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These are the user uploaded subtitles that are being translated: 1 00:00:21,300 --> 00:00:24,420 The night sky is ablaze with stars... 2 00:00:26,900 --> 00:00:29,980 ..hundreds of billions in our galaxy alone... 3 00:00:31,900 --> 00:00:35,940 ..many larger, brighter and more majestic than our sun. 4 00:00:42,460 --> 00:00:46,260 On the scale of galaxies and stars, the planets of our solar system 5 00:00:46,260 --> 00:00:49,260 are little more than grains of sand caught momentarily 6 00:00:49,260 --> 00:00:51,060 in the light of the sun. 7 00:00:51,060 --> 00:00:55,300 But on those motes of dust, for over four billion years, 8 00:00:55,300 --> 00:00:57,860 great stories have played out unseen. 9 00:01:01,900 --> 00:01:03,820 Stories of worlds born... 10 00:01:05,460 --> 00:01:07,180 ..and worlds lost. 11 00:01:12,900 --> 00:01:15,220 Planets forged amongst the calm... 12 00:01:17,980 --> 00:01:19,020 ..and the chaos. 13 00:01:22,460 --> 00:01:26,380 Their destinies more entwined than we ever imagined. 14 00:01:30,340 --> 00:01:32,860 We know this... 15 00:01:32,860 --> 00:01:35,820 ..because in the last few decades... 16 00:01:35,820 --> 00:01:39,260 ..we've sent spacecraft to all seven... 17 00:01:39,260 --> 00:01:41,380 ..of the worlds beyond our own. 18 00:01:45,540 --> 00:01:48,620 These are the stories that they return to Earth, 19 00:01:48,620 --> 00:01:50,580 the stories of the planets. 20 00:02:17,540 --> 00:02:20,780 For the first few million years after the sun formed, 21 00:02:20,780 --> 00:02:23,500 there were no planets to see it rise... 22 00:02:30,900 --> 00:02:34,580 ..just clouds of dust and the gas... 23 00:02:38,900 --> 00:02:41,540 ..the leftovers from the birth of the sun. 24 00:02:46,780 --> 00:02:52,300 Over tens of millions of years, the dust began to stick together... 25 00:02:52,300 --> 00:02:54,260 ..and form the first rocks. 26 00:02:59,980 --> 00:03:02,900 Eventually, gravity assembled the rocks 27 00:03:02,900 --> 00:03:05,180 to create planetary embryos... 28 00:03:08,620 --> 00:03:12,860 ..that in time formed the four closest planets to the sun. 29 00:03:31,700 --> 00:03:35,060 Today, Mercury is the closest of all, 30 00:03:35,060 --> 00:03:37,380 enduring the sun's full glare. 31 00:03:40,180 --> 00:03:42,020 Further out... 32 00:03:42,020 --> 00:03:43,380 ..lies Venus... 33 00:03:46,140 --> 00:03:49,100 ..chocked by a thick atmosphere. 34 00:03:51,140 --> 00:03:53,700 Then Venus's neighbour, Earth. 35 00:03:56,620 --> 00:03:59,740 And farthest of all, Mars... 36 00:04:04,220 --> 00:04:06,620 ..a cold desert world. 37 00:04:09,180 --> 00:04:13,500 Together, they formed the only rocky so-called terrestrial planets 38 00:04:13,500 --> 00:04:14,820 in the solar system. 39 00:04:16,300 --> 00:04:18,020 And of the four... 40 00:04:18,020 --> 00:04:20,300 ..one is unique. 41 00:04:33,460 --> 00:04:34,860 Just look at this... 42 00:04:35,860 --> 00:04:37,620 ..and listen to it. 43 00:04:37,620 --> 00:04:40,180 This is what a planet looks and sounds like 44 00:04:40,180 --> 00:04:44,300 after four billion years of evolution by natural selection. 45 00:04:44,300 --> 00:04:46,740 There is nowhere else in the solar system 46 00:04:46,740 --> 00:04:48,620 that looks and sounds like this, 47 00:04:48,620 --> 00:04:51,300 which is interesting, when you think about it, 48 00:04:51,300 --> 00:04:54,940 because all the planets and moons and are made out of the same stuff - 49 00:04:54,940 --> 00:04:58,420 they're carbon, nitrogen, oxygen, iron. 50 00:04:58,420 --> 00:05:01,980 All those atoms were present in the cloud that collapsed 51 00:05:01,980 --> 00:05:05,380 to form the solar system, four-and-a-half billion years ago. 52 00:05:06,860 --> 00:05:10,300 And yet Earth appears to be exceptional - 53 00:05:10,300 --> 00:05:14,460 a lone living planet in an otherwise desolate solar system. 54 00:05:16,020 --> 00:05:20,100 So what is it that makes this place so special? 55 00:05:20,100 --> 00:05:22,500 Is it fate? Is it chance? 56 00:05:22,500 --> 00:05:24,300 These are important questions 57 00:05:24,300 --> 00:05:26,620 because Earth is the only place we know of 58 00:05:26,620 --> 00:05:30,460 where the most complex phenomena in the universe exists, 59 00:05:30,460 --> 00:05:34,340 the thing that brings meaning to the universe - life. 60 00:05:56,980 --> 00:06:00,860 Earth is a special world in our solar system 61 00:06:00,860 --> 00:06:04,500 and perhaps even for thousands of light years beyond. 62 00:06:12,420 --> 00:06:15,700 Our world certainly has unique properties. 63 00:06:17,140 --> 00:06:20,180 It's the right size and distance from the sun 64 00:06:20,180 --> 00:06:24,300 to have retained an atmosphere that's protected its oceans 65 00:06:24,300 --> 00:06:27,380 of life-giving water for billions of years. 66 00:06:32,740 --> 00:06:35,620 But as we've left the blue planet, 67 00:06:35,620 --> 00:06:37,860 and explored our sister worlds... 68 00:06:42,580 --> 00:06:47,500 ..we've discovered that each appears to have had a moment 69 00:06:47,500 --> 00:06:50,900 when it enjoyed almost Earth-like conditions. 70 00:07:00,900 --> 00:07:06,580 Every one of our rocky neighbours has a story of what might have been. 71 00:07:26,820 --> 00:07:29,980 Mercury is a small, tortured world. 72 00:07:34,420 --> 00:07:36,180 More than any other planet, 73 00:07:36,180 --> 00:07:41,260 it's endured the unflinching glare of the sun for billions of years. 74 00:07:45,500 --> 00:07:49,700 Mercury is a world of mystery and apparent contradictions. 75 00:07:49,700 --> 00:07:51,420 It's in quite an elliptical orbit, 76 00:07:51,420 --> 00:07:55,300 which means it can be as far away from the sun as 70 million km, 77 00:07:55,300 --> 00:07:58,300 but as close as 46 million. 