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These are the user uploaded subtitles that are being translated: 1 00:00:09,540 --> 00:00:12,060 We live on a world of wonders. 2 00:00:14,180 --> 00:00:18,860 A place of astonishing beauty and complexity. 3 00:00:18,860 --> 00:00:21,060 We have vast oceans 4 00:00:21,060 --> 00:00:23,380 and incredible weather. 5 00:00:24,740 --> 00:00:28,940 Giant mountains and spectacular landscapes. 6 00:00:28,940 --> 00:00:31,460 If you think that this is all there is, 7 00:00:31,460 --> 00:00:34,740 that our planet exists in magnificent isolation, 8 00:00:34,740 --> 00:00:35,780 then you're wrong. 9 00:00:37,300 --> 00:00:39,340 We're part of a much wider eco-system, 10 00:00:39,340 --> 00:00:43,460 that extends way beyond the top of our atmosphere. 11 00:00:47,180 --> 00:00:49,340 I think we are living through 12 00:00:49,340 --> 00:00:53,540 the greatest age of discovery our civilisation has ever known. 13 00:00:53,540 --> 00:00:56,900 We've voyaged to the farthest reaches of the solar system, 14 00:00:56,900 --> 00:01:00,900 photographed strange new worlds, 15 00:01:00,900 --> 00:01:03,180 stood in unfamiliar landscapes, 16 00:01:03,180 --> 00:01:05,540 tasted alien air. 17 00:01:10,580 --> 00:01:13,140 Amongst all these wonders sits our Earth - 18 00:01:13,140 --> 00:01:17,380 an oasis of calm amidst the violence of the solar system. 19 00:01:19,420 --> 00:01:20,740 And all that separates us 20 00:01:20,740 --> 00:01:27,460 from what's out there is a thin, flimsy envelope of gas - 21 00:01:27,460 --> 00:01:29,180 our atmosphere. 22 00:01:30,660 --> 00:01:32,820 And it's thanks to this "thin blue line" 23 00:01:32,820 --> 00:01:35,900 that we have the air that we breathe, 24 00:01:35,900 --> 00:01:37,540 the water that we drink 25 00:01:37,540 --> 00:01:40,340 and the landscape that surrounds us. 26 00:01:42,700 --> 00:01:46,180 Atmospheres define all the planets in the solar system. 27 00:01:48,380 --> 00:01:53,980 They have the power to create dynamic worlds that are alien and chaotic. 28 00:01:56,020 --> 00:02:00,180 But, remarkably, in the frozen wastes of the solar system... 29 00:02:02,100 --> 00:02:07,620 ..one atmosphere has created the most unexpected wonder - 30 00:02:07,620 --> 00:02:10,780 a moon that looks a lot like home. 31 00:02:45,900 --> 00:02:47,900 I've come to Cape Town in South Africa 32 00:02:47,900 --> 00:02:50,940 to do something that I have always wanted to do, 33 00:02:50,940 --> 00:02:54,940 but never thought I would get the chance. 34 00:02:59,620 --> 00:03:03,180 I'm about to fly incredibly high, 35 00:03:03,180 --> 00:03:05,820 to the very edge of the Earth's atmosphere. 36 00:03:09,500 --> 00:03:14,780 From here, I am hoping to see something that only a handful of people have ever seen - 37 00:03:14,780 --> 00:03:17,500 the thin blue line, 38 00:03:17,500 --> 00:03:23,460 the fragile strip of gas that surrounds our whole planet. 39 00:03:23,460 --> 00:03:26,460 And this is what's going to take me there. 40 00:03:31,340 --> 00:03:34,460 This is an English Electric Lightning, 41 00:03:34,460 --> 00:03:37,820 the most beautiful fighter aircraft ever built. 42 00:03:40,420 --> 00:03:44,180 This is when England built the best aircraft in the world. 43 00:03:46,540 --> 00:03:48,740 The Lightning is no longer in service, 44 00:03:48,740 --> 00:03:54,420 but this piece of magnificently overpowered engineering 45 00:03:54,420 --> 00:03:57,300 is going to take me 18 kilometres, straight up. 46 00:03:58,340 --> 00:04:01,700 Actually, I read somewhere that when you read about the altitude 47 00:04:01,700 --> 00:04:05,900 of the Lightning, it says "Altitude: Estimated, 60,000 feet. 48 00:04:05,900 --> 00:04:08,340 "Ceiling: Classified." 49 00:04:08,340 --> 00:04:10,540 So I don't know how high these can go. 50 00:04:10,540 --> 00:04:15,300 I have heard rumours they can go to 80,000 feet, which is amazing. 51 00:04:18,660 --> 00:04:24,500 My journey will take me beyond almost all the molecules of gas that make up our atmosphere. 52 00:04:24,500 --> 00:04:31,180 If you feel you're going to get sick... Yeah? ..use a bag, OK? Right. Hopefully not. 53 00:04:37,700 --> 00:04:39,700 To get there, I'm going to experience 54 00:04:39,700 --> 00:04:42,460 what made the Lightning famous - 55 00:04:42,460 --> 00:04:44,300 a vertical take-off. 56 00:04:56,620 --> 00:04:57,700 Whoo-hoo! 57 00:05:16,140 --> 00:05:20,380 'It takes just seconds to reach nine kilometres up, 58 00:05:20,380 --> 00:05:23,260 'but I'm still in the thickest layer of the atmosphere, 59 00:05:23,260 --> 00:05:24,660 'called the troposphere. 60 00:05:25,620 --> 00:05:30,140 'But the further I climb, the thinner the atmosphere becomes.' 61 00:05:36,020 --> 00:05:42,420 Up at 58,000 feet. 90% of the atmosphere is below me. 62 00:05:43,980 --> 00:05:46,700 The only people above me are on the space station. 63 00:05:51,660 --> 00:05:53,420 So beautiful. 64 00:05:59,540 --> 00:06:01,780 I'm now at 60,000 feet. 65 00:06:01,780 --> 00:06:04,740 '18 kilometres up. 66 00:06:04,740 --> 00:06:07,340 'And the highest I can go.' 67 00:06:11,700 --> 00:06:16,700 Above me, the sky is a deep, dark blue. 68 00:06:23,340 --> 00:06:27,100 'And that is what I've come to see - ' 69 00:06:27,100 --> 00:06:28,140 our atmosphere. 70 00:06:34,540 --> 00:06:38,420 That really is the thin blue line that protects us. 71 00:06:40,460 --> 00:06:44,260 So...fragile and so tenuous. 72 00:06:45,780 --> 00:06:48,620 Just a tiny sliver of blue. 73 00:06:52,300 --> 00:06:53,380 Amazing. 74 00:07:04,020 --> 00:07:07,980 Between 55 and 60,000 feet, inverted, 75 00:07:07,980 --> 00:07:10,580 the curvature of the Earth there. 76 00:07:10,580 --> 00:07:13,460 5G, vertical ascent. 77 00:07:13,460 --> 00:07:17,220 That is just a ride! 78 00:07:17,220 --> 00:07:18,780 It is remarkable to see that. 79 00:07:18,780 --> 00:07:19,820 You can see 80 00:07:19,820 --> 00:07:23,500 the...thinness and fragility, 81 00:07:23,500 --> 00:07:27,300 you can see the atmosphere going from light blue, to dark blue, to black. 82 00:07:28,540 --> 00:07:30,340 It really is astonishing. 83 00:07:36,220 --> 00:07:41,340 The thin blue line makes the Earth the wonderfully diverse place it is. 84 00:07:46,220 --> 00:07:51,060 It acts as a soothing blanket, that traps the warmth of the sun... 85 00:07:52,940 --> 00:07:56,140 ..yet protects us from the harshness of its radiation. 86 00:08:00,300 --> 00:08:04,500 Its movements can be traced in the gentlest breeze. 87 00:08:06,620 --> 00:08:09,260 And the most devastating hurricane. 88 00:08:15,260 --> 00:08:18,580 The oxygen and water the atmosphere holds 89 00:08:18,580 --> 00:08:21,500 plays a fundamental role in the ongoing survival 90 00:08:21,500 --> 00:08:26,020 of millions of different species living on the planet. 