1 00:00:00,000 --> 00:00:05,426 Now with the tools that we've acquired for how to study a planet, let's turn our 2 00:00:05,426 --> 00:00:10,920 attention to the three other terrestrial planets in the solar system, and see how 3 00:00:10,920 --> 00:00:14,159 well they fall into place, with the way we have of 4 00:00:14,159 --> 00:00:18,358 understanding how a planet works. first thing you want to look at is the 5 00:00:18,358 --> 00:00:22,556 way they orbit and the way they spin. And, Mercury is an interesting case. 6 00:00:22,556 --> 00:00:26,979 Mercury orbits very close to the Sun, so it's subject to very powerful tidal 7 00:00:26,979 --> 00:00:30,450 forces from the Sun. We would not be surprised to find Mercury 8 00:00:30,450 --> 00:00:35,208 locked into a resonance between spin and orbit, just as the Moon, is locked when 9 00:00:35,208 --> 00:00:38,679 it orbits the Earth. It turns out things are a little bit more 10 00:00:38,679 --> 00:00:41,423 subtle. And the reason is that Mercury's orbit is 11 00:00:41,423 --> 00:00:44,502 eccentric. the following demo will explain what goes 12 00:00:44,502 --> 00:00:47,368 on. Mercury's eccentric orbit means that it 13 00:00:47,368 --> 00:00:52,901 can't quite synchronize spin and orbit, because the rate at which Mercury spins 14 00:00:52,901 --> 00:00:57,152 cannot adjust to the rate of its angular motion around the Sun. 15 00:00:57,152 --> 00:01:01,132 It moves faster around the Sun when it's nearer to the Sun. 16 00:01:01,132 --> 00:01:04,303 It moves slower when it's farther from the Sun. 17 00:01:04,303 --> 00:01:10,133 the way Mercury [COUGH] resolves this is that tidal forces are most intense when 18 00:01:10,133 --> 00:01:14,674 it's closest to the Sun. So it essentially matches its rotation 19 00:01:14,674 --> 00:01:20,081 rate to the rate at which it moves about the Sun near perihelion, when it's 20 00:01:20,081 --> 00:01:23,830 closest. what this means is that, Mercury spins 21 00:01:23,830 --> 00:01:28,300 faster than it orbits. And it turns out that the ratio that it 22 00:01:28,300 --> 00:01:31,616 manages to do this in is a three to two ratio. 23 00:01:31,616 --> 00:01:37,311 In other words, Mercury completes one and a half rotations about its axis every 24 00:01:37,311 --> 00:01:40,700 time it orbits the Sun once. This allows its, 25 00:01:40,700 --> 00:01:47,677 rate of spin to match the rate at which it moves around the Sun at its nearest 26 00:01:47,677 --> 00:01:53,483 approach, and this is the way in which Mercury is tidally locked between 27 00:01:53,483 --> 00:01:56,751 rotation and spin. It orbits the Sun every 88 days. 28 00:01:56,751 --> 00:02:01,785 Venus orbits the Sun every 224 days. Its orbit is tilted very little, about 29 00:02:01,785 --> 00:02:06,558 three degrees to the ecliptic. the rate at which Venus spins about its 30 00:02:06,558 --> 00:02:11,853 axis was very hard to determine, because as we shall see, Venus is covered by a 31 00:02:11,853 --> 00:02:15,449 deck of clouds that makes it impossible to the surface. 32 00:02:15,449 --> 00:02:20,941 eventually radio, radio telescopes from Earth managed to bounce right off the two 33 00:02:20,941 --> 00:02:25,060 sides of Venus, use the Doppler shift of the reflected light to, 34 00:02:25,060 --> 00:02:30,129 managed to figure out the velocity which with the sides of the planets were 35 00:02:30,129 --> 00:02:35,332 moving, and it turns out, surprisingly, that Mercury rotates about its axis very 36 00:02:35,332 --> 00:02:39,917 slowly, once every 243 Earth days is one sidereal rotation. 37 00:02:39,917 --> 00:02:45,023 And the spin is essentially opposite in direction to the orbit. 38 00:02:45,023 --> 00:02:48,508 Mercury is almost unique among the planets. 