78 00:07:58,300 --> 00:08:01,100 That means that temperatures at midday 79 00:08:01,100 --> 00:08:04,020 can rise to 430 degrees Celsius on the surface, 80 00:08:04,020 --> 00:08:08,020 but, at night, because it's a small planet and it's got no atmosphere, 81 00:08:08,020 --> 00:08:11,740 temperatures fall to minus 170 degrees. 82 00:08:11,740 --> 00:08:14,980 It's also locked into what's called the spin-orbit resonance, 83 00:08:14,980 --> 00:08:19,980 which means the planet spins precisely three times on its axis 84 00:08:19,980 --> 00:08:21,820 for every two orbits 85 00:08:21,820 --> 00:08:26,900 and that in turn means that its day is twice as long as its year. 86 00:08:26,900 --> 00:08:29,580 That means that I could walk over the surface like this 87 00:08:29,580 --> 00:08:31,220 about 2mph 88 00:08:31,220 --> 00:08:34,140 and keep the sun at the same point in the sky. 89 00:08:34,140 --> 00:08:36,820 I could stroll in eternal twilight. 90 00:08:41,380 --> 00:08:45,420 Mercury is the least explored of the inner rocky worlds... 91 00:08:48,100 --> 00:08:52,820 ..because getting to a planet in such a strange oval-shaped orbit, 92 00:08:52,820 --> 00:08:54,500 so close to the sun... 93 00:08:55,500 --> 00:08:58,100 ..is a tremendous challenge. 94 00:08:58,100 --> 00:09:01,260 Five, four, three, 95 00:09:01,260 --> 00:09:04,780 main engine start, two, one and zero. 96 00:09:04,780 --> 00:09:08,620 And lift off of Messenger on Nasa's mission to Mercury... 97 00:09:12,500 --> 00:09:15,340 ..a planetary enigma in our inner solar system. 98 00:09:20,780 --> 00:09:23,020 Now going through the sound barrier. 99 00:09:24,300 --> 00:09:28,180 A direct route to Mercury is impractical. 100 00:09:28,180 --> 00:09:32,780 Now going through the period of maximum dynamic pressure. 101 00:09:32,780 --> 00:09:35,180 A spacecraft would arrive with so much speed 102 00:09:35,180 --> 00:09:38,620 that it would need vast amounts of fuel to slow down 103 00:09:38,620 --> 00:09:40,860 and enter orbit around Mercury. 104 00:09:40,860 --> 00:09:43,300 We just had spacecraft separation. 105 00:09:49,580 --> 00:09:52,420 So Messenger controlled its trajectory 106 00:09:52,420 --> 00:09:56,100 by stepping from one planet to the next, 107 00:09:56,100 --> 00:09:58,420 using gravity to slow itself, 108 00:09:58,420 --> 00:10:01,540 spiralling inwards towards its target. 109 00:10:16,580 --> 00:10:21,060 Even so, Messenger approached Mercury at such high speed 110 00:10:21,060 --> 00:10:24,940 that it was forced to fly past the planet three times... 111 00:10:27,140 --> 00:10:28,700 ..slowing on each pass... 112 00:10:38,580 --> 00:10:42,900 ..until, after almost seven years of flawless navigation, 113 00:10:42,900 --> 00:10:45,100 it arrived safely in orbit. 114 00:11:07,580 --> 00:11:11,420 Messenger set about its mission to map Mercury's surface... 115 00:11:15,500 --> 00:11:19,060 ..and began revealing the secrets of the most cratered planet 116 00:11:19,060 --> 00:11:23,060 in the solar system in exquisite new detail. 117 00:11:49,660 --> 00:11:51,700 Messenger was able to do much more 118 00:11:51,700 --> 00:11:54,460 than just take images of Mercury's surface. 119 00:11:56,020 --> 00:11:58,620 By tracking radio signals emitted by the spacecraft, 120 00:11:58,620 --> 00:12:02,940 we're able to see very slight changes in the orbital path 121 00:12:02,940 --> 00:12:05,540 around Mercury, as seen from Earth, 122 00:12:05,540 --> 00:12:08,820 and that allows us to map out Mercury's gravitational field. 123 00:12:08,820 --> 00:12:10,500 There are also instruments that allow us 124 00:12:10,500 --> 00:12:13,300 to see how the planet wobbles around 125 00:12:13,300 --> 00:12:14,860 as it spins on its axis. 126 00:12:14,860 --> 00:12:17,060 And putting all these measurements together 127 00:12:17,060 --> 00:12:20,940 allows us to take a cross section through the planet, 128 00:12:20,940 --> 00:12:22,860 to see what it's made of. 129 00:12:22,860 --> 00:12:25,980 And when we do that, we find something very strange. 130 00:12:25,980 --> 00:12:29,220 Mercury's core extends out about 85% 131 00:12:29,220 --> 00:12:32,860 from the centre of the planet to the surface. 132 00:12:32,860 --> 00:12:37,060 It's almost entirely an exposed planetary core. 133 00:12:37,060 --> 00:12:41,340 It's as if the rocks of the surface were smashed away 134 00:12:41,340 --> 00:12:43,420 and removed at some point in its past. 135 00:12:47,940 --> 00:12:49,340 And there was more. 136 00:12:50,500 --> 00:12:55,060 The tiny probe began detecting chemical elements in concentrations 137 00:12:55,060 --> 00:12:59,540 that no-one had thought possible this close to the sun. 138 00:13:05,660 --> 00:13:09,180 The discovery of relatively large concentrations of elements 139 00:13:09,180 --> 00:13:12,420 like sulphur and potassium on Mercury's surface 140 00:13:12,420 --> 00:13:14,340 was a huge surprise. 141 00:13:14,340 --> 00:13:17,820 If you think back to the time when the planets were forming, 142 00:13:17,820 --> 00:13:21,820 you don't expect high concentrations of those elements close to the sun, 143 00:13:21,820 --> 00:13:26,020 where Mercury orbits today, because they're so-called volatile elements. 144 00:13:26,020 --> 00:13:27,540 They boil away easily, 145 00:13:27,540 --> 00:13:30,900 so you only find high concentrations further out, 146 00:13:30,900 --> 00:13:33,580 in the colder reaches of the solar system. 147 00:13:33,580 --> 00:13:36,460 So Mercury is an enigma 148 00:13:36,460 --> 00:13:40,180 and discoveries like these have forced us to completely rethink 149 00:13:40,180 --> 00:13:42,940 our theories about the formation of the planet. 150 00:13:55,300 --> 00:13:58,340 Just a few million years after its formation, 151 00:13:58,340 --> 00:14:02,580 Mercury was still seething with the heat of its violent birth. 152 00:14:13,140 --> 00:14:16,500 Slowly, it cooled and a crust formed. 153 00:14:24,380 --> 00:14:27,980 Over time, the crust became enriched 154 00:14:27,980 --> 00:14:31,780 in the volatile elements that were escaping Mercury's interior. 