1 00:00:02,680 --> 00:00:06,760 This week, our skies will see a rare daytime astronomical event. 2 00:00:08,000 --> 00:00:10,600 On Monday, if the weather's better than this, 3 00:00:10,600 --> 00:00:13,240 we'll be able to watch a transit of Mercury 4 00:00:13,240 --> 00:00:16,040 when the solar system's smallest planet passes in front of the sun. 5 00:00:17,520 --> 00:00:20,520 Centuries of work allow us to predict precisely 6 00:00:20,520 --> 00:00:21,960 when this event will occur. 7 00:00:23,080 --> 00:00:26,080 But although we know exactly where Mercury is, 8 00:00:26,080 --> 00:00:28,880 it's a planet that we know surprisingly little about. 9 00:00:31,120 --> 00:00:34,080 I like to think of it as the solar system's problem child 10 00:00:34,080 --> 00:00:36,800 cos it so often confounds our expectations. 11 00:00:38,800 --> 00:00:42,520 A planet this close to the sun should be baked dry 12 00:00:42,520 --> 00:00:44,880 and yet there's ice on its surface. 13 00:00:46,960 --> 00:00:51,120 It is expect such a tiny world to have solidified into 14 00:00:51,120 --> 00:00:52,680 an inactive ball of rock... 15 00:00:53,800 --> 00:00:56,240 ..but Mercury is geologically alive. 16 00:00:58,760 --> 00:01:00,720 It even appears to be shrinking. 17 00:01:03,800 --> 00:01:05,960 On this month's programme, 18 00:01:05,960 --> 00:01:09,560 we'll be investigating Mercury's curious behaviour. 19 00:01:11,280 --> 00:01:14,480 And Pete Lawrence will be here to explain how you can watch 20 00:01:14,480 --> 00:01:15,760 the transit in safety. 21 00:01:16,840 --> 00:01:20,280 So, tonight, in all its glory, we give you Mercury - 22 00:01:20,280 --> 00:01:23,080 the most puzzling planet in the solar system. 23 00:01:23,080 --> 00:01:24,920 Welcome to The Sky At Night. 24 00:01:56,920 --> 00:02:00,120 At first glance, the thing that strikes you about Mercury 25 00:02:00,120 --> 00:02:02,520 is that it looks a lot like our moon - 26 00:02:02,520 --> 00:02:05,560 a bare, pale rock covered in craters. 27 00:02:06,720 --> 00:02:08,440 It's even a similar size. 28 00:02:10,160 --> 00:02:13,720 But in reality, Mercury is not at all like the moon. 29 00:02:13,720 --> 00:02:16,880 In fact, it's not like anywhere else in the solar system. 30 00:02:19,400 --> 00:02:23,440 Mercury has long been seen as an enigmatic planet, 31 00:02:23,440 --> 00:02:26,480 but it's difficult to observe because it's orbit is so close to the sun. 32 00:02:26,480 --> 00:02:29,720 So, whenever we look at it, it's lost in the sun's glare. 33 00:02:31,680 --> 00:02:34,600 Even so, with observations from Earth, 34 00:02:34,600 --> 00:02:37,480 we can map out Mercury's unusual orbit. 35 00:02:38,800 --> 00:02:42,280 Unsurprisingly, Mercury, being the closest planet to the sun, 36 00:02:42,280 --> 00:02:43,880 has the shortest orbit. 37 00:02:43,880 --> 00:02:46,720 It takes just 88 days to go all the way round. 38 00:02:46,720 --> 00:02:48,840 It also has the most elliptical orbit 39 00:02:48,840 --> 00:02:50,840 of all the planets in the solar system. 40 00:02:51,960 --> 00:02:55,120 At its closest approach, it's just 46 million kilometres 41 00:02:55,120 --> 00:02:58,960 away from the sun, and, at its furthest, 70 million kilometres. 42 00:02:58,960 --> 00:03:01,840 Now, that's unusual, but things get really weird 43 00:03:01,840 --> 00:03:04,400 when you take into account Mercury's rotation. 44 00:03:07,920 --> 00:03:10,080 Mercury rotates very slowly. 45 00:03:10,080 --> 00:03:13,320 It actually takes 59 Earth days for it to complete one revolution. 46 00:03:14,720 --> 00:03:18,640 But as it rotates, Mercury is also moving around its orbit. 47 00:03:19,800 --> 00:03:21,880 And the combination of rotation and orbit 48 00:03:21,880 --> 00:03:25,360 causes the sun to move very slowly across the sky. 49 00:03:26,520 --> 00:03:30,560 So slowly in fact that from the planet's surface, a day 50 00:03:30,560 --> 00:03:34,440 from sunrise to sunrise actually lasts two complete orbits. 51 00:03:34,440 --> 00:03:38,320 So, on Mercury, a day is twice as long as its year. 52 00:03:38,320 --> 00:03:41,920 That means that, on the surface, daylight lasts the equivalent of 53 00:03:41,920 --> 00:03:46,960 three Earth months and temperatures rise to around 450 Celsius. 54 00:03:46,960 --> 00:03:49,440 That's followed by three months of night-time, 55 00:03:49,440 --> 00:03:51,920 where the temperatures plummet to minus 180. 56 00:03:54,640 --> 00:03:57,640 This temperature difference of over 600 degrees 57 00:03:57,640 --> 00:04:00,680 is the highest experienced anywhere in the solar system. 58 00:04:03,440 --> 00:04:07,160 That made it all the more surprising when astronomers at the Arecibo 59 00:04:07,160 --> 00:04:10,040 radio telescope bounced radar pulses off the planet. 60 00:04:14,040 --> 00:04:17,840 The signals that came back showed bright spots near the poles 61 00:04:17,840 --> 00:04:20,840 that bore the unmistakable signature of water ice. 62 00:04:22,360 --> 00:04:25,520 The shape of the ice deposits clearly showed that they were 63 00:04:25,520 --> 00:04:28,360 concealed in the depths of polar craters - 64 00:04:28,360 --> 00:04:32,600 the only areas on the planet that never receive any direct sunlight. 65 00:04:34,400 --> 00:04:36,400 That means that, on this planet, 66 00:04:36,400 --> 00:04:38,720 three times closer to the sun than Earth, 67 00:04:38,720 --> 00:04:40,680 you can still find water ice. 68 00:04:40,680 --> 00:04:42,000 That's pretty amazing. 