1 00:00:05,000 --> 00:00:08,320 Lift off of Messenger on Nasa's mission to Mercury. 2 00:00:08,320 --> 00:00:10,440 MUSIC: The Void by Muse 3 00:00:10,440 --> 00:00:17,160 # They'll say no-one can see us 4 00:00:17,160 --> 00:00:23,920 # That we're estranged and all alone 5 00:00:23,920 --> 00:00:29,440 # They believe nothing can reach us 6 00:00:30,880 --> 00:00:33,960 # And pull us out of 7 00:00:33,960 --> 00:00:37,680 # The boundless gloom 8 00:00:37,680 --> 00:00:44,400 # They're wrong 9 00:00:44,400 --> 00:00:47,440 # They're wrong. # 10 00:01:09,520 --> 00:01:12,120 Beyond the warm worlds of the inner solar system... 11 00:01:16,880 --> 00:01:19,520 ..beyond the gas giant Jupiter... 12 00:01:21,480 --> 00:01:24,400 ..in the freezing regions far beyond the Sun... 13 00:01:28,680 --> 00:01:30,480 ..lies Saturn... 14 00:01:37,160 --> 00:01:42,480 ..a planet made unique, thanks to a nearly 300,000km-wide ring 15 00:01:42,480 --> 00:01:44,720 of frozen water. 16 00:01:57,240 --> 00:02:01,800 Here, trillions of pieces of ice have been sculpted by gravitational 17 00:02:01,800 --> 00:02:06,040 forces into some of the solar system's most stunning vistas. 18 00:02:31,040 --> 00:02:35,360 Here on Earth, water takes many forms - oceans and clouds 19 00:02:35,360 --> 00:02:38,960 and rivers, but it's in its crystalline form, ice, 20 00:02:38,960 --> 00:02:41,520 that it's at its most beautiful. 21 00:02:41,520 --> 00:02:46,120 Here, ice just adorns the landscape, but in the outer solar system, 22 00:02:46,120 --> 00:02:49,920 where Saturn lives, ice is so abundant that, well, 23 00:02:49,920 --> 00:02:53,360 it's become a building material. 24 00:02:53,360 --> 00:02:56,200 Now, there's no more beautiful sight in the solar system than 25 00:02:56,200 --> 00:02:58,240 the ice rings of Saturn. 26 00:02:58,240 --> 00:03:02,120 It's almost as if a god had taken snowflakes and sprinkled them 27 00:03:02,120 --> 00:03:06,160 over a gravitational field, so we could see it, but how could 28 00:03:06,160 --> 00:03:11,880 something so intricate be sculpted out of something so simple? 29 00:03:11,880 --> 00:03:14,920 Well, as is always the case in science, 30 00:03:14,920 --> 00:03:17,320 the true beauty lies in the story. 31 00:03:23,400 --> 00:03:26,280 Saturn is a planet that has undergone some of the most 32 00:03:26,280 --> 00:03:28,720 radical transformations in the solar system. 33 00:03:37,800 --> 00:03:41,720 A planet that has seen destruction... 34 00:03:49,960 --> 00:03:51,680 ..and creation. 35 00:03:56,120 --> 00:03:58,960 A planet that harbours hidden worlds... 36 00:04:05,680 --> 00:04:07,800 ..where, just perhaps, 37 00:04:07,800 --> 00:04:10,360 there may exist a second home 38 00:04:10,360 --> 00:04:11,960 for life. 39 00:04:38,800 --> 00:04:42,240 Imagine a place with no surface to stand. 40 00:04:44,880 --> 00:04:46,720 Just an endless atmosphere. 41 00:05:01,480 --> 00:05:04,720 Compared to Earth, Saturn is so alien 42 00:05:04,720 --> 00:05:07,440 that it's hard to imagine how it could have grown 43 00:05:07,440 --> 00:05:08,800 from the same ingredients. 44 00:05:10,800 --> 00:05:13,120 But go far enough back in time 45 00:05:13,120 --> 00:05:16,040 and Saturn would appear to be surprisingly familiar. 46 00:05:31,800 --> 00:05:35,800 Saturn began life as tiny worlds of rock and ice... 47 00:05:37,680 --> 00:05:39,920 ..tumbling chaotically through space. 48 00:05:53,320 --> 00:05:56,600 Just like Earth, Mars, Venus and Mercury, 49 00:05:56,600 --> 00:06:00,240 they began to merge and clump together and grow. 50 00:06:09,880 --> 00:06:13,640 But unlike the planets of the inner solar system, Saturn was 51 00:06:13,640 --> 00:06:18,360 forming on the other side of a boundary we call the Snow Line... 52 00:06:22,800 --> 00:06:27,040 ..a place where the Sun is so distant and it's so cold, 53 00:06:27,040 --> 00:06:30,720 water exists only in its frozen, solid state. 54 00:06:33,360 --> 00:06:38,640 Out here, trillions of particles of ice behave just like rock, 55 00:06:38,640 --> 00:06:42,880 providing huge amounts of extra planetary building material. 56 00:06:49,200 --> 00:06:53,440 Under the force of gravity, this abundant ice and rock collided 57 00:06:53,440 --> 00:06:55,160 and combined... 58 00:06:57,920 --> 00:07:01,240 ..helping the young Saturn to grow into a giant. 59 00:07:18,880 --> 00:07:21,800 The details of how the planets formed in the solar system is 60 00:07:21,800 --> 00:07:25,360 not fully understood. It's cutting-edge scientific research. 61 00:07:25,360 --> 00:07:30,040 We're talking about events that happened over 4.5 billion years ago, 62 00:07:30,040 --> 00:07:35,080 but it seems likely that Saturn was once a world of rock and ice, 63 00:07:35,080 --> 00:07:39,800 perhaps ten or even 20 times the mass of the Earth, and it's almost 64 00:07:39,800 --> 00:07:45,040 possible, in your dreams, to imagine standing on its surface. 65 00:07:45,040 --> 00:07:47,920 But because of its immense mass and its place 66 00:07:47,920 --> 00:07:53,000 in the solar system, Saturn didn't remain a rocky, icy world for long. 67 00:08:02,240 --> 00:08:07,000 As it grew older, Saturn became a radically different kind of planet. 