155 00:14:34,180 --> 00:14:38,300 But this could only happen if Mercury started out 156 00:14:38,300 --> 00:14:40,380 not in the position we see it today... 157 00:14:41,940 --> 00:14:43,620 ..but much further out. 158 00:14:45,340 --> 00:14:51,340 We now think Mercury was born perhaps 170 million km 159 00:14:51,340 --> 00:14:54,300 further away, close to the orbit of Mars... 160 00:14:55,740 --> 00:14:58,900 ..a place where, if it had stayed, 161 00:14:58,900 --> 00:15:01,780 its destiny could have been very different. 162 00:15:19,620 --> 00:15:21,420 But it wasn't to be. 163 00:15:22,700 --> 00:15:26,740 The young planetary embryo was ripped from its promising position 164 00:15:26,740 --> 00:15:28,900 long before it could mature. 165 00:15:39,580 --> 00:15:42,140 Today, it's hard to imagine the planets 166 00:15:42,140 --> 00:15:45,580 in orbits other than the ones we see in the night sky. 167 00:15:45,580 --> 00:15:48,900 They feel eternal, permanent. 168 00:15:50,500 --> 00:15:52,540 It's natural to think of the solar system 169 00:15:52,540 --> 00:15:56,180 as a piece of celestial clockwork, almost like a Swiss watch. 170 00:15:56,180 --> 00:15:59,900 So if we knew where all the planets were at some point in time, 171 00:15:59,900 --> 00:16:03,260 let's say today, then we could imagine calculating 172 00:16:03,260 --> 00:16:07,260 exactly where they're going to be at any point in time. 173 00:16:07,260 --> 00:16:11,860 Now, that is true if there's only one planet and one star. 174 00:16:11,860 --> 00:16:14,740 So imagine that's the sun and this is Mercury. 175 00:16:14,740 --> 00:16:18,660 Now, we know the gravitational force between Mercury and the sun. 176 00:16:18,660 --> 00:16:21,460 And, indeed, if that's all there is, 177 00:16:21,460 --> 00:16:24,780 then we can calculate its orbit around the sun 178 00:16:24,780 --> 00:16:27,340 with essentially infinite precision. 179 00:16:27,340 --> 00:16:32,460 But add in one more planet, let's say Jupiter over there. 180 00:16:32,460 --> 00:16:36,900 Now there's a gravitational force between all three of these objects 181 00:16:36,900 --> 00:16:40,220 and it turns out that, even in principle, 182 00:16:40,220 --> 00:16:42,820 it is not possible to calculate exactly 183 00:16:42,820 --> 00:16:45,620 where they're all going to be in the future 184 00:16:45,620 --> 00:16:49,140 or where they were at some point in the past. 185 00:16:50,380 --> 00:16:54,180 This means that any uncertainty, even of a few metres 186 00:16:54,180 --> 00:16:57,020 in our knowledge of the position of the planets, 187 00:16:57,020 --> 00:16:59,700 can lead to radically different predictions. 188 00:17:01,380 --> 00:17:05,940 And that's because the system itself, the orbits of the planets, 189 00:17:05,940 --> 00:17:08,900 are not stable over very long timescales. 190 00:17:12,700 --> 00:17:17,260 So planets don't necessarily remain in the same orbits forever. 191 00:17:24,660 --> 00:17:26,380 And the evidence we've gathered 192 00:17:26,380 --> 00:17:28,620 from the volatiles on Mercury's surface, 193 00:17:28,620 --> 00:17:31,060 and the unusual size of its core, 194 00:17:31,060 --> 00:17:34,580 suggests that this may have been what happened. 195 00:17:43,580 --> 00:17:45,780 If Mercury began its life 196 00:17:45,780 --> 00:17:49,300 170 million km further away from the sun... 197 00:17:51,660 --> 00:17:53,660 ..then it would have been in a region of space 198 00:17:53,660 --> 00:17:55,900 where the young Mars was also forming. 199 00:18:02,780 --> 00:18:06,780 This region was full of scores of planetary embryos, 200 00:18:06,780 --> 00:18:08,700 all fighting for position. 201 00:18:17,540 --> 00:18:22,260 Amongst the chaos, something large kicked Mercury inwards 202 00:18:22,260 --> 00:18:23,860 towards the sun. 203 00:18:48,180 --> 00:18:51,140 Mercury collided with another embryo. 204 00:19:00,820 --> 00:19:03,980 A glancing blow saw much of its crust 205 00:19:03,980 --> 00:19:06,020 and mantle lost to space. 206 00:19:15,220 --> 00:19:18,260 Much of this material remained behind, 207 00:19:18,260 --> 00:19:21,180 perhaps helping to form the early Venus. 208 00:19:25,140 --> 00:19:28,140 If this theory is correct, then Mercury, 209 00:19:28,140 --> 00:19:33,220 now little more than a planetary core, continued towards the sun 210 00:19:33,220 --> 00:19:38,140 and ended up in the peculiar elliptical orbit we see today. 211 00:19:49,900 --> 00:19:52,940 The idea that Mercury's outer layers were stripped away 212 00:19:52,940 --> 00:19:56,020 in some violent collision many billions of years ago 213 00:19:56,020 --> 00:19:58,340 is a superficially attractive one, 214 00:19:58,340 --> 00:20:01,300 but the theory does have problems. 215 00:20:01,300 --> 00:20:04,980 Any collision violent enough to do that heats up the planet 216 00:20:04,980 --> 00:20:07,500 and that boils away the volatiles. 217 00:20:07,500 --> 00:20:11,740 So you have to think of a very specific kind of collision, 218 00:20:11,740 --> 00:20:15,180 or perhaps even multiple more delicate collisions, 219 00:20:15,180 --> 00:20:17,660 in order to fit the data. 220 00:20:17,660 --> 00:20:21,940 So I think it's fair to say that the precise nature 221 00:20:21,940 --> 00:20:26,220 of Mercury's formation is still one of the great unsolved mysteries 222 00:20:26,220 --> 00:20:27,900 in planetary science. 223 00:20:34,300 --> 00:20:36,620 After four years of observation, 224 00:20:36,620 --> 00:20:41,420 and its discoveries that hint at Mercury's turbulent past, 225 00:20:41,420 --> 00:20:43,900 Messenger finally ran out of fuel... 226 00:20:52,500 --> 00:20:56,140 ..and added yet another crater to this tiny world 227 00:20:56,140 --> 00:21:00,580 that just perhaps could've had a different story to tell. 228 00:21:25,100 --> 00:21:28,540 50 million km beyond Mercury, 229 00:21:28,540 --> 00:21:33,060 shrouded by an unbroken blanket of cloud... 