0:00:08 > 0:00:10We live on a world of wonders.
0:00:12 > 0:00:17A place of astonishing beauty and complexity.
0:00:17 > 0:00:19We have vast oceans
0:00:19 > 0:00:22and incredible weather.
0:00:23 > 0:00:27Giant mountains and spectacular landscapes.
0:00:27 > 0:00:30If you think that this is all there is,
0:00:30 > 0:00:33that our planet exists in magnificent isolation,
0:00:33 > 0:00:34then you're wrong.
0:00:36 > 0:00:38We're part of a much wider eco-system,
0:00:38 > 0:00:42that extends way beyond the top of our atmosphere.
0:00:45 > 0:00:48I think we are living through
0:00:48 > 0:00:52the greatest age of discovery our civilisation has ever known.
0:00:52 > 0:00:55We've voyaged to the farthest reaches of the solar system,
0:00:55 > 0:00:59photographed strange new worlds,
0:00:59 > 0:01:01stood in unfamiliar landscapes,
0:01:01 > 0:01:04tasted alien air.
0:01:09 > 0:01:11Amongst all these wonders sits our Earth -
0:01:11 > 0:01:16an oasis of calm amidst the violence of the solar system.
0:01:18 > 0:01:19And all that separates us
0:01:19 > 0:01:26from what's out there is a thin, flimsy envelope of gas -
0:01:26 > 0:01:27our atmosphere.
0:01:29 > 0:01:31And it's thanks to this "thin blue line"
0:01:31 > 0:01:34that we have the air that we breathe,
0:01:34 > 0:01:36the water that we drink
0:01:36 > 0:01:39and the landscape that surrounds us.
0:01:41 > 0:01:44Atmospheres define all the planets in the solar system.
0:01:47 > 0:01:52They have the power to create dynamic worlds that are alien and chaotic.
0:01:54 > 0:01:58But, remarkably, in the frozen wastes of the solar system...
0:02:00 > 0:02:06..one atmosphere has created the most unexpected wonder -
0:02:06 > 0:02:09a moon that looks a lot like home.
0:02:44 > 0:02:46I've come to Cape Town in South Africa
0:02:46 > 0:02:49to do something that I have always wanted to do,
0:02:49 > 0:02:53but never thought I would get the chance.
0:02:58 > 0:03:01I'm about to fly incredibly high,
0:03:01 > 0:03:04to the very edge of the Earth's atmosphere.
0:03:08 > 0:03:13From here, I am hoping to see something that only a handful of people have ever seen -
0:03:13 > 0:03:16the thin blue line,
0:03:16 > 0:03:22the fragile strip of gas that surrounds our whole planet.
0:03:22 > 0:03:25And this is what's going to take me there.
0:03:30 > 0:03:33This is an English Electric Lightning,
0:03:33 > 0:03:36the most beautiful fighter aircraft ever built.
0:03:39 > 0:03:42This is when England built the best aircraft in the world.
0:03:45 > 0:03:47The Lightning is no longer in service,
0:03:47 > 0:03:53but this piece of magnificently overpowered engineering
0:03:53 > 0:03:56is going to take me 18 kilometres, straight up.
0:03:57 > 0:04:00Actually, I read somewhere that when you read about the altitude
0:04:00 > 0:04:04of the Lightning, it says "Altitude: Estimated, 60,000 feet.
0:04:04 > 0:04:07"Ceiling: Classified."
0:04:07 > 0:04:09So I don't know how high these can go.
0:04:09 > 0:04:14I have heard rumours they can go to 80,000 feet, which is amazing.
0:04:17 > 0:04:23My journey will take me beyond almost all the molecules of gas that make up our atmosphere.
0:04:23 > 0:04:29- If you feel you're going to get sick...- Yeah?- ..use a bag, OK? - Right. Hopefully not.
0:04:36 > 0:04:38To get there, I'm going to experience
0:04:38 > 0:04:41what made the Lightning famous -
0:04:41 > 0:04:43a vertical take-off.
0:04:55 > 0:04:56Whoo-hoo!
0:05:14 > 0:05:19'It takes just seconds to reach nine kilometres up,
0:05:19 > 0:05:21'but I'm still in the thickest layer of the atmosphere,
0:05:21 > 0:05:23'called the troposphere.
0:05:24 > 0:05:28'But the further I climb, the thinner the atmosphere becomes.'
0:05:34 > 0:05:41Up at 58,000 feet. 90% of the atmosphere is below me.
0:05:42 > 0:05:45The only people above me are on the space station.
0:05:50 > 0:05:52So beautiful.
0:05:58 > 0:06:00I'm now at 60,000 feet.
0:06:00 > 0:06:03'18 kilometres up.
0:06:03 > 0:06:06'And the highest I can go.'
0:06:10 > 0:06:15Above me, the sky is a deep, dark blue.
0:06:22 > 0:06:25'And that is what I've come to see - '
0:06:25 > 0:06:26our atmosphere.
0:06:33 > 0:06:37That really is the thin blue line that protects us.
0:06:39 > 0:06:42So...fragile and so tenuous.
0:06:44 > 0:06:47Just a tiny sliver of blue.
0:06:51 > 0:06:52Amazing.
0:07:02 > 0:07:06Between 55 and 60,000 feet, inverted,
0:07:06 > 0:07:09the curvature of the Earth there.
0:07:09 > 0:07:125G, vertical ascent.
0:07:12 > 0:07:15That is just a ride!
0:07:15 > 0:07:17It is remarkable to see that.
0:07:17 > 0:07:18You can see
0:07:18 > 0:07:22the...thinness and fragility,
0:07:22 > 0:07:26you can see the atmosphere going from light blue, to dark blue, to black.
0:07:27 > 0:07:29It really is astonishing.
0:07:34 > 0:07:40The thin blue line makes the Earth the wonderfully diverse place it is.
0:07:44 > 0:07:49It acts as a soothing blanket, that traps the warmth of the sun...
0:07:51 > 0:07:54..yet protects us from the harshness of its radiation.
0:07:59 > 0:08:03Its movements can be traced in the gentlest breeze.
0:08:05 > 0:08:07And the most devastating hurricane.
