you're looking into the core of the planet.
Yeah, it's a window into the interior of the Earth.
So, magma's rising up from some kilometres down, circulating through the surface, and then sinking back down again.
It's very difficult to breathe, it's very acidic, very bitter.
The magma has gases in it.
As it comes up to the surface, just like when you pour out a bottle of Coca Cola, the gases come out.
And what we have here is sulphur dioxide, hydrogen sulphide, water vapour, carbon dioxide, coming out of the magma before the magma then cools and sinks down, and that's what we're breathing now, it's kind of unpleasant.
This volcanic phenomenon is a product of immense amounts of primordial heat escaping from inside our planet.
Yet we have seen something similar in the far reaches of the solar system.
Io is the same size as our Moon and should be a cold, dead world.
Yet our first glimpses of Io revealed it as seething with heat, alive with volcanic activity.
Just one of the many lava lakes on Io releases more heat than all Earth's volcanoes put together.
If we were to stand on the surface of Io now, what would be the similarities and what would be the differences? The lake will probably appear very, very similar to this, except for the scale.
The lava lakes in Io are vastly larger.
The biggest one we think is a180 kilometres in diameter.
COX: 180 kilometres? So that would stretch, way, obviously, way beyond the horizon on Earth.
Yeah, it's almost beyond description to see something that size and it's just this huge pool of molten lava.
COX: Io is the most volcanic place in the solar system, endlessly pumping out heat into the cold vacuum of space.
But what's really interesting is that it's so small that it shouldn't be volcanic at all.
I mean, it was one of the greatest surprises of planetary science when these massive volcanoes were discovered on Io.
Mighty planets, like Mars, have big volcanoes, they're not erupting any more.
They haven't erupted for a long time.
Venus, lots of volcanoes there, but they haven't been erupting for a long time, probably.
And here we have Io, which is just insanely volcanic.
It's just pumping out vast amounts of energy in a zone, in a part of the solar system, where it was thought that everything was dead.
COX: Everything we now know about Io comes from looking at it from a distance.
But measuring the heat pumping out of this lava lake will give us a better idea of the true scale of the heat loss on Io.
Far from being a benign, bubbling cauldron, this volcano has the power to kick off at a moment's notice.
(RUMBLING) I can just see pieces of molten rock rising up just below my chin.
And with it a cloud of heat, absolutely overpowering heat.
There must be a (COUGHING) Hell of an eruption coming on.
Seeing active volcanism like this on such a small moon like Io, changed our view of the workings of the solar system.
A world like Io having such a powerful internal heat source, cries out for an explanation.
See, Io is far too small a world to have retained any internal heat, to have the same heat source that powers the Earth's volcanoes.
So something else must be driving that powerful volcanism on Io.
New images sent back from recent space probes confirm that Io is a surprising and bizarre world.
Being so far from the Sun, lo's surface is mostly very cold.
It's covered in frozen sulphur, which gives it its yellow colour.
Yet Io is pockmarked by cauldrons of molten lava.
You know, I think it is remarkable and fortunate, in a sense, that you can come to a place like this on our planet and just get the tiniest sense of what it must be like to stand on the edge of one of those magnificent lava lakes on Io.
When we first saw hot volcanic action on Io, it was quite a surprise for most planetary scientists.
But by considering simple laws of physics, it didn't surprise everyone.
Just weeks before Voyager arrived at Jupiter, three scientists made a prediction.
And it was one of those predictions that, when you see it, is almost obvious.
It was using physics that had been known for hundreds of years but nobody had thought of it.
They predicted that Io should have an intense internal heat source because of its unique position in the solar system, very close to a giant planet and surrounded by other large moons.
Io sits about the same distance from Jupiter as our own Moon does from Earth.
But don't forget that orbiting outside Io are its sister moons, Europa and Ganymede.
Io is under the influence not just of the massive gravitational pull of Jupiter, but also the additional pull of its neighbouring moons.
It's this gravitational tug of war that conspires to breathe life into Io.
Now, Io has a very interesting relationship with Europa and Ganymede because for every four orbits that Io makes around the planet, Europa goes around almost exactly twice and Ganymede goes around just once.
Periodically, they line up together, bang, bang, bang.
And Io gets a powerful gravitational kick on a very regular basis.
And that has the effect of moving Io out of a nice circular orbit into an elliptical, or an eccentric orbit.
Io comes close to Jupiter and then far away from Jupiter and then close to Jupiter again.
And because Jupiter's gravity is so big, that has the effect of stretching and squashing Io.
Now, imagine it was a squash ball.
If you stretch and squash and stretch and squash, then it gets hot by friction and the same thing happens to this moon.