230 00:21:33,060 --> 00:21:36,140 ..lies a world which, at first sight, 231 00:21:36,140 --> 00:21:39,500 has the potential to be far more Earth-like. 232 00:21:55,220 --> 00:21:59,300 See that bright point of light out there in the evening sky? 233 00:21:59,300 --> 00:22:01,100 That's Venus. 234 00:22:01,100 --> 00:22:03,740 It's so bright because it's quite a large planet, 235 00:22:03,740 --> 00:22:06,900 about the same size as the Earth. It's not too far away. 236 00:22:06,900 --> 00:22:08,500 But, in particular, 237 00:22:08,500 --> 00:22:12,660 because it's shrouded in highly reflective clouds. 238 00:22:12,660 --> 00:22:17,020 That's the frustrating but also tantalising thing about Venus. 239 00:22:17,020 --> 00:22:19,700 Even through a big telescope, when you look at it, 240 00:22:19,700 --> 00:22:22,580 it is featureless. You never see the surface. 241 00:22:22,580 --> 00:22:24,820 And that meant that, even until the 1950s, 242 00:22:24,820 --> 00:22:28,940 astronomers speculated that it might be a living world, 243 00:22:28,940 --> 00:22:32,420 with jungles and forests and rivers and oceans. 244 00:22:32,420 --> 00:22:36,060 So much so, in fact, that when we first sent a spacecraft to land 245 00:22:36,060 --> 00:22:39,700 on the surface of Venus, we prepared for a splash landing. 246 00:23:22,100 --> 00:23:24,540 Throughout the 1960s and '70s, 247 00:23:24,540 --> 00:23:29,740 the Soviet's Venera programme sent multiple missions to explore Venus. 248 00:23:33,180 --> 00:23:34,460 Many failed... 249 00:23:36,780 --> 00:23:39,540 ..but, with each attempt, we learned a little more 250 00:23:39,540 --> 00:23:42,220 of the extreme conditions on the planet. 251 00:24:13,340 --> 00:24:18,540 After 20 years of trying, Venera 13 began its perilous descent. 252 00:24:45,820 --> 00:24:48,220 The craft was prepared to withstand pressures 253 00:24:48,220 --> 00:24:50,660 that could crush a car in seconds... 254 00:24:52,700 --> 00:24:55,340 ..in temperatures that would melt lead. 255 00:24:59,900 --> 00:25:02,740 On March 1st 1982... 256 00:25:05,660 --> 00:25:08,980 ..the Soviets took the first full colour picture... 257 00:25:10,860 --> 00:25:12,300 ..of the Venusian surface. 258 00:25:33,420 --> 00:25:36,300 Even under the most extreme of conditions, 259 00:25:36,300 --> 00:25:39,660 the probe sent its precious data home to Earth... 260 00:25:49,060 --> 00:25:54,860 ..until, 127 minutes after touchdown, it finally succumbed. 261 00:25:56,300 --> 00:25:58,460 Far from a benign ocean world... 262 00:25:59,820 --> 00:26:01,700 ..Venus is a vision of hell... 263 00:26:02,980 --> 00:26:05,340 ..where no life can survive. 264 00:26:15,540 --> 00:26:17,820 So where did it all go wrong for Venus? 265 00:26:17,820 --> 00:26:21,900 Well, that is a good question and it's an important one. 266 00:26:21,900 --> 00:26:24,380 It's been said that we won't fully understand the Earth 267 00:26:24,380 --> 00:26:25,980 until we understand Venus 268 00:26:25,980 --> 00:26:29,220 and that's because the planets are so similar. 269 00:26:29,220 --> 00:26:31,660 Venus is the same size as the Earth. 270 00:26:31,660 --> 00:26:35,500 It's the same composition, as far as we know. 271 00:26:35,500 --> 00:26:39,420 And although it's closer to the sun, it's not as close as Mercury. 272 00:26:39,420 --> 00:26:42,740 So why is it that one world remained heaven 273 00:26:42,740 --> 00:26:45,180 whilst the other became hell? 274 00:26:53,300 --> 00:26:57,980 Counterintuitively, the surface temperatures today on Venus 275 00:26:57,980 --> 00:27:00,700 are hotter than those on Mercury... 276 00:27:07,380 --> 00:27:10,980 ..and the story of Venus's climate is further complicated 277 00:27:10,980 --> 00:27:14,580 by the fact that, over the lifetime of the planet, 278 00:27:14,580 --> 00:27:17,620 the sun itself has been evolving. 279 00:27:20,540 --> 00:27:26,460 As the sun gets older, the star burns hotter and hotter and hotter. 280 00:27:26,460 --> 00:27:29,220 That means that, in the past, when the sun was younger, 281 00:27:29,220 --> 00:27:31,380 it must have been cooler. 282 00:27:31,380 --> 00:27:33,420 It's called the faint young sun 283 00:27:33,420 --> 00:27:36,020 and that has a big impact on the planets. 284 00:27:36,020 --> 00:27:40,060 At the time, when life was just about beginning on the Earth, 285 00:27:40,060 --> 00:27:42,580 three-and-a-half to four billion years ago, 286 00:27:42,580 --> 00:27:46,660 the sun was fainter and that means that Venus was cooler. 287 00:27:46,660 --> 00:27:49,340 In fact, temperatures on Venus at that time 288 00:27:49,340 --> 00:27:53,420 would have been like a pleasant spring day here on Earth. 289 00:28:04,700 --> 00:28:07,300 Within a few million years of its formation, 290 00:28:07,300 --> 00:28:09,500 the surface of Venus had cooled. 291 00:28:13,380 --> 00:28:16,900 The planet now found itself at just the right distance 292 00:28:16,900 --> 00:28:22,060 from the faint young sun for Venus to experience a sight 293 00:28:22,060 --> 00:28:24,420 familiar to us here on Earth. 294 00:28:42,860 --> 00:28:44,300 The heavens opened. 295 00:29:02,300 --> 00:29:04,820 Great torrents flooded the surface. 296 00:29:07,740 --> 00:29:09,980 Rivers of water flowed. 297 00:29:18,460 --> 00:29:21,460 Venus became an ocean world. 298 00:29:38,980 --> 00:29:42,780 The planet's atmosphere allowed it to hold on to the oceans... 299 00:29:46,820 --> 00:29:50,660 ..by acting as a blanket, keeping the surface temperate... 300 00:29:53,060 --> 00:29:55,900 ..thanks to the greenhouse effect. 301 00:30:01,980 --> 00:30:05,140 The greenhouse effect is pretty simple physics. 