0:08:13 > 0:08:17The oxygen and water the atmosphere holds
0:08:17 > 0:08:20plays a fundamental role in the ongoing survival
0:08:20 > 0:08:24of millions of different species living on the planet.
0:08:27 > 0:08:30In this film, I want to explain how the laws of physics
0:08:30 > 0:08:33that created our unique atmosphere
0:08:33 > 0:08:38are the same laws that created many diverse and different atmospheres
0:08:38 > 0:08:40across the solar system.
0:08:43 > 0:08:46When perfectly balanced, a world as familiar
0:08:46 > 0:08:50and beautiful as the Earth can evolve beneath the clouds.
0:08:53 > 0:08:58But the slightest changes can lead to alien and violent worlds.
0:09:00 > 0:09:04There are planets in our solar system that have been transformed
0:09:04 > 0:09:09into hellish worlds, by nothing more than the gases in their atmosphere.
0:09:13 > 0:09:16And just as atmospheres can choke a planet to death,
0:09:16 > 0:09:20they are also powerful enough to shape their surfaces.
0:09:26 > 0:09:30And there are worlds out there which are all atmosphere.
0:09:32 > 0:09:36Giant balls of churning gas, where storms three times
0:09:36 > 0:09:40the size of the Earth have raged for hundreds of years.
0:09:55 > 0:09:58All atmospheres in the solar system are unique,
0:09:58 > 0:10:03but the ingredients and forces that shape them are universal.
0:10:05 > 0:10:11At the heart of each is the glue which holds the solar system together,
0:10:11 > 0:10:14a fundamental force of nature -
0:10:14 > 0:10:15gravity.
0:10:17 > 0:10:21Gravity is, by far, the weakest known force in the universe.
0:10:23 > 0:10:28You can see that because it's really easy for me to pick a rock up off the ground,
0:10:28 > 0:10:33even though there's a whole planet, Earth, pulling the rock down.
0:10:33 > 0:10:35I can just lift it up.
0:10:35 > 0:10:37Incredibly weak,
0:10:37 > 0:10:41but incredibly important, because it's the only force there is
0:10:41 > 0:10:43to hold an atmosphere to the planet.
0:10:45 > 0:10:49The more massive the planet, the greater its gravitational force.
0:10:50 > 0:10:56Earth has enough mass to keep a tight grip of the gas molecules that make up our atmosphere.
0:10:57 > 0:11:00It holds them against the surface and allows us to breathe.
0:11:03 > 0:11:05Now, we don't really notice the presence
0:11:05 > 0:11:09of our atmosphere, I suppose, because we live in it, all the time.
0:11:09 > 0:11:11But there's a lot of it.
0:11:11 > 0:11:17There's five million billion tons of air surrounding the Earth.
0:11:17 > 0:11:21That's the equivalent of a weight of one kilogram
0:11:21 > 0:11:25pressing down on every square centimetre of our bodies.
0:11:25 > 0:11:28Or, put it another way, if I'm about a metre square,
0:11:28 > 0:11:32that's ten tons of weight pressing down.
0:11:33 > 0:11:36Now I say pressing down, but that's not entirely right,
0:11:36 > 0:11:38that's not how air pressure works.
0:11:38 > 0:11:43It presses in every direction at once. I can demonstrate that.
0:11:43 > 0:11:45This is a glass full of water,
0:11:45 > 0:11:51so if I put a piece of paper on there, turn it upside down.
0:11:51 > 0:11:53Now, if I'm right, then the air pressure is pushing
0:11:53 > 0:11:59in every direction on this glass of water, the air pressure is pushing up as well as down.
0:12:00 > 0:12:03And it has no problem in holding the water in the glass.
0:12:07 > 0:12:08Cool.
0:12:09 > 0:12:11Where did you get this water from?
0:12:11 > 0:12:13CREW LAUGH
0:12:15 > 0:12:19Life on the surface of this planet survives,
0:12:19 > 0:12:24surrounded by this enormous mass of gas. We're like lobsters,
0:12:24 > 0:12:26scuttling around on the ocean floor.
0:12:33 > 0:12:37But our atmosphere does more than allow us to breathe.
0:12:37 > 0:12:38It protects us
0:12:38 > 0:12:41from the most powerful force in the solar system...
0:12:44 > 0:12:46..our sun.
0:12:50 > 0:12:53If you ask yourself the question, "Why is Earth
0:12:53 > 0:12:55"the temperature that it is?"
0:12:55 > 0:12:57Then, the obvious answer might seem to be,
0:12:57 > 0:13:01"Well, because it's 150m kilometres away from the sun".
0:13:01 > 0:13:04But actually, things aren't quite that simple.
0:13:04 > 0:13:08This is the Namib desert in Namibia, in south-western Africa.
0:13:08 > 0:13:13And as the sun sinks below the horizon, the temperature change,
0:13:13 > 0:13:18from day to night, can be as much as 30 degrees Celsius.
0:13:18 > 0:13:20That's an immense amount in just a few hours,
0:13:20 > 0:13:25much more than in somewhere like Manchester, for example.
0:13:25 > 0:13:28The reason is that this is also one of the driest places on the planet
0:13:28 > 0:13:32and so there is very little water vapour in the atmosphere.
0:13:32 > 0:13:36That means that the atmosphere is not a very good insulator,
0:13:36 > 0:13:42so when the sun disappears, the heat just disappears quickly into space.
0:13:42 > 0:13:44Now, there's a planet in the solar system,
0:13:44 > 0:13:46somewhere over there, near the sun,
0:13:46 > 0:13:49where the temperature shift, from day to night,
0:13:49 > 0:13:53is not a mere 30 degrees Celsius, but an immense amount bigger.
0:14:14 > 0:14:17Roughly 58 million kilometres from the sun
0:14:17 > 0:14:21is the smallest planet in the solar system...
0:14:25 > 0:14:26..Mercury.
0:14:29 > 0:14:31This tortured piece of rock
0:14:31 > 0:14:35suffers the biggest temperature swings of all the planets,
0:14:35 > 0:14:38from 450 degrees Celsius in the day,
0:14:38 > 0:14:43to minus 180 degrees at night.
0:14:43 > 0:14:47And all because Mercury has been stripped naked.