69 00:04:45,520 --> 00:04:48,800 But there's only so much we can tell by observing from Earth. 70 00:04:50,080 --> 00:04:53,560 To learn more about Mercury, we need spacecraft data. 71 00:04:55,160 --> 00:04:58,000 Only two missions have ever visited the planet - 72 00:04:58,000 --> 00:05:01,240 the first was Mariner 10 in 1974. 73 00:05:03,800 --> 00:05:06,560 Its camera only produced grainy pictures, 74 00:05:06,560 --> 00:05:09,640 but the other instruments it carried began to reveal major 75 00:05:09,640 --> 00:05:13,280 differences between Mercury and the other rocky planets. 76 00:05:14,680 --> 00:05:18,080 I've come to the Open University in Milton Keynes to talk to 77 00:05:18,080 --> 00:05:20,080 Mercury expert David Rothery. 78 00:05:21,160 --> 00:05:23,600 When they see images of Mercury, like this one, 79 00:05:23,600 --> 00:05:25,680 you can't fail to be intrigued by it. 80 00:05:25,680 --> 00:05:27,600 It's got a wonderful landscape. 81 00:05:27,600 --> 00:05:31,080 It's a beautiful image as well, but we've only sent two spacecraft 82 00:05:31,080 --> 00:05:34,560 two Mercury, compared to many that have been to Venus and Mars. 83 00:05:34,560 --> 00:05:36,800 Why is that? Why is Mercury being neglected? 84 00:05:36,800 --> 00:05:39,840 Well, Mars and Venus are the low-hanging fruit - 85 00:05:39,840 --> 00:05:41,160 they are closer to the Earth. 86 00:05:41,160 --> 00:05:43,680 And at first sight, certainly they are more interesting. 87 00:05:43,680 --> 00:05:46,440 Mercury, when we first sent a probe by it, it's airless, 88 00:05:46,440 --> 00:05:48,880 it's heavily cratered, it's a little bit dull. 89 00:05:48,880 --> 00:05:51,320 Now we realise it is not dull at all. 90 00:05:51,320 --> 00:05:53,480 There's all kinds of things going on there. 91 00:05:53,480 --> 00:05:55,920 Even that first mission, the Mariner mission, 92 00:05:55,920 --> 00:05:59,000 that flew past made some remarkable discoveries. It did. 93 00:05:59,000 --> 00:06:01,080 It was equipped with a magnetometer 94 00:06:01,080 --> 00:06:03,200 to look at the interaction between the solar wind, 95 00:06:03,200 --> 00:06:05,840 the charged particles from the Sun and the planet's surface, 96 00:06:05,840 --> 00:06:09,640 but it found that the planet's generating its own magnetic field. 97 00:06:09,640 --> 00:06:12,480 It's like a scaled-down version of the Earth's magnetic field. 98 00:06:12,480 --> 00:06:14,800 And that's surprising, for such a small planet 99 00:06:14,800 --> 00:06:16,400 to have a magnetic field. 100 00:06:16,400 --> 00:06:18,400 It's really, really unusual. Absolutely. 101 00:06:18,400 --> 00:06:21,560 Mars, the Moon and Venus don't generate a magnetic field, 102 00:06:21,560 --> 00:06:22,880 but Mercury does. 103 00:06:22,880 --> 00:06:24,960 And, you know, it caught everybody by surprise. 104 00:06:24,960 --> 00:06:27,360 And thank goodness Mariner 10 carried its magnetometer. 105 00:06:27,360 --> 00:06:28,520 But what does it tell us? 106 00:06:28,520 --> 00:06:31,400 What do we need to be able to generate this magnetic field? 107 00:06:31,400 --> 00:06:33,920 OK, well, some people say, "Well, iron in the core. 108 00:06:33,920 --> 00:06:36,520 "You've got a core made of iron - it will be a magnet." 109 00:06:36,520 --> 00:06:38,560 But that doesn't work. It has to be fluid. 110 00:06:39,640 --> 00:06:42,760 You have to have an electrically conducting fluid churning around, 111 00:06:42,760 --> 00:06:46,880 so we think the outer part of Mercury's interior core 112 00:06:46,880 --> 00:06:50,760 is made of molten iron, and that's the explanation that holds for the Earth as well. 113 00:06:50,760 --> 00:06:54,600 Well, that would make sense, apart from the fact that Mercury's such a small world. 114 00:06:54,600 --> 00:06:57,440 On Mars, for example, we think that there's no magnetic field 115 00:06:57,440 --> 00:07:00,200 because it's cooled down and there's no longer any fluid core. 116 00:07:00,200 --> 00:07:02,960 Yeah, well, Mercury should be cooling down as well 117 00:07:02,960 --> 00:07:05,800 cos it's got a large surface compared to its volume. 118 00:07:05,800 --> 00:07:09,520 It was probably quite hot to begin with, but there is also clearly some 119 00:07:09,520 --> 00:07:12,800 way of generating heat in the core to stop it having frozen, 120 00:07:12,800 --> 00:07:15,360 and also something to reduce the melting 121 00:07:15,360 --> 00:07:17,280 temperature of the outer part of the core - 122 00:07:17,280 --> 00:07:19,000 we think that is probably sulphur. 123 00:07:19,000 --> 00:07:21,080 Mixed in with the iron? Mixed in with the iron. 124 00:07:21,080 --> 00:07:23,880 There has to be enough iron to make an electrical conductor. 125 00:07:23,880 --> 00:07:28,120 So what we think is happening is iron is still today sinking 126 00:07:28,120 --> 00:07:33,080 inwards to join the frozen inner core of solid iron 127 00:07:33,080 --> 00:07:35,040 and, as the iron sinks inwards, 128 00:07:35,040 --> 00:07:37,960 it's turning gravitational energy into heat 129 00:07:37,960 --> 00:07:41,080 and that's heating the outer part of the core, which is gradually 130 00:07:41,080 --> 00:07:44,080 becoming richer in sulphur, reducing its melting temperature 131 00:07:44,080 --> 00:07:46,920 and enabling it to churn round and generate a magnetic field. 