302 00:30:05,140 --> 00:30:09,620 The gases like carbon dioxide, water vapour and the planetary atmosphere 303 00:30:09,620 --> 00:30:11,780 are transparent to visible light 304 00:30:11,780 --> 00:30:14,940 and it's obvious because there's a source of visible light, 305 00:30:14,940 --> 00:30:16,940 the sun, and I can see it. 306 00:30:16,940 --> 00:30:20,580 So that radiation falls onto the surface of the planet 307 00:30:20,580 --> 00:30:24,700 and it heats it up. The rocks then re-radiate that 308 00:30:24,700 --> 00:30:26,300 out into the atmosphere again, 309 00:30:26,300 --> 00:30:29,820 but, this time, not as visible light, but as infrared, 310 00:30:29,820 --> 00:30:31,860 which my eyes can't see. 311 00:30:31,860 --> 00:30:35,620 Now, carbon dioxide and water vapour absorb infrared, 312 00:30:35,620 --> 00:30:39,540 and so they trap that energy and the planet heats up. 313 00:30:39,540 --> 00:30:42,660 Now, that's not necessarily a bad thing. 314 00:30:42,660 --> 00:30:45,340 The Earth would be at an average temperature 315 00:30:45,340 --> 00:30:49,700 of around minus 18 degrees Celsius without the greenhouse effect, 316 00:30:49,700 --> 00:30:55,060 but there is a thin line between heating the planet up and frying it. 317 00:31:01,260 --> 00:31:06,820 Gradually, over two billion years, the young sun grew brighter. 318 00:31:16,820 --> 00:31:21,340 Temperatures began to rise, lifting more and more water vapour 319 00:31:21,340 --> 00:31:22,820 into the atmosphere. 320 00:31:29,580 --> 00:31:32,660 The greenhouse effect grew more intense. 321 00:31:53,460 --> 00:31:57,100 Rain evaporated long before reaching the ground. 322 00:32:04,540 --> 00:32:07,020 Venus had reached a tipping point. 323 00:32:08,380 --> 00:32:11,500 A runaway greenhouse effect had taken hold. 324 00:32:30,660 --> 00:32:34,420 Venus's moment in the sun was over. 325 00:32:42,180 --> 00:32:47,220 Its cracked surface today is even hotter than Mercury's, 326 00:32:47,220 --> 00:32:50,580 making Venus the hottest of all the planets. 327 00:33:01,540 --> 00:33:04,860 As the young sun's brightness continued to increase, 328 00:33:04,860 --> 00:33:08,220 the effects were felt across all the terrestrial planets. 329 00:33:21,020 --> 00:33:27,140 Mars, much further out than Venus, enjoyed its moment in the sun too. 330 00:33:34,420 --> 00:33:37,260 With an atmosphere rich in greenhouse gases, 331 00:33:37,260 --> 00:33:41,780 rivers flowed across its surface for hundreds of millions of years. 332 00:33:49,060 --> 00:33:54,020 But Mars, being smaller than Venus, couldn't hold on to its atmosphere. 333 00:34:05,500 --> 00:34:07,900 Much of its water evaporated and... 334 00:34:09,740 --> 00:34:11,420 ..and escaped into space... 335 00:34:21,340 --> 00:34:24,540 ..leaving only small traces behind, 336 00:34:24,540 --> 00:34:27,140 frozen in patches across the planet... 337 00:34:28,380 --> 00:34:30,940 ..where missions continue to search 338 00:34:30,940 --> 00:34:34,060 for the first signs of extraterrestrial life. 339 00:34:41,660 --> 00:34:46,180 There's a crater on Mars called the Hellas Basin, 340 00:34:46,180 --> 00:34:49,660 which is 1,500 km across and 9km deep. 341 00:34:49,660 --> 00:34:52,580 That means you could put Everest on the floor 342 00:34:52,580 --> 00:34:55,260 and the summit would not reach the rim. 343 00:34:55,260 --> 00:34:57,460 The air pressure is so high down there 344 00:34:57,460 --> 00:34:59,380 that liquid water can exist. 345 00:35:02,500 --> 00:35:06,060 So I suppose it's not impossible to imagine microbes 346 00:35:06,060 --> 00:35:10,820 coming up from deep below the surface to bask in the midday sun 347 00:35:10,820 --> 00:35:13,940 before disappearing back down below again 348 00:35:13,940 --> 00:35:16,980 to survive the cold of the Martian night. 349 00:35:16,980 --> 00:35:19,540 But if life does exist out there, 350 00:35:19,540 --> 00:35:22,540 it will certainly only be simple life. 351 00:35:22,540 --> 00:35:26,900 There will be nothing anywhere near as complex as you or me, 352 00:35:26,900 --> 00:35:28,780 or even this plant. 353 00:35:57,340 --> 00:36:00,180 The story of the solar system is, in a sense, 354 00:36:00,180 --> 00:36:02,900 a story of instability and constant change, 355 00:36:02,900 --> 00:36:05,180 at least for the inner rocky worlds. 356 00:36:05,180 --> 00:36:08,140 Mercury has changed its position radically. 357 00:36:08,140 --> 00:36:11,980 Its orbit now takes it close to the searing heat of the sun. 358 00:36:11,980 --> 00:36:15,460 Venus probably had water on its surface 359 00:36:15,460 --> 00:36:20,820 for around two billion years before it became hotter than Mercury. 360 00:36:20,820 --> 00:36:23,620 And Mars lost its oceans and rivers 361 00:36:23,620 --> 00:36:26,060 perhaps three-and-a-half billion years ago. 362 00:36:26,060 --> 00:36:29,020 But unique amongst those worlds is Earth 363 00:36:29,020 --> 00:36:32,700 because it's remained pretty much like this, 364 00:36:32,700 --> 00:36:36,900 liquid water on the surface, for four billion years, 365 00:36:36,900 --> 00:36:40,420 and that has allowed complex carbon chemistry to develop. 366 00:36:46,540 --> 00:36:49,140 Today, our planet is dominated by life. 367 00:36:49,140 --> 00:36:52,780 It's in every nook and cranny. I mean, look at this place. 368 00:36:52,780 --> 00:36:56,140 This is a volcano in the middle of the Atlantic Ocean 369 00:36:56,140 --> 00:36:58,660 and it is literally teeming with life. 370 00:36:58,660 --> 00:37:02,260 And think about all the chance events that had to happen 371 00:37:02,260 --> 00:37:03,900 over four billion years 372 00:37:03,900 --> 00:37:07,340 just to produce the little creatures in this rock pool. 373 00:37:12,660 --> 00:37:16,460 Life has woven itself into the fabric of the planet. 374 00:37:23,100 --> 00:37:27,220 It's an integral part of every continent and every ocean. 375 00:37:32,500 --> 00:37:34,140 It plays a crucial role 376 00:37:34,140 --> 00:37:37,420 in maintaining the balance of our atmosphere... 377 00:37:37,420 --> 00:37:39,740 ..that keeps our planet temperate. 