39 00:02:48,508 --> 00:02:54,830 In, well, completely unique in this sense, in spinning about its axis, at a 40 00:02:54,830 --> 00:02:59,697 effective axis tilt of 180 degrees. Its rotation is not part of the original 41 00:02:59,697 --> 00:03:04,437 rotation of the solar system. quite possibly some large collision early 42 00:03:04,437 --> 00:03:09,624 in the days of the solar system flipped the planet over and gave it this angular 43 00:03:09,624 --> 00:03:14,172 momentum in the wrong direction. Mars orbits the Sun every two years or 44 00:03:14,172 --> 00:03:16,670 so. its orbit is a little more, 45 00:03:16,670 --> 00:03:22,252 eccentric, than that of the Earth. its day however, is almost exactly an 46 00:03:22,252 --> 00:03:27,051 Earth day, 24.6 hours, and its tilt angle is 25 degrees, its axis is tilted 25 47 00:03:27,051 --> 00:03:31,471 degrees, so that seas, seasonal variations on Mars are somewhat similar 48 00:03:31,471 --> 00:03:36,648 to those on Earth, although accentuated as I said by the, increased eccentricity. 49 00:03:36,648 --> 00:03:42,078 it's interesting if you want that, on Mars, there's a north star like there is 50 00:03:42,078 --> 00:03:45,298 on Earth. But the direction in which Mars's axis is 51 00:03:45,298 --> 00:03:50,539 tilted is not the same as that on Earth. On Mars, Deneb would serve very nicely as 52 00:03:50,539 --> 00:03:53,536 a north star. What do we learn by looking on the 53 00:03:53,536 --> 00:03:57,020 surfaces of these three planets? Well, looking at Mercury, 54 00:03:57,020 --> 00:04:00,589 we see evidence of cratering on the surface, 55 00:04:00,589 --> 00:04:06,350 so we have a relatively old surface, and we see no evidence of tectonic 56 00:04:06,350 --> 00:04:09,920 activity. We do see the graben and the 57 00:04:09,920 --> 00:04:16,330 rilles, that reflect compression of the crust, the shrinking of the crust as the 58 00:04:16,330 --> 00:04:20,314 interior cools. But we don't see any evidence of tectonic 59 00:04:20,314 --> 00:04:23,369 activity. Observing the surface of Venus is 60 00:04:23,369 --> 00:04:27,287 difficult for reasons that will become clear shortly. 61 00:04:27,287 --> 00:04:32,732 But we do have radar imaging through the clouds that gives us an understanding of 62 00:04:32,732 --> 00:04:38,376 the topography and are again, we don't see evidence of plate tectonics in the 63 00:04:38,376 --> 00:04:44,192 sense in which this takes place on Earth. The entire planet seems to be one huge 64 00:04:44,192 --> 00:04:50,056 plate if you wish. The crust is very strong and, does not deform very easily. 65 00:04:50,056 --> 00:04:54,492 So we do not see the kind of features that we see on Earth. 66 00:04:54,492 --> 00:04:59,003 the age of the crust of Mercury is, alarmingly uniform. 67 00:04:59,003 --> 00:05:05,092 It seems that the entire planet underwent a global resurfacing maybe four or 600 68 00:05:05,092 --> 00:05:09,227 million years ago. And the features that we do see that 69 00:05:09,227 --> 00:05:13,489 indicate, the motion and the pass are indicated in 70 00:05:13,489 --> 00:05:17,992 these reconstructions. These are not images from the surface, we 71 00:05:17,992 --> 00:05:22,210 have precisely about three of those but these are 72 00:05:22,210 --> 00:05:27,064 reconstructions of models from three dimensional, radar measurements of the 73 00:05:27,064 --> 00:05:29,678 terrain. And since the terrain on Venus is 74 00:05:29,678 --> 00:05:34,471 extraordinarily flat, these have been enhanced in the vertical direction by a 75 00:05:34,471 --> 00:05:37,582 factor of about 22. And so this is a region called Ovda 76 00:05:37,582 --> 00:05:41,504 Regio, and it shows you these sort of, compression ridges, 77 00:05:41,504 --> 00:05:46,545 this complicated network of, compression ridges that occur when the surface is 78 00:05:46,545 --> 00:05:51,400 shrinking and the sea, other area called Gula Rift shows you these large, 79 00:05:51,400 --> 00:05:54,254 rifts, which, which indicate a extension motion. 