132 00:07:46,920 --> 00:07:50,280 So we know about one process that is happening inside Mercury - 133 00:07:50,280 --> 00:07:52,920 what do we know about the rest of its structure? 134 00:07:52,920 --> 00:07:55,800 We do know that the core must be very, 135 00:07:55,800 --> 00:07:59,200 very large compared to the size of the planet. Wow! 136 00:07:59,200 --> 00:08:03,080 Beyond the iron-rich inner core and outer core, you've got the rock. 137 00:08:03,080 --> 00:08:05,120 Most of the rock is what we call the mantle, 138 00:08:05,120 --> 00:08:08,080 there's a crust on the outside that is slightly chemically different. 139 00:08:08,080 --> 00:08:10,120 This rocky outer part is much, 140 00:08:10,120 --> 00:08:13,040 much thinner on Mercury than it is on any of the other rocky planets. 141 00:08:13,040 --> 00:08:16,400 It's the opposite of the earth, where we have quite a thick mantle and a thin core. 142 00:08:16,400 --> 00:08:19,120 So, to me, this is one of the great mysteries of Mercury. 143 00:08:19,120 --> 00:08:21,920 Why do we have this large core surrounded by a thin mantle? 144 00:08:21,920 --> 00:08:22,920 Absolutely. 145 00:08:24,040 --> 00:08:28,240 Mariner had started to reveal Mercury's inner secrets, 146 00:08:28,240 --> 00:08:31,640 but we had to wait another 35 years for a really good 147 00:08:31,640 --> 00:08:33,440 look at Mercury's surface. 148 00:08:35,520 --> 00:08:40,080 That finally came in 2011, when NASA's MESSENGER probe 149 00:08:40,080 --> 00:08:43,480 started returning high-definition images like these. 150 00:08:45,640 --> 00:08:49,000 The team here in Milton Keynes are using these images to create 151 00:08:49,000 --> 00:08:52,200 detailed geological maps of Mercury, 152 00:08:52,200 --> 00:08:56,640 but they're also finding plenty more evidence of Mercury's strangeness. 153 00:08:58,360 --> 00:09:01,240 So, Jack, if we can interrupt, what are you working on? 154 00:09:01,240 --> 00:09:04,440 I'm making a geological map of a region on Mercury, this area 155 00:09:04,440 --> 00:09:08,680 you see here. I'm using data from NASA's MESSENGER satellite, 156 00:09:08,680 --> 00:09:10,840 planetary images mosaiced together, 157 00:09:10,840 --> 00:09:13,680 and I'm interpreting the geological units I see at the surface. 158 00:09:13,680 --> 00:09:16,200 And so what I see when I look at this globe of Mercury is 159 00:09:16,200 --> 00:09:18,400 craters and I think that's what people think of. 160 00:09:18,400 --> 00:09:21,440 Like the moon, it's a grey body with a cratered surface. 161 00:09:21,440 --> 00:09:22,800 You're absolutely right. 162 00:09:22,800 --> 00:09:25,000 It is a heavily cratered surface in places. 163 00:09:25,000 --> 00:09:26,800 However, there is more to Mercury. 164 00:09:26,800 --> 00:09:30,040 For example, these lobate scarps that you see here, 165 00:09:30,040 --> 00:09:31,640 this one is called Carnegie Rupes. 166 00:09:31,640 --> 00:09:34,840 So that's this line running from top left to bottom, 167 00:09:34,840 --> 00:09:36,920 right across the image. Absolutely. 168 00:09:36,920 --> 00:09:40,120 This is an escarpment in the landscape caused by faulting 169 00:09:40,120 --> 00:09:41,800 within the crust of Mercury. 170 00:09:41,800 --> 00:09:43,880 So we see this kind of thing on Earth, 171 00:09:43,880 --> 00:09:46,800 these places where you have this sort of raised up area. 172 00:09:46,800 --> 00:09:48,760 Yes, you see these things on Earth. 173 00:09:48,760 --> 00:09:50,240 On Earth we have plate tectonics 174 00:09:50,240 --> 00:09:53,360 and, because of the collisions of plates, we find them building 175 00:09:53,360 --> 00:09:56,840 mountains and making escarpments in the landscape such as this one. 176 00:09:56,840 --> 00:10:00,280 However, there's no evidence to suggest Mercury has multiple 177 00:10:00,280 --> 00:10:03,240 tectonics plates that move around and collide with each other, 178 00:10:03,240 --> 00:10:06,520 so instead this is internal deformation within one plate 179 00:10:06,520 --> 00:10:08,240 that's being drawn in from within. 180 00:10:08,240 --> 00:10:11,120 So it's almost as if the whole planet is shrinking. 181 00:10:11,120 --> 00:10:13,680 Yes, the planet is in a state of global contraction. 182 00:10:13,680 --> 00:10:16,200 And people have looked at the distribution of these lobate 183 00:10:16,200 --> 00:10:18,560 scarps over the planet and added up all their effects, 184 00:10:18,560 --> 00:10:20,840 and they calculate that perhaps the planet has 185 00:10:20,840 --> 00:10:24,040 lost as much as 7km of its planetary radius. That's enormous. 186 00:10:24,040 --> 00:10:29,400 That's Everest-ish... It's hard to imagine a planet shrinking, 187 00:10:29,400 --> 00:10:31,680 but Mercury demonstrates that it has. 188 00:10:31,680 --> 00:10:35,200 So what's causing this? Why is there this shrinking? 189 00:10:35,200 --> 00:10:37,960 As the planet loses heat, this causes a volume reduction, 190 00:10:37,960 --> 00:10:41,720 particularly because as the liquid part of Mercury's iron core freezes, 191 00:10:41,720 --> 00:10:44,680 that causes a volume reduction, which pulls the crust in everywhere. 192 00:10:44,680 --> 00:10:46,560 So it's quite a simple process but, Dave, 193 00:10:46,560 --> 00:10:49,840 this isn't the only thing that's happening on the surface. 194 00:10:49,840 --> 00:10:52,720 If we look elsewhere, we can find evidence of other processes. 195 00:10:52,720 --> 00:10:56,120 Far from it. There's all kinds of things that have gone on on Mercury. 