0:14:47 > 0:14:51It has virtually no atmosphere at all.
0:15:03 > 0:15:06Like all the rocky inner planets of the solar system,
0:15:06 > 0:15:09Mercury had an atmosphere when it was formed,
0:15:09 > 0:15:11but it lost it very quickly.
0:15:11 > 0:15:12Here on Earth,
0:15:12 > 0:15:16at sea level, then...
0:15:16 > 0:15:20Well, in a volume about the size of this pebble,
0:15:20 > 0:15:26there are 10 billion billion molecules of gas.
0:15:26 > 0:15:32On Mercury, in the same volume, there would be around a 100,000,
0:15:32 > 0:15:34that's 10 million million times less.
0:15:34 > 0:15:39Now, planets hang on to their atmosphere by the force of gravity.
0:15:39 > 0:15:41It's the only way they can
0:15:41 > 0:15:47stop that thin blue line of gas disappearing off into space.
0:15:47 > 0:15:50So, the bigger the planet, the more massive the planet,
0:15:50 > 0:15:55the stronger the gravitational pull and the easier it is for the planet to keep hold of its atmosphere.
0:15:55 > 0:16:02So, Mercury was just too small and too hot to hang onto its atmosphere
0:16:02 > 0:16:07and the consequences for the planet were absolutely devastating.
0:16:11 > 0:16:15Atmospheres may be just a thin strip of molecules,
0:16:15 > 0:16:18but they are a planet's first line of defence.
0:16:19 > 0:16:23Without them, a planet like Mercury
0:16:23 > 0:16:27is at the mercy of our violent solar system.
0:16:40 > 0:16:46This is Saskatchewan in western Canada and it is a cold place to be in November.
0:16:46 > 0:16:52About a year ago, in November 2008, a piece of asteroid,
0:16:52 > 0:16:54a space rock, weighing about ten tons,
0:16:54 > 0:16:59entered the atmosphere right over here and actually landed
0:16:59 > 0:17:02about 30 kilometres that way, at a place called Buzzard Coulee.
0:17:02 > 0:17:06Now, it's not unusual for rocks that big to hit the Earth.
0:17:06 > 0:17:08On average, that happens about once a month.
0:17:08 > 0:17:13What was unusual about this one was that it was over quite a densely-populated area.
0:17:13 > 0:17:18So tens of thousands, if not hundreds of thousands, of people saw it and heard it.
0:17:18 > 0:17:24But most spectacularly, it was captured by a lot of CCTV cameras,
0:17:24 > 0:17:28including that one, in this garage.
0:17:34 > 0:17:38These are the actual CCTV images captured around the city.
0:17:38 > 0:17:40They show the meteorite,
0:17:40 > 0:17:44as it streaked across the sky at 20 kilometres per second.
0:17:44 > 0:17:49The fireball was brighter than the moon and turned the night sky blue.
0:17:53 > 0:17:56Scientists used these remarkable images
0:17:56 > 0:18:01to triangulate the impact site of the meteorite.
0:18:01 > 0:18:06They traced it to a field, just outside the city of Lloydminster.
0:18:06 > 0:18:12A team of meteorite hunters have been searching the debris left by the enormous explosion.
0:18:14 > 0:18:17They are led by Dr Alan Hildebrand.
0:18:19 > 0:18:20How much energy
0:18:20 > 0:18:23does a rock like this have, then?
0:18:23 > 0:18:27You know, what is it, a ten-ton rock travelling at 50 times the speed of sound?
0:18:27 > 0:18:29You know, it would be like if you'd stocked up,
0:18:29 > 0:18:33say, 400 tons of TNT to explode.
0:18:33 > 0:18:35I mean, it's really quite dramatic.
0:18:35 > 0:18:39- 400 tons that just dissipates away in the Earth's atmosphere?- Yes.
0:18:39 > 0:18:42Atmosphere slowing it down, of course, causing it to break up.
0:18:42 > 0:18:47In just five seconds, it's almost all over and, of course,
0:18:47 > 0:18:50you know it's an extreme friction, makes the light show.
0:18:50 > 0:18:5610% of the energy goes in light and it's like a billion-watt bulb shining high in the sky.
0:18:59 > 0:19:00So, what are we looking for?
0:19:00 > 0:19:03What does a piece of that asteroid look like?
0:19:04 > 0:19:07They... Going through the atmosphere,
0:19:07 > 0:19:10the surface has got melted, so you end up with a dark crust on them.
0:19:10 > 0:19:17So, essentially, you're looking for an oddly-sculpted dark rock.
0:19:17 > 0:19:23- Yeah.- Well, in all fairness, you've got to be able to tell it from, you know the cow patties and so on.
0:19:23 > 0:19:25- But...- I could probably manage that.
0:19:25 > 0:19:28Once you get your eye in, you'll have no trouble.
0:19:44 > 0:19:46We've got one right here.
0:19:47 > 0:19:49I'll pick that up.
0:19:49 > 0:19:52Astonishing.
0:19:52 > 0:19:55It's just been completely rounded off.
0:19:55 > 0:19:57Yeah, the heat melted the surface of the rock.
0:19:57 > 0:20:02- I mean, how hot does something have to be to do that?- Yeah.
0:20:02 > 0:20:056,000 degrees C would do it.
0:20:09 > 0:20:13So, this little rock has had an amazing history.
0:20:13 > 0:20:17I mean, it approached Earth as part of this bigger fragment,
0:20:17 > 0:20:22at about, what, 18, 19, 20 kilometres per second.
0:20:22 > 0:20:25It hit the Earth's atmosphere.
0:20:25 > 0:20:31About 85 kilometres up, it began to feel the effects
0:20:31 > 0:20:33of the Earth's atmosphere.
0:20:33 > 0:20:37It began to squash the air in front of it, creating a pressure wave,
0:20:37 > 0:20:41essentially, which, in turn, causes this thing to heat up.
0:20:41 > 0:20:46And it would have heated up to something like the temperature of the surface of the sun.
0:20:46 > 0:20:51It would have been 5 or 6,000 degrees Celsius as it plummeted through the atmosphere,
0:20:51 > 0:20:56lit up the sky over here and then, quite literally,
0:20:56 > 0:21:02exploded in a series of explosions and peppered these fields with lumps of rock this big.