80 00:05:54,254 --> 00:05:57,978 And, what we see in the background are two huge volcanoes. 81 00:05:57,978 --> 00:06:03,005 Mercury does exhibit, large volcanoes. the surface of Mars on the other hand, 82 00:06:03,005 --> 00:06:06,107 is, heavily cratered, but only in one hemisphere. 83 00:06:06,107 --> 00:06:11,010 There's this great dichotomy between the north and south hemisphere, suggesting 84 00:06:11,010 --> 00:06:15,614 that perhaps the planet. is constructed out of two plates though 85 00:06:15,614 --> 00:06:19,066 the origins of the dichotomy are not exactly clear. 86 00:06:19,066 --> 00:06:24,482 And right along the boundary runs this huge canyon the Valles Marineris 1500 87 00:06:24,482 --> 00:06:26,310 kilometers long. Dwarfs the. 88 00:06:26,310 --> 00:06:31,332 Grand Canyon by a large margin. And this might be the fault line between 89 00:06:31,332 --> 00:06:35,424 two plates would be reasonable. All three planets show evidence of 90 00:06:35,424 --> 00:06:39,082 volcanic activity. one is not surprised to find that all 91 00:06:39,082 --> 00:06:42,306 volcanoes on Mars are extinct. Mars is a small world, 92 00:06:42,306 --> 00:06:47,143 and as we saw on the moon, one would imagine that Mars would cool more quickly 93 00:06:47,143 --> 00:06:51,359 than the others. The fact that vulcanism on Mercury seems 94 00:06:51,359 --> 00:06:54,955 still to be active, is, perhaps, explained by tidal friction. 95 00:06:54,955 --> 00:06:59,730 The strong tidal forces of the sun are maintaining the heat in the planet. 96 00:06:59,730 --> 00:07:06,315 So, judging from this, we can try to understand something about the interior 97 00:07:06,315 --> 00:07:11,119 of the planet, so Mercury Based on what we know about its density, 98 00:07:11,119 --> 00:07:14,717 it has a huge core. Its uncompressed density, in other words 99 00:07:14,717 --> 00:07:19,535 if you relieved gravity, it would have almost the density of iron, 5300 kilo per 100 00:07:19,535 --> 00:07:24,664 meter cubed. Essentially the density of earth would be the density of Mercurym if 101 00:07:24,664 --> 00:07:29,705 you relieved gravitational pressure. there is a, the, some of the, core at 102 00:07:29,705 --> 00:07:35,257 least must be liquid, because of features we observe in the dynamics of 103 00:07:35,257 --> 00:07:38,350 the planet Also it has much too 104 00:07:38,350 --> 00:07:44,963 Astronomers' surprise missions to Mercury discovered dipolar magnetic field so 105 00:07:44,963 --> 00:07:48,525 there seems to be a geodynamo at work on Mercury. 106 00:07:48,525 --> 00:07:52,086 That is still something that people are studying. 107 00:07:52,086 --> 00:07:58,118 The conjecture for how it is that Mercury came to be, dominated by essentially a 108 00:07:58,118 --> 00:08:03,612 large core with a very thin mantle is that back in the days of, heavy 109 00:08:03,612 --> 00:08:10,064 bombardment, Mercury underwent a rather dramatic collision and, the effect of 110 00:08:10,064 --> 00:08:17,007 this collision was, to essentially strip the outer layers of the planet leaving, 111 00:08:17,007 --> 00:08:21,336 essentially an exposed core with a small, covering. 112 00:08:21,336 --> 00:08:25,420 And so here's Mercury orbiting. Here is another, 113 00:08:25,420 --> 00:08:30,043 proto-planet coming in. And the collision vaporizes the mantles 114 00:08:30,043 --> 00:08:34,443 of both planets. these, are ejected and what's left is 115 00:08:34,443 --> 00:08:40,111 basically an almost exposed core. This is our, conjecture to understanding 116 00:08:40,111 --> 00:08:45,630 of why it is that Mercury is so dense and is so dominated by a large core. 117 00:08:45,630 --> 00:08:48,917 What we know very little about the interior of Venus. 