91 00:08:28,940 --> 00:08:31,700 In this film, I want to explain how the laws of physics 92 00:08:31,700 --> 00:08:34,900 that created our unique atmosphere 93 00:08:34,900 --> 00:08:39,660 are the same laws that created many diverse and different atmospheres 94 00:08:39,660 --> 00:08:41,540 across the solar system. 95 00:08:44,460 --> 00:08:47,700 When perfectly balanced, a world as familiar 96 00:08:47,700 --> 00:08:51,820 and beautiful as the Earth can evolve beneath the clouds. 97 00:08:55,220 --> 00:08:59,500 But the slightest changes can lead to alien and violent worlds. 98 00:09:01,940 --> 00:09:05,300 There are planets in our solar system that have been transformed 99 00:09:05,300 --> 00:09:10,340 into hellish worlds, by nothing more than the gases in their atmosphere. 100 00:09:14,780 --> 00:09:18,100 And just as atmospheres can choke a planet to death, 101 00:09:18,100 --> 00:09:21,820 they are also powerful enough to shape their surfaces. 102 00:09:27,500 --> 00:09:31,860 And there are worlds out there which are all atmosphere. 103 00:09:33,540 --> 00:09:37,780 Giant balls of churning gas, where storms three times 104 00:09:37,780 --> 00:09:41,940 the size of the Earth have raged for hundreds of years. 105 00:09:56,780 --> 00:09:59,980 All atmospheres in the solar system are unique, 106 00:09:59,980 --> 00:10:04,740 but the ingredients and forces that shape them are universal. 107 00:10:06,820 --> 00:10:13,100 At the heart of each is the glue which holds the solar system together, 108 00:10:13,100 --> 00:10:15,300 a fundamental force of nature - 109 00:10:15,300 --> 00:10:16,780 gravity. 110 00:10:18,940 --> 00:10:22,820 Gravity is, by far, the weakest known force in the universe. 111 00:10:24,780 --> 00:10:29,900 You can see that because it's really easy for me to pick a rock up off the ground, 112 00:10:29,900 --> 00:10:34,900 even though there's a whole planet, Earth, pulling the rock down. 113 00:10:34,900 --> 00:10:36,780 I can just lift it up. 114 00:10:36,780 --> 00:10:39,220 Incredibly weak, 115 00:10:39,220 --> 00:10:42,540 but incredibly important, because it's the only force there is 116 00:10:42,540 --> 00:10:44,540 to hold an atmosphere to the planet. 117 00:10:46,340 --> 00:10:50,460 The more massive the planet, the greater its gravitational force. 118 00:10:52,060 --> 00:10:57,660 Earth has enough mass to keep a tight grip of the gas molecules that make up our atmosphere. 119 00:10:58,620 --> 00:11:01,700 It holds them against the surface and allows us to breathe. 120 00:11:04,420 --> 00:11:06,820 Now, we don't really notice the presence 121 00:11:06,820 --> 00:11:10,860 of our atmosphere, I suppose, because we live in it, all the time. 122 00:11:10,860 --> 00:11:12,860 But there's a lot of it. 123 00:11:12,860 --> 00:11:19,100 There's five million billion tons of air surrounding the Earth. 124 00:11:19,100 --> 00:11:23,180 That's the equivalent of a weight of one kilogram 125 00:11:23,180 --> 00:11:27,020 pressing down on every square centimetre of our bodies. 126 00:11:27,020 --> 00:11:29,820 Or, put it another way, if I'm about a metre square, 127 00:11:29,820 --> 00:11:34,100 that's ten tons of weight pressing down. 128 00:11:35,100 --> 00:11:38,020 Now I say pressing down, but that's not entirely right, 129 00:11:38,020 --> 00:11:39,820 that's not how air pressure works. 130 00:11:39,820 --> 00:11:44,620 It presses in every direction at once. I can demonstrate that. 131 00:11:44,620 --> 00:11:46,380 This is a glass full of water, 132 00:11:46,380 --> 00:11:52,620 so if I put a piece of paper on there, turn it upside down. 133 00:11:52,620 --> 00:11:55,260 Now, if I'm right, then the air pressure is pushing 134 00:11:55,260 --> 00:12:00,300 in every direction on this glass of water, the air pressure is pushing up as well as down. 135 00:12:02,020 --> 00:12:04,860 And it has no problem in holding the water in the glass. 136 00:12:08,780 --> 00:12:09,820 Cool. 137 00:12:10,780 --> 00:12:12,540 Where did you get this water from? 138 00:12:12,540 --> 00:12:15,020 CREW LAUGH 139 00:12:16,940 --> 00:12:20,740 Life on the surface of this planet survives, 140 00:12:20,740 --> 00:12:25,580 surrounded by this enormous mass of gas. We're like lobsters, 141 00:12:25,580 --> 00:12:28,100 scuttling around on the ocean floor. 142 00:12:34,860 --> 00:12:38,660 But our atmosphere does more than allow us to breathe. 143 00:12:38,660 --> 00:12:40,100 It protects us 144 00:12:40,100 --> 00:12:43,180 from the most powerful force in the solar system... 145 00:12:45,420 --> 00:12:47,340 ..our sun. 146 00:12:51,940 --> 00:12:54,340 If you ask yourself the question, "Why is Earth 147 00:12:54,340 --> 00:12:56,660 "the temperature that it is?" 148 00:12:56,660 --> 00:12:58,900 Then, the obvious answer might seem to be, 149 00:12:58,900 --> 00:13:03,100 "Well, because it's 150m kilometres away from the sun". 150 00:13:03,100 --> 00:13:05,780 But actually, things aren't quite that simple. 151 00:13:05,780 --> 00:13:09,900 This is the Namib desert in Namibia, in south-western Africa. 152 00:13:09,900 --> 00:13:14,580 And as the sun sinks below the horizon, the temperature change, 153 00:13:14,580 --> 00:13:19,420 from day to night, can be as much as 30 degrees Celsius. 154 00:13:19,420 --> 00:13:21,980 That's an immense amount in just a few hours, 155 00:13:21,980 --> 00:13:26,380 much more than in somewhere like Manchester, for example. 156 00:13:26,380 --> 00:13:30,020 The reason is that this is also one of the driest places on the planet 157 00:13:30,020 --> 00:13:33,620 and so there is very little water vapour in the atmosphere. 158 00:13:33,620 --> 00:13:37,660 That means that the atmosphere is not a very good insulator, 159 00:13:37,660 --> 00:13:43,380 so when the sun disappears, the heat just disappears quickly into space. 160 00:13:43,380 --> 00:13:45,660 Now, there's a planet in the solar system, 161 00:13:45,660 --> 00:13:47,540 somewhere over there, near the sun, 162 00:13:47,540 --> 00:13:50,780 where the temperature shift, from day to night, 163 00:13:50,780 --> 00:13:55,220 is not a mere 30 degrees Celsius, but an immense amount bigger. 164 00:14:15,700 --> 00:14:19,020 Roughly 58 million kilometres from the sun 165 00:14:19,020 --> 00:14:22,780 is the smallest planet in the solar system... 166 00:14:26,420 --> 00:14:27,660 ..Mercury. 167 00:14:30,300 --> 00:14:32,580 This tortured piece of rock 168 00:14:32,580 --> 00:14:36,620 suffers the biggest temperature swings of all the planets, 169 00:14:36,620 --> 00:14:40,140 from 450 degrees Celsius in the day, 170 00:14:40,140 --> 00:14:44,500 to minus 180 degrees at night. 171 00:14:44,500 --> 00:14:48,740 And all because Mercury has been stripped naked. 