196 00:10:56,120 --> 00:10:58,960 One thing that comes to mind is something that we didn't know about 197 00:10:58,960 --> 00:11:02,440 until MESSENGER got there, which is these areas called hollows. 198 00:11:02,440 --> 00:11:05,720 There's a view here that's about 20-30km across 199 00:11:05,720 --> 00:11:09,760 and it's showing an area of surface where the top 20m of material 200 00:11:09,760 --> 00:11:14,680 is just gone, it's been stripped away to leave that irregular area. 201 00:11:14,680 --> 00:11:16,400 There's some smaller hollows nearby. 202 00:11:16,400 --> 00:11:20,640 It looks a bit like mould or Swiss cheese, or something like that. 203 00:11:20,640 --> 00:11:22,440 Absolutely. How is it being removed? 204 00:11:22,440 --> 00:11:24,120 It's not falling into caverns, 205 00:11:24,120 --> 00:11:27,000 it's not being blowing away in the wind, there's no wind. 206 00:11:27,000 --> 00:11:30,480 It's turning to vapour somehow and just being lost to space, 207 00:11:30,480 --> 00:11:33,720 so something in the surface is volatile. 208 00:11:33,720 --> 00:11:36,440 Volatile enough to turn to vapour and just go. 209 00:11:36,440 --> 00:11:40,120 So what could that be? It's a big problem and we can't tell from this. 210 00:11:40,120 --> 00:11:42,080 It could be sulphur, could be chlorine. 211 00:11:42,080 --> 00:11:45,640 Is it driven by heat or charged particles breaking chemical bonds? 212 00:11:45,640 --> 00:11:48,640 But there is evidence of volatile richness in the planet, 213 00:11:48,640 --> 00:11:50,520 which was completely unexpected. 214 00:11:50,520 --> 00:11:53,880 Something this close to the sun ought not to be rich in volatiles. 215 00:11:53,880 --> 00:11:55,120 Why not? 216 00:11:55,120 --> 00:11:57,840 Close to the sun you should be losing volatiles as you're 217 00:11:57,840 --> 00:12:01,240 trying to grow a planet because it's hot and, because Mercury has 218 00:12:01,240 --> 00:12:05,120 a large core, people thought it's had a violent birth. 219 00:12:05,120 --> 00:12:07,760 How do you get such a large core and a thin, rocky area? 220 00:12:07,760 --> 00:12:09,240 You blast the rock away, 221 00:12:09,240 --> 00:12:12,320 but that should be stripping away the volatiles as well, 222 00:12:12,320 --> 00:12:14,720 and yet Mercury has retained it's volatiles 223 00:12:14,720 --> 00:12:16,280 and still got a large core. 224 00:12:16,280 --> 00:12:20,360 It doesn't fit. And it's a world that's changing now. 225 00:12:20,360 --> 00:12:22,680 Some of these processes are still happening. 226 00:12:22,680 --> 00:12:25,720 The hollow-forming processes are still going on today. 227 00:12:25,720 --> 00:12:28,640 When you look at fields of hollows, you don't find impact craters 228 00:12:28,640 --> 00:12:32,880 superimposed. We think the hollows are still growing in some areas. 229 00:12:32,880 --> 00:12:35,360 Who would have thought Mercury would be an active planet? 230 00:12:35,360 --> 00:12:38,720 That adds up to Mercury being a very exciting place. Thank you very much. 231 00:12:38,720 --> 00:12:39,920 Pleasure. 232 00:12:42,120 --> 00:12:45,360 One of the reasons that we knew so little about Mercury for 233 00:12:45,360 --> 00:12:49,000 so long was that it's difficult to observe from the Earth. 234 00:12:52,520 --> 00:12:55,880 But this week's transit will give everyone the chance to see 235 00:12:55,880 --> 00:12:57,240 this mysterious planet... 236 00:12:59,440 --> 00:13:03,160 ..and Peter's here to explain the best way to view the transit safely. 237 00:13:07,080 --> 00:13:09,720 As planets go, Mercury isn't far away. 238 00:13:10,800 --> 00:13:13,280 Only 48 million miles at closest approach. 239 00:13:16,080 --> 00:13:19,200 But it's surprisingly hard to observe... 240 00:13:19,200 --> 00:13:21,240 because being so close to the sun, 241 00:13:21,240 --> 00:13:23,760 it is only ever visible for a short period, 242 00:13:23,760 --> 00:13:25,960 just before sunrise or after sunset. 243 00:13:28,160 --> 00:13:31,120 But this week's transit is a great opportunity to see 244 00:13:31,120 --> 00:13:33,080 the planet during broad daylight. 245 00:13:34,720 --> 00:13:38,600 We won't get another chance as good as this until 2049. 246 00:13:40,640 --> 00:13:44,200 Eclipse glasses are a great way to look at the sun safely, 247 00:13:44,200 --> 00:13:47,120 but unfortunately Mercury is going to be too small to be seen 248 00:13:47,120 --> 00:13:48,800 with the naked eye. 249 00:13:48,800 --> 00:13:51,960 It's about 1/155th the apparent size of the sun, 250 00:13:51,960 --> 00:13:54,880 so to see it at all you're going to need something 251 00:13:54,880 --> 00:13:56,320 with a bit more power - 252 00:13:56,320 --> 00:14:00,320 say a telescope or a powerful pair of binoculars. 253 00:14:00,320 --> 00:14:02,400 But to be safe, these must be fitted 254 00:14:02,400 --> 00:14:04,720 with a certified solar safety filter. 255 00:14:04,720 --> 00:14:08,280 There are various filters available, but one of the easiest ways 256 00:14:08,280 --> 00:14:12,920 to achieve this is to get hold of an A4 sheet of solar safety film 257 00:14:12,920 --> 00:14:15,120 and then make your own filter, 258 00:14:15,120 --> 00:14:19,080 which slips on the front of the telescope - just like that. 259 00:14:19,080 --> 00:14:21,480 You can point the telescope at the sun... 260 00:14:23,720 --> 00:14:25,080 ..and you're good to go. 261 00:14:28,040 --> 00:14:29,520 With the filter in place, 262 00:14:29,520 --> 00:14:32,240 you should get a view of the whole of the sun's disk, 263 00:14:32,240 --> 00:14:35,040 on which it's possible to make out small sun spots. 