1 00:00:00,000 --> 00:00:05,584 We've looked at Earth there's another celestial body about which we know almost 2 00:00:05,584 --> 00:00:10,698 as much as we know about Earth. it shares with the Earth the amazing fact 3 00:00:10,698 --> 00:00:13,327 that it's been visited by humans. 4 00:00:13,327 --> 00:00:17,175 This is our moon. Twelve people, in total, have visited the 5 00:00:17,175 --> 00:00:20,146 moon. And this means that we have samples of 6 00:00:20,146 --> 00:00:23,116 lunar rocks and soil that were brought back. 7 00:00:23,116 --> 00:00:27,167 As we will see, this is not unique to the moon, in that sense. 8 00:00:27,167 --> 00:00:31,485 bits of other planets have been knocked off by impacts and have, some of them 9 00:00:31,485 --> 00:00:35,814 have eventually found their way to Earth, so that we have meteorites whose source 10 00:00:35,814 --> 00:00:40,020 can be traced back to Mars or other planets, but the samples brought back 11 00:00:40,020 --> 00:00:42,483 from the moon were brought back from precise positions. 12 00:00:42,483 --> 00:00:45,170 We know where they came from and this will become important. 13 00:00:45,170 --> 00:00:49,428 And there are experiments that the astronauts left upon the moon and so we 14 00:00:49,428 --> 00:00:54,197 should know quite a bit about the moon. It's a good place to test what we learned 15 00:00:54,197 --> 00:00:57,547 on Earth and apply it to another similar smaller object. 16 00:00:57,547 --> 00:01:01,691 And let's start by looking at the moon. The moon as we know has two sides. 17 00:01:01,691 --> 00:01:04,984 as it orbits the Earth one side always faces the Earth. 18 00:01:04,984 --> 00:01:09,697 Looking at these images it's pretty easy to recognize the familiar man in the moon 19 00:01:09,697 --> 00:01:14,012 face on the left, that's the near side. The far side is missing those dark 20 00:01:14,012 --> 00:01:16,960 splotches. They're called maria because originally 21 00:01:16,960 --> 00:01:21,003 they were assumed to be oceans. We now know that there is no water on the 22 00:01:21,003 --> 00:01:23,494 moon. We talked about there could be no water 23 00:01:23,494 --> 00:01:26,374 on the moon. These are not oceans, they're lava flows 24 00:01:26,374 --> 00:01:30,810 but they're named Maria styx. regions of the moon that are not maria 25 00:01:30,810 --> 00:01:36,470 are called highlands, and we see from this image that they're heavily cratered 26 00:01:36,470 --> 00:01:39,286 This is true. the far side is all highlands. 27 00:01:39,286 --> 00:01:43,755 And, the distinction, the dichotomy between the far side and the near side. 28 00:01:43,755 --> 00:01:49,081 the causes for why one side is different from the other are not, I think, clear in 29 00:01:49,081 --> 00:01:53,550 any sense that I can explain it. And so, we'll just leave the statement at 30 00:01:53,550 --> 00:01:56,427 that. It certainly has something to do with the 31 00:01:56,427 --> 00:02:00,957 presence of the earth on one side, but what exactly is far from obvious. 32 00:02:00,957 --> 00:02:04,080 let's take a closer look. There are beautiful 33 00:02:04,080 --> 00:02:08,727 Lunar maps, let's take a closer look at the surface of the Moon and see what we 34 00:02:08,727 --> 00:02:12,199 can see there. This is one of the benefits of having had 35 00:02:12,199 --> 00:02:14,670 so many spacecraft orbit the Moon, we have. 36 00:02:14,670 --> 00:02:18,677 Brilliant photographic maps of the moon, I'll post a link to this one. 37 00:02:18,677 --> 00:02:23,324 You can play with it if you would like. And here we are looking at the region of 38 00:02:23,324 --> 00:02:27,913 the Maria Crisium, one of those maria that I spoke about and we can see that 39 00:02:27,913 --> 00:02:31,688 while it's not an ocean it does share the properties of an ocean. 