118 00:08:48,917 --> 00:08:53,941 We can barely see the surface of Venus. As I said it's covered with clouds and, 119 00:08:53,941 --> 00:08:58,655 we can see through them with radar but, we do not have good on-the-ground 120 00:08:58,655 --> 00:09:02,624 measurements on Venus. We presume that the internal structure is 121 00:09:02,624 --> 00:09:06,408 rather similar to Earth. The density and the size are similar. 122 00:09:06,408 --> 00:09:09,943 But, Venus does not show any geodynamic magnetic field. 123 00:09:09,943 --> 00:09:13,090 there are. Various conjectures as to why this might 124 00:09:13,090 --> 00:09:15,487 be. one of them, as demonstrated in this 125 00:09:15,487 --> 00:09:19,825 image, is that much of its core has already solidified, and therefore there's 126 00:09:19,825 --> 00:09:23,250 no condensation of a liquid core and no room for convection. 127 00:09:23,250 --> 00:09:27,816 It may be that temperatures throughout the core are uniform because resurfacing 128 00:09:27,816 --> 00:09:31,754 of the crust, as we saw, has now basically sealed the crust completely. 129 00:09:31,754 --> 00:09:35,407 There are no venting. There's no room for energy to be vented, 130 00:09:35,407 --> 00:09:39,973 so the entire core is heated to a uniform temperature and there's no convection 131 00:09:39,973 --> 00:09:43,106 currents going. On, our understanding of the interior 132 00:09:43,106 --> 00:09:47,699 geology of Mercury, of Venus is incomplete in general. Venus is a hard 133 00:09:47,699 --> 00:09:52,367 planet to study. moving on to our friend Mars Mars has a, 134 00:09:52,367 --> 00:09:56,650 very low density, essentially, slightly higher than the. 135 00:09:56,650 --> 00:10:00,294 Density of rocks. It has a mantle that seems to be 136 00:10:00,294 --> 00:10:03,794 inactive. The interpretation is that its core is 137 00:10:03,794 --> 00:10:09,280 small, it never accumulated lots of iron. but there are traces on Mars of the 138 00:10:09,280 --> 00:10:14,488 existence of a geodynamo in the past. We can see the magnetic field frozen into 139 00:10:14,488 --> 00:10:17,783 the rocks. we'll talk about why Mars is such an 140 00:10:17,783 --> 00:10:22,595 underdeveloped planet in as, as we go on with the our investigation. 141 00:10:22,595 --> 00:10:28,132 But the similarity in many ways between Mars and Earth, in terms of orbits and so 142 00:10:28,132 --> 00:10:32,483 on, is belied by the fact that Mars is both smaller and less dense. 143 00:10:32,483 --> 00:10:37,624 So for example, the gravitational field on Mars is much weaker than that on 144 00:10:37,624 --> 00:10:41,020 Earth. This is what we see on the sur, on the 145 00:10:41,020 --> 00:10:45,497 interior of the planets, this is what we know about it, we can move on and talk 146 00:10:45,497 --> 00:10:49,630 about atmospheres. That was very important to our discussion of Earth 147 00:10:49,630 --> 00:10:53,481 Mercury is an easy answer. There's not much of an atmosphere. 148 00:10:53,481 --> 00:10:58,166 Mercury actually is a very dark planet. Albedo is much smaller than that of 149 00:10:58,166 --> 00:11:01,247 Earth. on the other hand, absent an atmosphere 150 00:11:01,247 --> 00:11:06,061 and with the slow rotation, you get this huge dichotomy between daytime and 151 00:11:06,061 --> 00:11:09,463 nighttime temperatures. 700 kelvin on the day side. 152 00:11:09,463 --> 00:11:12,287 100 kelvin, rather cold, in, on the night side. 153 00:11:12,287 --> 00:11:15,946 And again there are craters that are in perpetual shadow. 