264 00:14:37,320 --> 00:14:39,520 This one is about the size that Mercury will 265 00:14:39,520 --> 00:14:41,360 appear during the transit. 266 00:14:44,680 --> 00:14:48,320 The transit will begin just after noon, with the sun high in the sky. 267 00:14:51,720 --> 00:14:54,600 You will see Mercury make first contact with the eastern 268 00:14:54,600 --> 00:14:57,760 edge of the sun and will then track southwest 269 00:14:57,760 --> 00:15:00,880 until the transit finishes at 7:42 in the evening. 270 00:15:04,080 --> 00:15:07,280 If you don't have a filter then there is another way to get 271 00:15:07,280 --> 00:15:11,040 a decent view of the sun and that's to project it. 272 00:15:11,040 --> 00:15:14,520 Now, projection is only really suitable for small refracting 273 00:15:14,520 --> 00:15:17,560 or lens-based telescopes. But using a small refractor, 274 00:15:17,560 --> 00:15:20,120 if you point this directly at the sun with 275 00:15:20,120 --> 00:15:23,840 an eyepiece in the eyepiece holder, it's then possible to project 276 00:15:23,840 --> 00:15:26,880 an image of the sun onto a piece of white card 277 00:15:26,880 --> 00:15:29,440 and it actually gives you a really good view. 278 00:15:29,440 --> 00:15:32,440 One final warning, if you're using this method, 279 00:15:32,440 --> 00:15:36,800 is to not keep the telescope pointed at the sun for too long a period. 280 00:15:36,800 --> 00:15:39,560 If you do, you run a risk of damaging the internal 281 00:15:39,560 --> 00:15:41,080 components of the telescope. 282 00:15:41,080 --> 00:15:44,560 If it's got plastic bits inside, for example, they may melt 283 00:15:44,560 --> 00:15:47,080 and plastic eyepieces may melt, as well. 284 00:15:47,080 --> 00:15:48,440 Also, be careful 285 00:15:48,440 --> 00:15:52,200 because the temperature just behind the eyepiece is really hot. 286 00:15:52,200 --> 00:15:55,680 I can demonstrate that with this little piece of black card. 287 00:15:55,680 --> 00:15:58,560 Look at that. That didn't take very long at all. 288 00:15:58,560 --> 00:16:01,520 Makes Bear Grylls look pretty pathetic, doesn't it? 289 00:16:01,520 --> 00:16:02,760 HE LAUGHS 290 00:16:08,760 --> 00:16:10,720 Hopefully, on the day of the transit, 291 00:16:10,720 --> 00:16:13,480 the skies will be lovely and clear like they are today. 292 00:16:13,480 --> 00:16:15,400 But if the clouds do come, 293 00:16:15,400 --> 00:16:20,200 in the event is so long at 7.5 hours from beginning to end that we do 294 00:16:20,200 --> 00:16:23,520 stand at least a decent possibility of some clear breaks 295 00:16:23,520 --> 00:16:25,480 where we'll see something of it. 296 00:16:25,480 --> 00:16:27,880 The end of the transit occurs with the sun just 297 00:16:27,880 --> 00:16:30,280 nine degrees above the northwest horizon, 298 00:16:30,280 --> 00:16:34,160 so if you do intend to watch the entire event then make sure 299 00:16:34,160 --> 00:16:36,960 you've got a clear view in that particular direction. 300 00:16:36,960 --> 00:16:39,080 So clear skies and good luck. 301 00:16:42,280 --> 00:16:44,040 If you don't have the right equipment, 302 00:16:44,040 --> 00:16:46,720 there will be lots of events happening all over the country, 303 00:16:46,720 --> 00:16:49,840 like here at the Open University, where you can watch the transit 304 00:16:49,840 --> 00:16:51,880 in the company of your local astronomers. 305 00:16:51,880 --> 00:16:54,400 And if it does happen to be cloudy, like today, 306 00:16:54,400 --> 00:16:57,520 then Isa are streaming the transit live from space using 307 00:16:57,520 --> 00:17:00,120 satellites that will get a great view whatever the weather, 308 00:17:00,120 --> 00:17:02,120 and we'll have details of that stream 309 00:17:02,120 --> 00:17:04,480 and the list of events on our website. 310 00:17:04,480 --> 00:17:07,280 But transits are more than just rare and remarkable events - 311 00:17:07,280 --> 00:17:09,680 they also have significant scientific value. 312 00:17:09,680 --> 00:17:11,840 We asked public astronomer Marek Kukula 313 00:17:11,840 --> 00:17:14,720 from the Royal Observatory Greenwich to investigate. 314 00:17:18,880 --> 00:17:21,280 To understand the importance of transits, 315 00:17:21,280 --> 00:17:23,520 we need to geo back to the 17th century, 316 00:17:23,520 --> 00:17:26,480 to the time just after the founding of the Royal Observatory. 317 00:17:29,240 --> 00:17:32,440 This is what was known as the solar system in the second 318 00:17:32,440 --> 00:17:36,880 half of the 1600s. The six inner planets all orbiting around the sun. 319 00:17:36,880 --> 00:17:40,080 The outer planets, of course, hadn't been discovered yet. 320 00:17:40,080 --> 00:17:43,400 It was a model we'd had since the time of Copernicus and Kepler. 321 00:17:43,400 --> 00:17:47,320 We knew the order of the planets and we knew the shape of their orbits, 322 00:17:47,320 --> 00:17:50,480 but there was one thing about the solar system we didn't know - 323 00:17:50,480 --> 00:17:52,120 we didn't know how big it was. 324 00:17:52,120 --> 00:17:55,400 Although the relative sizes of the orbits were understood, 325 00:17:55,400 --> 00:17:57,760 for instance, we knew that the Earth was three times 326 00:17:57,760 --> 00:17:59,840 further from the sun than Mercury, 327 00:17:59,840 --> 00:18:01,960 the actual distances weren't known. 