1 00:00:56,727 --> 00:01:00,117 MANNING: This lush sub-tropical island off the coast of Africa 2 00:01:00,447 --> 00:01:03,325 emerged from the Atlantic two million years ago. 3 00:01:08,287 --> 00:01:11,962 Today, La Palma rises over two kilometres out of the sea. 4 00:01:15,327 --> 00:01:18,205 At its summit, the rock suddenly falls away. 5 00:01:28,807 --> 00:01:32,083 The middle of La Palma is a huge rocky chasm, 6 00:01:32,407 --> 00:01:34,079 shaped like a horseshoe. 7 00:01:34,807 --> 00:01:38,277 This whole vast natural amphitheatre 8 00:01:39,207 --> 00:01:41,801 was first investigated by the great German geologist 9 00:01:41,887 --> 00:01:44,162 Leopold von Buch in the 1820s. 10 00:01:45,167 --> 00:01:49,683 He came here and worked out that this had once been a giant active volcano. 11 00:01:52,967 --> 00:01:56,755 Half a million years ago, there was a colossal cataclysm here. 12 00:01:57,407 --> 00:02:01,161 The whole centre of the volcano, 200 cubic kilometres of rock, 13 00:02:01,327 --> 00:02:03,124 just collapsed into the sea. 14 00:02:04,447 --> 00:02:08,440 This created a huge tidal wave that raced across the oceans, 15 00:02:08,647 --> 00:02:11,923 leaving a trail of devastation halfway around the globe. 16 00:02:15,887 --> 00:02:19,516 But events in La Palma are just one example of the vast forces 17 00:02:19,607 --> 00:02:22,405 that are periodically unleashed by our planet. 18 00:02:22,767 --> 00:02:25,918 And these are an inevitable part of the way it works. 19 00:02:32,487 --> 00:02:34,523 In the course of making this series, 20 00:02:34,607 --> 00:02:37,599 I've discovered just how dynamic our planet is. 21 00:02:40,287 --> 00:02:42,721 The Earth's surface is in constant motion, 22 00:02:42,807 --> 00:02:45,116 driven by its internal store of heat. 23 00:02:50,247 --> 00:02:54,160 As continents collide, they create great mountain ranges. 24 00:02:59,807 --> 00:03:03,277 As mountains rise and fall, they influence our climate. 25 00:03:03,647 --> 00:03:06,923 And the climate shapes the life which inhabits the planet. 26 00:03:10,807 --> 00:03:13,799 It's all this change which makes the Earth what it is. 27 00:03:17,127 --> 00:03:19,322 With all this happening around and beneath us, 28 00:03:19,407 --> 00:03:22,080 we are bound to find the Earth a special place. 29 00:03:22,247 --> 00:03:24,681 But just how special is it as a planet? 30 00:03:24,807 --> 00:03:28,197 And what's even more interesting, why is it the way that it is? 31 00:03:38,207 --> 00:03:40,926 To begin to understand what sets Earth apart, 32 00:03:41,007 --> 00:03:44,079 scientists need to find out how it was formed. 33 00:03:46,287 --> 00:03:48,926 Clustered on La Palma's volcanic summit 34 00:03:49,007 --> 00:03:51,760 are some of Europe's most powerful telescopes. 35 00:03:53,527 --> 00:03:56,997 Here, astronomers are beginning to throw light on Earth's origins 36 00:03:57,087 --> 00:03:59,362 by looking out, deep into space. 37 00:04:01,567 --> 00:04:04,445 They now believe that our entire solar system 38 00:04:04,567 --> 00:04:07,001 formed from the debris of dead stars, 39 00:04:07,487 --> 00:04:09,955 as Mark Kidger explained to me. 40 00:04:13,847 --> 00:04:16,998 I think it's a beautiful idea that the solar system 41 00:04:17,407 --> 00:04:21,002 condenses out of star dust, the death of a star, as you put it. 42 00:04:21,207 --> 00:04:26,281 But is that a unique event or are there many such systems? 43 00:04:26,487 --> 00:04:30,196 Good heavens, no. The Hubble Space Telescope 44 00:04:30,367 --> 00:04:34,360 has been searching for other solar systems that are in process of forming, 45 00:04:34,487 --> 00:04:37,001 and at the last count, 46 00:04:37,087 --> 00:04:40,363 it had found something around 140 of them. 47 00:04:40,447 --> 00:04:45,567 KIDGER: Solar systems, clouds of dust and gas in distinct phases of forming into solar systems. 48 00:04:46,847 --> 00:04:49,805 MANNING: Astronomers now think that solar systems form 49 00:04:50,007 --> 00:04:52,567 when those clouds of stellar debris collapse. 50 00:04:54,047 --> 00:04:56,641 You've got clouds of dust and gas in space, 51 00:04:56,727 --> 00:05:00,640 but those clouds don't collapse without something to help them. 52 00:05:00,847 --> 00:05:02,644 And what is that process? 53 00:05:03,167 --> 00:05:05,556 Well, one idea which is particularly interesting 54 00:05:05,647 --> 00:05:09,799 is the idea that a large star explodes as a supernova 55 00:05:09,887 --> 00:05:13,482 and the shock wave from that supernova slams into the cloud 56 00:05:13,567 --> 00:05:17,719 and that starts the cloud contracting and then gravity takes over from there. 57 00:05:21,527 --> 00:05:23,563 MANNING: Here, far out in space, 58 00:05:23,727 --> 00:05:27,766 scientists are observing new solar systems condensing from star dust, 59 00:05:28,407 --> 00:05:29,965 just like ours did. 60 00:05:35,447 --> 00:05:38,120 About four-and-a-half billion years ago, 61 00:05:38,287 --> 00:05:42,280 the cloud of star dust and gas that was to become our solar system 62 00:05:42,367 --> 00:05:43,959 began to spin. 63 00:05:45,527 --> 00:05:48,405 As the gravity pulled the material into the centre, 64 00:05:48,487 --> 00:05:49,966 it heated up. 65 00:05:50,927 --> 00:05:53,077 Suddenly, the young sun ignited. 66 00:05:54,247 --> 00:05:57,637 The lighter gas was swept far out into the solar system 67 00:05:57,887 --> 00:06:00,845 leaving heavier, rocky material near the sun 68 00:06:01,047 --> 00:06:03,607 which soon condensed to form rocky planets. 69 00:06:05,007 --> 00:06:09,285 Mars, furthest from the sun, then the Earth, then Venus. 70 00:06:11,167 --> 00:06:14,921 At this stage, was there anything to distinguish one from the others? 71 00:06:15,807 --> 00:06:18,765 KIDGER: They're very, very similar. They've all got an iron core, 72 00:06:18,847 --> 00:06:23,602 they've all got lighter silicate stuff floating on top, they seem very similar. 73 00:06:24,807 --> 00:06:28,516 MANNING: So, when the solar system condensed out of this gas, 74 00:06:28,607 --> 00:06:32,361 what's special about the Earth in the solar system? 75 00:06:32,447 --> 00:06:34,119 Well, really, not a lot. 76 00:06:34,207 --> 00:06:37,119 The Earth has exactly the same components 77 00:06:37,247 --> 00:06:39,556 as the sun and all the other planets. 78 00:06:43,287 --> 00:06:48,315 MANNING: But from early on, there was one thing which made Earth different from Venus and Mars, 79 00:06:48,487 --> 00:06:50,876 it alone had a large satellite. 80 00:06:51,767 --> 00:06:56,363 Visiting the moon was the first step in unravelling the history of the inner planets. 81 00:06:58,447 --> 00:07:01,962 MAN: 17 Houston, you are go for orbit, go for orbit. 82 00:07:02,607 --> 00:07:07,362 MANNING: Dr Harrison Schmitt is the only geologist to visit an alien world. 