The power of the gravitational interaction between Jupiter and Io is extraordinary.
It contorts the shape of this tiny moon, moving rock as if it were nothing more than water.
Now, this crater is about, what, 30 metres from the base that you can see down there, up to the edge of the rim.
Now, Io, when it orbits around Jupiter every 1.
8 days, flexes by something like 100 metres.
That's three times the height of that crater.
Remember, lo's surface is pretty much like this, solid rock.
So imagine how much energy that takes, and all that energy comes from Jupiter's gravitational field and that is the energy that powers the volcanoes.
Io is a world beyond our imagination.
Its unique gravitational connections provide a seemingly inexhaustible supply of heat.
As well as its huge lava lakes, the heat also powers the largest volcanic eruptions in the solar system.
(EXPLODING) Molten rock and gas blasts out from the frigid surface, the gas expands, shattering lava into a giant fountain of fine particles.
With weak gravity and a sparse atmosphere, lo's volcanic plumes can reach 500 kilometres above the moon's surface.
(BUBBLING) Now, this incredible phenomena, volcanism, comes from the simplest of laws of physics, the law that says that heat contained in a planet must, eventually, find a way to escape into the coldness of space.
But what a spectacular way for the laws of physics to play out.
In the most unexpected of places, in the coldest reaches of the solar system, the laws of physics created a fiery world of wonder.
And Io is not alone.
Many of the hundreds of moons in the solar system are not dead, barren and uninteresting worlds, but active, often violent, and always beautiful worlds of wonder.
Io is fascinating.
It doesn't derive its energy from an internal heat source in the same way that the Earth does.
It extracts energy from its orbit around its giant parent planet Jupiter.
And, for all those reasons, Io is a wonder of the solar system.
(RUMBLING) Our exploration of the planets and moons orbiting our star has given us valuable insights into the nature of our own world.
Our view of the Earth's place in space has been turned on its head.
Out there are many truly violent and hostile worlds.
But they're driven by the same laws that shape and control our own world.
And so, I suppose, it's in many ways a miracle that we exist at all.
Our solar system is like a cosmic laboratory.
Until we went there, we had no idea of what the laws of nature could produce.
I think one of the most important lessons that our exploration of the solar system has taught us is that the laws of nature can create vastly different worlds with the tiniest of changes.
We now see how the life and death of planets and moons is governed by interconnections which span the solar system.
And we wouldn't be here if it wasn't for those connections.
We don't live on a planet isolated from the rest of the solar system, from the rest of the universe.
We live in a place that is intimately connected with everywhere else.
We have vast oceans, incredible weather, (THUNDER RUMBLING) 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.
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.
Now, the laws of physics are simple and they're universal.
What applies here, applies everywhere else up there.
But, it's fascinating that they can manifest themselves in so many different ways across the solar system.
(NARRATING) In this programme, I'm going to look at how the universal forces of nature that created all this can also wreak devastation across the solar system.
How they can be the death of a planet and how they can keep other worlds alive.
I can just see pieces of molten rock rising up just below my chin.
These forces are so far reaching, they bridge the depths of space and transform a world long thought dead into a world of perpetual change and everlasting youth.
The intense source of heat that powers that eternal change, also drives one of the most spectacular sights in the solar system.
I've come to one of Earth's natural wonders, the Grand Canyon in Arizona.
It's a place to be humbled by nature.
Oh, this is undoubtedly one of the most beautiful places I've seen on Earth.
From sunrise to sunset, the changing light brings this immense landscape to life.
I hear stories of people coming here at sunrise with tears in their eyes at the majesty of the view, and I can see why.
It's incredible to think that this enormous valley was etched and carved by the action of running water over just a few million years.
Our knowledge of natural wonders like the Grand Canyon was once limited to our own planet.
But the Space Age has brought new worlds into view.
This is Mars, the Red Planet.
Fourth rock out from the Sun, it has a canyon so vast you could fit our own Grand Canyon into one of its side channels.
Named after the space probe that first saw it, this is the Valles Marineris.
Eight kilometres deep and over three thousand kilometres long, on Earth it would run all the way from Los Angeles to New York.
We're beginning to get a deep and quite profound understanding of the way that Mars has evolved geologically because we are now there, because we now have eyes and ears on the surface.
And there really is no substitute for actual exploration, for actually going somewhere and touching it and taking pictures of it.
Look at this picture.
This is a picture of a sunset.
These are pictures of clouds.
It's amazing to think these incredibly familiar looking photographs were taken on the surface of Mars.
This picture, or this amazing colour picture.
I could have got a camera here and just snapped any picture and it would have looked exactly like this one.