68 00:08:10,080 --> 00:08:13,040 In the region in which the young world was forming, there was 69 00:08:13,040 --> 00:08:16,760 more than just ice and rock - there were large amounts of gas. 70 00:08:20,400 --> 00:08:22,640 And Saturn began to gain an atmosphere... 71 00:08:25,360 --> 00:08:28,720 ..but on a scale unlike anything we experience here on Earth. 72 00:08:37,280 --> 00:08:40,920 It's not until you see the Earth's atmosphere from space that 73 00:08:40,920 --> 00:08:43,520 you appreciate just how thin it really is. 74 00:08:54,720 --> 00:08:56,960 And it's a similar story for the other 75 00:08:56,960 --> 00:08:58,760 worlds of the inner solar system. 76 00:09:19,760 --> 00:09:23,240 Despite being relatively thin, these atmospheres can exert 77 00:09:23,240 --> 00:09:26,400 powerful forces on the planetary surfaces beneath. 78 00:09:36,800 --> 00:09:39,840 On Earth, you can see the effects of an atmosphere with nothing 79 00:09:39,840 --> 00:09:41,640 more than a sealed container. 80 00:09:44,920 --> 00:09:48,680 It's easy to forget that the atmosphere has a weight. 81 00:09:48,680 --> 00:09:54,040 There's a column of air stretching up 100km above my head. 82 00:09:54,040 --> 00:09:57,080 That's a lot of molecules and it exerts a pressure. 83 00:09:59,160 --> 00:10:02,760 You really see it if you look at what happens to this bottle. 84 00:10:02,760 --> 00:10:05,840 We've filled it with air from the top of the mountain, which is 85 00:10:05,840 --> 00:10:08,400 about 3,000 metres. 86 00:10:08,400 --> 00:10:12,120 Now, up there, the atmosphere is about 25% less dense 87 00:10:12,120 --> 00:10:15,360 than it is down there at sea level. 88 00:10:15,360 --> 00:10:20,720 So, as we descend, the atmospheric pressure outside is increasing 89 00:10:20,720 --> 00:10:23,720 and the pressure in here is just what it 90 00:10:23,720 --> 00:10:27,960 was at the top of the mountain, and so you see visually that 91 00:10:27,960 --> 00:10:31,640 force in action and that is squashing the bottle. 92 00:10:32,880 --> 00:10:34,880 Physics in action! 93 00:10:34,880 --> 00:10:36,400 It's brilliant, isn't it? 94 00:10:41,200 --> 00:10:44,440 While just a few hundred metres of atmosphere can crush a bottle... 95 00:10:47,080 --> 00:10:51,320 ..if an atmosphere gets big enough, it can transform an entire planet. 96 00:11:01,280 --> 00:11:03,920 Within just a few million years of its birth... 97 00:11:05,680 --> 00:11:09,800 ..Saturn had grown as large as it could, from rock and ice alone. 98 00:11:12,440 --> 00:11:15,520 And now it turned to another building material - 99 00:11:15,520 --> 00:11:20,560 the hydrogen and helium gas left over from the formation of the Sun. 100 00:11:28,000 --> 00:11:30,600 This gas would have been too light for the smaller 101 00:11:30,600 --> 00:11:32,920 worlds of the inner solar system to hold on to. 102 00:11:34,480 --> 00:11:38,600 But Saturn's great mass created gravitational forces powerful 103 00:11:38,600 --> 00:11:39,880 enough to draw it in. 104 00:11:44,560 --> 00:11:47,640 Trillions upon trillions of tonnes of hydrogen 105 00:11:47,640 --> 00:11:51,040 and helium gas began to envelope the planet. 106 00:11:56,760 --> 00:11:58,760 And as this new atmosphere grew, 107 00:11:58,760 --> 00:12:01,200 it transformed the surface below. 108 00:12:08,120 --> 00:12:12,160 Enormous pressures, created by the weight of this gas, heated rock 109 00:12:12,160 --> 00:12:16,520 and ice so much, they began to glow. 110 00:12:20,080 --> 00:12:23,440 As Saturn matured, the pressure at the surface rose 111 00:12:23,440 --> 00:12:27,000 to ten million times the atmospheric pressure on Earth. 112 00:12:27,000 --> 00:12:28,640 Under those kind of pressures, 113 00:12:28,640 --> 00:12:31,240 matter behaves in extremely strange ways. 114 00:12:31,240 --> 00:12:36,120 The very idea of a surface becomes meaningless, and Saturn was 115 00:12:36,120 --> 00:12:38,000 transformed from an icy, 116 00:12:38,000 --> 00:12:41,800 rocky world into a completely different class of planet - 117 00:12:41,800 --> 00:12:43,240 a gas giant. 118 00:12:52,560 --> 00:12:54,360 Saturn was now so big, 119 00:12:54,360 --> 00:12:57,880 it could contain nearly 5,000 Earth-size worlds. 120 00:13:00,960 --> 00:13:06,160 But this vast, wild protoplanet was totally unlike the Saturn 121 00:13:06,160 --> 00:13:07,600 we see today. 122 00:13:10,640 --> 00:13:12,840 To discover how that world emerged, 123 00:13:12,840 --> 00:13:17,720 we had to send spacecraft to meet it close up. 124 00:13:28,360 --> 00:13:32,000 We have ignition and we have liftoff of the Titan Centaur carrying 125 00:13:32,000 --> 00:13:34,840 the first of two Voyager spacecraft to extend Man's senses 126 00:13:34,840 --> 00:13:37,280 farther into the solar system than ever before. 127 00:13:54,920 --> 00:13:58,240 Reports coming back indicate those twin large solid motors are 128 00:13:58,240 --> 00:14:02,120 functioning perfectly, producing 1.2 million pounds of thrust each. 129 00:14:11,800 --> 00:14:15,080 The guidance data coming back shows that we're right on the money. 130 00:14:24,400 --> 00:14:26,080 It's there! 131 00:14:26,080 --> 00:14:29,520 CHEERING 132 00:14:29,520 --> 00:14:33,000 Voyager 2 has reached Saturn after four years in space. 