83 00:07:07,447 --> 00:07:09,961 SCHMITT: ... the corner and get that content, 84 00:07:10,887 --> 00:07:12,639 if I can get it. 85 00:07:14,727 --> 00:07:17,002 MANNING: While working on the moon, he looked back 86 00:07:17,087 --> 00:07:20,443 and saw how much our planet now differs from the others. 87 00:07:21,767 --> 00:07:23,644 HARRISON: We were in a deep mountain valley, 88 00:07:23,727 --> 00:07:29,085 deeper than the Grand Canyon of Colorado of the United States, 7000 feet on either side. 89 00:07:29,167 --> 00:07:32,159 We could see mountains rising above us, 90 00:07:32,367 --> 00:07:36,519 brilliantly illuminated by a sun brighter than any desert sun that you can imagine. 91 00:07:36,607 --> 00:07:39,075 But the only real colour that we could see, 92 00:07:39,167 --> 00:07:43,399 other than the spacecraft and maybe your colleague wandering around in the distance, 93 00:07:43,487 --> 00:07:46,365 was this beautiful blue and white marbled Earth 94 00:07:46,447 --> 00:07:48,244 with a desert beacon here and there 95 00:07:48,327 --> 00:07:52,798 that hung always over the same part of the mountain complex. 96 00:07:53,407 --> 00:07:56,160 In an absolutely stark, black background. 97 00:07:56,287 --> 00:07:59,996 It was something that one cannot really describe adequately. 98 00:08:00,087 --> 00:08:03,966 It's something that everyone should have a chance to see for themselves. 99 00:08:10,167 --> 00:08:14,683 MANNING: Something must have happened to make Earth so different from its neighbours. 100 00:08:15,167 --> 00:08:19,524 The moon enabled scientists to investigate the solar system's distant past. 101 00:08:20,127 --> 00:08:23,836 The moon has given us a window into the early history of the Earth 102 00:08:23,927 --> 00:08:27,363 that we never really expected to have, I don't think. 103 00:08:27,447 --> 00:08:30,996 We hoped it would be that way, but when we actually explored it, 104 00:08:31,087 --> 00:08:34,636 we found we had this beautiful window into that early history. 105 00:08:34,727 --> 00:08:39,118 Pitted and dusty in some respects, but still one that we could now 106 00:08:39,367 --> 00:08:42,564 understand better what kind of Earth we had 107 00:08:43,647 --> 00:08:45,842 three billion years ago and later. 108 00:08:46,607 --> 00:08:49,599 The crust as it formed began to record 109 00:08:49,687 --> 00:08:52,918 an extraordinarily violent period of solar system history 110 00:08:53,007 --> 00:08:57,125 in which the debris left over from the formation of the solar system 111 00:08:57,407 --> 00:09:00,843 was impacting the moon and the Earth at an incredible rate. 112 00:09:02,647 --> 00:09:06,037 MANNING: The surface provided scientists with a sort of clock. 113 00:09:06,167 --> 00:09:09,603 The more meteorite impacts, the older a surface was. 114 00:09:10,727 --> 00:09:14,197 This was true not just on the moon, but on all the inner planets. 115 00:09:15,927 --> 00:09:18,202 Meteorite craters were to prove crucial 116 00:09:18,287 --> 00:09:21,643 in working out the history of Earth's planetary neighbours. 117 00:09:23,527 --> 00:09:27,315 How Mars became a cold, dead world. 118 00:09:30,807 --> 00:09:33,685 How Venus, beneath a permanent cloud cover, 119 00:09:33,967 --> 00:09:36,686 developed a surface temperature of molten lead. 120 00:09:37,487 --> 00:09:40,638 And only on Earth are the conditions right for life. 121 00:09:42,807 --> 00:09:46,720 How is it then that the three rocky planets that began so similarly 122 00:09:46,807 --> 00:09:49,196 have ended up along such different paths? 123 00:09:49,607 --> 00:09:51,757 It's a bit like the Goldilocks story. 124 00:09:51,847 --> 00:09:56,079 You remember Goldilocks first tasted one porridge and it was too cold, 125 00:09:56,367 --> 00:09:57,800 that's like Mars. 126 00:09:58,007 --> 00:10:01,636 She then tasted another, that was too hot, that's like Venus. 127 00:10:01,967 --> 00:10:05,880 Then she tasted the baby bear's porridge and that was just right. 128 00:10:06,007 --> 00:10:07,884 And that's what Earth is now. 129 00:10:08,047 --> 00:10:11,084 The puzzle is how did Earth come to be just right? 130 00:10:14,847 --> 00:10:17,998 MANNING: As scientists continue to explore the solar system, 131 00:10:18,087 --> 00:10:20,920 answers to the Goldilocks problem are beginning to emerge 132 00:10:21,007 --> 00:10:24,363 from studies of our planetary neighbours, Mars and Venus. 133 00:10:29,847 --> 00:10:32,281 Mike Carr is an expert on Mars. 134 00:10:33,527 --> 00:10:36,917 He noticed that some geological features on the surface of that planet 135 00:10:37,007 --> 00:10:39,077 look surprisingly Earth-like. 136 00:10:39,647 --> 00:10:41,763 And that lead him to conclude that Mars 137 00:10:41,847 --> 00:10:44,964 was not always as cold and dry as it is today. 138 00:10:47,727 --> 00:10:51,322 We do have evidence that Mars and the Earth 139 00:10:51,407 --> 00:10:54,001 started off relatively similarly, 140 00:10:54,087 --> 00:10:55,315 two rocky planets. 141 00:10:55,927 --> 00:11:00,159 That's right. When we look back at early Mars, 142 00:11:00,247 --> 00:11:03,205 it appears that early Mars was warm and wet 143 00:11:03,287 --> 00:11:05,198 and then it changed later. 144 00:11:05,287 --> 00:11:09,360 CARR: The evidence for that is that when we look at the oldest terrains on Mars, 145 00:11:09,447 --> 00:11:12,439 what we see are little river valleys all over the place. 146 00:11:12,527 --> 00:11:15,997 And they appear to have formed by slow erosion of running water. 147 00:11:16,287 --> 00:11:20,280 Well, to have water at the surface, you had to have warm, warm conditions. 148 00:11:20,407 --> 00:11:23,285 How do we know that was early in Mars' history? 149 00:11:23,687 --> 00:11:26,076 Well, it's earliest Mars because... 150 00:11:27,527 --> 00:11:32,647 Very early in Mars' history, there was what we call heavy bombardment, 151 00:11:32,847 --> 00:11:35,441 when the rate of impact of meteorites was very high, 152 00:11:35,527 --> 00:11:38,599 and so all these old terrains are very heavily cratered. 153 00:11:38,967 --> 00:11:43,324 And then on the crater rims in-between the craters we see all these river valleys. 154 00:11:49,487 --> 00:11:52,843 MANNING: Four billion years ago, Mars was not unlike Earth. 155 00:11:55,847 --> 00:11:58,884 The sky would have been full of clouds of water vapour. 156 00:12:03,687 --> 00:12:06,155 From the clouds, rain fell. 157 00:12:07,927 --> 00:12:11,124 Streams and rivers formed, eroding valleys. 158 00:12:15,927 --> 00:12:18,919 But what kept the young Mars so warm and wet? 159 00:12:20,327 --> 00:12:24,400 Clues to that puzzle can also be found on the surface of Mars today. 160 00:12:25,367 --> 00:12:29,155 CARR: The volcanoes on Mars are huge, absolutely huge. 161 00:12:29,687 --> 00:12:33,805 The crust is stable, the volcano just keeps growing and growing and growing 162 00:12:33,887 --> 00:12:35,718 to these enormous sizes. 