In the Grand Canyon, you can see the Colorado River running in the bottom of the valley.
So you can understand how this landscape was made.
Whereas, here on Mars, there's no sign of any water.
So Mars is a is a puzzling place.
Despite all the similarities between Mars and Earth, it's the differences between these two planets that are most telling.
Mars is now a desolate, dead wasteland.
A world where the processes that sculpted its familiar landscapes seized up long ago.
It's fascinating for me as a physicist to see how the same basic, simple laws of nature can play out in such radically different ways and produce such astonishingly varied, and beautiful, and violent, and dead worlds out there across the solar system.
(MEN TALKING OVER HELICOPTER RADIO) The Big Island of Hawaii, in the middle of the Pacific Ocean, holds the key to what happened to Mars.
This is the perfect place to witness how a planet can be kept alive by nothing more than the simple flow of heat.
You can smell the volcanic ash coming in to the helicopter.
And everywhere you look, it looks like a (LAUGHING) It's almost like an apocalyptic scene.
This volcano is Kilauea, which means "spewing".
It's been erupting almost continuously since 1983.
In fact, you can see molten rock flowing down the side of the mountain just cutting a sway through the trees.
This might look like widespread destruction, but volcanic eruptions are Earth's geological heartbeat.
Active volcanoes make our planet a vibrant, living world.
That is the most spectacular demonstration of our planet being geologically alive that I've ever seen.
A few kilometres north of Kilauea, you can see just what volcanic action can produce given enough time.
The islands of Hawaii were built entirely by volcanoes.
Today, these mountains are the largest volcanoes on our planet.
And we've seen landscapes just like this on Mars.
It's quite an experience being four kilometres high.
Makes you out of breath, you sniff a lot because your nose becomes (SNIFFLING) And makes you feel like you've had a few drinks.
Well, this is Mauna Kea, one of the five volcanoes that make up the Big Island of Hawaii.
I've known about it since I was very little because it's one of the most famous observatories on the planet.
Everywhere you look, you're surrounded by our eyes to the cosmos.
Now, though this mountain is four kilometres above the surface of the Pacific, it's actually 10 kilometres above the surface of the Pacific floor.
That makes it the highest mountain on Earth, over a kilometre higher than Mount Everest.
But it is a tiny volcano compared to the biggest volcano on the surface of Mars.
This is Olympus Mons, named after Mount Olympus, the mythical home of the Greek gods.
This vast outpouring of lava stretches over 550 kilometres across.
But it's the height of this volcano that is breathtaking.
It soars 25 kilometres into the Martian sky, nearly three times the full height of Mauna Kea.
But Olympus Mons isn't just the tallest volcano in the solar system, it's the highest mountain we know.
There are striking similarities between the volcanic landscapes here on Hawaii and the giant volcanoes found on Mars.
These similarities can be traced back billions of years to the fiery birth of the solar system.
All this heat you can see driving this spectacular volcanic activity is a relic, a hangover of the Earth's formation.
Now, all the rocky planets, Earth, Mars, Venus, Mercury, were formed in the same way.
They came from a collapsing dust cloud over four and a half billion years ago.
(EXPLOSION) With the ignition of the Sun, our solar system was born.
Little by little, the rocky bodies grew, falling together under their own gravity.
This process not only generated immense amounts of heat, it also delivered radioactive material to the cores.
These two ancient sources of heat power Earth's volcanoes to this day.
But the volcanoes on Mars are little more than a petrified memory of a distant past.
When we look down on the surface of Mars, we see no evidence of this kind of volcanic activity.
As far as we can tell, Mars is a dead world.
Mars must have had similar inner heat to Earth to build its volcanoes.
Yet something obviously happened to stop the Red Planet's geological heartbeat.
Now, as everyone who's left a hot cup of tea sat on the kitchen table knows, hot things lose heat to their cooler surroundings.
And what's true for cups of tea, in physics, is also true for planets.
This is hot, and that up there, space, is cold.
And so, planets lose heat to space.
And Mars is a much smaller planet than the Earth.
It's about, well, half the diameter, it's an eighth of the volume, so there's much less heat trapped in there to begin with.
Now, planets lose heat to space through their surfaces.
And smaller things have a larger surface area in relation to their volume than big things.
So that means that Mars will lose its heat to space much quicker than the Earth does.
When the interior of Mars grew cold, the mighty volcanoes lost their lifeblood.
The Red Planet's geological heart died and, millions of years ago, the surface of Mars ground to a halt.
The fate of a whole planet was destined by the simplest of laws of physics.
Since the dawn of human history, we've been able to gaze up into the night sky, but we're lucky because we're the first generation that's been able to build machines to actually go to those planets and moons.