133 00:14:33,000 --> 00:14:37,080 It got there just a bit early, 2.7 seconds early, to be exact, 134 00:14:37,080 --> 00:14:40,200 after a flight of a billion and a quarter miles. 135 00:14:54,480 --> 00:14:59,120 The Voyager probes gave us our first detailed look at Saturn, 136 00:14:59,120 --> 00:15:02,800 revealing the planet in exquisite resolution. 137 00:15:07,440 --> 00:15:10,320 There's just an amazing harvest of pictures. 138 00:15:10,320 --> 00:15:13,160 You know, there's tens of thousands of photographs of Saturn 139 00:15:13,160 --> 00:15:16,800 and its rings and its moons taken by the Voyager 2 spacecraft. 140 00:15:20,520 --> 00:15:22,520 For the very first time, 141 00:15:22,520 --> 00:15:27,160 we were able to study Saturn's vast atmosphere in detail. 142 00:15:29,400 --> 00:15:33,880 Saturn itself is composed of hydrogen and helium. 143 00:15:35,320 --> 00:15:38,560 And we are seeing an exotic meteorology here. 144 00:15:45,280 --> 00:15:49,400 Voyager showed, beyond doubt, that Saturn's upper atmosphere was 145 00:15:49,400 --> 00:15:52,360 made almost entirely of helium and hydrogen... 146 00:15:55,400 --> 00:15:59,440 ..the very same gases so abundant in the early solar system. 147 00:16:06,800 --> 00:16:10,240 But this once chaotic gas was now organised 148 00:16:10,240 --> 00:16:12,040 into intricate weather systems. 149 00:16:15,960 --> 00:16:19,800 And one, above all others, took the Voyager team by surprise. 150 00:16:22,840 --> 00:16:25,880 A huge hexagonal structure in the clouds, 151 00:16:25,880 --> 00:16:29,760 so big it could fit our entire planet within it, 152 00:16:29,760 --> 00:16:32,160 nearly four times over. 153 00:16:34,240 --> 00:16:37,840 The atmosphere of Saturn was revealing itself to be stranger 154 00:16:37,840 --> 00:16:40,680 and more dynamic than we could ever have imagined. 155 00:17:02,640 --> 00:17:05,480 Atmospheres are some of the most spectacular 156 00:17:05,480 --> 00:17:07,320 and complex planetary environments. 157 00:17:15,040 --> 00:17:18,880 Not many people see the Alps like this! Seriously! It's so great! 158 00:17:20,800 --> 00:17:26,000 I was afraid that you'd just get too sick or something. It's awesome! 159 00:17:26,000 --> 00:17:28,840 Man! It's fun, man! 160 00:17:28,840 --> 00:17:32,440 HE LAUGHS This IS fun! 161 00:17:38,000 --> 00:17:42,320 The atmosphere of every planet takes on its own unique character. 162 00:17:44,880 --> 00:17:48,760 But what powers them and the weather systems within 163 00:17:48,760 --> 00:17:51,240 depends on where you are in the solar system. 164 00:17:53,840 --> 00:17:56,920 Most of the weather on the Earth is driven by the Sun. 165 00:17:56,920 --> 00:17:59,520 It heats the ground up down there and that, in turn, 166 00:17:59,520 --> 00:18:02,600 heats the air up close to the ground, makes it expand, 167 00:18:02,600 --> 00:18:06,640 and that means that the air becomes less dense and the hot air rises. 168 00:18:08,280 --> 00:18:10,520 That means you get thermals, so energetic 169 00:18:10,520 --> 00:18:13,480 movements of air from the ground into the upper atmosphere. 170 00:18:13,480 --> 00:18:16,160 And that's what we're exploiting now, to glide. 171 00:18:25,880 --> 00:18:32,160 All this beautiful weather is driven by the Sun heating the land 172 00:18:32,160 --> 00:18:33,840 and causing air to move around. 173 00:18:35,880 --> 00:18:37,920 So we're sailing around... 174 00:18:37,920 --> 00:18:40,280 Sailing around in a turbulent, 175 00:18:40,280 --> 00:18:42,520 moving ocean of air. 176 00:18:49,920 --> 00:18:52,560 But travel further out into the solar system 177 00:18:52,560 --> 00:18:57,000 and the driving forces within an atmosphere must change. 178 00:18:59,480 --> 00:19:02,680 Now, there's been weather on Saturn for over four billion years, 179 00:19:02,680 --> 00:19:05,560 but Saturn is a long way from the Sun, and so 180 00:19:05,560 --> 00:19:08,920 those weather systems are not primarily driven by the Sun's heat. 181 00:19:10,840 --> 00:19:14,680 And yet the storms on Saturn are amongst the most violent 182 00:19:14,680 --> 00:19:16,720 found anywhere in the solar system. 183 00:19:27,280 --> 00:19:31,480 With sunlight almost 100 times weaker on Saturn than it is 184 00:19:31,480 --> 00:19:33,160 here on Earth... 185 00:19:35,000 --> 00:19:37,640 ..it means something else is helping drive its weather... 186 00:19:41,200 --> 00:19:44,760 ..and the huge, complex forms that arise in its atmosphere. 187 00:19:56,520 --> 00:20:01,400 By studying the cloud tops of Saturn and its great storm systems, 188 00:20:01,400 --> 00:20:07,080 we can now infer a great deal about the strange world that lies beneath. 189 00:20:15,040 --> 00:20:19,000 And the energy source that helps power this planet. 190 00:20:32,240 --> 00:20:34,200 Below the upper atmosphere, 191 00:20:34,200 --> 00:20:36,640 great clouds of water grow. 192 00:20:42,040 --> 00:20:46,800 Lightning 10,000 times more powerful than on Earth illuminates the sky. 193 00:21:02,360 --> 00:21:08,440 This lightning transforms methane gas into huge clouds of soot. 194 00:21:17,000 --> 00:21:18,880 Deeper still, 195 00:21:18,880 --> 00:21:21,560 the pressures grow so great that these 196 00:21:21,560 --> 00:21:27,320 chunks of soot are likely transformed into diamonds. 