163 00:12:35,927 --> 00:12:38,566 Well, Olympus Mons, which is the highest volcano, 164 00:12:38,647 --> 00:12:40,478 it is 84,000 feet high. 165 00:12:40,567 --> 00:12:44,355 It's 550 kilometres across. It really is a huge volcano. 166 00:12:46,447 --> 00:12:50,042 MANNING: Olympus Mons is almost three times the height of Mount Everest 167 00:12:50,127 --> 00:12:51,958 and half the size of France. 168 00:12:52,647 --> 00:12:55,286 It is the highest volcano in the solar system. 169 00:12:56,887 --> 00:12:59,799 On Earth, volcanoes produce a lot of gases, 170 00:12:59,887 --> 00:13:01,639 including carbon dioxide, 171 00:13:01,967 --> 00:13:04,037 which get added to the atmosphere. 172 00:13:05,167 --> 00:13:07,556 And carbon dioxide has a decisive effect 173 00:13:07,647 --> 00:13:09,797 on the planet's surface temperature. 174 00:13:10,087 --> 00:13:12,362 It's a so-called greenhouse gas, 175 00:13:12,647 --> 00:13:16,720 acting like a thermal blanket, keeping a planet warm. 176 00:13:19,487 --> 00:13:22,877 So four billion years ago, the huge Martian volcanoes 177 00:13:22,967 --> 00:13:25,720 must have been pumping out enough carbon dioxide 178 00:13:25,807 --> 00:13:28,526 to keep the planet's surface warm and wet. 179 00:13:33,447 --> 00:13:35,244 But Mars today is dry, 180 00:13:35,327 --> 00:13:37,397 with no sign of running water. 181 00:13:37,807 --> 00:13:40,116 Something dramatic must have happened. 182 00:13:44,807 --> 00:13:48,243 The explanation may lie in some mysterious features on Mars. 183 00:13:49,127 --> 00:13:51,402 Mike Carr estimates these were created 184 00:13:51,487 --> 00:13:55,082 500 million years after the Martian river valleys formed. 185 00:14:08,167 --> 00:14:10,727 Here were cliffs hundreds of metres high, 186 00:14:11,047 --> 00:14:12,844 suggesting water erosion, 187 00:14:13,087 --> 00:14:14,759 but no sign of rivers. 188 00:14:17,367 --> 00:14:20,359 They reminded him of a place called Dry Falls 189 00:14:20,447 --> 00:14:22,085 in America's northwest. 190 00:14:25,927 --> 00:14:28,361 Mike Carr's satellite photographs of Mars 191 00:14:28,527 --> 00:14:30,802 show what the two sites have in common. 192 00:14:31,647 --> 00:14:35,925 CARR: First, there's evidence of deep erosion such as cut this valley here 193 00:14:36,127 --> 00:14:37,196 and we see that here. 194 00:14:37,287 --> 00:14:38,356 MANNING: Yes, these are big... 195 00:14:38,447 --> 00:14:40,483 - These are big cliffs. - Big, big cliffs. 196 00:14:40,567 --> 00:14:43,684 - Yes. - We also see scour marks on the ground here 197 00:14:43,767 --> 00:14:45,723 and you can trace it all the way through here. 198 00:14:45,807 --> 00:14:48,367 And just above the Dry Falls over here, 199 00:14:48,447 --> 00:14:52,645 you can see these same scour marks that just go northward from Dry Falls. 200 00:14:53,407 --> 00:14:55,967 So, all the things that we see here 201 00:14:56,487 --> 00:14:59,479 on this map, we also see here in this location. 202 00:15:01,447 --> 00:15:03,642 MANNING: The explanation for Dry Falls 203 00:15:03,727 --> 00:15:06,605 should provide some clues to what happened on Mars. 204 00:15:10,007 --> 00:15:13,317 Geologists have worked out that Dry Falls were created 205 00:15:13,407 --> 00:15:17,161 when the last Ice Age ended, 12,000 years ago. 206 00:15:25,167 --> 00:15:27,123 As the climate gently recovered, 207 00:15:27,207 --> 00:15:30,882 a vast lake of melt water, as big as today's Great Lakes, 208 00:15:30,967 --> 00:15:33,037 formed behind a thick dam of ice. 209 00:15:37,927 --> 00:15:39,679 When the dam finally broke, 210 00:15:39,767 --> 00:15:43,282 the lake emptied, creating a cataclysmic flood. 211 00:15:46,567 --> 00:15:50,879 The flow of the water was greater than all the rivers of the world combined. 212 00:15:52,207 --> 00:15:55,483 Enormous boulders and lumps of ice scoured the land. 213 00:16:06,247 --> 00:16:08,636 Their force was so great at Dry Falls 214 00:16:08,727 --> 00:16:11,799 that the erosion formed cliffs like Niagara Falls. 215 00:16:19,767 --> 00:16:22,327 This was briefly a giant waterfall. 216 00:16:23,207 --> 00:16:26,040 Could something similar have happened on Mars? 217 00:16:26,847 --> 00:16:30,806 Mike Carr's Mars images also show evidence of a vast flood, 218 00:16:31,087 --> 00:16:33,885 but no sign of giant lakes to hold the water. 219 00:16:35,247 --> 00:16:37,886 Now, where did the water come from? 220 00:16:38,247 --> 00:16:40,317 Well, that's something of a puzzle. 221 00:16:40,407 --> 00:16:43,365 If you follow this large valley here 222 00:16:43,447 --> 00:16:46,120 it ends up in a large depression full of rubble. 223 00:16:46,207 --> 00:16:49,119 And the same... We see all these depressions here, full of rubble, 224 00:16:49,207 --> 00:16:50,959 out of which come very large channels. 225 00:16:51,047 --> 00:16:54,722 - MANNING: There's nothing flowing into them. - That's right, there's nothing flowing into them. 226 00:16:54,807 --> 00:16:59,164 And so what must have happened is the water must have come out of the ground. 227 00:17:00,287 --> 00:17:02,482 MANNING: Somehow the water must have been trapped, 228 00:17:02,567 --> 00:17:04,797 causing the pressure to build up. 229 00:17:07,007 --> 00:17:10,682 Well, one way of keeping the water under high pressure 230 00:17:10,967 --> 00:17:13,003 is by having a cap 231 00:17:13,207 --> 00:17:15,767 and the frozen ground is that cap. 232 00:17:15,847 --> 00:17:18,645 So the temperatures on Mars when these large floods formed 233 00:17:18,727 --> 00:17:20,365 were probably very cold. 234 00:17:20,447 --> 00:17:22,403 So you had a frozen layer 235 00:17:22,847 --> 00:17:25,964 at the surface, half a kilometre to a kilometre thick. 236 00:17:26,047 --> 00:17:28,356 - Permafrost. - Permafrost. Permafrost. 237 00:17:29,247 --> 00:17:33,286 And that prevented water leaking out onto the surface. 238 00:17:33,567 --> 00:17:35,319 And it became kind of unstable. 239 00:17:35,407 --> 00:17:37,921 Be rather like an oil well, in other words. 240 00:17:38,007 --> 00:17:41,761 Very much like an oil well. Of course, if you drill into an oil well, 241 00:17:41,967 --> 00:17:43,685 the oil comes gushing out. 242 00:17:43,767 --> 00:17:46,998 Here, a meteorite drills through that permafrost, 243 00:17:47,087 --> 00:17:50,682 the water comes gushing out in enormous volumes. 244 00:17:54,607 --> 00:17:56,404 MANNING: So, by three billion years ago, 245 00:17:56,487 --> 00:17:59,479 Mars had already entered a permanent Ice Age. 246 00:18:00,087 --> 00:18:04,126 Its water was locked up below the surface and was only occasionally released 247 00:18:04,207 --> 00:18:08,200 when a giant meteorite punched through the outer layer of permafrost. 248 00:18:10,007 --> 00:18:13,317 Something must have turned Mars from a warm, wet planet 249 00:18:13,407 --> 00:18:15,238 into a frozen world. 