And we found that they're more beautiful, more violent, more magnificent and fascinating than we could've possibly imagined.
The more worlds we study, the more we realise that our solar system is a cosmic laboratory.
Even the slightest differences in size or position can create a world radically different from its neighbours.
So, here on Earth, we have one beautiful manifestation of how those simple laws of nature can play out, how they can build a planet.
In Mars, we have another example.
What happens when you take a planet that's smaller than Earth and move it further away from the Sun? Well, it loses its heat more quickly and it becomes geologically inactive.
But what would happen if we took a planet just like the Earth and moved it a little closer to the Sun? Well, we know of such a planet.
It's the brightest point of light in our night sky.
So similar in size to our own world, this planet has been called Earth's twin.
This is Venus.
Orbiting closer to the Sun, Venus was named for its shining beauty.
But our planetary twin hides its true identity beneath a thick blanket of cloud.
Over the last 4.
5 billion years, Venus has turned into an unimaginably oppressive world.
The atmosphere is so dense that the pressure is crushing.
It's 90 times atmospheric pressure here on Earth.
Now, Venus takes 243 days to rotate once on its axis, that means its day is longer than its year.
Venus has the hottest average surface temperature, other than Sun's, anywhere in the solar system.
470 Celsius.
I've come to India to a place called the Deccan Traps.
Hidden in this lush green landscape are tantalising clues to understanding how immense heat caused Venus to choke to death.
You know, when you look this landscape today, it's incredibly peaceful and beautiful, rolling green hills.
But, I think it's astonishing to think that everything you see down there is lava.
This whole landscape, half a million square kilometres of it, was created in an ongoing volcanic eruption that lasted for a million years.
If you take away the green foliage, the underlying landscape of lava is actually very similar to what we see on Venus.
Using radar to peer down through the clouds, the surface of Venus was finally revealed.
It is covered with floods of solid lava, just like we see in India, but on a scale many thousands of times larger.
We've also counted over 50,000 volcanoes, the most on any planet in the solar system.
Venus is a similar size to Earth, so it may still have a hot geological heart powering its volcanoes.
But, as yet, we haven't witnessed any eruptions.
The ancient floods of lava we see on Venus and here in India were created in much the same way.
For both planets, this was volcanic activity in overdrive.
The eruptions here in India 65 million years ago affected the Earth's climate so much that they've thought to have played a major role in the mass extinction event at the end of the Cretaceous Period which wiped out over two-thirds of the species on Earth.
Now, life on Earth recovered, but Venus wasn't so lucky.
The intense volcanic activity on both planets didn't just blast out molten rock.
It also released copious amounts of gases, like carbon dioxide.
But slight differences in the way the laws of physics played out on Venus helped push our cosmic twin down a path of no return.
Venus and Earth reacted very differently to the same kind of volcanic cataclysm.
And the reason for that is something that happens so often on Earth that we take it for granted, and just moan about it.
(THUNDER RUMBLING) Rain plays a significant role in keeping our planet a pleasant place to live.
Acting as part of a global recycling system, rain keeps our atmosphere in balance, washing out the potent greenhouse gas, carbon dioxide, ready to be locked away in rocks in our oceans.
But on Venus, the laws of physics have made it impossible for rainfall to cleanse its atmosphere.
In fact, there's no liquid water at all.
Venus lost its water essentially because it's hotter than the Earth.
The temperature is just a measure of how fast things are moving around.
So on Venus, the oceans would have evaporated into the atmosphere, and that water in the atmosphere would be moving around extremely quickly simply because it's hot.
And Venus is so close to the Sun and so hot, those water molecules are moving so fast that the gravity of the planet can't continue to hold them in the atmosphere.
And so they simply escape off into space.
With no water, there is no rain on Venus.
For billions of years there has been nothing to temper the build-up of volcanic gases in its atmosphere.
Venus ended up cocooned in a thick, high-pressured dense blanket of carbon dioxide.
And that made the temperature rise, rise and rise.
Turning Venus into the hell-like world we see today.
Compared to scorched Venus and frozen Mars, our home is a very special ball of rock.
Although governed by the same universal set of rules, our planet is not too big, not too small, not too hot, not too cold.
Earth has been called the Goldilocks planet because everything is just right.
Our world is unique but it doesn't exist in splendid isolation.
It is intimately connected with its cosmic neighbours.
Earth is not the master of its own destiny, and it never has been.
The life and death of our planet is influenced by forces emanating from the very depths of space.
As we better understand our place in space, we've come to realise that our sharing of the same physical laws with the other worlds in the solar system isn't the only connection we have with them.