197 00:21:35,880 --> 00:21:39,520 But even these diamonds will succumb to the pressures of Saturn, 198 00:21:39,520 --> 00:21:41,720 eventually liquefying. 199 00:21:55,560 --> 00:21:59,200 It's not until 40,000km down 200 00:21:59,200 --> 00:22:03,120 that the major source of Saturn's energy is revealed. 201 00:22:05,720 --> 00:22:09,600 Here, pressures are so intense, gases behave like liquid metal. 202 00:22:09,600 --> 00:22:14,960 Molten helium falls like rain through hydrogen... 203 00:22:16,480 --> 00:22:19,200 ..and releases incredible amounts of heat. 204 00:22:26,160 --> 00:22:29,360 And we think that this is the extraordinary heat source 205 00:22:29,360 --> 00:22:32,720 that has been helping to drive Saturn's weather. 206 00:22:56,160 --> 00:22:59,720 Within just a few hundred million years of its birth, 207 00:22:59,720 --> 00:23:02,160 Saturn had lived a life of drama. 208 00:23:03,600 --> 00:23:08,080 Now, it remained largely unchanged for billions of years, 209 00:23:08,080 --> 00:23:11,520 although still very different from the planet we know today. 210 00:23:17,400 --> 00:23:19,680 But in time, this great size 211 00:23:19,680 --> 00:23:23,480 would lead to one more iconic transformation. 212 00:23:34,400 --> 00:23:37,240 Saturn is a planet defined by its rings. 213 00:23:37,240 --> 00:23:40,560 You get a child to draw a planet and quite 214 00:23:40,560 --> 00:23:43,800 a lot of the time, they'll draw something that looks like Saturn. 215 00:23:43,800 --> 00:23:48,280 Close up, they are almost impossibly delicate and intricate. 216 00:23:48,280 --> 00:23:52,520 They're made up of trillions of ice crystals, most of them 217 00:23:52,520 --> 00:23:56,400 no bigger than snowflakes, but some of them as large as houses. 218 00:23:56,400 --> 00:23:58,640 Now, if you look through a telescope, 219 00:23:58,640 --> 00:24:00,240 they're highly reflective. 220 00:24:00,240 --> 00:24:05,040 They reflect sunlight back just as powerfully as the planet itself. 221 00:24:05,040 --> 00:24:07,480 But that raises an interesting question, 222 00:24:07,480 --> 00:24:10,840 because the solar system is full of dust. 223 00:24:10,840 --> 00:24:13,280 Any ice crystals that hang around 224 00:24:13,280 --> 00:24:16,000 for any amount of time will get dirty, 225 00:24:16,000 --> 00:24:20,160 and so they won't reflect sunlight back, they won't be as bright. 226 00:24:20,160 --> 00:24:25,040 So, the question is, why then are Saturn's rings so reflective? 227 00:24:29,720 --> 00:24:33,000 The answer came from one of the most audacious 228 00:24:33,000 --> 00:24:36,840 and successful missions to the outer solar system. 229 00:24:36,840 --> 00:24:43,760 Five, four, three, two, one, and liftoff of the Cassini spacecraft 230 00:24:43,760 --> 00:24:46,000 on a billion-mile trek to Saturn. 231 00:24:48,640 --> 00:24:52,880 We have cleared the tower. T plus 20 seconds. All systems are go. 232 00:24:52,880 --> 00:24:56,600 The Cassini probe was designed to study Saturn and its moons 233 00:24:56,600 --> 00:25:00,400 and rings up close, to shed new light on their origins. 234 00:25:00,400 --> 00:25:03,640 All systems continuing to be go. 235 00:25:03,640 --> 00:25:06,920 But whereas the Voyager probes spent only days at Saturn, 236 00:25:06,920 --> 00:25:11,600 as they sped by, Cassini was built to enter Saturnian orbit... 237 00:25:11,600 --> 00:25:14,840 Solid rocket boosters have been jettisoned. 238 00:25:14,840 --> 00:25:18,000 Velocity 4,378 miles per hour. 239 00:25:18,000 --> 00:25:20,160 ..and explore the system for years. 240 00:25:27,040 --> 00:25:30,560 Many times more massive than Voyager, Cassini had to take 241 00:25:30,560 --> 00:25:33,520 a different route to enter orbit around Saturn. 242 00:25:37,200 --> 00:25:40,400 To conserve fuel, it flew by multiple planets, 243 00:25:40,400 --> 00:25:43,640 slingshotting past them to pick up speed 244 00:25:43,640 --> 00:25:47,120 and change direction over the course of seven years. 245 00:25:53,920 --> 00:25:56,560 As Cassini finally approached its target, 246 00:25:56,560 --> 00:26:00,680 it was travelling at almost 100,000 kilometres per hour 247 00:26:00,680 --> 00:26:04,200 and had to slow itself with exquisite precision. 248 00:26:19,000 --> 00:26:21,680 INDISTINCT RADIO COMMUNICATION 249 00:26:23,600 --> 00:26:26,320 CHEERING 250 00:26:38,280 --> 00:26:41,760 After its 3.5 billion-kilometre journey, 251 00:26:41,760 --> 00:26:45,240 Cassini was now able to see Saturn's rings like never before. 252 00:26:57,840 --> 00:27:02,320 What's more, during its long stay at Saturn, Cassini would be able 253 00:27:02,320 --> 00:27:05,520 to take physical samples for the very first time. 254 00:27:24,960 --> 00:27:28,280 By measuring the way that the dust from the solar system falls 255 00:27:28,280 --> 00:27:32,040 onto the rings, Cassini made a startling discovery. 256 00:27:39,040 --> 00:27:42,760 If the rings have been around Saturn for billions of years, 257 00:27:42,760 --> 00:27:46,560 they should have been darkened and dimmed by the dust. 258 00:27:48,400 --> 00:27:50,760 But they are pristine and bright. 259 00:27:55,320 --> 00:27:58,560 And the reason is because they are young. 