154 00:11:15,946 --> 00:11:21,238 And recently, it seems quite clear that there is substantial quantities of ice. 155 00:11:21,238 --> 00:11:25,814 In Merc-, on Mercury, very close the sun, in these, shadowed craters. 156 00:11:25,814 --> 00:11:28,303 A, a calculation, if you followed our 157 00:11:28,303 --> 00:11:32,072 calculation for the mean surface temperature of Mercury, 158 00:11:32,072 --> 00:11:35,369 treating it as, as a black body closer to the sun. 159 00:11:35,369 --> 00:11:39,070 We would get 428 degrees. So the average is about right. 160 00:11:39,070 --> 00:11:44,421 Venus is a more interesting object. Venus has an extremely dense atmosphere. 161 00:11:44,421 --> 00:11:50,253 the mass of Venus's atmosphere is almost 100 times the mass of Earth's atmosphere, 162 00:11:50,253 --> 00:11:55,536 which means pressure on the surface is about 100 times the pressure on Earth. 163 00:11:55,536 --> 00:11:59,172 this corresponds to being about a mile underwater. 164 00:11:59,172 --> 00:12:02,946 there is, the atmosphere is mostly carbon dioxide. 165 00:12:02,946 --> 00:12:06,308 So we predict a substantial, greenhouse effect. 166 00:12:06,308 --> 00:12:10,063 There are, this dense deck of clouds on the surface 167 00:12:10,063 --> 00:12:13,554 of Mercury, of Venus. This is what Venus looks like. 168 00:12:13,554 --> 00:12:16,627 It's covered with this dense deck of clouds. 169 00:12:16,627 --> 00:12:22,283 The, these clouds are actually sulfur dioxide. They are not water ice clouds. 170 00:12:22,283 --> 00:12:25,495 It turns out there's almost no water on Venus. 171 00:12:25,495 --> 00:12:31,570 These clouds give Venus a huge albedo, lowering its temperature the temperature 172 00:12:31,570 --> 00:12:37,226 is because of the thick atmosphere, almost uniform around everywhere on the 173 00:12:37,226 --> 00:12:42,934 planet at about 730 degrees. notice that given that albedo, the fact 174 00:12:42,934 --> 00:12:49,109 that only 10% of the Sun's, radiation, penetrates the clouds, our calculation 175 00:12:49,109 --> 00:12:53,827 would have predicted 184 degrees Kelvin as the planet's temperature. 176 00:12:53,827 --> 00:12:57,157 So this is a serious greenhouse effect going on. 177 00:12:57,157 --> 00:13:01,736 We'll talk about this. these, sulphur dioxide clouds actually 178 00:13:01,736 --> 00:13:06,523 lead to, lightning storms and rain. what rains is sulphuric acid. 179 00:13:06,523 --> 00:13:10,131 the surface of Venus is a very harsh environment. 180 00:13:10,131 --> 00:13:15,964 the Russian spacecraft, Venera which landed on Venus, survived for about 25 181 00:13:15,964 --> 00:13:21,378 minutes, before succumbing both to a lightning strike and to the corrosive, 182 00:13:21,378 --> 00:13:26,581 and high pressure atmosphere. And this makes studying the surface of 183 00:13:26,581 --> 00:13:31,500 Venus a very difficult task. Mars, on the other hand. 184 00:13:31,500 --> 00:13:35,509 It goes the other way. There is a very tenuous atmosphere. 185 00:13:35,509 --> 00:13:40,502 The mass of Mars's atmosphere is half a percent of the mass of Earth's 186 00:13:40,502 --> 00:13:43,949 atmosphere. And again the atmosphere is primarily 187 00:13:43,949 --> 00:13:49,857 carbon dioxide, presumably the oxygen generating, processes happened on neither 188 00:13:49,857 --> 00:13:53,936 Venus nor Mars. given, Mars's albedo of 25,. and its 189 00:13:53,936 --> 00:13:58,789 distance from the Sun, we predict a temperature of 210 degrees Kelvin. 190 00:13:58,789 --> 00:14:04,556 In fact, again absent a dense atmosphere, there are huge differences between day 191 00:14:04,556 --> 00:14:08,816 and night. And, temperatures range from 308 on the 192 00:14:08,816 --> 00:14:12,881 equator during the day to 130, near the poles. 