0:21:02 > 0:21:07Can you imagine standing here on that night and having this, these things -
0:21:07 > 0:21:10and this is heavy, right -
0:21:10 > 0:21:11raining down from the sky?
0:21:11 > 0:21:13It must have been quite incredible.
0:21:16 > 0:21:19If the meteorite had hit the ground intact,
0:21:19 > 0:21:24the explosion would have been been equivalent to 400 tons of TNT
0:21:24 > 0:21:26and left a crater 20 metres wide.
0:21:29 > 0:21:35The Earth was spared this colossal impact by nothing more
0:21:35 > 0:21:38than the tenuous strip of gases that surrounds us.
0:21:38 > 0:21:42But not all planets have this protective blanket.
0:21:47 > 0:21:50When a meteorite hits naked Mercury,
0:21:50 > 0:21:55there is no atmosphere to break it up or slow it down.
0:21:57 > 0:22:02It strikes the ground at full speed and completely intact.
0:22:03 > 0:22:06For the last 4.6 billion years,
0:22:06 > 0:22:12Mercury has been bombarded with countless asteroids and comets.
0:22:14 > 0:22:21The whole history of the planet's violent past is laid out on its surface,
0:22:21 > 0:22:26a world pitted with hundreds of thousands of craters.
0:22:27 > 0:22:32Craters inside craters, inside craters.
0:22:55 > 0:22:57Mercury was damned from the start.
0:22:57 > 0:23:00It's simply too small and too hot
0:23:00 > 0:23:04to have retained any meaningful traces of atmosphere.
0:23:04 > 0:23:06We, on the other hand,
0:23:06 > 0:23:09are big enough and cold enough
0:23:09 > 0:23:12to have retained this envelope of gases.
0:23:16 > 0:23:18That, in turn, allows
0:23:18 > 0:23:21living things, like me, to evolve
0:23:21 > 0:23:25and to use that atmosphere, to breathe and to live.
0:23:25 > 0:23:29But there are places out there in the solar system
0:23:29 > 0:23:33whose atmospheres have the same ingredients as our own,
0:23:33 > 0:23:37but when the formula is even slightly remixed,
0:23:37 > 0:23:41it leads to worlds that couldn't be more different.
0:23:52 > 0:23:55Roughly 108 million kilometres from the sun
0:23:55 > 0:24:00sits the brightest planet in the solar system, Venus.
0:24:01 > 0:24:04This footage shows the luminescent world appear
0:24:04 > 0:24:07from behind our cratered moon.
0:24:10 > 0:24:13Venus and Earth share many similarities.
0:24:13 > 0:24:16We sit next to each other in space,
0:24:16 > 0:24:19we were formed from the same material
0:24:19 > 0:24:25and we're roughly the same size and share a similar mass and gravity.
0:24:26 > 0:24:29But that's where any similarities end.
0:24:29 > 0:24:32Venus is a tortured world,
0:24:32 > 0:24:38where thick clouds of sulphuric acid are driven along by high winds
0:24:38 > 0:24:42and temperatures are hot enough to melt lead.
0:24:42 > 0:24:48All because this planet's atmosphere created a runaway greenhouse effect.
0:24:50 > 0:24:53The "greenhouse effect" has become a well-known phrase.
0:24:53 > 0:24:57You know, it's synonymous with global warming.
0:24:57 > 0:24:59But what is it?
0:24:59 > 0:25:05Well, a planet, like the Earth, absorbs energy from the sun as visible light.
0:25:05 > 0:25:12Now, atmospheres don't absorb much visible light, as you can see, because you can see the sun.
0:25:12 > 0:25:18The ground absorbs the visible light, heats up and then re-radiates it.
0:25:18 > 0:25:23But it re-radiates it as infrared radiation, heat radiation, if you want.
0:25:23 > 0:25:27And atmospheric gases, particularly carbon dioxide, are very good
0:25:27 > 0:25:33at absorbing in the infrared and so they trap the heat and the planet heats up.
0:25:37 > 0:25:41On Earth, greenhouse gases are essential to our survival.
0:25:41 > 0:25:45Without them our planet would be 30 degrees colder,
0:25:45 > 0:25:49too cold to support life as we know it.
0:25:55 > 0:26:01But Venus's atmosphere was flooded with greenhouse gases.
0:26:01 > 0:26:05The nearby sun slowly boiled away its oceans,
0:26:05 > 0:26:08pumping water vapour into the atmosphere.
0:26:09 > 0:26:13And carbon dioxide, from thousands of erupting volcanoes,
0:26:13 > 0:26:16added to the stifling mix.
0:26:17 > 0:26:21Venus grew hotter and hotter.
0:26:21 > 0:26:24The planet was slowly choked to death.
0:26:26 > 0:26:31Venus is a planet with an atmosphere in overdrive,
0:26:31 > 0:26:36but Earth's other rocky neighbour tells quite a different story.
0:27:00 > 0:27:02Get it!
0:27:07 > 0:27:09These are the dunes in the Namib desert.
0:27:09 > 0:27:13It's an absolutely spectacular place.
0:27:20 > 0:27:21This place is not the hottest,
0:27:21 > 0:27:25nor the driest, desert in the world, but these dunes
0:27:25 > 0:27:28are some of the oldest sand dunes in the world.
0:27:32 > 0:27:34And the reason we're here in the Namib desert
0:27:34 > 0:27:37is that this is a great analogue
0:27:37 > 0:27:39for the surface of Mars.
0:27:39 > 0:27:42This is what the surface of Mars looks like and these dunes,
0:27:42 > 0:27:45called barchan dunes, these crescent-shaped dunes,
0:27:45 > 0:27:49are the same as the sand dunes on Mars.
0:27:56 > 0:28:00So, if you want to get a feel for what it would be like on the surface of Mars,
0:28:00 > 0:28:06and you want to know what driving a 4x4 around on it would be like, then this is the place to come.
0:28:11 > 0:28:16Incredibly, there is a vehicle driving across the surface of the "red planet" today...
0:28:21 > 0:28:25..a space rover, named Opportunity.