378 00:37:44,980 --> 00:37:47,380 Of all the terrestrial planets, 379 00:37:47,380 --> 00:37:50,900 Earth has enjoyed the longest moment of them all... 380 00:37:59,740 --> 00:38:01,300 ..but it can't last. 381 00:38:04,540 --> 00:38:09,460 Earth will ultimately follow the fate of the other rocky planets 382 00:38:09,460 --> 00:38:12,820 because, even though we don't feel it day-to-day, 383 00:38:12,820 --> 00:38:16,580 the sun's ageing process is relentless. 384 00:38:18,980 --> 00:38:21,860 We can say with confidence what's going to happen to the sun 385 00:38:21,860 --> 00:38:23,380 towards the end of its life, 386 00:38:23,380 --> 00:38:25,700 partly because we understand physics 387 00:38:25,700 --> 00:38:29,780 and the nuclear physics of what happens inside the cores of stars, 388 00:38:29,780 --> 00:38:32,820 but also because the life cycle of stars 389 00:38:32,820 --> 00:38:35,340 is written across the night sky. 390 00:38:35,340 --> 00:38:38,180 Take that bright star there, for example. 391 00:38:38,180 --> 00:38:40,060 It's called Arcturus. 392 00:38:40,060 --> 00:38:43,980 It's around the mass of the sun, perhaps a little bit heavier, 393 00:38:43,980 --> 00:38:47,780 but it's between six and eight billion years old, 394 00:38:47,780 --> 00:38:51,180 perhaps three billion years older than the sun - 395 00:38:51,180 --> 00:38:53,980 and it is now a red giant star. 396 00:38:53,980 --> 00:38:56,820 It's exhausted the hydrogen fuelled in its core, 397 00:38:56,820 --> 00:38:58,900 and it's swollen up and cooled. 398 00:39:01,020 --> 00:39:03,940 And that is what we think will happen to the sun 399 00:39:03,940 --> 00:39:05,980 in about five billion years' time. 400 00:39:16,340 --> 00:39:19,860 As the sun exhausts its hydrogen fuel in the core, 401 00:39:19,860 --> 00:39:22,220 its outer edge will inflate. 402 00:39:27,660 --> 00:39:30,020 It will enter a red giant phase, 403 00:39:30,020 --> 00:39:33,860 expanding millions of kilometres out into space. 404 00:39:38,300 --> 00:39:41,260 Mercury will be the first to be engulfed. 405 00:39:52,540 --> 00:39:55,500 Then Venus's fate will be sealed. 406 00:40:09,020 --> 00:40:12,900 Earth may just escape the fiery fate of its neighbours... 407 00:40:16,140 --> 00:40:20,540 ..hanging on with Mars beyond the edge of the dying star. 408 00:40:34,340 --> 00:40:38,820 The era of the four terrestrial planets will be over. 409 00:40:42,020 --> 00:40:44,580 The lives lived on the surface of one of them 410 00:40:44,580 --> 00:40:47,380 nothing more than a distant memory. 411 00:40:55,540 --> 00:40:59,020 But that's not quite the end of the story. 412 00:41:01,620 --> 00:41:05,460 Right at the end of the sun's life, something wonderful will happen. 413 00:41:08,860 --> 00:41:11,620 A collection of icy worlds that have lain dormant 414 00:41:11,620 --> 00:41:15,500 for the entire history of the solar system will awake. 415 00:41:15,500 --> 00:41:18,620 These are the worlds that orbit the outer planets, 416 00:41:18,620 --> 00:41:20,700 the moons of Jupiter and Saturn. 417 00:41:26,660 --> 00:41:31,060 These distant worlds that circle the outer gas giants 418 00:41:31,060 --> 00:41:32,900 will begin to warm... 419 00:41:37,940 --> 00:41:40,860 ..like Saturn's moon Enceladus... 420 00:41:44,260 --> 00:41:46,460 ..or Jupiter's moon Europa. 421 00:41:53,140 --> 00:41:56,580 Amongst all these moons, there is one above all others 422 00:41:56,580 --> 00:41:59,340 that we think perhaps has the best chance 423 00:41:59,340 --> 00:42:02,140 of becoming a place that we'd recognise. 424 00:42:14,980 --> 00:42:19,020 Way out in the cold, distant reaches of the solar system... 425 00:42:20,740 --> 00:42:22,180 ..past Jupiter... 426 00:42:24,500 --> 00:42:27,860 ..around the icy-ringed planet Saturn, 427 00:42:27,860 --> 00:42:29,340 orbits a gem... 428 00:42:30,700 --> 00:42:33,500 And the Cassini spacecraft is on its way to Saturn. 429 00:42:41,660 --> 00:42:43,060 ..Titan... 430 00:42:55,060 --> 00:42:58,580 ..a planet-sized moon bigger than Mercury... 431 00:43:04,100 --> 00:43:08,220 ..surrounded by a thick atmosphere of nitrogen and methane... 432 00:43:09,900 --> 00:43:13,340 ..with a surface that has long remained a mystery. 433 00:43:19,220 --> 00:43:22,380 The Huygens probe was our first chance to explore 434 00:43:22,380 --> 00:43:24,100 beneath the clouds... 435 00:43:25,260 --> 00:43:29,860 ..and its camera sent back these first glimpses of the distant moon. 436 00:44:07,060 --> 00:44:11,020 Wonderfully, the craft made a soft landing 437 00:44:11,020 --> 00:44:14,540 and continued to beam back what it saw. 438 00:44:19,980 --> 00:44:23,180 This is a remarkable photograph and, as is always the case in science, 439 00:44:23,180 --> 00:44:26,620 the more you know about it, the more wonderful it gets. 440 00:44:26,620 --> 00:44:29,820 This is a photograph from the surface of the moon 441 00:44:29,820 --> 00:44:33,900 orbiting around a planet over a billion kilometres away, 442 00:44:33,900 --> 00:44:37,660 so we've got a camera down onto the surface of a world 443 00:44:37,660 --> 00:44:41,140 in the frozen far reaches of the solar system. 444 00:44:41,140 --> 00:44:44,260 What you see here is something that looks like a flood plain, 445 00:44:44,260 --> 00:44:47,300 or a riverbed, very much like this, actually. 446 00:44:47,300 --> 00:44:50,340 And we can say that this is a riverbed or a flood plain 447 00:44:50,340 --> 00:44:53,220 because these rocks on the surface... 448 00:44:53,220 --> 00:44:54,580 ..look like this. 449 00:44:54,580 --> 00:44:58,820 They've been smoothed and eroded by flowing liquid. 450 00:45:01,620 --> 00:45:07,300 We know these are in fact boulders of frozen water. 451 00:45:07,300 --> 00:45:11,100 They're frozen solid because the temperature on the surface 452 00:45:11,100 --> 00:45:15,500 of this moon is minus 180 degrees Celsius. 453 00:45:15,500 --> 00:45:17,460 That raises an interesting question. 