193 00:14:12,881 --> 00:14:17,285 there are these, very serious seasonal changes. 194 00:14:17,285 --> 00:14:22,959 So it's 130 in winter at the pole, and 308 in summer at the equator. 195 00:14:22,959 --> 00:14:29,057 Mars has large polar icecaps, made, we think, primarily of water ice. 196 00:14:29,057 --> 00:14:32,255 Now, These are, this is a beautiful image, 197 00:14:32,255 --> 00:14:35,528 There's some, carbon dioxide ices in the poles. 198 00:14:35,528 --> 00:14:39,002 But mostly, there's an active hydrosphere on Mars. 199 00:14:39,002 --> 00:14:43,678 again, with such a tenuous atmosphere, you don't expect liquid water, 200 00:14:43,678 --> 00:14:47,352 but you do get ice and water vapor in Mars' atmosphere. 201 00:14:47,352 --> 00:14:50,007 And these polar ice caps, sublimate. 202 00:14:50,007 --> 00:14:54,816 The ice evaporates in the summer, to a large extent, and then reforms in the 203 00:14:54,816 --> 00:14:57,766 winter, and we can follow the seasonal changes. 204 00:14:57,766 --> 00:15:02,960 The atmosphere is very dusty, because the surface is dusty, and we have these huge 205 00:15:02,960 --> 00:15:04,370 dust storms. And. 206 00:15:04,370 --> 00:15:11,373 There are tantalizing hints of something like water, this sequence of images from 207 00:15:11,373 --> 00:15:16,902 the Mars orbiter, shows something that. Were it on Earth, would definitely look 208 00:15:16,902 --> 00:15:20,925 like water flowing. You cannot possibly have an actual liquid 209 00:15:20,925 --> 00:15:25,937 water flowing on the surface of Mars. We saw that water does not exist under 210 00:15:25,937 --> 00:15:31,214 such low pressures, but it may be that some heavily salted brines are able to 211 00:15:31,214 --> 00:15:36,160 flow either on or just below the surface, and the, the attempt to understand 212 00:15:36,160 --> 00:15:41,238 exactly how the hydrosphere of Mars operates is of course, a area of intense 213 00:15:41,238 --> 00:15:46,317 interest about which we are hoping to learn from the current curiosity rover 214 00:15:46,317 --> 00:15:52,599 and many Mars orbiting systems. The difference in these atmospheres, 215 00:15:52,599 --> 00:15:58,451 the fact that, Venus has this very dense atmosphere of carbon dioxide, and these 216 00:15:58,451 --> 00:16:02,947 extremely high temperatures. Mars has essentially no atmosphere, 217 00:16:02,947 --> 00:16:08,157 and in between them Earth has an atmosphere, is something that's, worth 218 00:16:08,157 --> 00:16:12,296 understanding. a comparative history of the atmospheres 219 00:16:12,296 --> 00:16:17,149 of the three terrestrial planets will, probably be worth pursuing. 220 00:16:17,149 --> 00:16:22,218 So we have to imagine that initially. All three planets would have lost 221 00:16:22,218 --> 00:16:28,134 hydrogen and helium, and there would have been some outgassing of carbon dioxide as 222 00:16:28,134 --> 00:16:31,773 rocks cooked. And there would be some water import as, 223 00:16:31,773 --> 00:16:36,647 heavy bombardment, imported ice from the outer parts of the solar system. 224 00:16:36,647 --> 00:16:41,456 So the initial conditions for Venus, Mars, and Earth are reasonably similar. 225 00:16:41,456 --> 00:16:46,525 The differences are, the smaller gravity on Mars and the slightly different 226 00:16:46,525 --> 00:16:50,815 distance from the Sun. But note, these are distances of a factor 227 00:16:50,815 --> 00:16:53,540 of two at most. And so, 228 00:16:53,540 --> 00:16:57,813 despite this rather modest change, this makes a huge difference in the history, 229 00:16:57,813 --> 00:17:01,430 and it explains what I meant when I said there's this sensitivity, 230 00:17:01,430 --> 00:17:06,494 to non-linear sensitivity, to details of the greenhouse mechanism effect. 