250 00:18:26,767 --> 00:18:30,203 There's one place on Earth which suggests how this happened. 251 00:18:48,767 --> 00:18:52,555 Here, at Mono Lake, in the desert of central California, 252 00:18:52,807 --> 00:18:55,719 Mike Rampino believes there are clues 253 00:18:55,807 --> 00:19:00,278 to what happened to the blanket of greenhouse gas which kept Mars warm. 254 00:19:04,207 --> 00:19:09,679 Why is Mars too cold, Venus too warm and the Earth just right for life? 255 00:19:10,487 --> 00:19:12,796 It turns out that the key to that question 256 00:19:12,887 --> 00:19:15,765 could be seen in these rocks here at Mono Lake. 257 00:19:16,087 --> 00:19:17,884 These rocks are solid rock 258 00:19:17,967 --> 00:19:20,606 but they contain a gas, carbon dioxide. 259 00:19:20,727 --> 00:19:24,766 If we drop a small piece of this rock in acid, it will bubble. 260 00:19:24,847 --> 00:19:27,839 The bubbles coming out of the rock are carbon dioxide. 261 00:19:31,327 --> 00:19:36,276 MANNING: The carbon dioxide in the rocks here has been drawn out of the Earth's atmosphere. 262 00:19:38,847 --> 00:19:40,678 What's happening here at Mono Lake 263 00:19:40,767 --> 00:19:44,396 is very similar to what happened on Mars three and half billion years ago. 264 00:19:44,487 --> 00:19:45,966 These rocks are here 265 00:19:46,247 --> 00:19:50,035 because rainwater picks up carbon dioxide from the atmosphere, 266 00:19:50,487 --> 00:19:51,966 forming a weak acid, 267 00:19:52,367 --> 00:19:55,882 which begins to dissolve away the granite rocks of the mountains, 268 00:19:56,007 --> 00:20:00,319 washing the mineral material and the carbon dioxide into the lake waters. 269 00:20:01,887 --> 00:20:05,516 MANNING: As carbon dioxide is washed from the atmosphere into the lake, 270 00:20:05,607 --> 00:20:07,962 conditions are set up for a chemical reaction 271 00:20:08,047 --> 00:20:10,686 that traps it into newly formed rocks. 272 00:20:12,367 --> 00:20:15,006 RAMPINO: When that material gets there, it's concentrated 273 00:20:15,087 --> 00:20:18,477 by the evaporation of the water in this very dry environment. 274 00:20:19,127 --> 00:20:22,722 Eventually, the material in Mono Lake 275 00:20:22,807 --> 00:20:24,445 produces the deposits 276 00:20:24,527 --> 00:20:27,644 that form those strange rock formations we were looking at 277 00:20:27,727 --> 00:20:30,685 - and trap the carbon dioxide. - MANNING: Right. 278 00:20:36,447 --> 00:20:38,915 MANNING: Mike Rampino showed me springs in the lake 279 00:20:39,007 --> 00:20:41,885 where you can actually see this process in action. 280 00:20:43,087 --> 00:20:44,679 Goodness. 281 00:20:44,767 --> 00:20:47,156 You can see the spring water bubbling up 282 00:20:47,247 --> 00:20:49,203 - through the lake bottom. - Yeah. 283 00:20:49,447 --> 00:20:51,677 And that's where you get the deposition 284 00:20:51,767 --> 00:20:55,555 of the carbonate trapping rock that forms around the lake. 285 00:20:55,647 --> 00:20:58,366 And out there, is that a lump which is actually forming? 286 00:20:58,447 --> 00:21:02,440 Yes, that's where the deposition of this limestone rock is taking place, 287 00:21:02,567 --> 00:21:06,116 which is the material that traps the carbon dioxide from the atmosphere. 288 00:21:06,207 --> 00:21:09,085 Now imagine this going on on a planetary scale, 289 00:21:09,167 --> 00:21:12,603 in the early history of the solar system, on the Earth or on Mars. 290 00:21:12,687 --> 00:21:15,406 Bodies of water like Mono Lake, maybe even larger 291 00:21:15,487 --> 00:21:18,877 where this process of formation of limestone is taking place, 292 00:21:19,007 --> 00:21:21,362 trapping carbon dioxide from the atmosphere, 293 00:21:21,447 --> 00:21:25,759 and taking the carbon dioxide out of the atmosphere permanently, in some cases. 294 00:21:34,967 --> 00:21:39,279 MANNING: When Mars was young, the carbon dioxide being locked up in its rocks 295 00:21:39,487 --> 00:21:43,116 would have been replaced by gas erupting from the huge volcanoes. 296 00:21:44,167 --> 00:21:47,955 But it's clear that eventually this volcanic activity ceased. 297 00:21:50,727 --> 00:21:53,878 One can look at the volcanoes on Mars from orbit 298 00:21:54,447 --> 00:21:57,325 and see impact craters on the surface of those volcanoes. 299 00:21:57,407 --> 00:21:58,999 Ancient impact craters. 300 00:21:59,127 --> 00:22:02,836 It shows that that surface has been sitting there for a billion years 301 00:22:02,927 --> 00:22:05,521 without any new flows of lava covering it up. 302 00:22:06,167 --> 00:22:08,397 MANNING: Mars is much smaller than Earth 303 00:22:08,487 --> 00:22:11,684 and its internal store of heat powering the volcanoes 304 00:22:11,767 --> 00:22:13,405 was quickly reduced. 305 00:22:15,207 --> 00:22:17,516 Eventually the volcanic activity stopped 306 00:22:17,607 --> 00:22:20,997 and all of the carbon dioxide in the atmosphere, or almost all of it, 307 00:22:21,087 --> 00:22:24,841 was removed from the Martian atmosphere by this process. 308 00:22:25,927 --> 00:22:29,966 MANNING: Because its greenhouse atmosphere was soon locked up in its rocks, 309 00:22:30,047 --> 00:22:31,958 Mars quickly froze. 310 00:22:32,287 --> 00:22:34,926 And it remains frozen to this day. 311 00:22:40,407 --> 00:22:44,605 MANNING: What about Venus, its surface a mystery beneath the layer of clouds? 312 00:22:53,807 --> 00:22:58,562 In the 1980s, the Russians pulled off an amazing feat of space exploration. 313 00:23:02,647 --> 00:23:07,084 They actually managed to land a probe on the scorching surface of Venus. 314 00:23:08,847 --> 00:23:12,965 Before it burnt up, it sent back this one extraordinary image, 315 00:23:13,447 --> 00:23:15,915 looking just like volcanic lava on Earth. 316 00:23:20,247 --> 00:23:23,523 In the 1990s, NASA's satellite Magellan 317 00:23:23,767 --> 00:23:27,521 penetrated the clouds with radar and mapped the entire surface. 318 00:23:30,007 --> 00:23:32,362 The images were clearly of volcanoes. 319 00:23:33,287 --> 00:23:37,599 Suddenly, Venus began to appear like parts of Earth, only hotter. 320 00:23:47,447 --> 00:23:50,803 Ellen Stofan was a chief scientist on the Magellan mission. 321 00:23:52,407 --> 00:23:57,322 The satellite gave her important information about the types of volcanism on Venus. 322 00:24:00,247 --> 00:24:02,761 Almost everywhere on the surface of Venus, 323 00:24:02,847 --> 00:24:04,678 you see some sort of volcanic landform. 324 00:24:04,767 --> 00:24:08,282 There was huge lava flows, all kinds of small volcanoes, large volcanoes. 325 00:24:08,367 --> 00:24:11,723 So in that sense, this is very similar to Venus. 326 00:24:12,687 --> 00:24:16,282 But certain aspects of the volcanism on Venus are quite different. 327 00:24:17,047 --> 00:24:21,484 MANNING: The cinder cones so common here on La Palma appear to be rare on Venus. 