Because those same laws lead to a direct physical connection between the Earth and the other worlds out there that is subtle, is complicated, but can sometimes be extremely powerful.
Out in the farthest reaches of the solar system are vast worlds that have a direct impact on our very existence.
This is our Sun from 10 billion kilometres away, just another star in a sea of stars.
But as you head towards the light, you enter a realm of giants.
The furthest planet from the Sun is icy Neptune, with its thick, blue methane atmosphere.
Uranus comes next, the only planet that lies on its back.
Further in towards the Sun, and the planets get even bigger.
Saturn, with its beautiful rings of ice.
Finally, we reach the king of the giants, Jupiter.
Jupiter is the largest planet in the solar system, so big you could fit Earth inside it over 1,000 times.
It's made up of the same stuff as our Sun, hydrogen and helium, the most common elements in the universe.
In its thick churning atmosphere, gigantic storms have raged for centuries.
Now, astrologers have said for years that Jupiter influences our lives.
But we now have scientific evidence that this mighty planet does have a significant connection with our own small world.
Now, Jupiter is so different to our planet, you know, a big ball of gas half a billion kilometres away.
It's difficult to see how it could have anything to do with us all.
But despite the fact that astrology is a load of rubbish, Jupiter can, in fact, have a profound influence on our planet.
And it's through a force that, well, surrounds us and penetrates us and binds the galaxy together, gravity.
Gravity is one of the fundamental forces of nature.
It exists between all objects.
And the effects of a gravitational field extend way beyond the planet that creates it.
(WIND HOWLING) Gravity is by far the weakest force of nature.
I mean, I can pick this rock up off the ground and this entire planet, Planet Earth, will try to pull it down.
But it's the only force that has the influence across the entire solar system.
And that's because, although it's weak, it has an infinite range, it never quite goes away.
So, no matter how far you go, you feel the force of gravity of a planet, although it drops and drops and drops away.
Jupiter has the most powerful gravitational field of all the planets.
And it's the gas giant's gravity that can directly influence the orbits of asteroids and other wandering space debris.
Jupiter is so massive, I mean, by far the most massive planet in the solar system, that its gravitational field can have a profound influence on passing interplanetary stuff.
It can do three things.
Firstly, it can capture the stuff, literally, hoover it up.
Secondly, it can deflect the stuff as such that it throws it out of the solar system.
But thirdly, and most importantly for us, it can deflect stuff onto a direct collision course with our planet.
Influenced by Jupiter's gravity, today, right now, we're highly vulnerable from asteroid impacts.
But we do have sentinels standing guard.
On top of the mountain of Haleakala on the Hawaiian honeymoon island of Maui, I've come to see Professor Nick Kaiser, who's searching our solar system for potentially hostile space debris.
The prime task is to try and find killer asteroids, things that are out there in the solar system that might hit the Earth.
There's an air of Hollywood about it, isn't there, in some sense? Well, that's right.
I would say a lot more resources have been spent on making movies about killer asteroids than actually finding them.
Anything that's a kilometre in size, if it hit the Earth it would be devastating, right? It would probably kill nearly everyone on the planet.
COX: Each night, using a revolutionary billion-pixel digital sensor, the team scans a vast swathe of the sky.
NICK: Mind your head.
They're looking for any unidentified objects that might be heading our way.
So, any one of these points of light could, in fact, be an asteroid.
That's right.
And it's very likely, in fact it's almost certain, that there are asteroids in that image.
Right, the problem is how do you figure out which ones they are.
Yeah.
The camera captures several images of the same patch of sky taken minutes apart.
The team can then see if anything has moved relative to the background of stars.
What we've done here is taken two images and subtracted them, and you see the stars have nearly all gone away, right? But there's a couple of interesting things left.
If you look over here, you see a dark thing and a white thing.
So that's something which was there in the first image - and there in the second image.
- Yeah.
NICK: And there's another one here.
So, even on this tiny patch of sky we've already detected two objects.
What that means is, when we do that kind of analysis on a whole field of view, we'll detect hundreds of objects.
COX: The beauty of our night sky belies the potential danger it holds in store.
Over 2,000 objects have been identified that pass close to the Earth, with something like 400 that could be on a future collision course.
And all of these menacing lumps of rock, at some point, come under Jupiter's gravitational influence.
Now, if you ever needed a demonstration of how congested the space is near the Earth, just look at this movie of the near-Earth objects.
So here's Mercury, Venus, the Earth, Mars, and out here is Jupiter.
Here's the asteroid belt.
But look at the congestion in there.
Every one of those points of light is an asteroid that we know of.
Just look at the Earth swimming through them.