260 00:28:01,280 --> 00:28:06,200 Nearly 4.5 billion years younger than Saturn itself. 261 00:28:15,840 --> 00:28:19,880 The mystery of the origin of the rings is an active area of research. 262 00:28:21,520 --> 00:28:24,960 And the evidence that Cassini delivered hints that the solution 263 00:28:24,960 --> 00:28:28,000 lies not with the planet itself, 264 00:28:28,000 --> 00:28:31,840 but with the worlds tracked in orbit around it - 265 00:28:31,840 --> 00:28:33,680 Saturn's moons. 266 00:28:38,760 --> 00:28:41,920 Saturn has 62 large moons and countless smaller ones - 267 00:28:41,920 --> 00:28:43,840 we're still discovering them. 268 00:28:43,840 --> 00:28:47,200 So it's like a mini solar system in itself. 269 00:28:47,200 --> 00:28:50,360 And those moons range in size from planetary-size objects, 270 00:28:50,360 --> 00:28:54,200 like Titan, to small irregular-shaped lumps of rock. 271 00:28:54,200 --> 00:28:57,680 And there's so much diversity in size and form 272 00:28:57,680 --> 00:29:01,760 and surface features, that the whole system is like a history book. 273 00:29:01,760 --> 00:29:05,200 And if we learn to read it then we see that Saturn 274 00:29:05,200 --> 00:29:09,240 has had a very violent and changeable past. 275 00:29:09,240 --> 00:29:12,920 Look at this, this is Mimas, which is often called the Death Star moon 276 00:29:12,920 --> 00:29:14,720 for obvious reasons. 277 00:29:14,720 --> 00:29:20,200 It has this huge impact crater, which tells you that once, long ago, 278 00:29:20,200 --> 00:29:23,040 there was a very violent impact 279 00:29:23,040 --> 00:29:26,080 that almost - but not quite - destroyed the Moon. 280 00:29:27,560 --> 00:29:30,000 And this is Iapetus. 281 00:29:30,000 --> 00:29:32,960 It's a very large moon, almost planetary-size. 282 00:29:32,960 --> 00:29:35,720 It's about 1,500 kilometres across. 283 00:29:35,720 --> 00:29:39,480 And half of it is white, and half of it is dark. 284 00:29:42,120 --> 00:29:45,120 Iapetus also has another distinctive feature 285 00:29:45,120 --> 00:29:48,440 which is this ridge all the way around the equator, and you can see 286 00:29:48,440 --> 00:29:50,680 the scale of it from this photograph. 287 00:29:50,680 --> 00:29:52,520 And this is clearly visible. 288 00:29:52,520 --> 00:29:55,960 It's one of the highest mountain ranges in the solar system, 289 00:29:55,960 --> 00:29:59,640 20km from peak to the ground below. 290 00:30:05,800 --> 00:30:11,000 And it's thought that this formed because a ring that was once 291 00:30:11,000 --> 00:30:15,760 around the Moon, just like Saturn's rings, collapsed onto its surface. 292 00:30:23,440 --> 00:30:28,880 As Cassini continued its journey, it revealed in detail multiple moons 293 00:30:28,880 --> 00:30:31,120 made almost entirely of ice. 294 00:30:33,680 --> 00:30:37,080 And many of them had taken on extraordinary forms 295 00:30:37,080 --> 00:30:39,440 in and around the rings. 296 00:31:01,160 --> 00:31:05,320 As Cassini analysed the ice moons in ever greater detail, 297 00:31:05,320 --> 00:31:09,280 it became apparent that many of them were made of exactly 298 00:31:09,280 --> 00:31:12,320 the same material as the rings themselves. 299 00:31:15,440 --> 00:31:18,960 The rings and moons were profoundly linked. 300 00:31:23,160 --> 00:31:25,800 But for all the moons that Cassini saw, 301 00:31:25,800 --> 00:31:29,600 it now seemed likely that one was missing. 302 00:31:49,480 --> 00:31:51,720 When dinosaurs roamed the Earth, 303 00:31:51,720 --> 00:31:56,640 we now suspect Saturn had a moon in orbit that no longer exists. 304 00:32:00,880 --> 00:32:04,080 A moon perhaps 400km across 305 00:32:04,080 --> 00:32:06,760 and formed almost entirely of ice. 306 00:32:10,760 --> 00:32:12,720 But this world was doomed. 307 00:32:15,920 --> 00:32:20,200 It found itself orbiting too close to resist the immense forces 308 00:32:20,200 --> 00:32:22,200 of Saturn's gravity. 309 00:32:29,520 --> 00:32:32,040 WAVES ROAR 310 00:32:34,760 --> 00:32:38,680 Gravity is the sculptor of the Saturnian system 311 00:32:38,680 --> 00:32:42,120 but also the instigator of change within it. 312 00:32:42,120 --> 00:32:46,600 We tend to think of gravity as a force that pulls things together, 313 00:32:46,600 --> 00:32:50,240 but it can also act to rip things apart. 314 00:32:50,240 --> 00:32:53,720 Think of the tides here on Earth, they're caused by the difference 315 00:32:53,720 --> 00:32:57,560 in gravitational pull of the Moon from one side of the Earth 316 00:32:57,560 --> 00:32:59,320 to the other. 317 00:32:59,320 --> 00:33:03,040 And that difference, which can be quite subtle, is also powerful, 318 00:33:03,040 --> 00:33:05,080 it can move entire oceans. 319 00:33:05,080 --> 00:33:10,000 It's called tidal gravity, but the Earth also has a tidal effect 320 00:33:10,000 --> 00:33:13,400 on the Moon, and because the Earth is an entire planet 321 00:33:13,400 --> 00:33:16,440 that effect is much more powerful. 322 00:33:20,920 --> 00:33:24,560 The pull of the Earth is enough to deform the Moon's surface. 323 00:33:26,400 --> 00:33:30,640 The effect was particularly strong 4.5 billion years ago 324 00:33:30,640 --> 00:33:33,800 when the Moon was nearly 17 times closer. 