0:28:30 > 0:28:33The rovers and spacecraft that circle the planet
0:28:33 > 0:28:38have sent back images which reveal Mars in exquisite detail.
0:28:47 > 0:28:51Mars has vast dunes,
0:28:51 > 0:28:56enormous volcanoes
0:28:56 > 0:28:59and giant ice sheets.
0:29:02 > 0:29:06It has canyons and river valleys.
0:29:07 > 0:29:14Mars is a dry, frozen version of our home, covered in red dust and sand.
0:29:14 > 0:29:20And it's all due to the fact that Mars has virtually no atmosphere.
0:29:27 > 0:29:30But there are clues
0:29:30 > 0:29:32that things weren't always this way.
0:29:33 > 0:29:39These are pictures taken from the surface of Mars in August 2009.
0:29:42 > 0:29:47And they caused quite a bit of excitement, because of this,
0:29:47 > 0:29:51this rock sat on the surface of Mars in front of the rover.
0:29:51 > 0:29:54This rock is about...
0:29:54 > 0:29:55Well, here's a close-up.
0:29:55 > 0:29:58It's actually a nickel iron meteorite
0:29:58 > 0:30:01and it's about, what, 60 centimetres across,
0:30:01 > 0:30:03weighs half a ton.
0:30:03 > 0:30:07It came from space,
0:30:07 > 0:30:10came through the Martian atmosphere and landed on the ground.
0:30:10 > 0:30:15But the mystery is that a meteorite this big, if it hit Mars today,
0:30:15 > 0:30:17would disintegrate when it hit the surface.
0:30:17 > 0:30:20It would be travelling too fast and that's because
0:30:20 > 0:30:24Mars's atmosphere is too thin, too diffuse to slow it down.
0:30:26 > 0:30:28But that meteorite is very definitely there
0:30:28 > 0:30:32so how could it have made it to the ground?
0:30:32 > 0:30:36Well, it must be that, in the past, when this meteorite hit Mars,
0:30:36 > 0:30:39Mars' atmosphere was significantly denser,
0:30:39 > 0:30:43dense enough to slow this piece of rock down enough
0:30:43 > 0:30:47that it could land on the surface intact.
0:30:51 > 0:30:57But why did Mars lose its thick atmosphere and become the barren planet we see today?
0:30:59 > 0:31:05There are so many ways for planets to lose their atmospheres
0:31:05 > 0:31:08that it feels like a miracle that we've still got ours.
0:31:08 > 0:31:10But with Mars, it's thought that one of
0:31:10 > 0:31:13the dominant mechanisms was interaction with solar winds.
0:31:18 > 0:31:24The solar wind is a stream of super-heated, electrically-charged particles
0:31:24 > 0:31:30that constantly stream away from the sun at over one million kilometres per hour.
0:31:34 > 0:31:40This wave of smashed atoms has the power to strip a planet of its atmosphere.
0:31:47 > 0:31:51On Earth, we're protected from this onslaught by an invisible shield
0:31:51 > 0:31:57that completely surrounds our planet, known as the magnetosphere.
0:32:02 > 0:32:08The magnetosphere is created deep within the Earth's molten iron core.
0:32:08 > 0:32:12As the core spins, it generates a powerful magnetic field
0:32:12 > 0:32:17which shoots out of the pole and cocoons the whole planet.
0:32:18 > 0:32:25This magnetic shield is strong enough to deflect most of the solar wind that comes our way.
0:32:27 > 0:32:31Now, we know that at some point in the past, Mars
0:32:31 > 0:32:35would also have had a molten core and did have a magnetic field.
0:32:35 > 0:32:38But because Mars is a smaller planet than the Earth,
0:32:38 > 0:32:42it lost its heat more quickly and the core solidified.
0:32:42 > 0:32:46Electric currents could no longer flow and its field vanished.
0:32:46 > 0:32:52And that was a major factor in the solar wind being allowed to
0:32:52 > 0:32:55blast the planet and strip away its atmosphere.
0:33:04 > 0:33:09With no atmosphere to insulate it, this once Earth-like world
0:33:09 > 0:33:13transformed into the frozen desert we see today.
0:33:15 > 0:33:18A shadow of its former self.
0:33:26 > 0:33:30Although Mars has lost most of its atmosphere,
0:33:30 > 0:33:36those few molecules that remain still have the power to sculpt and transform the surface.
0:33:36 > 0:33:41And that power, that transformative effect,
0:33:41 > 0:33:45is present on every planet in the solar system that has an atmosphere.
0:33:47 > 0:33:53You can see it transforming the surface of the Namibian desert today as we speak.
0:33:53 > 0:34:00It is, of course, the force of nature that we call weather.
0:34:06 > 0:34:11We've got to go. Wow!
0:34:11 > 0:34:14Weather is a feature of every planet with an atmosphere.
0:34:17 > 0:34:23Our world is transformed as this huge mass of air moves across its surface.
0:34:25 > 0:34:28But as we look out into the solar system,
0:34:28 > 0:34:34we see it only takes the slightest atmosphere to produce extraordinary weather.
0:34:37 > 0:34:43Every few years, Mars all but disappears under a maelstrom of dust.
0:34:47 > 0:34:52Global dust storms are so huge they dwarf Olympus Mons,
0:34:52 > 0:34:55a volcano three times bigger than Everest.
0:34:59 > 0:35:04But to experience the most extreme and violent weather in the solar system,
0:35:04 > 0:35:06we need to travel to Jupiter.
0:35:06 > 0:35:13This banded gas giant is over 140,000 kilometres in diameter.
0:35:14 > 0:35:20Its atmosphere isn't a thin blue line, it's many thousand of kilometres thick
0:35:20 > 0:35:24and in a constant state of seething motion.
0:35:27 > 0:35:31The whole surface boils with gigantic storms.
0:35:34 > 0:35:38Yet, this most alien world shares a feature with our own planet.
0:35:40 > 0:35:43RUMBLING
0:35:43 > 0:35:48Jupiter crackles to the sound of electrical storms.
0:35:48 > 0:35:53The bolts of lightning are thousands of times brighter than lightning here on Earth.