40 00:02:31,688 --> 00:02:36,255 It looks kind of like it, it's flat and straight and dark and whereas the 41 00:02:36,255 --> 00:02:42,772 highlands around it appear to be peppered with craters, and we see that this Maria 42 00:02:42,772 --> 00:02:47,269 appears to be a crater it's in it's own right, except a 43 00:02:47,269 --> 00:02:51,814 very large crater. Indeed when astronauts investigated, the 44 00:02:51,814 --> 00:02:56,210 Maria became clear that what these are, are old lava flows. 45 00:02:56,210 --> 00:03:01,352 This is are, these are traces of old volcanic activity on the moon. 46 00:03:01,352 --> 00:03:07,834 not in the form of large dome volcanoes, like on earth, though there are traces of 47 00:03:07,834 --> 00:03:13,721 that but mostly lava flows that essentially, flowed and filled a existing 48 00:03:13,721 --> 00:03:17,255 large crater. you will notice that the highlands 49 00:03:17,255 --> 00:03:21,778 surrounding the Maria are heavily scarred with craters of all sizes. 50 00:03:21,778 --> 00:03:26,639 But few as large as the aria. And, so we begin to be developing and 51 00:03:26,639 --> 00:03:30,420 understanding of time. And we see that there are craters. 52 00:03:30,420 --> 00:03:33,949 Inside the Maria but there are few and far between. 53 00:03:33,949 --> 00:03:39,422 what we take this to mean is that the terrain of the aria is more recent. 54 00:03:39,422 --> 00:03:45,040 The moon has been under constant bombardment and the mare only has craters 55 00:03:45,040 --> 00:03:50,585 reflecting those impacts that have occurred since the time it was resurfaced 56 00:03:50,585 --> 00:03:54,835 by the lava flow. This is why terrain in the Maria is by 57 00:03:54,835 --> 00:03:59,180 and large smooth, because it is younger terrain. 58 00:03:59,180 --> 00:04:03,376 Who. We've learned something about how to 59 00:04:03,376 --> 00:04:09,169 learn about the age of terrain older terrain will have more craters, the 60 00:04:09,169 --> 00:04:13,274 longer something is sitting there the more impacts it will absorb. 61 00:04:13,274 --> 00:04:18,001 This of course terminates at some point, you reach some saturation where new 62 00:04:18,001 --> 00:04:21,048 impacts destroys as many craters as they produce. 63 00:04:21,048 --> 00:04:26,708 you can get a little more refined sense of dating we focused here a little bit to 64 00:04:26,708 --> 00:04:30,750 the north of the Maria Crisium on this crater called Cleomedes. 65 00:04:30,750 --> 00:04:35,103 We've made a little bit of an enlargement, and what we can see is that 66 00:04:35,103 --> 00:04:39,589 Cleomedes itself are. Large, and rather old crater, and it's 67 00:04:39,589 --> 00:04:44,572 been degraded, its rim is not very sharp and not very high. 68 00:04:44,572 --> 00:04:48,010 And indeed, on its western rim, I'm using. 69 00:04:48,010 --> 00:04:53,199 Conventional east and west notations here, is this crater Tralles which 70 00:04:53,199 --> 00:04:58,023 clearly post dates Cleomedes. The impact that created Tralles post 71 00:04:58,023 --> 00:05:04,236 dates the impact that created Cleomedes because Tralles obliterated a part of the 72 00:05:04,236 --> 00:05:08,695 rim of Cleomedes. And similarly going up here to the north 73 00:05:08,695 --> 00:05:13,799 we see that Cleomedes E post dates Cleomedes and Cleomedes A, this nice 74 00:05:13,799 --> 00:05:19,144 sharp round one, post dates Cleomedes E. And so because we have craters upon 75 00:05:19,144 --> 00:05:24,845 craters upon craters, and if we magnify further we can see yet more craters upon 76 00:05:24,845 --> 00:05:30,546 them, what we can see is that we can begin to order things in terms of the 77 00:05:30,546 --> 00:05:35,820 order in which they occurred by not only the degree of cratering but also 78 00:05:35,820 --> 00:05:39,740 precisely ordering the craters one on top of the other. 79 00:05:39,740 --> 00:05:46,091 Enlarging still further, we see that when I say that the interior of Cleomedes was 80 00:05:46,091 --> 00:05:50,443 relatively unscarred, relatively was the operative word 81 00:05:50,443 --> 00:05:54,960 indeed this is a region that is less cratered than the surrounding. 82 00:05:54,960 --> 00:05:57,721 But, there have been impacts in Cleomedes. 