328 00:18:03,680 --> 00:18:06,040 It was a solar system without scale. 329 00:18:07,960 --> 00:18:11,560 And that's where this guy comes in. He's Edmund Halley. 330 00:18:11,560 --> 00:18:15,360 He'd later become Astronomer Royal himself, but in 1677 he was 331 00:18:15,360 --> 00:18:19,240 just an assistant astronomer, here at the observatory in Greenwich. 332 00:18:19,240 --> 00:18:22,360 And he'd been sent to St Helena in the South Atlantic to observe 333 00:18:22,360 --> 00:18:23,600 the southern skies. 334 00:18:23,600 --> 00:18:26,440 While he was there, he watched a transitive Mercury, 335 00:18:26,440 --> 00:18:29,000 just like the one that's due this week. 336 00:18:29,000 --> 00:18:32,160 Watching Mercury crawl across the face of the sun, 337 00:18:32,160 --> 00:18:35,360 Halley realised how a transit could be used to measure 338 00:18:35,360 --> 00:18:37,200 the size of the solar system. 339 00:18:39,880 --> 00:18:42,120 Halley's breakthrough was to understand that 340 00:18:42,120 --> 00:18:45,960 if you viewed the transit from two widely spaced locations, 341 00:18:45,960 --> 00:18:49,800 thousands of miles apart, then you'll see the transit differently. 342 00:18:50,920 --> 00:18:53,200 Viewed from here, south of the equator, 343 00:18:53,200 --> 00:18:56,400 the planet will appear here against the disk of the sun. 344 00:18:56,400 --> 00:18:58,520 But viewed from north of the equator, 345 00:18:58,520 --> 00:19:01,560 the planet will appear further down on the sun's disk. 346 00:19:02,680 --> 00:19:04,320 What Halley realised was that 347 00:19:04,320 --> 00:19:07,280 if you could measure the apparent separation between the points, 348 00:19:07,280 --> 00:19:09,400 the parallax, then you could work out 349 00:19:09,400 --> 00:19:11,760 the distance between the earth and the sun. 350 00:19:14,240 --> 00:19:16,240 It was a calculation that required 351 00:19:16,240 --> 00:19:18,480 some fiendishly complicated geometry. 352 00:19:20,080 --> 00:19:23,160 But to produce an accurate figure, it also needed a number 353 00:19:23,160 --> 00:19:26,840 of extremely precise measurements to be made during the transit... 354 00:19:28,040 --> 00:19:29,760 ..including, most crucially, 355 00:19:29,760 --> 00:19:32,720 the exact time it takes for the planet to cross the sun. 356 00:19:35,520 --> 00:19:37,600 But here Halley faced a problem. 357 00:19:38,800 --> 00:19:42,720 Mercury was so small and travelled so fast that it would be almost 358 00:19:42,720 --> 00:19:45,400 impossible to make the measurements required. 359 00:19:47,440 --> 00:19:49,400 A more suitable target was Venus, 360 00:19:49,400 --> 00:19:52,840 which appears both bigger and slower as it transits the sun. 361 00:19:55,480 --> 00:19:58,920 But the next transit of Venus wasn't due for another 84 years. 362 00:20:00,240 --> 00:20:03,160 Halley knew he'd be long dead by then, 363 00:20:03,160 --> 00:20:06,120 so he laid down the gauntlet for future generations. 364 00:20:08,120 --> 00:20:12,160 Almost a century later, the world's astronomers rose to that challenge. 365 00:20:14,320 --> 00:20:17,760 There were transits of Venus in 1761 and 1769, 366 00:20:17,760 --> 00:20:21,640 and expeditions were sent out all around the world to observe them. 367 00:20:21,640 --> 00:20:24,480 These expeditions were among the first great international 368 00:20:24,480 --> 00:20:26,200 scientific collaborations 369 00:20:26,200 --> 00:20:29,280 and, in many ways, they were like the Large Hadron Collider 370 00:20:29,280 --> 00:20:31,680 or International Space Station of their day. 371 00:20:42,240 --> 00:20:44,880 The French, British and Austrians went to Siberia 372 00:20:44,880 --> 00:20:46,080 and Northern Canada, 373 00:20:46,080 --> 00:20:49,080 where they had to brave polar bears and hostile locals. 374 00:20:49,080 --> 00:20:51,680 They went to the Indian and Pacific Oceans. 375 00:20:51,680 --> 00:20:55,360 Captain James Cook was sent to Tahiti in 1769. 376 00:20:55,360 --> 00:20:57,680 And these were major expeditions for the time, 377 00:20:57,680 --> 00:21:01,040 involving perilous sea voyages sometimes lasting several years. 378 00:21:01,040 --> 00:21:04,320 There was one French expedition to Mexico from which only one 379 00:21:04,320 --> 00:21:05,800 person returned alive. 380 00:21:05,800 --> 00:21:08,160 And to make matters worse, during some of this time, 381 00:21:08,160 --> 00:21:09,680 France and Britain were at war 382 00:21:09,680 --> 00:21:12,800 and special arrangements had to be made to give safe passage 383 00:21:12,800 --> 00:21:14,400 to scientists from each side. 384 00:21:18,080 --> 00:21:20,760 Once they'd arrived, each team had to spend weeks 385 00:21:20,760 --> 00:21:23,400 calculating their latitude and longitude. 386 00:21:23,400 --> 00:21:26,560 And this is dated from Cook's voyage in 1769. 387 00:21:26,560 --> 00:21:28,080 You can see how detailed it is. 388 00:21:28,080 --> 00:21:32,040 They're even using the moons of Jupiter to calculate their longitude. 389 00:21:32,040 --> 00:21:34,640 And once they'd done that, they had to hope for clear 390 00:21:34,640 --> 00:21:37,120 skies for the transit itself. 391 00:21:37,120 --> 00:21:40,800 The crucial measurement was to time how long it took Venus to 392 00:21:40,800 --> 00:21:44,720 cross the disk of the sun to within a couple of seconds. 