172 00:14:48,740 --> 00:14:52,300 It has virtually no atmosphere at all. 173 00:15:04,620 --> 00:15:07,580 Like all the rocky inner planets of the solar system, 174 00:15:07,580 --> 00:15:10,580 Mercury had an atmosphere when it was formed, 175 00:15:10,580 --> 00:15:13,180 but it lost it very quickly. 176 00:15:13,180 --> 00:15:14,220 Here on Earth, 177 00:15:14,220 --> 00:15:17,900 at sea level, then... 178 00:15:17,900 --> 00:15:21,620 Well, in a volume about the size of this pebble, 179 00:15:21,620 --> 00:15:28,100 there are 10 billion billion molecules of gas. 180 00:15:28,100 --> 00:15:33,500 On Mercury, in the same volume, there would be around a 100,000, 181 00:15:33,500 --> 00:15:35,980 that's 10 million million times less. 182 00:15:35,980 --> 00:15:41,140 Now, planets hang on to their atmosphere by the force of gravity. 183 00:15:41,140 --> 00:15:43,220 It's the only way they can 184 00:15:43,220 --> 00:15:48,540 stop that thin blue line of gas disappearing off into space. 185 00:15:48,540 --> 00:15:51,540 So, the bigger the planet, the more massive the planet, 186 00:15:51,540 --> 00:15:57,180 the stronger the gravitational pull and the easier it is for the planet to keep hold of its atmosphere. 187 00:15:57,180 --> 00:16:03,340 So, Mercury was just too small and too hot to hang onto its atmosphere 188 00:16:03,340 --> 00:16:08,660 and the consequences for the planet were absolutely devastating. 189 00:16:12,540 --> 00:16:16,740 Atmospheres may be just a thin strip of molecules, 190 00:16:16,740 --> 00:16:20,180 but they are a planet's first line of defence. 191 00:16:21,260 --> 00:16:24,340 Without them, a planet like Mercury 192 00:16:24,340 --> 00:16:28,540 is at the mercy of our violent solar system. 193 00:16:41,420 --> 00:16:47,380 This is Saskatchewan in western Canada and it is a cold place to be in November. 194 00:16:47,380 --> 00:16:53,860 About a year ago, in November 2008, a piece of asteroid, 195 00:16:53,860 --> 00:16:56,060 a space rock, weighing about ten tons, 196 00:16:56,060 --> 00:17:00,500 entered the atmosphere right over here and actually landed 197 00:17:00,500 --> 00:17:04,140 about 30 kilometres that way, at a place called Buzzard Coulee. 198 00:17:04,140 --> 00:17:07,700 Now, it's not unusual for rocks that big to hit the Earth. 199 00:17:07,700 --> 00:17:10,060 On average, that happens about once a month. 200 00:17:10,060 --> 00:17:15,140 What was unusual about this one was that it was over quite a densely-populated area. 201 00:17:15,140 --> 00:17:19,940 So tens of thousands, if not hundreds of thousands, of people saw it and heard it. 202 00:17:19,940 --> 00:17:25,620 But most spectacularly, it was captured by a lot of CCTV cameras, 203 00:17:25,620 --> 00:17:29,540 including that one, in this garage. 204 00:17:36,140 --> 00:17:40,020 These are the actual CCTV images captured around the city. 205 00:17:40,020 --> 00:17:41,940 They show the meteorite, 206 00:17:41,940 --> 00:17:45,820 as it streaked across the sky at 20 kilometres per second. 207 00:17:45,820 --> 00:17:50,860 The fireball was brighter than the moon and turned the night sky blue. 208 00:17:55,020 --> 00:17:58,100 Scientists used these remarkable images 209 00:17:58,100 --> 00:18:02,820 to triangulate the impact site of the meteorite. 210 00:18:02,820 --> 00:18:07,420 They traced it to a field, just outside the city of Lloydminster. 211 00:18:07,420 --> 00:18:14,020 A team of meteorite hunters have been searching the debris left by the enormous explosion. 212 00:18:15,540 --> 00:18:18,340 They are led by Dr Alan Hildebrand. 213 00:18:20,580 --> 00:18:21,620 How much energy 214 00:18:21,620 --> 00:18:24,420 does a rock like this have, then? 215 00:18:24,420 --> 00:18:28,500 You know, what is it, a ten-ton rock travelling at 50 times the speed of sound? 216 00:18:28,500 --> 00:18:30,900 You know, it would be like if you'd stocked up, 217 00:18:30,900 --> 00:18:34,540 say, 400 tons of TNT to explode. 218 00:18:34,540 --> 00:18:36,740 I mean, it's really quite dramatic. 219 00:18:36,740 --> 00:18:40,540 400 tons that just dissipates away in the Earth's atmosphere? Yes. 220 00:18:40,540 --> 00:18:44,140 Atmosphere slowing it down, of course, causing it to break up. 221 00:18:44,140 --> 00:18:48,740 In just five seconds, it's almost all over and, of course, 222 00:18:48,740 --> 00:18:51,820 you know it's an extreme friction, makes the light show. 223 00:18:51,820 --> 00:18:57,420 10% of the energy goes in light and it's like a billion-watt bulb shining high in the sky. 224 00:19:00,540 --> 00:19:01,980 So, what are we looking for? 225 00:19:01,980 --> 00:19:04,980 What does a piece of that asteroid look like? 226 00:19:06,220 --> 00:19:08,620 They... Going through the atmosphere, 227 00:19:08,620 --> 00:19:12,260 the surface has got melted, so you end up with a dark crust on them. 228 00:19:12,260 --> 00:19:18,300 So, essentially, you're looking for an oddly-sculpted dark rock. 229 00:19:18,300 --> 00:19:24,420 Yeah. Well, in all fairness, you've got to be able to tell it from, you know the cow patties and so on. 230 00:19:24,420 --> 00:19:26,340 But... I could probably manage that. 231 00:19:26,340 --> 00:19:29,540 Once you get your eye in, you'll have no trouble. 232 00:19:45,460 --> 00:19:47,940 We've got one right here. 233 00:19:49,180 --> 00:19:51,260 I'll pick that up. 234 00:19:51,260 --> 00:19:53,300 Astonishing. 235 00:19:53,300 --> 00:19:56,860 It's just been completely rounded off. 236 00:19:56,860 --> 00:19:59,260 Yeah, the heat melted the surface of the rock. 237 00:19:59,260 --> 00:20:03,900 I mean, how hot does something have to be to do that? Yeah. 238 00:20:03,900 --> 00:20:07,180 6,000 degrees C would do it. 239 00:20:10,340 --> 00:20:14,460 So, this little rock has had an amazing history. 240 00:20:14,460 --> 00:20:18,620 I mean, it approached Earth as part of this bigger fragment, 241 00:20:18,620 --> 00:20:23,900 at about, what, 18, 19, 20 kilometres per second. 242 00:20:23,900 --> 00:20:27,260 It hit the Earth's atmosphere. 243 00:20:27,260 --> 00:20:32,500 About 85 kilometres up, it began to feel the effects 244 00:20:32,500 --> 00:20:34,340 of the Earth's atmosphere. 245 00:20:34,340 --> 00:20:38,940 It began to squash the air in front of it, creating a pressure wave, 246 00:20:38,940 --> 00:20:43,140 essentially, which, in turn, causes this thing to heat up. 247 00:20:43,140 --> 00:20:47,660 And it would have heated up to something like the temperature of the surface of the sun. 248 00:20:47,660 --> 00:20:52,460 It would have been 5 or 6,000 degrees Celsius as it plummeted through the atmosphere, 249 00:20:52,460 --> 00:20:57,780 lit up the sky over here and then, quite literally, 250 00:20:57,780 --> 00:21:03,780 exploded in a series of explosions and peppered these fields with lumps of rock this big. 251 00:21:03,780 --> 00:21:08,540 Can you imagine standing here on that night and having this, these things - 252 00:21:08,540 --> 00:21:11,460 and this is heavy, right - 253 00:21:11,460 --> 00:21:13,020 raining down from the sky? 254 00:21:13,020 --> 00:21:15,260 It must have been quite incredible. 