325 00:33:36,160 --> 00:33:39,920 Back then, the pull from the Earth caused a tide of solid rock 326 00:33:39,920 --> 00:33:41,920 to rise and fall. 327 00:33:46,720 --> 00:33:50,360 If the Moon had been any nearer, it would have crossed what we call 328 00:33:50,360 --> 00:33:55,480 the Roche limit, a place where tidal gravitational forces are so strong, 329 00:33:55,480 --> 00:33:58,200 moons can get ripped apart. 330 00:34:01,480 --> 00:34:03,600 WAVES ROAR 331 00:34:07,640 --> 00:34:11,920 The Romans named the planet Saturn after their god of time and harvest. 332 00:34:11,920 --> 00:34:15,760 And in one of the more gruesome tales from classical mythology, 333 00:34:15,760 --> 00:34:20,240 Saturn actually ate his newborn babies in order to prevent them 334 00:34:20,240 --> 00:34:22,080 from taking his power. 335 00:34:22,080 --> 00:34:25,720 What's sort of interesting is, that what we've learned from Cassini 336 00:34:25,720 --> 00:34:30,040 is that, at least metaphorically speaking, they weren't far wrong. 337 00:34:43,400 --> 00:34:47,520 Just beyond Saturn's atmosphere, our leading theory suggests 338 00:34:47,520 --> 00:34:50,880 an icy moon must have approached close to 339 00:34:50,880 --> 00:34:54,560 or even just inside the planet's Roche limit. 340 00:34:58,680 --> 00:35:01,680 As Saturn's immense tidal gravitational forces 341 00:35:01,680 --> 00:35:05,360 acted across the Moon, it began to rupture. 342 00:35:09,960 --> 00:35:13,240 Saturn began to devour its child. 343 00:36:02,400 --> 00:36:05,680 Up to 15,000 trillion tonnes of ice 344 00:36:05,680 --> 00:36:09,200 broke apart in orbit around Saturn. 345 00:36:21,880 --> 00:36:25,360 Because of the speeds the ice fragments were travelling, 346 00:36:25,360 --> 00:36:28,800 it's likely that in just a few days they spread out 347 00:36:28,800 --> 00:36:30,840 to encircle the great giant. 348 00:36:37,120 --> 00:36:40,800 Saturn's iconic ring had been born. 349 00:37:01,280 --> 00:37:03,720 Today, the giant ring has evolved. 350 00:37:06,000 --> 00:37:09,520 Saturn's powerful gravitational field has helped keep 351 00:37:09,520 --> 00:37:11,880 its near perfect circular shape. 352 00:37:13,080 --> 00:37:16,560 But collisions within have caused it to flatten out. 353 00:37:21,320 --> 00:37:24,880 Now this debris forms a disc wider than Jupiter, 354 00:37:24,880 --> 00:37:28,360 yet, on average, just ten metres thick. 355 00:37:33,800 --> 00:37:37,880 Within, moon-sized chunks of ice orbit the structure 356 00:37:37,880 --> 00:37:40,720 clearing great voids. 357 00:37:42,400 --> 00:37:44,360 Turning one ring... 358 00:37:45,800 --> 00:37:47,600 ..into many. 359 00:37:53,120 --> 00:37:57,800 In places, moons have pulled particles of ice upwards 360 00:37:57,800 --> 00:38:01,640 to create strange peaks over 2km high. 361 00:38:04,280 --> 00:38:06,520 And in just the right conditions, 362 00:38:06,520 --> 00:38:09,880 they cast spectacular shadows across the rings. 363 00:38:18,080 --> 00:38:21,360 This once tiny world of rock and ice... 364 00:38:23,800 --> 00:38:27,000 ..that has undergone so many great transformations... 365 00:38:33,600 --> 00:38:37,000 ..has today become the solar system's jewel. 366 00:38:50,800 --> 00:38:54,440 Cassini had deepened our understanding of the origin 367 00:38:54,440 --> 00:38:56,280 and evolution of Saturn. 368 00:38:57,360 --> 00:39:00,080 But its mission was far from over. 369 00:39:00,080 --> 00:39:04,400 Because just beyond the rings it discovered another treasure. 370 00:39:06,040 --> 00:39:10,480 A place that may hold answers to some of our deepest questions 371 00:39:10,480 --> 00:39:14,640 about the possibility of life in the solar system and beyond. 372 00:39:35,040 --> 00:39:39,280 Over a billion kilometres from the warmth of the Sun, 373 00:39:39,280 --> 00:39:42,600 just on the outer edge of Saturn's rings, 374 00:39:42,600 --> 00:39:45,040 lies the icy moon Enceladus. 375 00:39:48,280 --> 00:39:50,120 Enceladus is quite a small moon, 376 00:39:50,120 --> 00:39:53,160 it's only about the size of Iceland actually. 377 00:39:53,160 --> 00:39:55,960 But that does not make it dull. 378 00:39:55,960 --> 00:39:59,480 It is the most reflective object in the solar system - 379 00:39:59,480 --> 00:40:03,720 over 90% of the light that hits it bounces back, 380 00:40:03,720 --> 00:40:07,760 and that's because its surface is covered in pure ice. 381 00:40:07,760 --> 00:40:10,760 It's also crisscrossed by crevasses 382 00:40:10,760 --> 00:40:13,320 like this one you see in this glacier, 383 00:40:13,320 --> 00:40:15,680 but on a much grander scale. 384 00:40:15,680 --> 00:40:18,480 The largest, known as the tiger stripes, 385 00:40:18,480 --> 00:40:20,240 are over 100km long. 386 00:40:20,240 --> 00:40:22,840 But for all its surface beauty, 387 00:40:22,840 --> 00:40:25,480 it's what's going on below the ice 388 00:40:25,480 --> 00:40:27,920 that makes Enceladus so special. 389 00:40:33,960 --> 00:40:37,840 What we found beneath Enceladus's shell of ice must rank 390 00:40:37,840 --> 00:40:41,760 as one of the greatest discoveries in 21st-century space exploration. 391 00:40:43,720 --> 00:40:47,480 But it was a discovery that had been hiding in plain sight. 