83 00:05:57,721 --> 00:06:00,803 We also see another typical lunar feature. 84 00:06:00,803 --> 00:06:05,555 This Rima Cleomedes, these are these ridges, which are thought to have formed 85 00:06:05,555 --> 00:06:08,894 when the interior of the Moon cooled and contracted. 86 00:06:08,894 --> 00:06:11,720 The crust having already solidified. This is. 87 00:06:11,720 --> 00:06:16,868 The closest analog the moon has to plate tectonics, we do not find on the moon, 88 00:06:16,868 --> 00:06:21,159 aside from the impact craters, any indication of end of volcanism. 89 00:06:21,159 --> 00:06:26,308 But any indication of linear features that would indicate plate tectonics, the 90 00:06:26,308 --> 00:06:31,259 moon's crust is all one plate. The moon is geologically inactive now not 91 00:06:31,259 --> 00:06:34,559 completely. Its geologic activity such as it is, is 92 00:06:34,559 --> 00:06:38,410 that as the moon is shrinking. We get these 93 00:06:38,410 --> 00:06:43,209 linear features that are these ridges, which are basically the result of the 94 00:06:43,209 --> 00:06:46,240 surface cool shrinking as the interior cools. 95 00:06:46,240 --> 00:06:51,970 But the moon shows traces of past geologic activity, but certainly no 96 00:06:51,970 --> 00:06:55,380 current geologic activity. What we saw 97 00:06:55,380 --> 00:07:02,352 lunar craters are created by impacts, this is the mechanism of by which, the 98 00:07:02,352 --> 00:07:07,090 moon is weathered, a continuous barrage of impacts. 99 00:07:07,090 --> 00:07:12,863 No atmosphere to protect it, and so even small rocks make it all the way down to 100 00:07:12,863 --> 00:07:17,049 the lunar surface. The maria or lava plains, often they end 101 00:07:17,049 --> 00:07:22,100 up filling old large craters. We see that there are old craters and 102 00:07:22,100 --> 00:07:27,327 then it seems that with time the size of impactors has become smaller. Large 103 00:07:27,327 --> 00:07:32,239 impacts are always contained in, inside smaller, contained smaller ones that are 104 00:07:32,239 --> 00:07:35,354 more recent. So there's a progression in the size of 105 00:07:35,354 --> 00:07:37,930 the things that are crashing into the moon. 106 00:07:37,930 --> 00:07:42,961 We small these Rima Rilles and Graben that result from the shrinking of the 107 00:07:42,961 --> 00:07:45,776 interior. There's no volcanism, currently, on the 108 00:07:45,776 --> 00:07:47,094 Moon. This makes sense. 109 00:07:47,094 --> 00:07:49,730 The moon is a smaller object than the Earth. 110 00:07:49,730 --> 00:07:54,222 It will have cooled, the interior will have managed to radiate out its heat 111 00:07:54,222 --> 00:07:55,720 sooner. And, at some point, 112 00:07:55,720 --> 00:07:59,301 given enough time, the Earth too, will become volcanically inactive. 113 00:07:59,301 --> 00:08:03,913 On the Moon this has already happened so small planets we expect will cool faster, 114 00:08:03,913 --> 00:08:07,222 just because of scale. The moon has essentially no atmosphere, 115 00:08:07,222 --> 00:08:09,827 its gravity is not enough to bind an atmosphere. 116 00:08:09,827 --> 00:08:15,089 such molecules of gas as are found on the moon will be exposed in the absence of 117 00:08:15,089 --> 00:08:19,885 both a global magnetic field, as we will see, and atmospheric protection will be 118 00:08:19,885 --> 00:08:24,074 dissociated by the sun's ultraviolet radiation, and so, for example, if 119 00:08:24,074 --> 00:08:28,748 there's any water vapor on the moon it'll, the molecules will be broken up by 120 00:08:28,748 --> 00:08:32,451 ultraviolet radiation. The hydrogen atoms will be unbound and 121 00:08:32,451 --> 00:08:36,720 they will escape to space. The temperature on the moon, 122 00:08:36,720 --> 00:08:40,874 well, the moon is kind of like the easy model of the Earth we studied before. 