255 00:21:17,660 --> 00:21:20,340 If the meteorite had hit the ground intact, 256 00:21:20,340 --> 00:21:26,180 the explosion would have been been equivalent to 400 tons of TNT 257 00:21:26,180 --> 00:21:28,020 and left a crater 20 metres wide. 258 00:21:31,220 --> 00:21:36,300 The Earth was spared this colossal impact by nothing more 259 00:21:36,300 --> 00:21:39,340 than the tenuous strip of gases that surrounds us. 260 00:21:39,340 --> 00:21:43,420 But not all planets have this protective blanket. 261 00:21:48,620 --> 00:21:51,340 When a meteorite hits naked Mercury, 262 00:21:51,340 --> 00:21:56,580 there is no atmosphere to break it up or slow it down. 263 00:21:58,780 --> 00:22:03,340 It strikes the ground at full speed and completely intact. 264 00:22:04,900 --> 00:22:08,100 For the last 4.6 billion years, 265 00:22:08,100 --> 00:22:14,180 Mercury has been bombarded with countless asteroids and comets. 266 00:22:15,500 --> 00:22:22,580 The whole history of the planet's violent past is laid out on its surface, 267 00:22:22,580 --> 00:22:28,100 a world pitted with hundreds of thousands of craters. 268 00:22:29,060 --> 00:22:33,940 Craters inside craters, inside craters. 269 00:22:56,380 --> 00:22:58,700 Mercury was damned from the start. 270 00:22:58,700 --> 00:23:01,860 It's simply too small and too hot 271 00:23:01,860 --> 00:23:05,700 to have retained any meaningful traces of atmosphere. 272 00:23:05,700 --> 00:23:07,580 We, on the other hand, 273 00:23:07,580 --> 00:23:11,020 are big enough and cold enough 274 00:23:11,020 --> 00:23:14,140 to have retained this envelope of gases. 275 00:23:17,660 --> 00:23:19,460 That, in turn, allows 276 00:23:19,460 --> 00:23:22,700 living things, like me, to evolve 277 00:23:22,700 --> 00:23:27,220 and to use that atmosphere, to breathe and to live. 278 00:23:27,220 --> 00:23:30,780 But there are places out there in the solar system 279 00:23:30,780 --> 00:23:34,820 whose atmospheres have the same ingredients as our own, 280 00:23:34,820 --> 00:23:39,220 but when the formula is even slightly remixed, 281 00:23:39,220 --> 00:23:42,900 it leads to worlds that couldn't be more different. 282 00:23:53,740 --> 00:23:57,100 Roughly 108 million kilometres from the sun 283 00:23:57,100 --> 00:24:01,980 sits the brightest planet in the solar system, Venus. 284 00:24:02,900 --> 00:24:05,500 This footage shows the luminescent world appear 285 00:24:05,500 --> 00:24:08,420 from behind our cratered moon. 286 00:24:11,860 --> 00:24:15,060 Venus and Earth share many similarities. 287 00:24:15,060 --> 00:24:18,100 We sit next to each other in space, 288 00:24:18,100 --> 00:24:21,100 we were formed from the same material 289 00:24:21,100 --> 00:24:26,580 and we're roughly the same size and share a similar mass and gravity. 290 00:24:28,020 --> 00:24:30,820 But that's where any similarities end. 291 00:24:30,820 --> 00:24:33,900 Venus is a tortured world, 292 00:24:33,900 --> 00:24:39,740 where thick clouds of sulphuric acid are driven along by high winds 293 00:24:39,740 --> 00:24:43,300 and temperatures are hot enough to melt lead. 294 00:24:43,300 --> 00:24:49,380 All because this planet's atmosphere created a runaway greenhouse effect. 295 00:24:52,140 --> 00:24:55,140 The "greenhouse effect" has become a well-known phrase. 296 00:24:55,140 --> 00:24:58,620 You know, it's synonymous with global warming. 297 00:24:58,620 --> 00:25:00,700 But what is it? 298 00:25:00,700 --> 00:25:07,060 Well, a planet, like the Earth, absorbs energy from the sun as visible light. 299 00:25:07,060 --> 00:25:13,820 Now, atmospheres don't absorb much visible light, as you can see, because you can see the sun. 300 00:25:13,820 --> 00:25:19,580 The ground absorbs the visible light, heats up and then re-radiates it. 301 00:25:19,580 --> 00:25:24,300 But it re-radiates it as infrared radiation, heat radiation, if you want. 302 00:25:24,300 --> 00:25:29,060 And atmospheric gases, particularly carbon dioxide, are very good 303 00:25:29,060 --> 00:25:34,460 at absorbing in the infrared and so they trap the heat and the planet heats up. 304 00:25:39,060 --> 00:25:43,140 On Earth, greenhouse gases are essential to our survival. 305 00:25:43,140 --> 00:25:47,140 Without them our planet would be 30 degrees colder, 306 00:25:47,140 --> 00:25:50,900 too cold to support life as we know it. 307 00:25:57,060 --> 00:26:02,340 But Venus's atmosphere was flooded with greenhouse gases. 308 00:26:02,340 --> 00:26:06,580 The nearby sun slowly boiled away its oceans, 309 00:26:06,580 --> 00:26:09,380 pumping water vapour into the atmosphere. 310 00:26:10,540 --> 00:26:15,020 And carbon dioxide, from thousands of erupting volcanoes, 311 00:26:15,020 --> 00:26:17,420 added to the stifling mix. 312 00:26:19,140 --> 00:26:22,740 Venus grew hotter and hotter. 313 00:26:22,740 --> 00:26:25,380 The planet was slowly choked to death. 314 00:26:28,180 --> 00:26:33,100 Venus is a planet with an atmosphere in overdrive, 315 00:26:33,100 --> 00:26:37,580 but Earth's other rocky neighbour tells quite a different story. 316 00:27:01,540 --> 00:27:03,300 Get it! 317 00:27:08,300 --> 00:27:11,260 These are the dunes in the Namib desert. 318 00:27:11,260 --> 00:27:14,740 It's an absolutely spectacular place. 319 00:27:21,500 --> 00:27:23,060 This place is not the hottest, 320 00:27:23,060 --> 00:27:26,460 nor the driest, desert in the world, but these dunes 321 00:27:26,460 --> 00:27:29,900 are some of the oldest sand dunes in the world. 322 00:27:33,420 --> 00:27:35,780 And the reason we're here in the Namib desert 323 00:27:35,780 --> 00:27:39,060 is that this is a great analogue 324 00:27:39,060 --> 00:27:40,340 for the surface of Mars. 325 00:27:40,340 --> 00:27:44,060 This is what the surface of Mars looks like and these dunes, 326 00:27:44,060 --> 00:27:46,900 called barchan dunes, these crescent-shaped dunes, 327 00:27:46,900 --> 00:27:51,100 are the same as the sand dunes on Mars. 328 00:27:57,300 --> 00:28:01,340 So, if you want to get a feel for what it would be like on the surface of Mars, 329 00:28:01,340 --> 00:28:07,420 and you want to know what driving a 4x4 around on it would be like, then this is the place to come. 330 00:28:12,980 --> 00:28:18,140 Incredibly, there is a vehicle driving across the surface of the "red planet" today... 331 00:28:23,260 --> 00:28:26,300 ..a space rover, named Opportunity. 332 00:28:31,580 --> 00:28:35,100 The rovers and spacecraft that circle the planet 333 00:28:35,100 --> 00:28:39,340 have sent back images which reveal Mars in exquisite detail. 