0:36:11 > 0:36:16The physics of storms on Jupiter is, of course, the same as the physics of storms on Earth.
0:36:16 > 0:36:23The warm moist air deep in the atmosphere starts to rise, and as it rises it cools.
0:36:23 > 0:36:26And the moisture condenses out to form clouds.
0:36:26 > 0:36:31Now, that rising air leaves a gap beneath it, a low pressure area,
0:36:31 > 0:36:36and so more warm, moist air is sucked in and that fuels the rise of the storm.
0:36:36 > 0:36:41Now, on Earth, those storm systems are driven by the power of the sun.
0:36:41 > 0:36:46But therein lies a mystery because the storm systems on Jupiter are far more powerful
0:36:46 > 0:36:51and yet Jupiter is five times further away from the sun than the Earth is,
0:36:51 > 0:36:56which means it receives 25 times less solar energy.
0:36:56 > 0:37:01So, what mechanism could it be that powers those intensely violent storms on Jupiter?
0:37:07 > 0:37:13The secret to Jupiter's storm-tossed atmosphere lies hidden deep within the gas giant.
0:37:22 > 0:37:27On Earth, we have clear boundaries between the gaseous sky,
0:37:27 > 0:37:31the liquid oceans and the solid ground.
0:37:31 > 0:37:34But on Jupiter, there are no such boundaries.
0:37:39 > 0:37:44It's a gas giant, made of the two lightest and most abundant elements in the universe,
0:37:44 > 0:37:46hydrogen and helium.
0:37:46 > 0:37:49But as you go deep into Jupiter's atmosphere,
0:37:49 > 0:37:54something very strange and interesting happens to those gases.
0:37:58 > 0:38:02Jupiter's atmosphere is so thick and its gravitational pull so strong
0:38:02 > 0:38:06that 20,000 kilometres beneath the cloud tops,
0:38:06 > 0:38:12the pressure is 2,000,000 times greater than the surface pressure here on Earth.
0:38:14 > 0:38:18Under such immense pressure, the hydrogen gas in the atmosphere
0:38:18 > 0:38:22is transformed into a strange metallic liquid.
0:38:24 > 0:38:29As the gases are squeezed, a vast amount of energy is released,
0:38:29 > 0:38:34enough energy to fuel some of the biggest storms in the solar system.
0:38:37 > 0:38:41The biggest of them all is the Great Red Spot.
0:38:45 > 0:38:50This giant anti-cyclone has raged for hundreds of years
0:38:50 > 0:38:54and is large enough to swallow the Earth three times over.
0:38:57 > 0:39:01The Great Red Spot is an awesome sight.
0:39:01 > 0:39:06But this giant isn't one of my wonders.
0:39:06 > 0:39:09My wonder is a much smaller world.
0:39:11 > 0:39:17A moon that orbits the gas giant Saturn, 1.5 billion kilometres from Earth.
0:39:19 > 0:39:25What we have found on this small world is simply astonishing.
0:39:48 > 0:39:55If you thought of our moon as the archetypal moon of the solar system, if you like,
0:39:55 > 0:40:01then... Well, you might think that all the other moons out there,
0:40:01 > 0:40:06hundreds of them, would be dead, uninteresting worlds.
0:40:06 > 0:40:09I mean not uninteresting places to visit.
0:40:09 > 0:40:14I mean that is, in my view, the greatest thing that humans have ever achieved,
0:40:14 > 0:40:22landing on the surface of the moon but it's a dead and lifeless place.
0:40:22 > 0:40:26But as we've begun to visit those worlds, as we've flown spacecraft
0:40:26 > 0:40:33to within hundreds of miles of their surfaces, we've found that the moons in the outer solar system
0:40:33 > 0:40:40are of an astonishingly interesting and varied and fascinated bunch of worlds.
0:40:40 > 0:40:44This is Jupiter's moon, Europa.
0:40:44 > 0:40:49This is Jupiter's moon, Io, the most volcanic object in the solar system.
0:40:49 > 0:40:56But of all the worlds out there, this one - Saturn's moon, Titan - is unique, because of that.
0:40:56 > 0:41:00That is an atmosphere, and what an atmosphere it is!
0:41:00 > 0:41:07It's 1,000 kilometres deep, it's four times denser than the atmosphere of the Earth.
0:41:07 > 0:41:09I mean imagine that,
0:41:09 > 0:41:15a moon around a distant planet in the icy, distant reaches of the solar system
0:41:15 > 0:41:21with an atmosphere denser and thicker than our own.
0:41:28 > 0:41:34Titan has the most Earth-like atmosphere in the entire solar system,
0:41:34 > 0:41:40a thick blue line, rich in nitrogen and containing methane.
0:41:41 > 0:41:48At first sight, a world this small shouldn't be able to hold onto such a dense atmosphere,
0:41:48 > 0:41:53except Titan lies in one of the coldest regions of the solar system,
0:41:53 > 0:41:56and that makes all the difference.
0:42:02 > 0:42:07Temperature for gases like this, the gases in our atmosphere,
0:42:07 > 0:42:12is really a measure of how fast the molecules of the gas are moving around,
0:42:12 > 0:42:18and I can demonstrate that with this thing, which is a Chinese lantern.
0:42:20 > 0:42:23If I light this fuel,
0:42:23 > 0:42:25then what's going to happen...
0:42:27 > 0:42:31is that the gas inside is going to heat up.
0:42:33 > 0:42:37And as you heat up a gas,
0:42:37 > 0:42:41what that basically means is that you speed all the molecules up.
0:42:43 > 0:42:48As the molecules of air heat up and move faster,
0:42:48 > 0:42:51the air pressure inside the lantern begins to increase.
0:42:51 > 0:42:58That means that molecules are forced out, making the air inside less dense than the air outside,
0:42:58 > 0:43:01and the lantern gets lighter.
0:43:03 > 0:43:07And eventually the lantern is so light...
0:43:09 > 0:43:15..that it will just float away in the atmosphere of our planet.
0:43:35 > 0:43:42Hot gases have more energy to escape a planet's gravitational pull than cold gases.
0:43:42 > 0:43:50Now Titan is a much smaller body than the Earth. It has much weaker gravitational pull,
0:43:50 > 0:43:53and if it were in the same region of the solar system as we are,
0:43:53 > 0:43:56then it would not be able to hold onto its atmosphere.