123 00:08:40,874 --> 00:08:44,645 It has an actually, a very small albedo, and it's roughly a black body. 124 00:08:44,645 --> 00:08:48,362 There's no greenhouse effect. So the temperature we expect will be a 125 00:08:48,362 --> 00:08:52,188 little bit under 300 degrees. It's in the same orbit around the sun as 126 00:08:52,188 --> 00:08:54,752 Earth is. The thing that the, moon, that the 127 00:08:54,752 --> 00:08:59,776 absence of an atmosphere does is that there's no mechanism for shifting heat 128 00:08:59,776 --> 00:09:04,224 from the day side of the moon to the, night side of the moon, remember that 129 00:09:04,224 --> 00:09:08,954 lunar days and nights are two weeks long. And so you develop a huge temperature 130 00:09:08,954 --> 00:09:12,330 differential. Temperature is as high as 370 Kelvin on 131 00:09:12,330 --> 00:09:15,643 the day side, as low as 100 Kelvin on the night side. 132 00:09:15,643 --> 00:09:20,420 And in the deep dark shadows of some craters where the sun literally never 133 00:09:20,420 --> 00:09:25,134 shines the coldest temperatures known in the inner solar system have been 134 00:09:25,134 --> 00:09:29,019 measured, down to 35 degrees Kelvin which is very cold indeed. 135 00:09:29,019 --> 00:09:33,669 there's clearly no water on the moon because water as we saw requires 136 00:09:33,669 --> 00:09:38,765 atmospheric pressure to maintain it, but we have very good evidence that in some 137 00:09:38,765 --> 00:09:41,199 of these craters of eternal shadow. 138 00:09:41,199 --> 00:09:46,207 There are significant quantities of water ice, presumably imported on the same 139 00:09:46,207 --> 00:09:50,830 large bombardment that imported water into the time of the Earth and 140 00:09:50,830 --> 00:09:56,830 the crust of the moon is very old. We see that it carries the traces of 141 00:09:56,830 --> 00:10:01,098 impacts generations in billions of years of impact on Earth. 142 00:10:01,098 --> 00:10:06,434 That same amount of impacts at least the larger ones must have occurred. 143 00:10:06,434 --> 00:10:10,560 On Earth this record has been erased by tectonic activity. 144 00:10:10,560 --> 00:10:16,252 Since the Moon ceased geological activity it's in some sense is a museum of the 145 00:10:16,252 --> 00:10:20,947 history of the solar system for the past four billion years or so. 146 00:10:20,947 --> 00:10:26,476 And in the Highlands the effect of these impacts been basically to cover the 147 00:10:26,476 --> 00:10:31,256 entire surface of the moon with a thick layer of very fine dust, which is 148 00:10:31,256 --> 00:10:37,018 basically rocks that have been pounded down to dust by impacts and then also the 149 00:10:37,018 --> 00:10:42,257 effect of ultraviolet radiation and this is the regulate, the fine dust in which 150 00:10:42,257 --> 00:10:47,430 that beautiful footprint we saw was, created and so we can use the moon as a 151 00:10:47,430 --> 00:10:52,276 museum of history and when we put together the ability to date crater and 152 00:10:52,276 --> 00:10:56,140 order craters by their size and their relative position. 153 00:10:56,140 --> 00:11:02,289 Together with the fact that we have lunar samples whose provenance is known, and we 154 00:11:02,289 --> 00:11:07,777 can radio date them we have we can now identify which craters were 155 00:11:07,777 --> 00:11:13,893 created when, and then use that to relate that, find the, dates of other craters, 156 00:11:13,893 --> 00:11:17,554 and we have Bombardment history we can construct a 157 00:11:17,554 --> 00:11:22,777 history of the rate at which the moon was bombarded starting from the present and 158 00:11:22,777 --> 00:11:27,873 the most recent craters that we see and into the older craters that we found. 159 00:11:27,873 --> 00:11:32,905 And we see that the rate of lunar bombardment and you see that none of this 160 00:11:32,905 --> 00:11:37,237 is perfect. This is, science is an approximation to truth, has been 161 00:11:37,237 --> 00:11:42,269 relatively constant for the past three billion years or so but it appears that 162 00:11:42,269 --> 00:11:46,928 about 3.9 billion years ago there was phenomenon first described as a lunar 163 00:11:46,928 --> 00:11:50,462 cataclysm, now it's known as the age of heavy bombardment. 