334 00:28:49,180 --> 00:28:52,700 Mars has vast dunes, 335 00:28:52,700 --> 00:28:57,300 enormous volcanoes 336 00:28:57,300 --> 00:29:00,420 and giant ice sheets. 337 00:29:03,620 --> 00:29:07,420 It has canyons and river valleys. 338 00:29:08,620 --> 00:29:15,900 Mars is a dry, frozen version of our home, covered in red dust and sand. 339 00:29:15,900 --> 00:29:21,420 And it's all due to the fact that Mars has virtually no atmosphere. 340 00:29:29,260 --> 00:29:31,460 But there are clues 341 00:29:31,460 --> 00:29:34,220 that things weren't always this way. 342 00:29:35,180 --> 00:29:40,900 These are pictures taken from the surface of Mars in August 2009. 343 00:29:43,300 --> 00:29:48,860 And they caused quite a bit of excitement, because of this, 344 00:29:48,860 --> 00:29:53,100 this rock sat on the surface of Mars in front of the rover. 345 00:29:53,100 --> 00:29:55,540 This rock is about... 346 00:29:55,540 --> 00:29:56,860 Well, here's a close-up. 347 00:29:56,860 --> 00:29:59,300 It's actually a nickel iron meteorite 348 00:29:59,300 --> 00:30:03,100 and it's about, what, 60 centimetres across, 349 00:30:03,100 --> 00:30:04,980 weighs half a ton. 350 00:30:04,980 --> 00:30:08,500 It came from space, 351 00:30:08,500 --> 00:30:11,980 came through the Martian atmosphere and landed on the ground. 352 00:30:11,980 --> 00:30:16,820 But the mystery is that a meteorite this big, if it hit Mars today, 353 00:30:16,820 --> 00:30:19,180 would disintegrate when it hit the surface. 354 00:30:19,180 --> 00:30:21,820 It would be travelling too fast and that's because 355 00:30:21,820 --> 00:30:26,060 Mars's atmosphere is too thin, too diffuse to slow it down. 356 00:30:27,580 --> 00:30:29,940 But that meteorite is very definitely there 357 00:30:29,940 --> 00:30:33,500 so how could it have made it to the ground? 358 00:30:33,500 --> 00:30:37,900 Well, it must be that, in the past, when this meteorite hit Mars, 359 00:30:37,900 --> 00:30:41,180 Mars' atmosphere was significantly denser, 360 00:30:41,180 --> 00:30:45,060 dense enough to slow this piece of rock down enough 361 00:30:45,060 --> 00:30:48,660 that it could land on the surface intact. 362 00:30:52,340 --> 00:30:58,740 But why did Mars lose its thick atmosphere and become the barren planet we see today? 363 00:31:01,260 --> 00:31:06,780 There are so many ways for planets to lose their atmospheres 364 00:31:06,780 --> 00:31:09,620 that it feels like a miracle that we've still got ours. 365 00:31:09,620 --> 00:31:11,660 But with Mars, it's thought that one of 366 00:31:11,660 --> 00:31:14,860 the dominant mechanisms was interaction with solar winds. 367 00:31:20,140 --> 00:31:25,780 The solar wind is a stream of super-heated, electrically-charged particles 368 00:31:25,780 --> 00:31:31,620 that constantly stream away from the sun at over one million kilometres per hour. 369 00:31:35,620 --> 00:31:41,300 This wave of smashed atoms has the power to strip a planet of its atmosphere. 370 00:31:48,340 --> 00:31:52,940 On Earth, we're protected from this onslaught by an invisible shield 371 00:31:52,940 --> 00:31:59,100 that completely surrounds our planet, known as the magnetosphere. 372 00:32:03,900 --> 00:32:09,820 The magnetosphere is created deep within the Earth's molten iron core. 373 00:32:09,820 --> 00:32:14,180 As the core spins, it generates a powerful magnetic field 374 00:32:14,180 --> 00:32:18,700 which shoots out of the pole and cocoons the whole planet. 375 00:32:20,220 --> 00:32:26,540 This magnetic shield is strong enough to deflect most of the solar wind that comes our way. 376 00:32:28,820 --> 00:32:32,340 Now, we know that at some point in the past, Mars 377 00:32:32,340 --> 00:32:36,660 would also have had a molten core and did have a magnetic field. 378 00:32:36,660 --> 00:32:39,780 But because Mars is a smaller planet than the Earth, 379 00:32:39,780 --> 00:32:43,980 it lost its heat more quickly and the core solidified. 380 00:32:43,980 --> 00:32:48,100 Electric currents could no longer flow and its field vanished. 381 00:32:48,100 --> 00:32:53,340 And that was a major factor in the solar wind being allowed to 382 00:32:53,340 --> 00:32:57,180 blast the planet and strip away its atmosphere. 383 00:33:05,300 --> 00:33:10,740 With no atmosphere to insulate it, this once Earth-like world 384 00:33:10,740 --> 00:33:14,540 transformed into the frozen desert we see today. 385 00:33:17,100 --> 00:33:19,420 A shadow of its former self. 386 00:33:28,060 --> 00:33:31,300 Although Mars has lost most of its atmosphere, 387 00:33:31,300 --> 00:33:38,220 those few molecules that remain still have the power to sculpt and transform the surface. 388 00:33:38,220 --> 00:33:42,700 And that power, that transformative effect, 389 00:33:42,700 --> 00:33:46,780 is present on every planet in the solar system that has an atmosphere. 390 00:33:48,300 --> 00:33:54,900 You can see it transforming the surface of the Namibian desert today as we speak. 391 00:33:54,900 --> 00:34:02,060 It is, of course, the force of nature that we call weather. 392 00:34:07,940 --> 00:34:12,420 We've got to go. Wow! 393 00:34:12,420 --> 00:34:16,100 Weather is a feature of every planet with an atmosphere. 394 00:34:18,780 --> 00:34:24,460 Our world is transformed as this huge mass of air moves across its surface. 395 00:34:27,020 --> 00:34:29,820 But as we look out into the solar system, 396 00:34:29,820 --> 00:34:35,820 we see it only takes the slightest atmosphere to produce extraordinary weather. 397 00:34:38,980 --> 00:34:45,140 Every few years, Mars all but disappears under a maelstrom of dust. 398 00:34:48,460 --> 00:34:53,540 Global dust storms are so huge they dwarf Olympus Mons, 399 00:34:53,540 --> 00:34:56,940 a volcano three times bigger than Everest. 400 00:35:00,940 --> 00:35:05,500 But to experience the most extreme and violent weather in the solar system, 401 00:35:05,500 --> 00:35:08,180 we need to travel to Jupiter. 402 00:35:08,180 --> 00:35:14,300 This banded gas giant is over 140,000 kilometres in diameter. 403 00:35:16,020 --> 00:35:21,460 Its atmosphere isn't a thin blue line, it's many thousand of kilometres thick 404 00:35:21,460 --> 00:35:25,820 and in a constant state of seething motion. 405 00:35:28,300 --> 00:35:32,780 The whole surface boils with gigantic storms. 406 00:35:35,620 --> 00:35:40,060 Yet, this most alien world shares a feature with our own planet. 407 00:35:42,180 --> 00:35:44,580 RUMBLING 408 00:35:44,580 --> 00:35:49,460 Jupiter crackles to the sound of electrical storms. 409 00:35:49,460 --> 00:35:54,820 The bolts of lightning are thousands of times brighter than lightning here on Earth. 410 00:36:12,540 --> 00:36:18,180 The physics of storms on Jupiter is, of course, the same as the physics of storms on Earth. 411 00:36:18,180 --> 00:36:24,940 The warm moist air deep in the atmosphere starts to rise, and as it rises it cools. 