0:43:56 > 0:44:00But it's a lot further away from the sun than we are
0:44:00 > 0:44:07and so that means that it's colder, its atmospheric molecules are moving around much more slowly than ours.
0:44:07 > 0:44:14That means that its weak gravity is enough to hold on to that thick dense atmosphere.
0:44:17 > 0:44:21Titan's thick atmosphere was an unexpected discovery,
0:44:21 > 0:44:24but it took an audacious mission
0:44:24 > 0:44:28to reveal the world that lies beneath the blanket of clouds.
0:44:32 > 0:44:36We have lift off of the Cassini spacecraft on a billion-mile trek to Saturn.
0:44:36 > 0:44:41In 1997, Cassini began its journey to Titan.
0:44:44 > 0:44:51It carried with it the Huygens probe, a lander designed to set down on this frozen moon.
0:44:57 > 0:44:59On Christmas Day 2004,
0:44:59 > 0:45:04Huygens was released from Cassini and it began the bumpy ride
0:45:04 > 0:45:08through one of the most intriguing atmospheres in the solar system.
0:45:11 > 0:45:14And then, for the first time,
0:45:14 > 0:45:19the thick clouds parted and the surface of Titan was revealed.
0:45:27 > 0:45:31These are the actual images taken by Huygens
0:45:31 > 0:45:35as it slowly parachuted to the surface.
0:45:40 > 0:45:42The world it revealed
0:45:42 > 0:45:46was more familiar than we could have possibly imagined.
0:45:59 > 0:46:01One of the first people to see
0:46:01 > 0:46:05these incredible images was a man who helped design the probe,
0:46:05 > 0:46:07Ralph Lorenz.
0:46:07 > 0:46:11It was amazing because we just had no idea what to expect.
0:46:11 > 0:46:18We didn't know whether it would be, you know, cratered like the moon or just sort of a flat expanse of sand
0:46:18 > 0:46:22and then these first pictures came back and it was just astonishingly familiar.
0:46:22 > 0:46:27Did that picture, that initial series of pictures...
0:46:27 > 0:46:30- I suppose it did look somewhat like this, didn't it?- It did.
0:46:30 > 0:46:34- It could have been there. - It could have been right here. - I do see that.
0:46:34 > 0:46:36I could sit here,
0:46:36 > 0:46:38look at that and that's what that picture looks like.
0:46:38 > 0:46:40I could take it with a camera.
0:46:40 > 0:46:44The camera on the probe was about the height of your knee, so yeah,
0:46:44 > 0:46:47the view the Huygens probe had is just like this.
0:46:47 > 0:46:50Rounded stones dot the landscape.
0:46:50 > 0:46:56They're smooth and look like they have been eroded by tumbling water,
0:46:56 > 0:46:59similar to stones found on river beds, here on Earth.
0:47:01 > 0:47:05It sounds to me like this was one of the easiest pictures to interpret
0:47:05 > 0:47:07in the history of space exploration.
0:47:07 > 0:47:14You know, the way you tell it, it's just that's a river bed with these stones. I mean, is it that simple?
0:47:14 > 0:47:16Because you can be misled easily, with...
0:47:16 > 0:47:18The devil is always in the details,
0:47:18 > 0:47:21but I think there were very few people
0:47:21 > 0:47:24disputed the interpretation of a river channel.
0:47:24 > 0:47:29I mean it's just such a familiar thing to so many people on Earth, there really wasn't much doubt.
0:47:32 > 0:47:35It was an extraordinary discovery.
0:47:35 > 0:47:40Evidence of flowing rivers had never been found before on a moon.
0:47:41 > 0:47:46But it wasn't the only surprise Titan held in store.
0:48:05 > 0:48:08This is the Matanuska glacier in Alaska.
0:48:10 > 0:48:15It really is one of the most astonishing places I've ever seen.
0:48:18 > 0:48:25And this whole landscape is testament to the erosive power of this stuff,
0:48:25 > 0:48:30this mixture of ice and rock
0:48:30 > 0:48:34as it rolls down this valley over hundreds of thousands of years
0:48:34 > 0:48:37and creates this astonishing landscape.
0:48:46 > 0:48:48But the reason it can do that
0:48:48 > 0:48:52is because of the delicate balance of the Earth's atmosphere.
0:48:52 > 0:49:00You see, our planet is just at the right temperature and pressure to allow water to exist as solid,
0:49:00 > 0:49:05as liquid and as gas, as vapour in the clouds.
0:49:05 > 0:49:12And so the sun can heat up the oceans and it can move the water over the top of the mountains.
0:49:12 > 0:49:17It can fall as rain, turn to ice, become a glacier
0:49:17 > 0:49:23and then sweep down the valley to sculpt this astonishing landscape.
0:49:34 > 0:49:39Just as our atmosphere allows all this to exist,
0:49:39 > 0:49:45the atmosphere of Titan is the perfect temperature and pressure to allow something to exist
0:49:45 > 0:49:50that has never been seen before on a world beyond Earth.
0:49:56 > 0:50:00This is a picture taken of the south pole of Titan
0:50:00 > 0:50:02by Cassini in June 2005,
0:50:02 > 0:50:08and it's become one of the most important and fascinating pictures
0:50:08 > 0:50:12in the history of space exploration.
0:50:12 > 0:50:15The interesting thing is this black blob, here.
0:50:17 > 0:50:22Now this fascinated the Cassini scientists but the explanation as to what that is
0:50:22 > 0:50:26had to wait just over a year till July 2006,
0:50:26 > 0:50:28when this picture was taken,
0:50:28 > 0:50:33and it's a radar image, this time of the north pole of Titan,
0:50:33 > 0:50:37and you see, again, these huge black areas.
0:50:37 > 0:50:44The black in this case means that the radar waves that bounced onto them didn't come back
0:50:44 > 0:50:49so they're completely black, and there's only one really good explanation for that.
0:50:49 > 0:50:54That is that they are incredibly flat surfaces.