328 00:24:23,527 --> 00:24:27,486 For example, here we have what are called cinders. 329 00:24:27,567 --> 00:24:30,365 Now, this has come off what's called an effusive eruption, 330 00:24:30,447 --> 00:24:34,076 where you have little clumps of magma or liquid rock 331 00:24:34,167 --> 00:24:36,761 that are thrown up in the air, propelled by gas. 332 00:24:36,847 --> 00:24:41,637 Those red blobs, they cool as they come down and they form these little pieces of lava. 333 00:24:41,727 --> 00:24:45,197 MANNING: So it's a real spurting type of eruption? STOFAN: Exactly. 334 00:24:48,527 --> 00:24:51,837 STOFAN: So these would be the glowing hot pieces and as they fall, they cool 335 00:24:51,927 --> 00:24:54,646 and turn into this feature which is a cinder cone. 336 00:24:54,727 --> 00:24:57,161 Now, we think we're going to see less of this on Venus 337 00:24:57,247 --> 00:25:01,559 because of that high, high atmospheric pressure. It just sort of suppresses that ability 338 00:25:01,647 --> 00:25:04,366 of volcanoes to throw things up in the air. 339 00:25:04,447 --> 00:25:08,486 What's going to be more typical are just lava flows coming out on the surface. 340 00:25:08,567 --> 00:25:13,197 More typical to this sort of eruption where you're just getting a lava flow coming out. 341 00:25:23,247 --> 00:25:25,522 MANNING: It'd be much smoother on Venus, would it? 342 00:25:25,607 --> 00:25:30,476 Very smooth, and in fact there are some lava flows that we've seen in the Magellan-data Venus 343 00:25:30,567 --> 00:25:33,479 that are as smooth as parking lots, they're just amazingly smooth, 344 00:25:33,567 --> 00:25:36,604 which is something you really don't see here on Earth. 345 00:25:37,127 --> 00:25:41,279 MANNING: Venus' surface is lava flattened by an incredibly high pressure. 346 00:25:41,807 --> 00:25:45,561 The pressure comes from an atmosphere crammed full of carbon dioxide. 347 00:25:46,087 --> 00:25:47,566 What does this mean? 348 00:25:50,927 --> 00:25:53,919 Clues come from the earliest history of Venus. 349 00:25:54,367 --> 00:25:56,085 It's the same size as Earth, 350 00:25:56,167 --> 00:25:59,842 so it wouldn't cool down as quickly as the smaller Mars did. 351 00:26:00,927 --> 00:26:02,599 STOFAN: At about three-and-a-half billion years ago, 352 00:26:02,687 --> 00:26:05,360 at the end of this early bombardment that had taken place, 353 00:26:05,447 --> 00:26:09,520 Venus and the Earth are not that different. Their atmospheres are very similar at this point. 354 00:26:09,607 --> 00:26:11,677 But there is a significant difference. 355 00:26:11,767 --> 00:26:14,281 Venus is closer to the sun and it's warmer. 356 00:26:14,367 --> 00:26:17,359 It's actually warmer than it is right now on Earth. 357 00:26:18,767 --> 00:26:22,123 Water, instead of being a liquid on the surface, 358 00:26:22,567 --> 00:26:25,684 is right at the point where it's wanting to be a gas. 359 00:26:25,767 --> 00:26:28,725 It's wanting to form water vapour in the atmosphere. 360 00:26:33,607 --> 00:26:37,725 As that happens, in the meantime, you've got volcanoes erupting. 361 00:26:37,967 --> 00:26:40,845 As a volcano erupts, it not only makes nice lava flows, 362 00:26:40,927 --> 00:26:46,081 it's also pumping carbon dioxide, silicon dioxide. It's putting a lot of gas into the atmosphere. 363 00:26:48,887 --> 00:26:54,405 Water vapour and carbon dioxide are both a type of gas that if they're in the atmosphere, 364 00:26:54,647 --> 00:26:57,639 they start to keep the heat from the sun in. 365 00:26:57,727 --> 00:27:00,719 They're not letting the heat escape. So, the surface starts warming up 366 00:27:00,807 --> 00:27:02,684 and warming up and warming up. 367 00:27:13,647 --> 00:27:16,719 MANNING: Ellen Stofan believes that as Venus got hotter, 368 00:27:16,807 --> 00:27:19,162 its climate became unstable. 369 00:27:24,207 --> 00:27:26,641 It goes back to the whole water question. 370 00:27:26,727 --> 00:27:31,243 Water actually helps to remove carbon dioxide from the atmosphere. 371 00:27:31,527 --> 00:27:34,758 On Venus again, the problem is, it's just a little too warm. 372 00:27:34,847 --> 00:27:37,839 There is not that liquid water that's wearing down the rocks. 373 00:27:37,927 --> 00:27:40,839 This whole chemical reaction just isn't taking place. 374 00:27:40,927 --> 00:27:43,839 So there's no way to get the carbon dioxide out of the atmosphere. 375 00:27:43,927 --> 00:27:48,682 The carbon dioxide is building up, building up, building up. It's not being fixed into rocks. 376 00:27:48,767 --> 00:27:50,962 And that's where this runaway greenhouse comes from. 377 00:27:51,047 --> 00:27:54,676 Those processes ran away, the surface heated up and heated up and heated up 378 00:27:54,767 --> 00:27:57,361 - till it's 500 degrees centigrade on the surface. - Right. 379 00:27:57,447 --> 00:28:00,996 And it all started at that point when Venus was just a little bit too close to the sun, 380 00:28:01,087 --> 00:28:04,636 a little bit too warm, and you end up with the Venus we have now. 381 00:28:05,047 --> 00:28:10,519 Without the ability to bring the carbon dioxide out of the atmosphere, the planet ran away. 382 00:28:11,087 --> 00:28:13,396 And that's why it's so hot there today. 383 00:28:16,207 --> 00:28:18,516 MANNING: The Magellan radar images were put together 384 00:28:18,607 --> 00:28:22,043 to form this computerised flyover of Venus. 385 00:28:22,327 --> 00:28:26,718 They show a surface locked into a temperature of nearly 500 degrees. 386 00:28:40,167 --> 00:28:44,957 So the climates of both Earth's neighbours have changed dramatically since they were young 387 00:28:45,047 --> 00:28:47,117 and are now at opposite extremes. 388 00:28:48,287 --> 00:28:52,724 What strikes me about the Earth and makes it so special 389 00:28:52,807 --> 00:28:56,720 is its stability over a long, long period of time. 390 00:28:57,447 --> 00:29:00,405 For example, there's been liquid water on the surface 391 00:29:00,487 --> 00:29:02,955 for over three and a half billion years. 392 00:29:03,607 --> 00:29:06,519 Yet if I've learnt one thing from this series, 393 00:29:06,727 --> 00:29:11,642 it's the compelling fact of how dynamic and variable our planet's been, 394 00:29:11,727 --> 00:29:14,400 constantly changing and recycling. 395 00:29:16,647 --> 00:29:19,844 Is it possible that this constant geological change 396 00:29:19,927 --> 00:29:23,363 may even be responsible for the Earth's climatic stability? 397 00:29:24,327 --> 00:29:27,637 Earth's surface is made up of a few large rocky plates, 398 00:29:28,167 --> 00:29:33,116 their edges marked by chains of volcanoes, earthquakes and mountain ranges. 399 00:29:34,447 --> 00:29:38,281 This moving system of rocky plates, called plate tectonics, 400 00:29:38,407 --> 00:29:41,160 is fundamental to the functioning of our planet. 401 00:29:45,007 --> 00:29:48,556 It was natural to assume that Venus had a similar system. 