412 00:36:24,940 --> 00:36:27,940 And the moisture condenses out to form clouds. 413 00:36:27,940 --> 00:36:33,100 Now, that rising air leaves a gap beneath it, a low pressure area, 414 00:36:33,100 --> 00:36:37,940 and so more warm, moist air is sucked in and that fuels the rise of the storm. 415 00:36:37,940 --> 00:36:42,980 Now, on Earth, those storm systems are driven by the power of the sun. 416 00:36:42,980 --> 00:36:47,900 But therein lies a mystery because the storm systems on Jupiter are far more powerful 417 00:36:47,900 --> 00:36:52,900 and yet Jupiter is five times further away from the sun than the Earth is, 418 00:36:52,900 --> 00:36:57,900 which means it receives 25 times less solar energy. 419 00:36:57,900 --> 00:37:02,980 So, what mechanism could it be that powers those intensely violent storms on Jupiter? 420 00:37:08,780 --> 00:37:14,940 The secret to Jupiter's storm-tossed atmosphere lies hidden deep within the gas giant. 421 00:37:23,740 --> 00:37:28,900 On Earth, we have clear boundaries between the gaseous sky, 422 00:37:28,900 --> 00:37:32,660 the liquid oceans and the solid ground. 423 00:37:32,660 --> 00:37:35,660 But on Jupiter, there are no such boundaries. 424 00:37:40,380 --> 00:37:45,500 It's a gas giant, made of the two lightest and most abundant elements in the universe, 425 00:37:45,500 --> 00:37:47,500 hydrogen and helium. 426 00:37:47,500 --> 00:37:51,260 But as you go deep into Jupiter's atmosphere, 427 00:37:51,260 --> 00:37:55,700 something very strange and interesting happens to those gases. 428 00:37:59,300 --> 00:38:03,940 Jupiter's atmosphere is so thick and its gravitational pull so strong 429 00:38:03,940 --> 00:38:07,820 that 20,000 kilometres beneath the cloud tops, 430 00:38:07,820 --> 00:38:13,460 the pressure is 2,000,000 times greater than the surface pressure here on Earth. 431 00:38:15,700 --> 00:38:19,420 Under such immense pressure, the hydrogen gas in the atmosphere 432 00:38:19,420 --> 00:38:23,700 is transformed into a strange metallic liquid. 433 00:38:25,700 --> 00:38:30,780 As the gases are squeezed, a vast amount of energy is released, 434 00:38:30,780 --> 00:38:35,500 enough energy to fuel some of the biggest storms in the solar system. 435 00:38:39,140 --> 00:38:42,900 The biggest of them all is the Great Red Spot. 436 00:38:47,220 --> 00:38:52,100 This giant anti-cyclone has raged for hundreds of years 437 00:38:52,100 --> 00:38:56,260 and is large enough to swallow the Earth three times over. 438 00:38:58,540 --> 00:39:02,860 The Great Red Spot is an awesome sight. 439 00:39:02,860 --> 00:39:07,420 But this giant isn't one of my wonders. 440 00:39:07,420 --> 00:39:10,580 My wonder is a much smaller world. 441 00:39:12,460 --> 00:39:18,620 A moon that orbits the gas giant Saturn, 1.5 billion kilometres from Earth. 442 00:39:20,540 --> 00:39:26,500 What we have found on this small world is simply astonishing. 443 00:39:49,580 --> 00:39:57,100 If you thought of our moon as the archetypal moon of the solar system, if you like, 444 00:39:57,100 --> 00:40:02,580 then... Well, you might think that all the other moons out there, 445 00:40:02,580 --> 00:40:07,660 hundreds of them, would be dead, uninteresting worlds. 446 00:40:07,660 --> 00:40:10,340 I mean not uninteresting places to visit. 447 00:40:10,340 --> 00:40:15,780 I mean that is, in my view, the greatest thing that humans have ever achieved, 448 00:40:15,780 --> 00:40:23,420 landing on the surface of the moon but it's a dead and lifeless place. 449 00:40:23,420 --> 00:40:27,540 But as we've begun to visit those worlds, as we've flown spacecraft 450 00:40:27,540 --> 00:40:34,340 to within hundreds of miles of their surfaces, we've found that the moons in the outer solar system 451 00:40:34,340 --> 00:40:42,100 are of an astonishingly interesting and varied and fascinated bunch of worlds. 452 00:40:42,100 --> 00:40:45,300 This is Jupiter's moon, Europa. 453 00:40:45,300 --> 00:40:51,020 This is Jupiter's moon, Io, the most volcanic object in the solar system. 454 00:40:51,020 --> 00:40:57,660 But of all the worlds out there, this one - Saturn's moon, Titan - is unique, because of that. 455 00:40:57,660 --> 00:41:01,820 That is an atmosphere, and what an atmosphere it is! 456 00:41:01,820 --> 00:41:08,340 It's 1,000 kilometres deep, it's four times denser than the atmosphere of the Earth. 457 00:41:08,340 --> 00:41:10,460 I mean imagine that, 458 00:41:10,460 --> 00:41:17,180 a moon around a distant planet in the icy, distant reaches of the solar system 459 00:41:17,180 --> 00:41:22,460 with an atmosphere denser and thicker than our own. 460 00:41:29,660 --> 00:41:35,700 Titan has the most Earth-like atmosphere in the entire solar system, 461 00:41:35,700 --> 00:41:41,540 a thick blue line, rich in nitrogen and containing methane. 462 00:41:43,060 --> 00:41:49,420 At first sight, a world this small shouldn't be able to hold onto such a dense atmosphere, 463 00:41:49,420 --> 00:41:54,580 except Titan lies in one of the coldest regions of the solar system, 464 00:41:54,580 --> 00:41:57,620 and that makes all the difference. 465 00:42:03,700 --> 00:42:08,420 Temperature for gases like this, the gases in our atmosphere, 466 00:42:08,420 --> 00:42:13,380 is really a measure of how fast the molecules of the gas are moving around, 467 00:42:13,380 --> 00:42:19,900 and I can demonstrate that with this thing, which is a Chinese lantern. 468 00:42:22,260 --> 00:42:25,180 If I light this fuel, 469 00:42:25,180 --> 00:42:26,860 then what's going to happen... 470 00:42:28,620 --> 00:42:32,940 is that the gas inside is going to heat up. 471 00:42:34,740 --> 00:42:38,500 And as you heat up a gas, 472 00:42:38,500 --> 00:42:42,980 what that basically means is that you speed all the molecules up. 473 00:42:45,220 --> 00:42:49,500 As the molecules of air heat up and move faster, 474 00:42:49,500 --> 00:42:52,940 the air pressure inside the lantern begins to increase. 475 00:42:52,940 --> 00:42:59,940 That means that molecules are forced out, making the air inside less dense than the air outside, 476 00:42:59,940 --> 00:43:02,660 and the lantern gets lighter. 477 00:43:04,500 --> 00:43:08,900 And eventually the lantern is so light... 478 00:43:10,420 --> 00:43:16,300 ..that it will just float away in the atmosphere of our planet. 479 00:43:36,980 --> 00:43:43,780 Hot gases have more energy to escape a planet's gravitational pull than cold gases. 480 00:43:43,780 --> 00:43:51,380 Now Titan is a much smaller body than the Earth. It has much weaker gravitational pull, 481 00:43:51,380 --> 00:43:54,620 and if it were in the same region of the solar system as we are, 482 00:43:54,620 --> 00:43:57,620 then it would not be able to hold onto its atmosphere. 