392 00:40:58,160 --> 00:41:02,520 This is a photograph taken by Voyager 1 of Enceladus in 1980. 393 00:41:02,520 --> 00:41:06,360 Now, nobody noticed anything remarkable about it at the time 394 00:41:06,360 --> 00:41:10,520 but this image has been enhanced and now you can see 395 00:41:10,520 --> 00:41:14,360 this sort of misty blob off one side of the Moon. 396 00:41:14,360 --> 00:41:17,000 Now, that is fascinating. 397 00:41:17,000 --> 00:41:21,720 It means something is rising up from inside Enceladus. 398 00:41:21,720 --> 00:41:26,720 Now, when Cassini got back there 24 years later it saw the same thing, 399 00:41:26,720 --> 00:41:29,600 but this time in much more detail. 400 00:41:33,200 --> 00:41:37,800 As Cassini approached Enceladus, the anomaly revealed itself. 401 00:41:41,320 --> 00:41:46,280 Giant plumes of water vapour and ice were erupting from its surface. 402 00:41:50,160 --> 00:41:55,080 Over 200kg of material was being released every second. 403 00:42:00,040 --> 00:42:03,120 But this material was not lost to space. 404 00:42:03,120 --> 00:42:06,600 It seemed to be feeding one of Saturn's outside rings, 405 00:42:06,600 --> 00:42:08,800 helping to replenish it. 406 00:42:16,240 --> 00:42:20,000 And these discoveries inspired an audacious risk. 407 00:42:22,480 --> 00:42:26,000 Piloted from over a billion kilometres away, 408 00:42:26,000 --> 00:42:29,760 Cassini was guided dangerously close to the plumes. 409 00:42:46,240 --> 00:42:50,640 The craft passed within just 48km of the surface 410 00:42:50,640 --> 00:42:52,240 of Enceladus. 411 00:42:55,160 --> 00:42:58,200 And Cassini was able to touch the plumes. 412 00:43:08,720 --> 00:43:12,000 What we discovered, thanks to a series of these flybys, 413 00:43:12,000 --> 00:43:13,920 was breathtaking. 414 00:43:15,600 --> 00:43:20,560 The ice in the plumes was actually frozen particles of salty water. 415 00:43:22,600 --> 00:43:26,640 We had found a subsurface ocean leaking into space. 416 00:43:31,360 --> 00:43:33,840 Far, far from the Sun, 417 00:43:33,840 --> 00:43:37,920 Enceladus was harbouring an ocean of liquid water. 418 00:43:39,240 --> 00:43:41,720 And it was there because of Saturn. 419 00:43:48,720 --> 00:43:52,400 Enceladus orbits Saturn in an elliptical orbit, 420 00:43:52,400 --> 00:43:55,320 which means the gravitational forces across the Moon 421 00:43:55,320 --> 00:43:57,320 are constantly changing. 422 00:44:00,160 --> 00:44:04,000 It's maintained in this orbit by the pull of another larger moon - 423 00:44:04,000 --> 00:44:05,440 Dione. 424 00:44:07,520 --> 00:44:11,720 These ever-shifting gravitational forces stretch and squeeze 425 00:44:11,720 --> 00:44:15,800 its heart, warming and melting its icy interior. 426 00:44:22,600 --> 00:44:26,360 But Cassini's data had one more surprise in store. 427 00:44:29,800 --> 00:44:33,880 As the plumes of water ice were analysed in ever greater detail, 428 00:44:33,880 --> 00:44:37,640 we discovered complex organic compounds 429 00:44:37,640 --> 00:44:40,760 and silica particles that could only have come 430 00:44:40,760 --> 00:44:43,600 from hot hydrothermal vents. 431 00:44:48,720 --> 00:44:52,960 In the frozen outer reaches of the solar system 432 00:44:52,960 --> 00:44:56,640 we had found a warm watery oasis. 433 00:44:58,880 --> 00:45:01,120 WATER GUSHES 434 00:45:06,560 --> 00:45:11,040 Cassini has given us a glimpse beneath the ice of Enceladus, 435 00:45:11,040 --> 00:45:16,520 and it is fascinating genuinely in a scientific sense because many 436 00:45:16,520 --> 00:45:20,920 biologists believe that hydrothermal vents like those that are almost 437 00:45:20,920 --> 00:45:24,040 certainly present on the floor of Enceladus's ocean 438 00:45:24,040 --> 00:45:26,680 were the cradle of life on Earth. 439 00:45:26,680 --> 00:45:28,920 All the ingredients are present - 440 00:45:28,920 --> 00:45:32,160 there's hot water in touch with ice and minerals. 441 00:45:32,160 --> 00:45:35,400 That's a reactive cauldron of chemistry. 442 00:45:35,400 --> 00:45:37,880 There are reactive gases - methane. 443 00:45:37,880 --> 00:45:41,520 Cassini found molecular hydrogen in the plumes, 444 00:45:41,520 --> 00:45:46,400 and that was one of the food sources of primitive organisms on Earth. 445 00:45:46,400 --> 00:45:50,240 So there really is a possibility that there is life 446 00:45:50,240 --> 00:45:53,280 in orbit around Saturn today. 447 00:46:01,600 --> 00:46:05,240 The prospect of life on Enceladus is exciting. 448 00:46:06,920 --> 00:46:10,160 But if it's there, it's likely to be only the simplest 449 00:46:10,160 --> 00:46:12,680 and most primitive of organisms. 450 00:46:12,680 --> 00:46:14,640 WATER ROARS 451 00:46:16,400 --> 00:46:21,320 And given how violent and changeable Saturn's past has been, 452 00:46:21,320 --> 00:46:24,360 this world of ice and liquid water may only have arisen 453 00:46:24,360 --> 00:46:26,600 relatively recently. 454 00:46:29,760 --> 00:46:31,680 Now, we don't know how long 455 00:46:31,680 --> 00:46:36,160 Enceladus has been geologically active, how long its had an ocean. 456 00:46:36,160 --> 00:46:40,240 If it was only tens of millions or even hundreds of millions of years, 457 00:46:40,240 --> 00:46:44,440 that may not have been enough time for life to get going. 