454 00:45:17,460 --> 00:45:19,540 If it's so cold... 455 00:45:19,540 --> 00:45:21,380 ..then what was the flowing liquid? 456 00:45:30,060 --> 00:45:34,260 Huygens detected significant amounts of methane... 457 00:45:34,260 --> 00:45:36,100 ..a flammable gas on Earth. 458 00:45:38,460 --> 00:45:41,100 But the relatively high atmospheric pressure 459 00:45:41,100 --> 00:45:44,060 and cold temperatures at the surface of Titan 460 00:45:44,060 --> 00:45:48,380 means that this methane exists as a liquid. 461 00:46:06,500 --> 00:46:08,500 Titan could be wet... 462 00:46:09,860 --> 00:46:14,060 ..not with water, but with liquid methane... 463 00:46:14,060 --> 00:46:18,580 ..driving rock-like chunks of ice down mountain channels 464 00:46:18,580 --> 00:46:20,860 and out into open flood plains. 465 00:46:26,780 --> 00:46:29,820 Huygens survived for just a few hours, 466 00:46:29,820 --> 00:46:32,140 but didn't detect any trace 467 00:46:32,140 --> 00:46:34,540 of liquid methane at its landing site. 468 00:46:41,900 --> 00:46:46,980 But the probe's mother ship Cassini remained in orbit around Saturn. 469 00:46:48,500 --> 00:46:51,100 A year after Huygens landed, 470 00:46:51,100 --> 00:46:55,220 Cassini again flew high above Titan's North Pole 471 00:46:55,220 --> 00:46:59,180 and discovered something seen nowhere else in the solar system 472 00:46:59,180 --> 00:47:00,900 beyond Earth. 473 00:47:10,820 --> 00:47:17,140 Liquid pooling into not just one, but scores of great lakes. 474 00:47:40,660 --> 00:47:42,780 Cassini discovered lakes... 475 00:47:43,940 --> 00:47:45,260 ..of liquid methane. 476 00:47:49,820 --> 00:47:53,940 Earth has a strange cold twin. 477 00:48:02,340 --> 00:48:07,060 In some ways, you could just imagine floating in a boat on those lakes 478 00:48:07,060 --> 00:48:08,820 and it'll look something like this, 479 00:48:08,820 --> 00:48:12,780 except this would be liquid methane gas 480 00:48:12,780 --> 00:48:17,340 and those mountains there would be mountains of frozen water ice 481 00:48:17,340 --> 00:48:18,820 as hard as rock. 482 00:48:20,740 --> 00:48:23,660 What's also fascinating, and in fact tantalising, 483 00:48:23,660 --> 00:48:26,860 is that Titan has a complex chemistry 484 00:48:26,860 --> 00:48:31,700 and that chemistry is carbon chemistry - the chemistry of life. 485 00:48:31,700 --> 00:48:34,020 So we found molecules like hydrogen cyanide, 486 00:48:34,020 --> 00:48:37,100 which are the building blocks of amino acids. 487 00:48:37,100 --> 00:48:40,020 We found molecules called vinyl cyanides, 488 00:48:40,020 --> 00:48:42,780 which chemists and biologists speculate 489 00:48:42,780 --> 00:48:46,060 could form some sort of cell membranes. 490 00:48:46,060 --> 00:48:50,020 And so all the ingredients for life are present on Titan. 491 00:48:52,580 --> 00:48:56,340 Now, very few scientists think there will be life on Titan today. 492 00:48:56,340 --> 00:49:01,260 It is, after all, minus 180 degrees Celsius at the surface, 493 00:49:01,260 --> 00:49:04,700 but, because of the presence of all those ingredients, 494 00:49:04,700 --> 00:49:08,540 it might be a very different story if you warm Titan up. 495 00:49:26,100 --> 00:49:29,100 In the light of the old expanding sun... 496 00:49:31,020 --> 00:49:33,580 ..the far reaches of the solar system 497 00:49:33,580 --> 00:49:35,860 will receive more solar energy. 498 00:49:37,180 --> 00:49:40,540 Titan's atmosphere will begin to warm. 499 00:49:55,580 --> 00:50:01,060 Mountains of ice will shrink and melt as temperatures rise... 500 00:50:05,340 --> 00:50:10,500 ..the frozen water they contain replacing the liquid methane. 501 00:50:42,140 --> 00:50:46,260 Mountains will become oceans of water. 502 00:50:55,740 --> 00:51:00,020 In a strange twist of fate, at the end the life of the sun... 503 00:51:01,500 --> 00:51:04,740 ..the solar system's last ocean world 504 00:51:04,740 --> 00:51:08,660 will wake up to its own biological possibilities. 505 00:51:14,540 --> 00:51:20,220 This distant moon will enjoy its brief moment in the sun. 506 00:51:27,100 --> 00:51:30,180 It's easy to think of habitability as a permanent feature 507 00:51:30,180 --> 00:51:33,620 of a world's defining characteristic, if you like. 508 00:51:33,620 --> 00:51:37,140 So the Earth is a living planet because it's in the Goldilocks zone 509 00:51:37,140 --> 00:51:40,780 around the sun - not too close and not too far away. 510 00:51:40,780 --> 00:51:42,660 But things are more complicated than that. 511 00:51:42,660 --> 00:51:45,060 Solar systems are dynamic places. 512 00:51:45,060 --> 00:51:49,500 Planetary orbits can change and stars can vary in brightness, 513 00:51:49,500 --> 00:51:53,220 so planets that were once heaven can become hell. 514 00:51:59,820 --> 00:52:03,260 We now understand that the Earth has been a fortunate world, 515 00:52:03,260 --> 00:52:06,060 an oasis of calm in an ever-changing solar system, 516 00:52:06,060 --> 00:52:09,940 that's maintained a stable climate, perhaps against the odds, 517 00:52:09,940 --> 00:52:14,540 for the four billion years it took complex living things to evolve. 518 00:52:22,500 --> 00:52:25,500 We don't know how many planets like Earth there are 519 00:52:25,500 --> 00:52:27,340 out there amongst the stars, 520 00:52:27,340 --> 00:52:30,220 places where the ingredients of solar systems 521 00:52:30,220 --> 00:52:32,940 have assembled themselves into structures 522 00:52:32,940 --> 00:52:35,180 that can dream of other worlds, 523 00:52:35,180 --> 00:52:38,300 but we have to take the possibility very seriously 524 00:52:38,300 --> 00:52:40,300 that there might be few. 525 00:52:40,300 --> 00:52:44,900 And that would make Earth, and us, extremely rare and precious. 526 00:53:16,500 --> 00:53:21,860 Mercury is the most enigmatic of all the planets and difficult to study. 527 00:53:26,540 --> 00:53:28,780 The real problem is that it's really hot. 528 00:53:28,780 --> 00:53:31,340 It's the planet closest to the sun 529 00:53:31,340 --> 00:53:34,300 and obviously that makes it challenging. 