164 00:11:50,462 --> 00:11:54,879 The Nice model very nicely explains it. As you can see, not everybody agrees 165 00:11:54,879 --> 00:11:59,001 that, that is what happened. But we have evidence there that the moon 166 00:11:59,001 --> 00:12:02,520 was heavily bombarded about 3.9 billion years ago and 167 00:12:02,520 --> 00:12:08,045 there are reasons to imagine that perhaps this was in fact a spike, as in these 168 00:12:08,045 --> 00:12:12,419 models would suggest. One of the main reasons we believe in the 169 00:12:12,419 --> 00:12:17,413 predictions, is that it agrees with this dating of moon rocks, there's more detail 170 00:12:17,413 --> 00:12:22,423 than is apparent in what I have said. careful geologists have been studying 171 00:12:22,423 --> 00:12:27,251 this thing very carefully over decades. This is what we learn from the lunar 172 00:12:27,251 --> 00:12:31,093 surface, what goes on inside. The moon melted, it's a round object, 173 00:12:31,093 --> 00:12:35,356 there's chemical differentiation. There is a core, but the lunar core is 174 00:12:35,356 --> 00:12:38,838 anomalously small. the moon's total density is not much 175 00:12:38,838 --> 00:12:43,040 larger than that of rock. once we conclude that it has a very small 176 00:12:43,040 --> 00:12:46,162 core. we study the moon's internal structure 177 00:12:46,162 --> 00:12:50,305 the same way we do the Earth's. There are moonquakes, in this case not 178 00:12:50,305 --> 00:12:54,928 generated by plate tectonics because that's not happening, but mostly by the 179 00:12:54,928 --> 00:12:57,810 tidal forces of Earth causing internal friction. 180 00:12:57,810 --> 00:13:02,343 and we see that the moon's core is anomalously small. 181 00:13:02,343 --> 00:13:07,168 And its mantle, very thick. the moon lacks a geodynamic magnetic 182 00:13:07,168 --> 00:13:10,605 field. There are traces of magnetization on the 183 00:13:10,605 --> 00:13:16,261 surface, but no global magnetic field. this, this has to do with both the slow 184 00:13:16,261 --> 00:13:20,489 rate of rotation, and the fact that it's essentially done cooling. 185 00:13:20,489 --> 00:13:25,367 And while there is a fluid inner core. A solid inner core and a fluid outer 186 00:13:25,367 --> 00:13:28,554 core. There's not enough convection heating, to 187 00:13:28,554 --> 00:13:31,930 drive a magnetic field. And 188 00:13:31,930 --> 00:13:36,881 the last important question obviously is where did this thing come from? 189 00:13:36,881 --> 00:13:41,970 How is it that earth of all the terrestrial planets has a moon that is so 190 00:13:41,970 --> 00:13:46,440 ominously large compared to the moons of other planets and 191 00:13:46,440 --> 00:13:51,120 even compared to the Jovian planets no planet has a moon so large by comparison 192 00:13:51,120 --> 00:13:54,104 to the planet. it turns out that the isotopic 193 00:13:54,104 --> 00:13:58,200 composition of the moon suggests that it's basically a piece of Earth. 194 00:13:58,200 --> 00:14:02,646 The relative concentrations of various isotopes of oxygen turns out to be a 195 00:14:02,646 --> 00:14:06,508 sensitive trace of where in the solar nebula something was formed. 196 00:14:06,508 --> 00:14:11,670 on the moon they track those on Earth so precisely that the conclusion is that 197 00:14:11,670 --> 00:14:17,261 moon is literally a piece of the Earth. how did a piece of the earth come to be 198 00:14:17,261 --> 00:14:20,222 in orbit? Well, the most likely theory is that 199 00:14:20,222 --> 00:14:25,550 there was what was called a giant impact early in the earth's history, before the 200 00:14:25,550 --> 00:14:29,915 solar system got cleaned up something either the size of Mars, or 201 00:14:29,915 --> 00:14:33,058 perhaps a little larger, crashed into the Earth, 202 00:14:33,058 --> 00:14:36,389 The impact vaporized large fractions of both objects. 