0:50:54 > 0:50:58In fact, they're surfaces of liquid
0:50:58 > 0:51:02so this picture combined with this picture
0:51:02 > 0:51:08means that this is the first observation of a liquid,
0:51:08 > 0:51:14a lake on the surface of a body other than the Earth in the solar system.
0:51:18 > 0:51:23But these lakes, of course, cannot be lakes of liquid water because
0:51:23 > 0:51:28the surface temperature on Titan is minus 180 degrees Celsius and, at those temperatures,
0:51:28 > 0:51:34water is frozen as hard as steel.
0:51:34 > 0:51:39So if these are not lakes of water, then what are they?
0:52:03 > 0:52:08This is Lake Eyak in Alaska, just on Prince William Sound,
0:52:08 > 0:52:15and I've come here to collect a molecule or a substance that's very abundant on Titan.
0:52:15 > 0:52:19In fact, it's abundant throughout the solar system, but here on Earth
0:52:19 > 0:52:24it exists as a gas and it bubbles up from the floor of this lake.
0:52:37 > 0:52:44The floor of Lake Eyak is covered in rotting vegetation, you know, dead leaves and bits of trees, twigs,
0:52:44 > 0:52:47and that's been broken down by bacteria which produce the gas
0:52:47 > 0:52:50that bubbles up from the floor of the lake.
0:52:50 > 0:52:55That gas is methane and we've been collecting it all night
0:52:55 > 0:52:57underneath this upturned boat
0:52:57 > 0:53:01so that I can take a sample of it in this bag.
0:53:03 > 0:53:09Now, on Earth, methane is very unstable.
0:53:09 > 0:53:10If you give it...
0:53:10 > 0:53:13a little kick...
0:53:13 > 0:53:18in the presence of oxygen, then you get what chemists call an exothermic reaction.
0:53:18 > 0:53:24Methane plus oxygen goes to water plus carbon dioxide, and...
0:53:24 > 0:53:26some energy.
0:53:28 > 0:53:31The Earth's temperature and atmospheric pressure
0:53:31 > 0:53:35means methane can only exist as a highly-flammable gas.
0:53:39 > 0:53:42But Titan's atmospheric pressure and temperature
0:53:42 > 0:53:48is perfect to allow methane to exist as a solid, a gas and, most importantly,
0:53:48 > 0:53:50a liquid.
0:53:54 > 0:54:00So the images Cassini captured were gigantic lakes of liquid methane...
0:54:02 > 0:54:05..the first ever liquid discovered
0:54:05 > 0:54:10pooling on the surface of another world in the solar system.
0:54:10 > 0:54:13This is Kraken Mare.
0:54:13 > 0:54:17At over 400,000 square kilometres,
0:54:17 > 0:54:21it's the biggest body of liquid on Titan.
0:54:22 > 0:54:26It's almost five times the size of Lake Superior,
0:54:26 > 0:54:29North America's greatest lake.
0:54:45 > 0:54:52On Titan, methane plays exactly the same role that water does here on Earth.
0:54:55 > 0:54:59So, where we have clouds of water,
0:54:59 > 0:55:04Titan has clouds of methane with methane rain.
0:55:04 > 0:55:12Whereas we have lakes and oceans of water, Titan has lakes of liquid methane.
0:55:12 > 0:55:18And whereas, here on Earth, the sun warms the water in the lakes and oceans,
0:55:18 > 0:55:21and fills our atmosphere with water vapour,
0:55:21 > 0:55:24on Titan the sun lifts the methane
0:55:24 > 0:55:29from the lakes and saturates the atmosphere with methane.
0:55:29 > 0:55:35So, whereas on Earth we have a hydrological cycle, on Titan there's a methanological cycle.
0:55:38 > 0:55:42And rain would be an absolutely magical sight on Titan.
0:55:42 > 0:55:47Because the atmosphere is so dense and the gravity of the moon is so weak,
0:55:47 > 0:55:52the drops of methane rain would grow to over a centimetre in size
0:55:52 > 0:56:00and they would fall to the ground as slowly as snowflakes fall onto the surface of our own planet.
0:56:02 > 0:56:07Thousands and thousands of gallons of liquid methane
0:56:07 > 0:56:11must have slowly rained down onto the surface,
0:56:11 > 0:56:16making rivers and streams swell and burst.
0:56:16 > 0:56:20Deep gullies were cut into the frozen water landscape...
0:56:20 > 0:56:23Which looks so familiar because it is familiar.
0:56:23 > 0:56:31It's this. You know, the atmosphere of Titan shapes the surface in exactly the same way
0:56:31 > 0:56:37that the atmosphere here on Earth shapes the surface of our planet.
0:56:48 > 0:56:54Titan is like a primordial Earth caught in a deep freeze.
0:56:54 > 0:56:58It's almost like looking back in time over four billion years
0:56:58 > 0:57:04and observing our planet before life began, and began to modify our atmosphere,
0:57:04 > 0:57:09to change it into the oxygen-rich atmosphere that we see today.
0:57:13 > 0:57:18In many ways, Titan looks so familiar.
0:57:18 > 0:57:24It's a place with rivers and lakes and clouds and rain.
0:57:24 > 0:57:28It's a place with water, albeit frozen as hard as steel,
0:57:28 > 0:57:35and a place of methane, albeit so cold that methane is now a liquid
0:57:35 > 0:57:41and flows and shapes the landscape just like water does here on Earth.
0:57:45 > 0:57:50For me, the most important thing about Titan
0:57:50 > 0:57:54is we now have two Earth-like worlds in our solar system
0:57:57 > 0:58:03One in this warm region, 93 million miles away from the sun,
0:58:03 > 0:58:08and the other in deep freeze, a billion miles away from our star
0:58:08 > 0:58:14in orbit around another planet, and that must greatly increase the probability
0:58:14 > 0:58:17that there are other Earth-like planets in orbit
0:58:17 > 0:58:24around the hundreds of billions of stars out there in the universe.
0:58:32 > 0:58:38# Somewhere over the rainbow
0:58:38 > 0:58:45# Skies are blue
0:58:45 > 0:58:53# And the dreams that you dare to dream
0:58:53 > 0:58:56# Really do come true. #
0:58:56 > 0:58:59E-mail subtitling@bbc.co.uk