530 00:53:35,860 --> 00:53:37,580 So you have to protect the spacecraft 531 00:53:37,580 --> 00:53:40,380 from the heat from the sun, and the way Messenger addressed this 532 00:53:40,380 --> 00:53:44,540 was to put the whole spacecraft behind a giant ceramic sunshade. 533 00:53:44,540 --> 00:53:47,060 It had this sunshade. If it didn't face the sun, 534 00:53:47,060 --> 00:53:50,100 it would have melted the instruments, literally. 535 00:53:52,540 --> 00:53:56,460 The other major challenge is, you have to protect a spacecraft 536 00:53:56,460 --> 00:53:58,780 from the heat reflected from the planet 537 00:53:58,780 --> 00:54:00,500 and the way we dealt with that 538 00:54:00,500 --> 00:54:02,940 was to be in an extremely elliptical orbit, 539 00:54:02,940 --> 00:54:07,380 where we flew in very close over the North Pole and took observations. 540 00:54:07,380 --> 00:54:10,300 The instruments would heat up and then we'd fly like 10,000km 541 00:54:10,300 --> 00:54:13,300 farther out from the planet... 542 00:54:13,300 --> 00:54:15,380 ..while we cooled off, and, in this way, 543 00:54:15,380 --> 00:54:17,940 heat up, cool down and kept everything below 544 00:54:17,940 --> 00:54:21,100 the danger temperatures, where instruments could be damaged. 545 00:54:31,860 --> 00:54:34,580 I was in charge of the camera team. 546 00:54:34,580 --> 00:54:36,900 I don't think I slept much of the night before. 547 00:54:38,420 --> 00:54:41,500 I was really anxious to get that first image back. 548 00:54:43,820 --> 00:54:45,220 When that image came back, 549 00:54:45,220 --> 00:54:47,220 we just started pointing at all the features 550 00:54:47,220 --> 00:54:50,860 that had never been seen before and saying, "Look at this, look at that." 551 00:54:55,340 --> 00:54:59,260 Over time, the close-up flights of Mercury's North Pole 552 00:54:59,260 --> 00:55:02,460 allowed the team to peer deep into the shadows 553 00:55:02,460 --> 00:55:04,340 of one particular crater. 554 00:55:08,180 --> 00:55:12,140 We realised that we could actually design a way 555 00:55:12,140 --> 00:55:15,540 to take a picture using a very long exposure 556 00:55:15,540 --> 00:55:18,580 to see inside these dark craters. 557 00:55:18,580 --> 00:55:20,420 Messenger was designed with instruments 558 00:55:20,420 --> 00:55:22,700 that could specifically look at their reflectance 559 00:55:22,700 --> 00:55:24,900 and also measure hydrogen. 560 00:55:24,900 --> 00:55:28,020 So we saw, "OK, there's a lot of hydrogen there," 561 00:55:28,020 --> 00:55:32,820 and there was like this fantastic case built for these ice deposits. 562 00:55:34,740 --> 00:55:38,380 Incredibly, Messenger detected hundreds of billions 563 00:55:38,380 --> 00:55:40,700 of tonnes of frozen water ice... 564 00:55:42,860 --> 00:55:46,900 ..scattered in the permanent shadows of the polar craters. 565 00:55:48,900 --> 00:55:52,860 The fact that water ice can survive for a very long period of time 566 00:55:52,860 --> 00:55:56,020 is a reflection of the fact that the Mercury is rotating 567 00:55:56,020 --> 00:55:59,340 almost perfectly straight up, so that there are craters near the pool 568 00:55:59,340 --> 00:56:02,140 that are so deep and completely shaded 569 00:56:02,140 --> 00:56:04,500 from sunlight ever hitting them. 570 00:56:04,500 --> 00:56:06,980 So ice could be stable in those polar regions 571 00:56:06,980 --> 00:56:09,780 that are permanently shadowed for billions of years. 572 00:56:15,780 --> 00:56:18,140 Messenger had confirmed the existence 573 00:56:18,140 --> 00:56:20,620 of an essential ingredient for life... 574 00:56:23,860 --> 00:56:27,340 ..on the surface at the closest planet to the sun. 575 00:56:30,020 --> 00:56:33,500 During Messenger's final orbits, all the fuel was depleted - 576 00:56:33,500 --> 00:56:35,340 there was nothing that we could do - 577 00:56:35,340 --> 00:56:39,300 and every orbit just brought it slightly closer to the planet. 578 00:56:39,300 --> 00:56:40,900 One of our engineers realised 579 00:56:40,900 --> 00:56:43,740 that we still had some helium on board the spacecraft 580 00:56:43,740 --> 00:56:47,140 and, if you blew helium out the back it would work like fuel, 581 00:56:47,140 --> 00:56:50,460 and that managed to extend us for a few extra weeks. 582 00:56:54,580 --> 00:56:58,020 But, of course, all things have to come to an end. 583 00:57:05,780 --> 00:57:08,580 The last day, many of us watched the signal 584 00:57:08,580 --> 00:57:10,820 as it went behind the planet and never came back 585 00:57:10,820 --> 00:57:12,100 and we knew it had crashed 586 00:57:12,100 --> 00:57:15,380 and it left us with a real bittersweet feeling, 587 00:57:15,380 --> 00:57:18,380 because we were happy at the success, 588 00:57:18,380 --> 00:57:20,300 but, of course, sad that it was over. 589 00:57:20,300 --> 00:57:24,260 We all took so much pride in this amazing spacecraft 590 00:57:24,260 --> 00:57:28,260 that lasted way longer than any of us had planned for it to last 591 00:57:28,260 --> 00:57:30,660 and that had told us so much about Mercury 592 00:57:30,660 --> 00:57:33,660 and really changed the way that we looked at this planet. 593 00:57:45,420 --> 00:57:48,980 Touchdown on the red planet... 594 00:57:48,980 --> 00:57:50,540 ..to uncover... 595 00:57:51,620 --> 00:57:52,940 ..the story of Mars. 596 00:57:59,340 --> 00:58:01,780 Journey through our solar system with this free poster 597 00:58:01,780 --> 00:58:03,980 produced by the Open University 598 00:58:03,980 --> 00:58:07,220 and discover more about its planets and moons. 599 00:58:08,340 --> 00:58:11,460 Order your free copy by calling... 600 00:58:15,300 --> 00:58:17,460 Or go to... 601 00:58:21,380 --> 00:58:24,580 ..and follow the links to the Open University. 50853

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