393 00:21:44,720 --> 00:21:48,840 And these are drawings by Captain Cook himself and they perfectly 394 00:21:48,840 --> 00:21:52,360 illustrate a really crucial problem that they discovered. 395 00:21:52,360 --> 00:21:56,080 It's an optical illusion - the so-called "black drop effect". 396 00:21:56,080 --> 00:21:59,080 And as Venus starts to cross the disk of the sun, 397 00:21:59,080 --> 00:22:02,200 the disk of Venus appears to stretch out and blur, 398 00:22:02,200 --> 00:22:04,840 and that makes it very difficult to measure 399 00:22:04,840 --> 00:22:07,480 the precise time at which the transit begins. 400 00:22:08,680 --> 00:22:11,640 The black drop effect made it impossible to record 401 00:22:11,640 --> 00:22:14,560 the length of the transit with the desired accuracy... 402 00:22:17,040 --> 00:22:20,000 ..but the data they did collect was enough for astronomers 403 00:22:20,000 --> 00:22:21,480 to start their calculations. 404 00:22:22,880 --> 00:22:26,520 In 1771, the French astronomer Jerome Lalande calculated 405 00:22:26,520 --> 00:22:28,960 a value for the astronomical unit, 406 00:22:28,960 --> 00:22:33,240 the distance between the earth and the sun, of 153 million km, 407 00:22:33,240 --> 00:22:36,960 which is impressively within 2.5% of the modern value. 408 00:22:36,960 --> 00:22:39,200 Suddenly, the solar system had a scale. 409 00:22:42,000 --> 00:22:44,800 That's why transits were important historically. 410 00:22:45,960 --> 00:22:48,720 But today, transits are still important in helping us 411 00:22:48,720 --> 00:22:51,120 understand our position in the universe 412 00:22:51,120 --> 00:22:54,160 because of the role they play in showing how many other planets 413 00:22:54,160 --> 00:22:56,200 there are outside the solar system. 414 00:22:59,480 --> 00:23:02,000 Whenever a planet passes in front of the sun, 415 00:23:02,000 --> 00:23:05,120 it blocks out a small but measurable amount of its light. 416 00:23:07,280 --> 00:23:10,360 The same principle applies when we look at other stars - 417 00:23:10,360 --> 00:23:13,920 it's very difficult to observe their planets directly - 418 00:23:13,920 --> 00:23:16,480 but we can see the tiny drop in brightness 419 00:23:16,480 --> 00:23:19,080 as the planet passes in front of the star. 420 00:23:19,080 --> 00:23:22,960 It's exactly this technique that the Kepler space telescope uses. 421 00:23:24,680 --> 00:23:29,640 It monitors 100,000 stars, looking for the telltale dip in luminance 422 00:23:29,640 --> 00:23:31,720 that indicates a transiting planet. 423 00:23:34,360 --> 00:23:37,640 By using this method, it has in the last seven years detected 424 00:23:37,640 --> 00:23:40,280 nearly 6,000 possible exoplanets. 425 00:23:43,120 --> 00:23:45,720 When you're watching a transit, like the one this week, 426 00:23:45,720 --> 00:23:47,720 you should bear in mind a couple of things. 427 00:23:47,720 --> 00:23:50,240 One is that what you are watching is a clear example 428 00:23:50,240 --> 00:23:51,920 of the solar system in action - 429 00:23:51,920 --> 00:23:55,000 planets actually moving along their orbits in real time. 430 00:23:55,000 --> 00:23:58,360 But also what you're seeing isn't just a pleasing spectacle 431 00:23:58,360 --> 00:24:01,200 because transits, perhaps more than any other phenomenon, 432 00:24:01,200 --> 00:24:05,000 have helped us to understand the scale and scope of our universe. 433 00:24:09,120 --> 00:24:12,400 Mercury is undoubtedly a strange world. 434 00:24:12,400 --> 00:24:15,240 With its large iron core and its thin mantle, 435 00:24:15,240 --> 00:24:19,120 it's not like any of the rest of the family of rocky planets. 436 00:24:19,120 --> 00:24:22,440 So what happened in Mercury's formation to make it this way? 437 00:24:23,840 --> 00:24:27,360 Maggie has been talking to planetary scientist Craig Agnor to 438 00:24:27,360 --> 00:24:29,000 discuss the latest ideas. 439 00:24:30,840 --> 00:24:32,560 Craig, can you describe to me 440 00:24:32,560 --> 00:24:35,000 the old theory of the formation of Mercury? 441 00:24:35,000 --> 00:24:38,160 One of the initial ideas was that Mercury's mantle was removed 442 00:24:38,160 --> 00:24:39,720 through a giant impact. 443 00:24:39,720 --> 00:24:43,600 And the initial modelling of this suggested a smaller object, 444 00:24:43,600 --> 00:24:48,160 maybe a third the mass of Mercury, smashed in at very high velocity, 445 00:24:48,160 --> 00:24:50,680 vaporised the mantle, blasted off into space 446 00:24:50,680 --> 00:24:54,840 and this would have predicted a very hot but iron-rich planet. 447 00:24:54,840 --> 00:24:58,400 OK, so an iron-rich planet, so a large core and a thin mantle, 448 00:24:58,400 --> 00:25:00,680 so that does tie in with what we see of Mercury. 449 00:25:00,680 --> 00:25:01,880 That's exactly right. 450 00:25:01,880 --> 00:25:04,240 The problem is that recent spacecraft data has shown 451 00:25:04,240 --> 00:25:06,720 that Mercury's mantle retains a significant 452 00:25:06,720 --> 00:25:09,440 inventory of volatiles that wouldn't have survived the extreme 453 00:25:09,440 --> 00:25:11,120 heating of this initial scenario. 454 00:25:11,120 --> 00:25:13,880 They would have been blown off into space with the temperature. 455 00:25:13,880 --> 00:25:16,680 That's right. OK, so there's a challenge there. That's right. 