483 00:43:57,620 --> 00:44:01,700 But it's a lot further away from the sun than we are 484 00:44:01,700 --> 00:44:09,020 and so that means that it's colder, its atmospheric molecules are moving around much more slowly than ours. 485 00:44:09,020 --> 00:44:16,060 That means that its weak gravity is enough to hold on to that thick dense atmosphere. 486 00:44:18,300 --> 00:44:23,100 Titan's thick atmosphere was an unexpected discovery, 487 00:44:23,100 --> 00:44:25,540 but it took an audacious mission 488 00:44:25,540 --> 00:44:30,180 to reveal the world that lies beneath the blanket of clouds. 489 00:44:33,420 --> 00:44:38,020 We have lift off of the Cassini spacecraft on a billion-mile trek to Saturn. 490 00:44:38,020 --> 00:44:42,500 In 1997, Cassini began its journey to Titan. 491 00:44:45,860 --> 00:44:53,220 It carried with it the Huygens probe, a lander designed to set down on this frozen moon. 492 00:44:58,380 --> 00:45:00,420 On Christmas Day 2004, 493 00:45:00,420 --> 00:45:06,020 Huygens was released from Cassini and it began the bumpy ride 494 00:45:06,020 --> 00:45:09,980 through one of the most intriguing atmospheres in the solar system. 495 00:45:13,100 --> 00:45:15,420 And then, for the first time, 496 00:45:15,420 --> 00:45:20,580 the thick clouds parted and the surface of Titan was revealed. 497 00:45:29,020 --> 00:45:33,020 These are the actual images taken by Huygens 498 00:45:33,020 --> 00:45:36,620 as it slowly parachuted to the surface. 499 00:45:41,980 --> 00:45:43,540 The world it revealed 500 00:45:43,540 --> 00:45:47,660 was more familiar than we could have possibly imagined. 501 00:46:00,900 --> 00:46:02,660 One of the first people to see 502 00:46:02,660 --> 00:46:06,540 these incredible images was a man who helped design the probe, 503 00:46:06,540 --> 00:46:08,740 Ralph Lorenz. 504 00:46:08,740 --> 00:46:13,020 It was amazing because we just had no idea what to expect. 505 00:46:13,020 --> 00:46:19,420 We didn't know whether it would be, you know, cratered like the moon or just sort of a flat expanse of sand 506 00:46:19,420 --> 00:46:24,020 and then these first pictures came back and it was just astonishingly familiar. 507 00:46:24,020 --> 00:46:28,420 Did that picture, that initial series of pictures... 508 00:46:28,420 --> 00:46:32,020 I suppose it did look somewhat like this, didn't it? It did. 509 00:46:32,020 --> 00:46:35,820 It could have been there. It could have been right here. I do see that. 510 00:46:35,820 --> 00:46:37,300 I could sit here, 511 00:46:37,300 --> 00:46:39,780 look at that and that's what that picture looks like. 512 00:46:39,780 --> 00:46:41,620 I could take it with a camera. 513 00:46:41,620 --> 00:46:45,780 The camera on the probe was about the height of your knee, so yeah, 514 00:46:45,780 --> 00:46:48,980 the view the Huygens probe had is just like this. 515 00:46:48,980 --> 00:46:51,540 Rounded stones dot the landscape. 516 00:46:51,540 --> 00:46:57,660 They're smooth and look like they have been eroded by tumbling water, 517 00:46:57,660 --> 00:47:01,060 similar to stones found on river beds, here on Earth. 518 00:47:02,940 --> 00:47:07,020 It sounds to me like this was one of the easiest pictures to interpret 519 00:47:07,020 --> 00:47:09,260 in the history of space exploration. 520 00:47:09,260 --> 00:47:15,300 You know, the way you tell it, it's just that's a river bed with these stones. I mean, is it that simple? 521 00:47:15,300 --> 00:47:17,460 Because you can be misled easily, with... 522 00:47:17,460 --> 00:47:19,740 The devil is always in the details, 523 00:47:19,740 --> 00:47:22,300 but I think there were very few people 524 00:47:22,300 --> 00:47:26,020 disputed the interpretation of a river channel. 525 00:47:26,020 --> 00:47:31,140 I mean it's just such a familiar thing to so many people on Earth, there really wasn't much doubt. 526 00:47:33,340 --> 00:47:36,660 It was an extraordinary discovery. 527 00:47:36,660 --> 00:47:41,300 Evidence of flowing rivers had never been found before on a moon. 528 00:47:42,900 --> 00:47:47,780 But it wasn't the only surprise Titan held in store. 529 00:48:06,340 --> 00:48:10,020 This is the Matanuska glacier in Alaska. 530 00:48:12,020 --> 00:48:17,180 It really is one of the most astonishing places I've ever seen. 531 00:48:19,540 --> 00:48:26,940 And this whole landscape is testament to the erosive power of this stuff, 532 00:48:26,940 --> 00:48:31,300 this mixture of ice and rock 533 00:48:31,300 --> 00:48:35,860 as it rolls down this valley over hundreds of thousands of years 534 00:48:35,860 --> 00:48:38,940 and creates this astonishing landscape. 535 00:48:48,140 --> 00:48:49,500 But the reason it can do that 536 00:48:49,500 --> 00:48:53,980 is because of the delicate balance of the Earth's atmosphere. 537 00:48:53,980 --> 00:49:01,580 You see, our planet is just at the right temperature and pressure to allow water to exist as solid, 538 00:49:01,580 --> 00:49:06,780 as liquid and as gas, as vapour in the clouds. 539 00:49:06,780 --> 00:49:13,900 And so the sun can heat up the oceans and it can move the water over the top of the mountains. 540 00:49:13,900 --> 00:49:18,980 It can fall as rain, turn to ice, become a glacier 541 00:49:18,980 --> 00:49:24,620 and then sweep down the valley to sculpt this astonishing landscape. 542 00:49:35,700 --> 00:49:40,460 Just as our atmosphere allows all this to exist, 543 00:49:40,460 --> 00:49:47,020 the atmosphere of Titan is the perfect temperature and pressure to allow something to exist 544 00:49:47,020 --> 00:49:51,500 that has never been seen before on a world beyond Earth. 545 00:49:57,580 --> 00:50:01,580 This is a picture taken of the south pole of Titan 546 00:50:01,580 --> 00:50:03,620 by Cassini in June 2005, 547 00:50:03,620 --> 00:50:10,020 and it's become one of the most important and fascinating pictures 548 00:50:10,020 --> 00:50:13,660 in the history of space exploration. 549 00:50:13,660 --> 00:50:16,820 The interesting thing is this black blob, here. 550 00:50:18,420 --> 00:50:24,060 Now this fascinated the Cassini scientists but the explanation as to what that is 551 00:50:24,060 --> 00:50:28,020 had to wait just over a year till July 2006, 552 00:50:28,020 --> 00:50:30,220 when this picture was taken, 553 00:50:30,220 --> 00:50:34,540 and it's a radar image, this time of the north pole of Titan, 554 00:50:34,540 --> 00:50:39,260 and you see, again, these huge black areas. 555 00:50:39,260 --> 00:50:45,580 The black in this case means that the radar waves that bounced onto them didn't come back 556 00:50:45,580 --> 00:50:50,700 so they're completely black, and there's only one really good explanation for that. 557 00:50:50,700 --> 00:50:55,620 That is that they are incredibly flat surfaces. 