402 00:29:51,887 --> 00:29:55,163 As on Earth, the inner heat powering its volcanoes 403 00:29:55,247 --> 00:29:57,886 could also drive a system of moving plates. 404 00:29:58,927 --> 00:30:04,160 The Magellan radar camera should have enabled Ellen Stofan's team to detect these plates. 405 00:30:04,607 --> 00:30:08,486 STOFAN: When we started getting the Magellan data set of Venus and we had a global view, 406 00:30:08,567 --> 00:30:11,877 we started wondering where were the geologic features located? 407 00:30:11,967 --> 00:30:15,880 The reason that we cared about this is because if you look at the Earth's surface, 408 00:30:15,967 --> 00:30:18,845 the Earth's surface is broken up into large plates 409 00:30:18,927 --> 00:30:23,842 and most of the geologic activity is concentrated at the edges of those plates. 410 00:30:23,927 --> 00:30:26,395 - So things like volcanoes and faults... - Earthquakes? 411 00:30:26,487 --> 00:30:32,323 Yeah, exactly. So we said okay, if you can look for patterns on the Earth and see them so clearly, 412 00:30:32,407 --> 00:30:36,719 if Venus has plates, we should be able to look for geologic features and patterns. 413 00:30:36,807 --> 00:30:39,162 So we mapped out all the volcanoes on Venus. 414 00:30:39,247 --> 00:30:40,885 We mapped out the faults. 415 00:30:41,007 --> 00:30:44,079 And you couldn't find any plates, you couldn't find any plate edges. 416 00:30:44,167 --> 00:30:46,806 MANNING: Why is that a crucial difference? 417 00:30:47,087 --> 00:30:49,647 It's telling you that the Earth and Venus 418 00:30:50,047 --> 00:30:52,641 again, just differ fundamentally in the way they work. 419 00:30:52,727 --> 00:30:55,605 It's not only hotter on Venus, it's not only higher pressure, 420 00:30:55,687 --> 00:30:58,963 it's also the whole planet is operating in a different way. 421 00:30:59,127 --> 00:31:03,723 So, is it activity at the edges of these plates, movements and so on, 422 00:31:03,807 --> 00:31:06,275 which are crucial for the way Earth has gone? 423 00:31:06,367 --> 00:31:10,997 We think so. We think it's part of the whole system that we have here on the Earth, 424 00:31:11,327 --> 00:31:15,240 that the plates and how they move is a critical part of that. 425 00:31:15,407 --> 00:31:18,797 And Venus is showing us how unique the Earth really is. 426 00:31:20,487 --> 00:31:24,446 MANNING: And then it began to emerge how this unique movement of rocky plates 427 00:31:24,527 --> 00:31:27,200 might maintain the climatic stability of Earth. 428 00:31:29,727 --> 00:31:33,083 As they move, one plate can slide beneath another. 429 00:31:33,527 --> 00:31:37,884 This process, subduction, buries rock beneath the surface. 430 00:31:38,807 --> 00:31:43,085 Some rock melts and erupts out of volcanoes back onto the surface. 431 00:31:44,007 --> 00:31:46,077 This recycles the rocks. 432 00:31:48,287 --> 00:31:52,883 The circulation of rocks is what links climate with plate tectonics. 433 00:31:53,687 --> 00:31:57,123 And there are processes on the Earth, geological processes, 434 00:31:57,207 --> 00:32:01,598 that take the rocks that form on the surface, that lock up the carbon dioxide, 435 00:32:01,687 --> 00:32:05,236 push it down to depth inside the Earth, those rocks are heated up 436 00:32:05,367 --> 00:32:08,200 and the carbon dioxide is released back through the volcanoes. 437 00:32:08,287 --> 00:32:10,847 There is a cycling of the carbon dioxide in the Earth, 438 00:32:10,927 --> 00:32:15,125 that takes place because the Earth is an active, geologically active planet 439 00:32:15,207 --> 00:32:18,438 and has been so for the past four-and-a-half billion years. 440 00:32:21,767 --> 00:32:26,045 MANNING: So on Earth, volcanoes pump carbon dioxide into the atmosphere, 441 00:32:26,127 --> 00:32:27,765 as they do on Venus. 442 00:32:30,767 --> 00:32:36,683 Rain dissolves the carbon dioxide, just as happened on Mars very early in its history. 443 00:32:37,927 --> 00:32:41,044 And the carbon dioxide is trapped in rocks. 444 00:32:42,927 --> 00:32:46,602 But on Earth, most of that rock ends up on ocean floors. 445 00:32:47,487 --> 00:32:51,241 Then it's subducted down as a slab, burying the carbon. 446 00:32:52,207 --> 00:32:54,960 The carbon dioxide erupts from volcanoes 447 00:32:55,447 --> 00:32:59,042 and so completes a cycle which stabilises the planet. 448 00:33:00,127 --> 00:33:02,800 This cycle happens only on Earth. 449 00:33:10,927 --> 00:33:13,839 But why does Venus have no plate tectonics? 450 00:33:14,567 --> 00:33:20,005 The critical pointer was that it had no oceans, where rocks on Earth get buried by subduction. 451 00:33:21,447 --> 00:33:25,076 Subduction is really the key to the whole process of plate tectonics. 452 00:33:25,167 --> 00:33:29,285 It turns out that if you calculate sort of all the forces of what's going on, 453 00:33:29,367 --> 00:33:31,562 the pull of the slab as it sinks 454 00:33:31,647 --> 00:33:35,720 really helps the whole process of plate tectonics go as a system. 455 00:33:36,567 --> 00:33:40,242 MANNING: And for subduction to keep working, it needs a lubricant. 456 00:33:41,807 --> 00:33:45,516 STOFAN: And there is one thing that we know on Earth, is we have plenty of water. 457 00:33:45,607 --> 00:33:48,405 The water lowers the melting temperature of basalt, 458 00:33:48,487 --> 00:33:50,443 makes it easier for the slab to subduct. 459 00:33:50,527 --> 00:33:53,678 Really does, as you say, it lubricates the whole process. 460 00:33:56,127 --> 00:34:00,678 MANNING: So, water, it seems, is a vital part of plate tectonics on Earth. 461 00:34:07,527 --> 00:34:11,725 Ever since our planet looked like this, three-and-a-half billion years ago, 462 00:34:11,967 --> 00:34:18,440 the carbon cycle, driven by plate tectonics and by water, has maintained a temperate climate. 463 00:34:19,807 --> 00:34:25,040 But as I discovered, keeping this astonishing system going over billions of years 464 00:34:25,127 --> 00:34:28,278 has required another surprising factor. 465 00:34:39,887 --> 00:34:42,845 To find out more, I went to Paris. 466 00:34:43,447 --> 00:34:45,483 I've come to the Paris Observatory, 467 00:34:45,567 --> 00:34:50,118 which for centuries has been a centre of research on the Earth's motion through the heavens, 468 00:34:50,207 --> 00:34:52,960 motion determined by the force of gravity. 469 00:34:58,327 --> 00:35:02,036 Here, Jacques Laskar uses a gyroscope 470 00:35:02,127 --> 00:35:06,325 to demonstrate the way gravity affects the Earth as it spins on its axis. 471 00:35:16,007 --> 00:35:19,363 I've always been delighted by gyroscopes. 472 00:35:19,567 --> 00:35:26,439 The gyroscope, it can help us to understand the motion of the axis of the Earth. 473 00:35:27,287 --> 00:35:32,361 You see, if I put the gyroscope here, without it rotating, it will just fall down 474 00:35:32,447 --> 00:35:34,039 due to the gravity. 