So, when you look up into the nice, clear night sky and you want to be reassured that we're all nice and safe, (CHUCKLING) just remember this movie.
Our planet is on a deadly journey.
Earth is trapped in a cosmic game of dodge ball as it orbits the Sun.
A game where the gravitational stranglehold of Jupiter regularly throws asteroids our way.
Jupiter's gravitational influence on passing space debris has made our planet a world under constant bombardment.
One of the most famous meteorite impact sites is the Barringer Crater in Arizona.
COX: Fifty thousand years ago, a 300,000 tonne, 50 metre in diameter lump of iron and nickel entered the Earth's atmosphere and made this crater.
And it should remind us that our environment doesn't just stop at the top of our atmosphere.
Our environment stretches out into the solar system to the very edges of the solar system to wherever this rock came from.
In the grand scheme of things, the asteroid that struck here was relatively small and innocuous.
But there are much larger impact craters hidden in Earth's landscapes that have far more devastating tales to tell.
Two hundred years ago, white settlers crossed the ancient Appalachian Mountains here in America, to seek new land out west.
Little did they know what they were walking into.
Well, this place to me feels like the very definition of small town America.
It's on the border between Virginia, Kentucky and Tennessee.
And it's the kind of town where you feel that nothing much has changed for the last 100 years.
But this place was the site, way back in history, of a violent cosmic intervention.
This is Middlesboro, Kentucky.
It's a town built inside a meteorite impact crater.
The asteroid that struck here would have been huge, around half a kilometre across, hitting the Earth well over 200 million years ago.
Yeah, I find it fascinating that when you look out at this view you don't really see an obvious crater, and, indeed, it wasn't until the 1960s that anybody had any idea that there was a colossal impact from space just over there, centred right on the 18th hole of the golf course.
(THUNDER RUMBLING) We now know where the giant asteroid that struck here could have come from.
Located between Jupiter and Mars is a vast reservoir of rocky debris that forms the asteroid belt.
And it's this ancient rubble that Jupiter, our neighbourhood giant, can nudge towards the Earth.
Well, here's my model of the solar system.
There's the Sun in the middle.
Then the Earth, Mars, Jupiter.
And then the asteroids sort of scattered in between, a big region, actually, between Mars and Jupiter.
In fact they extend over 150 million miles, which is further than the distance from the Earth to the Sun.
Now This is my coffee by the way, which doesn't represent anything.
I'll put it over there.
Now and again, because of collisions in the asteroid belt, a stray asteroid will get thrown into the position where they keep rhythmically meeting Jupiter over and over again.
And because Jupiter's such a massive planet, that means that it gets a kick, it gets a gravitational kick.
And that changes the orbit of these asteroids.
And, over time, their orbit can become, well, elongated or elliptical rather than circular.
That means that they can get thrown in to the inner solar system and cross the orbits of the inner planets, including the orbit of the Earth.
And you get a potentially catastrophic collision.
Jupiter was once thought to be our protector, its enormous gravity swallowing up dangerous asteroids.
Yet we now realise its gravitational influence can propel some of those asteroids in our direction.
But, surprisingly, catastrophic impacts with space debris might not be a bad thing, at least in Earth's past.
Impacts from space shaped our planet, they made our world what it is today.
Take life on Earth, for example.
Now, it's possible or probable even that impacts on a colossal scale changed the climate so much that huge swathes of life on Earth were wiped out, creating ecological niches into which other species could evolve.
Us, for example.
It's incredible to think that a planet half a billion kilometres away could dictate the fate of our world.
Jupiter's immense gravity bridges the depths of space.
And even though its power could one day devastate Earth, that same gravitational field breathes life into other corners of the solar system.
For better or worse, Jupiter's powerful gravitational field has had a direct influence on our planet.
Now, we're part of a much wider ecosystem that extends to the very edges of the solar system.
And that ecosystem is bound together by the force of gravity.
And it's gravity that has power to bring worlds to life.
Our understanding of the solar system began much closer to home.
Gazing down at us, it was our moon, with its regularly changing face, that first fired our interest in worlds beyond our own.
When we could look further out, we discovered the solar system was full of moons, each invisibly connected to their parent planet by gravity.
Our Moon is a cold, geologically dead world.
But the powerful gravitational bond that exists between another moon and its parent planet has done something astonishing.
It has brought this moon to life, making it the most violent place in the solar system.
Four hundred years ago, it was Galileo who first looked up at the night sky through a telescope.
Turning his attention to Jupiter, he noticed that this giant planet was not alone.
Oh, yeah.
That's absolutely magnificent.
You see a disc surrounded by Well, I can see three points of light.
Galileo, over several nights, saw four points of light.