231 00:17:06,494 --> 00:17:11,434 So, on Venus, what we think happens is the water lands on the surface and the 232 00:17:11,434 --> 00:17:14,608 planet is hot. the CO2 is generating a greenhouse 233 00:17:14,608 --> 00:17:16,301 effect. The water evaporates. 234 00:17:16,301 --> 00:17:19,990 Because Venus is closer to the Sun, the water never condenses. 235 00:17:19,990 --> 00:17:23,498 You never get rain. Water just evaporates right up through 236 00:17:23,498 --> 00:17:28,276 the atmosphere to the upper atmosphere, and there, ultraviolet light from the 237 00:17:28,276 --> 00:17:32,207 intense sunlight, dissociates it. The hydrogen is lost to space. 238 00:17:32,207 --> 00:17:36,380 The oxygen binds with, things like sulphur to make sulphur dioxide. 239 00:17:36,380 --> 00:17:43,454 without rains, you don't build up oceans. In fact, the oceans of Mercy, of Venus, 240 00:17:43,454 --> 00:17:45,926 evaporate. There's no tectonics. 241 00:17:45,926 --> 00:17:51,907 There's no continents building up. There's no way to recycle carbon dioxide 242 00:17:51,907 --> 00:17:54,220 into the rocks, which then 243 00:17:54,220 --> 00:17:59,253 dump it back into the magma so the carbon dioxide atmosphere continues to build up 244 00:17:59,253 --> 00:18:03,350 under volcanic activity out-gassing carbon dioxide from the mantle, and 245 00:18:03,350 --> 00:18:07,622 there's no way to get rid of it. Once all of the water has essentially 246 00:18:07,622 --> 00:18:12,187 boiled away and disappeared, you have a pure carbon dioxide atmosphere, and you 247 00:18:12,187 --> 00:18:17,122 have a runaway greenhouse effect which boils away the oceans and leave a baked, 248 00:18:17,122 --> 00:18:22,860 parched, high-pressure, high-temperature planet 249 00:18:22,860 --> 00:18:27,524 This is a story for Venus. Turn the dial a little bit down from 250 00:18:27,524 --> 00:18:31,368 where the Earth is and go out to Mars and start with it again. 251 00:18:31,368 --> 00:18:35,818 Similar initial conditions. Here, water that evaporates, it's cooler 252 00:18:35,818 --> 00:18:37,976 out on Mars. The water does rain. 253 00:18:37,976 --> 00:18:42,156 This removes some of the greenhouse gases from the atmosphere. 254 00:18:42,156 --> 00:18:45,797 Again Mars is not blessed with active plate tectonics. 255 00:18:45,797 --> 00:18:50,517 So what this means is that gas that gets trapped in rocks stays there. 256 00:18:50,517 --> 00:18:55,137 As Brain, er, er, removes carbon dioxide 257 00:18:55,137 --> 00:19:01,751 from the atmosphere, water that is the planet is cool enough that water trapped 258 00:19:01,751 --> 00:19:06,924 on the surface becomes trapped as permafrost we think, underneath the 259 00:19:06,924 --> 00:19:10,154 surface. Or if it evaporates up, again, evaporates 260 00:19:10,154 --> 00:19:15,222 out into space and is then dissociated by the ultraviolet light absent a strong 261 00:19:15,222 --> 00:19:20,290 magnetic field, there is no protection, and the solar wind removes the remnant of 262 00:19:20,290 --> 00:19:23,837 the atmosphere. In some sense you can describe the story 263 00:19:23,837 --> 00:19:29,032 of Mars as a runaway icehouse effect in the same way that Venus's atmosphere is a 264 00:19:29,032 --> 00:19:32,896 runaway greenhouse effect. If you start too hot you lose your 265 00:19:32,896 --> 00:19:35,659 oceans. There's no water, carbon dioxide builds 266 00:19:35,659 --> 00:19:40,240 up and the greenhouse effect takes away. You start too cold, the water freezes, 267 00:19:40,240 --> 00:19:44,078 does not generate liquid water to trap carbon dioxide. 268 00:19:44,078 --> 00:19:49,130 And again in both cases, the absence of tectonics makes the system unstable. 269 00:19:49,130 --> 00:19:53,508 We are very lucky to live on a planet that splits the difference.