475 00:35:35,487 --> 00:35:38,160 But if I make it rotate now, 476 00:35:38,647 --> 00:35:43,118 if I just take it like that and if I put it back, 477 00:35:43,207 --> 00:35:44,925 it is rotating now. 478 00:35:45,007 --> 00:35:47,601 Instead of falling down, it will 479 00:35:48,767 --> 00:35:51,600 - just slowly... - Slowly rotate. 480 00:35:51,687 --> 00:35:55,726 Slowly rotate like that, then the... So, its axis will slowly rotate. 481 00:35:57,247 --> 00:36:00,603 In the case of the Earth, it's roughly the same thing. 482 00:36:00,727 --> 00:36:03,878 The axis of the Earth is tilted 483 00:36:03,967 --> 00:36:08,165 and the moon and the sun are attracting this part 484 00:36:08,247 --> 00:36:11,603 to make it go back in the upright position. 485 00:36:11,967 --> 00:36:18,725 And it will make the axis of the Earth slowly rotate in a period of 26,000 years. 486 00:36:19,567 --> 00:36:24,721 So, the sun and the moon are both trying to pull the Earth upright 487 00:36:24,807 --> 00:36:29,483 and the Earth is trying to stay on its angle of rotation. 488 00:36:29,607 --> 00:36:34,078 And the result is that the axis is itself, turning. 489 00:36:34,207 --> 00:36:37,882 - Yes, it's turning, but it's a very slow motion. - Very slow. 490 00:36:40,327 --> 00:36:45,355 MANNING: This slow rotation is controlled mainly by the combined gravity of sun and moon. 491 00:36:45,887 --> 00:36:50,358 But the planets play a part, too, and that upsets Earth's stability. 492 00:36:53,247 --> 00:36:58,480 In Jacques Laskar's computer are the orbits of the planets over millions of years. 493 00:37:00,087 --> 00:37:04,638 These shifting orbits produce a fluctuating gravitational pull on the Earth. 494 00:37:09,887 --> 00:37:12,276 They make the Earth's own tilt shift. 495 00:37:12,447 --> 00:37:16,440 So the red circle drawn here by its axis changes in size. 496 00:37:18,607 --> 00:37:22,680 Although small, these shifts have a major influence on climate. 497 00:37:23,087 --> 00:37:28,241 As the Earth wobbles, vast ice sheets wax and wane over much of the globe. 498 00:37:33,287 --> 00:37:36,040 But without the moon at the Earth's side, 499 00:37:36,127 --> 00:37:40,996 it seems these fluctuations could cause catastrophe, worse than any Ice Age. 500 00:37:42,167 --> 00:37:45,045 MANNING: Now, here's the moon. You can take it away... 501 00:37:45,127 --> 00:37:48,403 That's something which is difficult to do in reality, 502 00:37:48,487 --> 00:37:51,604 - but on the computer simulation, it is quite easy. - Yes, on the computer... 503 00:37:51,687 --> 00:37:54,155 So, you see, I put the moon out. 504 00:37:54,247 --> 00:37:55,646 (MANNING CHUCKLING) 505 00:37:55,807 --> 00:37:58,765 And the immediate effect, you see immediately 506 00:37:58,847 --> 00:38:03,477 we are getting to much higher tilt, you see how it's changing. 507 00:38:04,327 --> 00:38:06,158 MANNING: With the moon's gravity removed, 508 00:38:06,247 --> 00:38:09,876 the Earth comes much more under the influence of the other planets. 509 00:38:10,007 --> 00:38:12,521 They cause chaos in the tilt of its axis 510 00:38:12,607 --> 00:38:15,883 and the red circle changes, apparently unpredictably. 511 00:38:19,207 --> 00:38:22,677 In reality, the moon being so close and large 512 00:38:22,767 --> 00:38:25,645 keeps the Earth's tilt within narrow limits. 513 00:38:26,567 --> 00:38:30,401 If the moon did go away, Earth could tip to vertical. 514 00:38:33,607 --> 00:38:35,723 The result would be no seasons. 515 00:38:37,407 --> 00:38:41,719 And it could go to any degree of tilt, even almost horizontal. 516 00:38:42,687 --> 00:38:45,884 At this angle, one hemisphere would stay light for months 517 00:38:46,047 --> 00:38:47,321 the other dark for months, 518 00:38:47,407 --> 00:38:50,638 as the Earth moved round its annual orbit of the sun. 519 00:38:53,367 --> 00:38:58,725 Effectively, a year would become a day, with colossal effects on climate. 520 00:39:04,727 --> 00:39:08,515 And events like this have happened to planets without big moons. 521 00:39:08,767 --> 00:39:14,000 Mars wobbles all the time and Venus appears to have flipped upside down. 522 00:39:17,207 --> 00:39:21,598 No one can be sure, but Jacques Laskar's calculations suggest, 523 00:39:21,687 --> 00:39:22,961 fascinatingly, 524 00:39:23,127 --> 00:39:27,678 that the moon's steadying influence has enabled the Earth's climate to remain stable. 525 00:39:37,487 --> 00:39:41,765 But there is a final surprising twist to the complex story of our climate. 526 00:39:45,967 --> 00:39:49,084 Something with the power to overwhelm the intricate system 527 00:39:49,167 --> 00:39:51,317 regulating the planet's temperature. 528 00:39:55,967 --> 00:40:00,279 Astronomers have discovered that the sun itself is not completely stable. 529 00:40:00,687 --> 00:40:02,882 It's gradually heating up. 530 00:40:03,687 --> 00:40:08,761 Today, it burns 25% more brightly than it did when the solar system was young. 531 00:40:12,087 --> 00:40:17,320 This colossal change should've had an absolutely catastrophic effect on the Earth, 532 00:40:17,807 --> 00:40:21,038 and yet it appears that our world has hardly been affected. 533 00:40:22,807 --> 00:40:25,367 The evidence for this comes from Greenland, 534 00:40:25,647 --> 00:40:29,356 where geologists have found the oldest rocks on the Earth's surface. 535 00:40:34,087 --> 00:40:37,477 As I discovered in the very first programme of this series, 536 00:40:37,607 --> 00:40:42,203 these rocks paint a vivid picture of the planet nearly four billion years ago. 537 00:40:45,887 --> 00:40:50,039 Among them is this deposit of rounded pebbles in a muddy matrix, 538 00:40:50,527 --> 00:40:53,564 the remains of an ancient beach or shoreline. 539 00:40:55,647 --> 00:41:01,882 This is unequivocal proof that then, as now, there was liquid water on the Earth. 540 00:41:02,767 --> 00:41:06,077 So, despite the steady increase in the sun's activity, 541 00:41:06,167 --> 00:41:09,716 Earth's temperature has changed little in four billion years. 542 00:41:11,567 --> 00:41:16,243 Something which controls the planetary climate must be reacting to the changing sun, 543 00:41:16,447 --> 00:41:18,278 so keeping the world cool. 544 00:41:19,407 --> 00:41:20,556 But what? 545 00:41:26,807 --> 00:41:31,358 The answer may lie here, in one of the most famous of all landmarks, 546 00:41:31,847 --> 00:41:35,522 the white cliffs that look out over the English Channel. 547 00:41:37,407 --> 00:41:39,716 As Rory Mortimore explained to me, 548 00:41:40,047 --> 00:41:43,437 these rocks have enormous significance for global climate. 549 00:41:44,087 --> 00:41:47,204 What sort of depth of rock is there beneath our feet here? 550 00:41:47,287 --> 00:41:50,757 It's got to be at least 200 metres. 551 00:41:59,327 --> 00:42:02,319 MANNING: But it's not only the height of the cliffs which matters. 