And he correctly surmised that those are actually moons, other worlds in orbit around Jupiter.
Jupiter's four largest moons are named after the four lovers of the Greek god Zeus.
Furthest out, is Callisto.
Then there's huge Ganymede, the largest moon in the solar system.
Next is icy Europa, and, finally, the small yellow-tinged moon nearest to Jupiter, Io.
So the legend goes, Zeus tried to protect his lover Io by turning her into a cow to hide her from the gaze of, the jealous gaze of his wife, Hera, but it didn't work, and Hera sent a gadfly to torment Io.
And it was prescient in a way to name that satellite Io the tormented moon, because we've since learned that it is indeed an incredibly tormented world.
For 400 years, we expected Io to be as dead as our own moon.
But in the late 1970s, when the first spacecraft passed by Jupiter, we finally saw Io up close.
But it didn't make any sense, as here was a moon with no meteorite impact craters.
Now, it is impossible to believe that Io could've escaped the bombardment that we see on our moon and practically every other body in the solar system.
So, the only explanation is, is that surface is young, it must have been recently produced.
And that, in turn, means that Io, that tiny moon of Jupiter out there, must be a geologically active world.
The truth about Io would blow us away.
We may not have stood on Io, but there are places we can go here on Earth to help unlock its secrets.
This is Ethiopia in east Africa.
We're being flown out by military helicopter to the very hot, very inhospitable Afar region in the northeast of the country.
And this is what I've come to see.
It's one of the rarest geological phenomena on our planet.
A volcano with a lake of molten lava.
As the Sun goes down, the lava lake comes to life.
This volcano is called Erta Ale by the local Afar people.
It means "smoking mountain".
For many, this place is a vision of hell.
Yet it holds the key to understanding Io, a world over half a billion kilometres away.
This hot, tortured landscape of lava will be our home for the next three days.
Temperatures here reach 50 Celsius in the shade, and we're camping right on top of the volcano.
But we're in good hands, as Io specialist Dr Ashley Davies is part of the team.
It is absolutely spectacular.
DAVIES: It is extraordinarily beautiful.
COX: You feel like you're looking into the core of the planet.
Yeah, it's a window into the interior of the Earth.
So, magma's rising up from some kilometres down, circulating through the surface, and then sinking back down again.
It's very difficult to breathe, it's very acidic, very bitter.
The magma has gases in it.
As it comes up to the surface, just like when you pour out a bottle of Coca Cola, the gases come out.
And what we have here is sulphur dioxide, hydrogen sulphide, water vapour, carbon dioxide, coming out of the magma before the magma then cools and sinks down, and that's what we're breathing now, it's kind of unpleasant.
This volcanic phenomenon is a product of immense amounts of primordial heat escaping from inside our planet.
Yet we have seen something similar in the far reaches of the solar system.
Io is the same size as our Moon and should be a cold, dead world.
Yet our first glimpses of Io revealed it as seething with heat, alive with volcanic activity.
Just one of the many lava lakes on Io releases more heat than all Earth's volcanoes put together.
If we were to stand on the surface of Io now, what would be the similarities and what would be the differences? The lake will probably appear very, very similar to this, except for the scale.
The lava lakes in Io are vastly larger.
The biggest one we think is a180 kilometres in diameter.
COX: 180 kilometres? So that would stretch, way, obviously, way beyond the horizon on Earth.
Yeah, it's almost beyond description to see something that size and it's just this huge pool of molten lava.
COX: Io is the most volcanic place in the solar system, endlessly pumping out heat into the cold vacuum of space.
But what's really interesting is that it's so small that it shouldn't be volcanic at all.
I mean, it was one of the greatest surprises of planetary science when these massive volcanoes were discovered on Io.
Mighty planets, like Mars, have big volcanoes, they're not erupting any more.
They haven't erupted for a long time.
Venus, lots of volcanoes there, but they haven't been erupting for a long time, probably.
And here we have Io, which is just insanely volcanic.
It's just pumping out vast amounts of energy in a zone, in a part of the solar system, where it was thought that everything was dead.
COX: Everything we now know about Io comes from looking at it from a distance.
But measuring the heat pumping out of this lava lake will give us a better idea of the true scale of the heat loss on Io.
Far from being a benign, bubbling cauldron, this volcano has the power to kick off at a moment's notice.
(RUMBLING) I can just see pieces of molten rock rising up just below my chin.
And with it a cloud of heat, absolutely overpowering heat.
There must be a (COUGHING) Hell of an eruption coming on.
Seeing active volcanism like this on such a small moon like Io, changed our view of the workings of the solar system.
A world like Io having such a powerful internal heat source, cries out for an explanation.