456 00:25:16,680 --> 00:25:19,520 So you have to look at the different types of giant impacts that 457 00:25:19,520 --> 00:25:21,160 occur during planet formation. 458 00:25:21,160 --> 00:25:23,880 One of the new ideas about this origin of Mercury is that 459 00:25:23,880 --> 00:25:27,680 maybe Mercury hit a larger object at slower velocity 460 00:25:27,680 --> 00:25:31,200 and this collision may be able to remove the mantle without 461 00:25:31,200 --> 00:25:33,240 the extreme heating of the earlier scenario. 462 00:25:33,240 --> 00:25:35,600 OK, so we keep the volatiles. 463 00:25:35,600 --> 00:25:36,680 Wonderful. 464 00:25:36,680 --> 00:25:39,920 So what we see in this animation here is kind of a proto-Mercury 465 00:25:39,920 --> 00:25:42,720 and a proto-Venus on crossing orbits that will eventually 466 00:25:42,720 --> 00:25:44,600 result in a giant impact. 467 00:25:44,600 --> 00:25:47,760 So the impact was actually between Venus and Mercury. Right. 468 00:25:47,760 --> 00:25:49,880 So what actually happens during impact? 469 00:25:49,880 --> 00:25:52,800 The way we study this is through computer simulations. 470 00:25:52,800 --> 00:25:54,760 You can model a planet in this 471 00:25:54,760 --> 00:25:58,400 simulation from Arizona State by Erik Asphaug and Andreas Reufer. 472 00:25:58,400 --> 00:26:02,600 An iron core shown in blue, rocky mantles are shown in red or orange. 473 00:26:02,600 --> 00:26:05,520 This type of collision is called a hit and run collision, 474 00:26:05,520 --> 00:26:08,000 where the two objects slam into each other, 475 00:26:08,000 --> 00:26:10,440 they sheer off a portion of their mantles 476 00:26:10,440 --> 00:26:12,640 and they leave the scene of the crime. 477 00:26:12,640 --> 00:26:15,440 The impact happens at a modest velocity, 478 00:26:15,440 --> 00:26:18,800 so this is quite a bit more gentle than the smaller, 479 00:26:18,800 --> 00:26:21,160 high velocity impact collisions. 480 00:26:21,160 --> 00:26:24,120 OK. So this could explain the core, the mantle, 481 00:26:24,120 --> 00:26:25,960 but the volatiles as well. 482 00:26:25,960 --> 00:26:28,840 That's right. So, in this scenario, what happens to Venus? 483 00:26:28,840 --> 00:26:33,360 Part of the proto-Mercury's mantle may have been deposited onto Venus 484 00:26:33,360 --> 00:26:36,560 and that may help to explain why Venus has a little more 485 00:26:36,560 --> 00:26:40,120 mantle material relative to the size of its core than the Earth. 486 00:26:40,120 --> 00:26:42,000 So this theory is looking pretty good now 487 00:26:42,000 --> 00:26:43,560 because we've got this collision, 488 00:26:43,560 --> 00:26:46,360 we've got Mercury left with a large iron core and a thin mantle, 489 00:26:46,360 --> 00:26:48,120 we've got Venus with an extra mantle, 490 00:26:48,120 --> 00:26:50,120 which is what we actually see in reality, 491 00:26:50,120 --> 00:26:52,560 so it does seem to stand up. So what happens next? 492 00:26:52,560 --> 00:26:54,280 It's not the end of the story 493 00:26:54,280 --> 00:26:56,600 because its orbit continues to evolve 494 00:26:56,600 --> 00:26:58,840 and, over the next five billion years, 495 00:26:58,840 --> 00:27:02,800 there's about a 1% chance that its orbit can become so eccentric 496 00:27:02,800 --> 00:27:05,440 that it again crosses the orbit of Venus. 497 00:27:05,440 --> 00:27:09,000 It can suffer giant impacts with Venus or Earth, 498 00:27:09,000 --> 00:27:11,360 or it may collide with the sun. 499 00:27:11,360 --> 00:27:14,200 Gosh. 1% probability is quite high, really, 500 00:27:14,200 --> 00:27:16,840 that our solar system could change radically. That's right. 501 00:27:16,840 --> 00:27:19,080 So it's not as static as I take for granted. 502 00:27:19,080 --> 00:27:22,320 No, the solar system is an amazingly dynamic place. Thanks very much. 503 00:27:22,320 --> 00:27:23,320 Thank you. 504 00:27:27,240 --> 00:27:31,080 We still don't understand everything about Mercury 505 00:27:31,080 --> 00:27:33,120 or how it became the planet it is today. 506 00:27:36,080 --> 00:27:38,760 It remains the problem child of the solar system. 507 00:27:40,200 --> 00:27:42,760 But by studying Mercury's peculiarities 508 00:27:42,760 --> 00:27:46,360 and troubled beginnings, we are producing new insights into 509 00:27:46,360 --> 00:27:50,440 the processes that formed and shaped the whole of the inner solar system. 510 00:27:57,520 --> 00:28:00,680 When we started working on this, I thought Mercury was the least 511 00:28:00,680 --> 00:28:01,920 interesting of planets, 512 00:28:01,920 --> 00:28:04,560 but the more you find out about it the more fascinating it is. 513 00:28:04,560 --> 00:28:07,160 I was particularly interested in the fact that the formation 514 00:28:07,160 --> 00:28:10,240 of Mercury tells us more about the dynamics of the early solar system. 515 00:28:10,240 --> 00:28:12,440 Well, that's all we've got time for this month. 516 00:28:12,440 --> 00:28:15,240 But if you're going to watch the transit tomorrow, good luck. 517 00:28:15,240 --> 00:28:17,760 If you're watching us on repeat, I hope it was clear. 518 00:28:17,760 --> 00:28:19,200 That's it for this month, 519 00:28:19,200 --> 00:28:22,280 but do check out the website, where we've got Pete's star guide. 520 00:28:22,280 --> 00:28:25,360 And in the meantime, get outside and get looking up... 521 00:28:25,360 --> 00:28:28,480 but never directly at the sun. Goodnight.