552 00:42:02,407 --> 00:42:05,479 It's what they're made of. Chalk. 553 00:42:05,927 --> 00:42:11,604 There's something special about chalk. It's not made by a purely geological process. 554 00:42:11,927 --> 00:42:14,077 It's made by living things. 555 00:42:15,487 --> 00:42:19,924 MORTIMER: It forms by a rain of plankton onto the seabed. 556 00:42:20,007 --> 00:42:24,205 They die in cycles and they come through in blooms in the oceans. 557 00:42:24,327 --> 00:42:28,605 So the chalk is made up of millions and millions of tiny coccoliths. 558 00:42:28,687 --> 00:42:31,565 They are so small that we cannot see them with the naked eye. 559 00:42:31,647 --> 00:42:35,686 And even with an ordinary light microscope, they're almost impossible to see. 560 00:42:37,247 --> 00:42:39,602 MANNING: As these tiny organisms grow, 561 00:42:39,727 --> 00:42:43,322 they use carbon dioxide from the atmosphere to build their shells. 562 00:42:44,367 --> 00:42:46,927 When they die and sink to the sea floor, 563 00:42:47,047 --> 00:42:51,563 these shells can become compressed to form chalk and other limestones. 564 00:42:57,727 --> 00:43:01,003 Let's look at this sample of chalk which we have here. 565 00:43:01,447 --> 00:43:04,086 Now, this weighs about six kilograms. 566 00:43:04,967 --> 00:43:10,837 And locked into it, we have about 1,300 litres of carbon dioxide. 567 00:43:10,927 --> 00:43:15,079 And that is an enormous amount of carbon dioxide locked into the rocks. 568 00:43:17,727 --> 00:43:20,924 MANNING: Today, almost all the carbon locked up in the rocks 569 00:43:21,007 --> 00:43:26,639 is found in deposits like limestone, chalk and coal, made from living things. 570 00:43:27,367 --> 00:43:29,198 In another words, on Earth today, 571 00:43:29,287 --> 00:43:32,484 it's life that pulls carbon dioxide from the atmosphere, 572 00:43:32,567 --> 00:43:35,559 locks it up and keeps our planet cool. 573 00:43:43,687 --> 00:43:47,965 So, at some point in Earth history, possibly billions of years ago, 574 00:43:48,247 --> 00:43:51,603 life must've taken over a key role in the carbon cycle 575 00:43:51,687 --> 00:43:54,247 which keeps the Earth's temperature stable. 576 00:44:00,367 --> 00:44:03,837 And of course, living things react to changes in sunlight. 577 00:44:04,527 --> 00:44:08,122 Perhaps as the energy from the sun increased, life flourished, 578 00:44:08,287 --> 00:44:11,404 drawing ever more carbon dioxide from the atmosphere. 579 00:44:12,207 --> 00:44:17,964 That gradually lessened the greenhouse effect and so kept the Earth from overheating. 580 00:44:28,727 --> 00:44:32,322 If life has played this crucial role in controlling the climate, 581 00:44:32,927 --> 00:44:36,203 we can perhaps begin to answer the question I started with. 582 00:44:36,967 --> 00:44:38,878 What makes our planet special 583 00:44:38,967 --> 00:44:43,836 is above all the unique partnership between the Earth and living things. 584 00:44:46,087 --> 00:44:50,478 That partnership can be seen in miniature here in La Palma. 585 00:44:53,247 --> 00:44:56,398 At its heart lies the planet's interior energy, 586 00:44:56,487 --> 00:44:59,877 which drives plate tectonics and the Earth's volcanism. 587 00:45:01,527 --> 00:45:03,995 This fantastic area here, 588 00:45:04,647 --> 00:45:08,845 which to me looks more like a giant rubbish heap of burnt plastic, 589 00:45:09,247 --> 00:45:14,446 is a graphic example of the Earth's volcanic activity. 590 00:45:14,847 --> 00:45:18,635 This was molten magma which flowed out of the volcanoes 591 00:45:18,967 --> 00:45:22,039 on June 24, 1949. 592 00:45:22,727 --> 00:45:26,003 It flowed down here and went down directly into the sea. 593 00:45:35,727 --> 00:45:39,322 Throughout Earth history, geological upheavals like these 594 00:45:39,407 --> 00:45:42,285 have been one of the driving forces of evolution. 595 00:45:55,687 --> 00:46:00,158 Living things today are the descendents of those that survived the many cataclysms 596 00:46:00,247 --> 00:46:02,044 unleashed by the planet. 597 00:46:03,167 --> 00:46:06,239 Like the pine trees on La Palma's volcanic slopes, 598 00:46:06,327 --> 00:46:11,447 whose insulating bark enables them to survive the fires triggered by volcanic eruptions. 599 00:46:18,207 --> 00:46:22,120 It's a remarkable thought that the planet's activity shapes life. 600 00:46:22,927 --> 00:46:27,125 But to me, even more astonishing is that through the carbon cycle, 601 00:46:27,207 --> 00:46:31,120 life plays a major role in maintaining Earth's activity. 602 00:46:33,807 --> 00:46:36,526 Whole hillsides of bananas on La Palma 603 00:46:36,767 --> 00:46:39,600 are also part of the planet's carbon cycle, 604 00:46:40,047 --> 00:46:44,518 which began here with gases released in the last volcanic eruption. 605 00:46:47,967 --> 00:46:52,006 Some of the carbon dioxide that came out of this volcano 27 years ago, 606 00:46:52,327 --> 00:46:57,447 is now fixed in these bananas and if I eat one, some of it will come out in my breath. 607 00:46:58,007 --> 00:47:01,602 Rainwater may dissolve it and it will end up in the ocean 608 00:47:02,047 --> 00:47:05,437 and get deposited in rocks deep in the ocean. 609 00:47:06,367 --> 00:47:10,042 Thence it'll move along on the plates and get subducted, 610 00:47:10,487 --> 00:47:13,001 and perhaps 100 million years from now, 611 00:47:13,087 --> 00:47:18,400 it will be erupted again from a volcano to complete this remarkable cycle. 612 00:47:22,087 --> 00:47:24,840 For millions of years, this partnership has controlled 613 00:47:24,927 --> 00:47:27,805 the level of carbon dioxide in our atmosphere, 614 00:47:27,927 --> 00:47:32,079 keeping the Earth's climate in the narrow zone where water remains liquid, 615 00:47:32,247 --> 00:47:36,798 neither freezing as on Mars nor boiling away as on Venus. 616 00:47:36,887 --> 00:47:41,005 In turn, liquid water has lubricated the motion of the plates 617 00:47:41,287 --> 00:47:44,199 and of course without it, life would be impossible. 618 00:47:46,047 --> 00:47:50,120 So every part of this astonishing system is essential. 619 00:47:52,687 --> 00:47:57,681 In Earth Story, I set out to explore the links between the Earth and life. 620 00:47:58,447 --> 00:48:03,157 Those links have turned out to be far more profound than I ever imagined. 621 00:48:04,607 --> 00:48:09,920 For me, this television series has been a scientific journey of discovery. 622 00:48:10,327 --> 00:48:13,842 I've learnt to look at the planet in a totally different way. 623 00:48:14,607 --> 00:48:19,476 As an intricate system in which changes to one part affect all the others. 624 00:48:20,567 --> 00:48:24,162 And as living beings, we're part of that system. 625 00:48:24,327 --> 00:48:29,037 And our own story is intimately tied up with the story of the Earth, 626 00:48:29,407 --> 00:48:33,195 our ancient, extraordinary living home.