See, Io is far too small a world to have retained any internal heat, to have the same heat source that powers the Earth's volcanoes.
So something else must be driving that powerful volcanism on Io.
New images sent back from recent space probes confirm that Io is a surprising and bizarre world.
Being so far from the Sun, lo's surface is mostly very cold.
It's covered in frozen sulphur, which gives it its yellow colour.
Yet Io is pockmarked by cauldrons of molten lava.
You know, I think it is remarkable and fortunate, in a sense, that you can come to a place like this on our planet and just get the tiniest sense of what it must be like to stand on the edge of one of those magnificent lava lakes on Io.
When we first saw hot volcanic action on Io, it was quite a surprise for most planetary scientists.
But by considering simple laws of physics, it didn't surprise everyone.
Just weeks before Voyager arrived at Jupiter, three scientists made a prediction.
And it was one of those predictions that, when you see it, is almost obvious.
It was using physics that had been known for hundreds of years but nobody had thought of it.
They predicted that Io should have an intense internal heat source because of its unique position in the solar system, very close to a giant planet and surrounded by other large moons.
Io sits about the same distance from Jupiter as our own Moon does from Earth.
But don't forget that orbiting outside Io are its sister moons, Europa and Ganymede.
Io is under the influence not just of the massive gravitational pull of Jupiter, but also the additional pull of its neighbouring moons.
It's this gravitational tug of war that conspires to breathe life into Io.
Now, Io has a very interesting relationship with Europa and Ganymede because for every four orbits that Io makes around the planet, Europa goes around almost exactly twice and Ganymede goes around just once.
Periodically, they line up together, bang, bang, bang.
And Io gets a powerful gravitational kick on a very regular basis.
And that has the effect of moving Io out of a nice circular orbit into an elliptical, or an eccentric orbit.
Io comes close to Jupiter and then far away from Jupiter and then close to Jupiter again.
And because Jupiter's gravity is so big, that has the effect of stretching and squashing Io.
Now, imagine it was a squash ball.
If you stretch and squash and stretch and squash, then it gets hot by friction and the same thing happens to this moon.
The power of the gravitational interaction between Jupiter and Io is extraordinary.
It contorts the shape of this tiny moon, moving rock as if it were nothing more than water.
Now, this crater is about, what, 30 metres from the base that you can see down there, up to the edge of the rim.
Now, Io, when it orbits around Jupiter every 1.
8 days, flexes by something like 100 metres.
That's three times the height of that crater.
Remember, lo's surface is pretty much like this, solid rock.
So imagine how much energy that takes, and all that energy comes from Jupiter's gravitational field and that is the energy that powers the volcanoes.
Io is a world beyond our imagination.
Its unique gravitational connections provide a seemingly inexhaustible supply of heat.
As well as its huge lava lakes, the heat also powers the largest volcanic eruptions in the solar system.
(EXPLODING) Molten rock and gas blasts out from the frigid surface, the gas expands, shattering lava into a giant fountain of fine particles.
With weak gravity and a sparse atmosphere, lo's volcanic plumes can reach 500 kilometres above the moon's surface.
(BUBBLING) Now, this incredible phenomena, volcanism, comes from the simplest of laws of physics, the law that says that heat contained in a planet must, eventually, find a way to escape into the coldness of space.
But what a spectacular way for the laws of physics to play out.
In the most unexpected of places, in the coldest reaches of the solar system, the laws of physics created a fiery world of wonder.
And Io is not alone.
Many of the hundreds of moons in the solar system are not dead, barren and uninteresting worlds, but active, often violent, and always beautiful worlds of wonder.
Io is fascinating.
It doesn't derive its energy from an internal heat source in the same way that the Earth does.
It extracts energy from its orbit around its giant parent planet Jupiter.
And, for all those reasons, Io is a wonder of the solar system.
(RUMBLING) Our exploration of the planets and moons orbiting our star has given us valuable insights into the nature of our own world.
Our view of the Earth's place in space has been turned on its head.
Out there are many truly violent and hostile worlds.
But they're driven by the same laws that shape and control our own world.
And so, I suppose, it's in many ways a miracle that we exist at all.
Our solar system is like a cosmic laboratory.
Until we went there, we had no idea of what the laws of nature could produce.
I think one of the most important lessons that our exploration of the solar system has taught us is that the laws of nature can create vastly different worlds with the tiniest of changes.
We now see how the life and death of planets and moons is governed by interconnections which span the solar system.
And we wouldn't be here if it wasn't for those connections.
We don't live on a planet isolated from the rest of the solar system, from the rest of the universe.
We live in a place that is intimately connected with everywhere else.