558 00:50:55,620 --> 00:50:59,420 In fact, they're surfaces of liquid 559 00:50:59,420 --> 00:51:03,620 so this picture combined with this picture 560 00:51:03,620 --> 00:51:09,780 means that this is the first observation of a liquid, 561 00:51:09,780 --> 00:51:15,580 a lake on the surface of a body other than the Earth in the solar system. 562 00:51:20,260 --> 00:51:25,180 But these lakes, of course, cannot be lakes of liquid water because 563 00:51:25,180 --> 00:51:30,140 the surface temperature on Titan is minus 180 degrees Celsius and, at those temperatures, 564 00:51:30,140 --> 00:51:35,740 water is frozen as hard as steel. 565 00:51:35,740 --> 00:51:40,540 So if these are not lakes of water, then what are they? 566 00:52:04,340 --> 00:52:10,260 This is Lake Eyak in Alaska, just on Prince William Sound, 567 00:52:10,260 --> 00:52:16,860 and I've come here to collect a molecule or a substance that's very abundant on Titan. 568 00:52:16,860 --> 00:52:21,100 In fact, it's abundant throughout the solar system, but here on Earth 569 00:52:21,100 --> 00:52:25,820 it exists as a gas and it bubbles up from the floor of this lake. 570 00:52:38,540 --> 00:52:46,060 The floor of Lake Eyak is covered in rotting vegetation, you know, dead leaves and bits of trees, twigs, 571 00:52:46,060 --> 00:52:49,260 and that's been broken down by bacteria which produce the gas 572 00:52:49,260 --> 00:52:52,020 that bubbles up from the floor of the lake. 573 00:52:52,020 --> 00:52:56,540 That gas is methane and we've been collecting it all night 574 00:52:56,540 --> 00:52:59,060 underneath this upturned boat 575 00:52:59,060 --> 00:53:02,500 so that I can take a sample of it in this bag. 576 00:53:05,220 --> 00:53:10,420 Now, on Earth, methane is very unstable. 577 00:53:10,420 --> 00:53:12,260 If you give it... 578 00:53:12,260 --> 00:53:15,140 a little kick... 579 00:53:15,140 --> 00:53:20,100 in the presence of oxygen, then you get what chemists call an exothermic reaction. 580 00:53:20,100 --> 00:53:25,780 Methane plus oxygen goes to water plus carbon dioxide, and... 581 00:53:25,780 --> 00:53:27,300 some energy. 582 00:53:29,340 --> 00:53:32,460 The Earth's temperature and atmospheric pressure 583 00:53:32,460 --> 00:53:36,500 means methane can only exist as a highly-flammable gas. 584 00:53:40,460 --> 00:53:43,980 But Titan's atmospheric pressure and temperature 585 00:53:43,980 --> 00:53:50,140 is perfect to allow methane to exist as a solid, a gas and, most importantly, 586 00:53:50,140 --> 00:53:52,140 a liquid. 587 00:53:55,580 --> 00:54:01,580 So the images Cassini captured were gigantic lakes of liquid methane... 588 00:54:04,260 --> 00:54:07,140 ..the first ever liquid discovered 589 00:54:07,140 --> 00:54:12,140 pooling on the surface of another world in the solar system. 590 00:54:12,140 --> 00:54:15,140 This is Kraken Mare. 591 00:54:15,140 --> 00:54:18,460 At over 400,000 square kilometres, 592 00:54:18,460 --> 00:54:22,460 it's the biggest body of liquid on Titan. 593 00:54:24,260 --> 00:54:28,220 It's almost five times the size of Lake Superior, 594 00:54:28,220 --> 00:54:30,900 North America's greatest lake. 595 00:54:46,620 --> 00:54:54,220 On Titan, methane plays exactly the same role that water does here on Earth. 596 00:54:56,740 --> 00:55:01,020 So, where we have clouds of water, 597 00:55:01,020 --> 00:55:05,700 Titan has clouds of methane with methane rain. 598 00:55:05,700 --> 00:55:13,340 Whereas we have lakes and oceans of water, Titan has lakes of liquid methane. 599 00:55:13,340 --> 00:55:19,540 And whereas, here on Earth, the sun warms the water in the lakes and oceans, 600 00:55:19,540 --> 00:55:22,740 and fills our atmosphere with water vapour, 601 00:55:22,740 --> 00:55:25,340 on Titan the sun lifts the methane 602 00:55:25,340 --> 00:55:30,300 from the lakes and saturates the atmosphere with methane. 603 00:55:30,300 --> 00:55:37,180 So, whereas on Earth we have a hydrological cycle, on Titan there's a methanological cycle. 604 00:55:39,340 --> 00:55:43,700 And rain would be an absolutely magical sight on Titan. 605 00:55:43,700 --> 00:55:48,980 Because the atmosphere is so dense and the gravity of the moon is so weak, 606 00:55:48,980 --> 00:55:54,220 the drops of methane rain would grow to over a centimetre in size 607 00:55:54,220 --> 00:56:01,620 and they would fall to the ground as slowly as snowflakes fall onto the surface of our own planet. 608 00:56:03,580 --> 00:56:09,020 Thousands and thousands of gallons of liquid methane 609 00:56:09,020 --> 00:56:12,620 must have slowly rained down onto the surface, 610 00:56:12,620 --> 00:56:17,300 making rivers and streams swell and burst. 611 00:56:17,300 --> 00:56:21,780 Deep gullies were cut into the frozen water landscape... 612 00:56:21,780 --> 00:56:25,260 Which looks so familiar because it is familiar. 613 00:56:25,260 --> 00:56:33,260 It's this. You know, the atmosphere of Titan shapes the surface in exactly the same way 614 00:56:33,260 --> 00:56:38,540 that the atmosphere here on Earth shapes the surface of our planet. 615 00:56:49,660 --> 00:56:55,780 Titan is like a primordial Earth caught in a deep freeze. 616 00:56:55,780 --> 00:57:00,180 It's almost like looking back in time over four billion years 617 00:57:00,180 --> 00:57:06,260 and observing our planet before life began, and began to modify our atmosphere, 618 00:57:06,260 --> 00:57:10,980 to change it into the oxygen-rich atmosphere that we see today. 619 00:57:14,900 --> 00:57:19,780 In many ways, Titan looks so familiar. 620 00:57:19,780 --> 00:57:25,580 It's a place with rivers and lakes and clouds and rain. 621 00:57:25,580 --> 00:57:30,100 It's a place with water, albeit frozen as hard as steel, 622 00:57:30,100 --> 00:57:37,260 and a place of methane, albeit so cold that methane is now a liquid 623 00:57:37,260 --> 00:57:42,860 and flows and shapes the landscape just like water does here on Earth. 624 00:57:47,220 --> 00:57:51,340 For me, the most important thing about Titan 625 00:57:51,340 --> 00:57:55,420 is we now have two Earth-like worlds in our solar system 626 00:57:58,820 --> 00:58:04,820 One in this warm region, 93 million miles away from the sun, 627 00:58:04,820 --> 00:58:10,020 and the other in deep freeze, a billion miles away from our star 628 00:58:10,020 --> 00:58:16,060 in orbit around another planet, and that must greatly increase the probability 629 00:58:16,060 --> 00:58:19,260 that there are other Earth-like planets in orbit 630 00:58:19,260 --> 00:58:25,900 around the hundreds of billions of stars out there in the universe. 631 00:58:33,420 --> 00:58:40,140 # Somewhere over the rainbow 632 00:58:40,140 --> 00:58:47,100 # Skies are blue 633 00:58:47,100 --> 00:58:55,100 # And the dreams that you dare to dream 634 00:58:55,100 --> 00:58:58,220 # Really do come true. # 635 00:58:58,220 --> 00:59:00,460 E-mail subtitling@bbc.co.uk 58397

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