203 00:14:36,389 --> 00:14:40,537 And if you get the dynamics of the collision right, as we will see. 204 00:14:40,537 --> 00:14:45,446 Then the, the impacting object, which is not part of Earth, and therefore doesn't 205 00:14:45,446 --> 00:14:50,349 have the right isotopic ratios, will be absorbed into the Earth's core, and 206 00:14:50,349 --> 00:14:55,252 mostly, bits of the mantle of Earth, will be spewed out into space, and this would 207 00:14:55,252 --> 00:14:59,397 explain both why the moon is. Poor in iron relatively, and not that 208 00:14:59,397 --> 00:15:04,452 dense because it is made up of the ejecta from this collision, whereas the core, 209 00:15:04,452 --> 00:15:09,762 the heavier materials sank into the core. We should look at this next simulation 210 00:15:09,762 --> 00:15:13,217 and see what it does. This is a very recent simulation. 211 00:15:13,217 --> 00:15:18,400 This is a recent version of the giant impact hypothesis that created the moon, 212 00:15:18,400 --> 00:15:23,502 and what we see here is that in this simulation, the Earth starts out with a 213 00:15:23,502 --> 00:15:26,761 rotation period, a day, a sidereal day, of 2.3 hours. 214 00:15:26,761 --> 00:15:31,976 This is something to pay attention to. Remember that near earth satellites orbit 215 00:15:31,976 --> 00:15:36,604 the earth every 90 minutes. the earth is rotating so fast that rocks 216 00:15:36,604 --> 00:15:40,320 are almost in orbit. The earth is on the verge of falling 217 00:15:40,320 --> 00:15:45,143 apart, in the way the simulation works. This is important because it is the 218 00:15:45,143 --> 00:15:50,619 reason that this impacting object with a mass, only 5% of Earth's mass, crashing 219 00:15:50,619 --> 00:15:55,926 into earth, essentially head on. leads to a cataclysmic collision the 220 00:15:55,926 --> 00:16:01,965 impactor is essentially absorbed by the earth and within 24 hours there's this 221 00:16:01,965 --> 00:16:08,309 cloud of what was formerly the earth's mantle scattered around the Earth the 222 00:16:08,309 --> 00:16:11,384 Earth's and, and out of this cloud, we now have 223 00:16:11,384 --> 00:16:16,598 protoplanets and the standard accretion disk model the moon will eventually form. 224 00:16:16,598 --> 00:16:20,632 Note that the moon forms in these models very close to the Earth. 225 00:16:20,632 --> 00:16:25,846 we've talked about George Darwin's theory about how the Earth is transferring 226 00:16:25,846 --> 00:16:28,702 angular momentum to the moon and slowing down. 227 00:16:28,702 --> 00:16:32,240 Earth's day, by the end of creating the moon was about 228 00:16:32,240 --> 00:16:36,581 2 and a half hours in this model and then over time it turns out the entire 229 00:16:36,581 --> 00:16:41,311 Earth-moon system exchanges angular momentum with the sun because of the 230 00:16:41,311 --> 00:16:45,393 sun's tidal forces over tens or hundreds of thousands of years. 231 00:16:45,393 --> 00:16:51,290 the moon settles down into it's nearly circular orbit about the Earth and then 232 00:16:51,290 --> 00:16:56,020 from this moment on, and the Earth has developed a day that is relatively 233 00:16:56,020 --> 00:17:00,426 generous five or six hours. And then the process from here on is the 234 00:17:00,426 --> 00:17:03,662 tidal a transfer of angular momentum from the 235 00:17:03,662 --> 00:17:09,032 Earth to the moon which mush, moves the Moon farther away from Earth and leads to 236 00:17:09,032 --> 00:17:12,480 the, 24 hour day that we observe today. one of the. 237 00:17:12,480 --> 00:17:16,283 Nice things that has been verified is that, astronauts on the moon left 238 00:17:16,283 --> 00:17:19,337 reflectors there. We can measure the distance to the moon 239 00:17:19,337 --> 00:17:22,122 very precisely. And indeed, the moon is receding from 240 00:17:22,122 --> 00:17:24,908 Earth at a rate of about three centimeters per year. 241 00:17:24,908 --> 00:17:29,462 as a told you once, total solar eclipses are something to enjoy because they won't 242 00:17:29,462 --> 00:17:29,730 last.