458 00:46:44,440 --> 00:46:49,320 But if there is life there then we can glimpse its future 459 00:46:49,320 --> 00:46:54,440 because there are still hydrothermal vent systems present on Earth today 460 00:46:54,440 --> 00:46:58,680 and life doesn't just survive there, it thrives. 461 00:47:17,320 --> 00:47:22,280 Often found in the deep oceans along fault lines in the Earth's crust, 462 00:47:22,280 --> 00:47:25,920 hydrothermal vents are some of the richest and most complex 463 00:47:25,920 --> 00:47:28,160 ecosystems on Earth. 464 00:47:29,920 --> 00:47:33,360 Feeding off the abundant bacteria that breed there, 465 00:47:33,360 --> 00:47:38,120 these habitats support a multitude of strange and complex organisms. 466 00:47:49,920 --> 00:47:54,000 Whilst creatures like this will never exist on Enceladus, 467 00:47:54,000 --> 00:47:58,960 billions of years from now, it is just possible that this 468 00:47:58,960 --> 00:48:03,200 distant world may become home to its own unique forms of life. 469 00:48:14,280 --> 00:48:18,600 But for Cassini, the brave new world under the ice of Enceladus 470 00:48:18,600 --> 00:48:21,160 was a bittersweet discovery. 471 00:48:35,040 --> 00:48:38,640 13 years after it arrived at Saturn, 472 00:48:38,640 --> 00:48:42,080 Cassini's fuel finally began to run dry. 473 00:48:44,040 --> 00:48:48,440 NASA couldn't risk letting this probe crash land on Enceladus 474 00:48:48,440 --> 00:48:52,240 and contaminate a potential habitat for life. 475 00:48:52,240 --> 00:48:53,880 OVER RADIO: Systems, ACS One. 476 00:48:53,880 --> 00:48:56,120 We've just had transition to high rate mode. 477 00:48:59,160 --> 00:49:03,640 And so the decision was made to send Cassini on one final journey. 478 00:49:07,080 --> 00:49:08,920 A journey that would take it 479 00:49:08,920 --> 00:49:11,440 into the atmosphere of Saturn itself. 480 00:49:12,960 --> 00:49:16,360 A journey from which it would never return. 481 00:49:21,760 --> 00:49:25,160 GROUND CONTROL: And we are in the atmosphere. 482 00:49:25,160 --> 00:49:27,200 Radio signal still holding. 483 00:49:27,200 --> 00:49:28,600 30 seconds. 484 00:49:49,520 --> 00:49:53,200 Just heard the signal from the spacecraft is gone 485 00:49:53,200 --> 00:49:57,320 and that, within the next 45 seconds, so will be the spacecraft. 486 00:50:01,080 --> 00:50:04,440 The planet that had borne witness to some of the greatest dramas 487 00:50:04,440 --> 00:50:08,600 in the history of the solar system was now consuming the craft 488 00:50:08,600 --> 00:50:11,600 that had told its extraordinary story. 489 00:50:24,280 --> 00:50:28,960 Cassini and Saturn had now become inseparable companions. 490 00:50:52,000 --> 00:50:55,480 We journeyed out to Saturn to experience and explore 491 00:50:55,480 --> 00:50:57,160 this great beauty. 492 00:51:03,480 --> 00:51:07,960 But in the end we were rewarded with insights closer to home. 493 00:51:16,760 --> 00:51:19,520 Cassini and her team of scientists and engineers 494 00:51:19,520 --> 00:51:22,520 should be counted amongst our greatest of explorers. 495 00:51:22,520 --> 00:51:24,520 The mission had all the ingredients - 496 00:51:24,520 --> 00:51:28,040 a vessel and a crew drawn by the prospects of beauty 497 00:51:28,040 --> 00:51:32,760 and new knowledge across an uncharted ocean to an alien world. 498 00:51:32,760 --> 00:51:36,000 And, as is so often the case in exploration, 499 00:51:36,000 --> 00:51:39,480 the real treasure was found in the unexpected shadows. 500 00:51:39,480 --> 00:51:42,240 In this case, in the shadow of the rings, 501 00:51:42,240 --> 00:51:45,520 where we found this tiny world, Enceladus, 502 00:51:45,520 --> 00:51:49,480 that may harbour life and provide us with reassurance 503 00:51:49,480 --> 00:51:52,400 that we are not alone in the universe. 504 00:51:52,400 --> 00:51:55,720 See? The beauty is in the story. 505 00:51:55,720 --> 00:51:59,120 Would it matter really if we found life on Enceladus? 506 00:51:59,120 --> 00:52:01,520 It would only be microbes after all. 507 00:52:01,520 --> 00:52:03,720 Yes, it would matter. 508 00:52:03,720 --> 00:52:08,480 We are at a time in our history, I think, where we need reassurance 509 00:52:08,480 --> 00:52:13,520 or perhaps a reminder that there is beauty and knowledge 510 00:52:13,520 --> 00:52:17,960 and perhaps even life and meaning beyond our shores. 511 00:52:29,000 --> 00:52:33,440 We will, I'm sure, go back to Saturn and its moons, 512 00:52:33,440 --> 00:52:36,080 explore deeper, stay longer, 513 00:52:36,080 --> 00:52:39,440 and maybe one day even visit ourselves. 514 00:52:42,920 --> 00:52:45,240 We don't know what we'll discover 515 00:52:45,240 --> 00:52:49,720 but we can be sure that the story has only just begun. 516 00:53:07,800 --> 00:53:11,240 Journey through our solar system with this free poster 517 00:53:11,240 --> 00:53:12,880 produced by the Open University, 518 00:53:12,880 --> 00:53:16,120 and discover more about its planets and moons. 519 00:53:16,120 --> 00:53:24,040 Order your free copy by calling... 520 00:53:24,040 --> 00:53:29,040 ..or go to... 521 00:53:29,040 --> 00:53:33,040 ..and follow the links to the Open University.