1 00:00:01,520 --> 00:00:06,818 We did pretty well understanding terrestrial worlds which are relatively 2 00:00:06,818 --> 00:00:10,862 similar to Earth. Now we can get ambitious, we can see what 3 00:00:10,862 --> 00:00:16,678 we can say about the Jovian planets and see how much of what we've understood 4 00:00:16,678 --> 00:00:19,766 translates to these more interesting systems. 5 00:00:19,766 --> 00:00:25,669 looking at their motion in the solar system the things I'd like to point out 6 00:00:25,669 --> 00:00:29,032 is the inclinations of the orbits are very small. 7 00:00:29,032 --> 00:00:32,120 the periods of the orbits are interesting. 8 00:00:32,120 --> 00:00:37,954 Jupiter and Saturn are locked, as you can see if you do the calculation, into a two 9 00:00:37,954 --> 00:00:41,270 to five resonance. So, the one to two resonance that 10 00:00:41,270 --> 00:00:46,188 destabilize things has locked into a stable two to five resonance which is 11 00:00:46,188 --> 00:00:50,349 anchoring if, in some sense, the stability of the rest of the solar 12 00:00:50,349 --> 00:00:53,658 system. in terms of the rotation periods, the 13 00:00:53,658 --> 00:00:57,340 spin periods, notice how fast all of these objects spin. 14 00:00:57,340 --> 00:01:00,821 The slowest is Uranus with a seventeen hour, spin. 15 00:01:00,821 --> 00:01:05,709 Jupiter and its entire mass rotates about its axis every ten hours. 16 00:01:05,709 --> 00:01:11,132 If we think about these giant planets forming out of a gravitational collapse, 17 00:01:11,132 --> 00:01:16,421 a gravitational instability then we remember that this will acquire angular 18 00:01:16,421 --> 00:01:19,501 momentum from more distant parts of the cloud. 19 00:01:19,501 --> 00:01:25,218 This explains the rather rapid spinning motion and will have consequences as we 20 00:01:25,218 --> 00:01:29,192 go along. the other thing to note is that the tilt 21 00:01:29,192 --> 00:01:34,514 angles are moderate except for Uranus, which has a axial tilt of 97 degrees, 22 00:01:34,514 --> 00:01:39,978 which means its axis essentially is not perpendicular to the ecliptic but is 23 00:01:39,978 --> 00:01:45,230 essentially in the plane of the ecliptic. this gives extraordinarily 24 00:01:45,230 --> 00:01:50,791 marked seasonal effects on Uranus and even from observations from Earth will be 25 00:01:50,791 --> 00:01:54,962 able to remark those. Notice that those seasons are rather long 26 00:01:54,962 --> 00:01:59,796 since the year is 84 years long. And so in some sense, Neptune with the 27 00:01:59,796 --> 00:02:02,775 longest year was discovered about a year ago. 28 00:02:02,775 --> 00:02:05,821 This is what we see in terms of their motions. 29 00:02:05,821 --> 00:02:11,051 We talked about how they got there. looking at the surface or the atmosphere, 30 00:02:11,051 --> 00:02:15,813 which in these cases is the same thing since this atmosphere, primarily 31 00:02:15,813 --> 00:02:20,927 hydrogen, molecular hydrogen and helium, is all we're going to see because there's 32 00:02:20,927 --> 00:02:25,737 a sufficiently deep atmosphere in all of these that we cannot see through them. 33 00:02:25,737 --> 00:02:28,937 the atmospheres are always hydrogen and helium. 34 00:02:28,937 --> 00:02:33,101 Uranus and Neptune acquire their blue tint from traces of methane. 35 00:02:33,101 --> 00:02:37,567 What do we see looking at the surface? Well, the surface is the same as the 36 00:02:37,567 --> 00:02:41,649 atmosphere because all of these planets have sufficiently thick 37 00:02:41,649 --> 00:02:47,571 high atmospheres of hydrogen and helium that we cannot see through them with any 38 00:02:47,571 --> 00:02:51,593 instrument. And so, the atmosphere is what we see the 39 00:02:51,593 --> 00:02:57,515 atmospheres are hydrogen and helium. In the case of Uranus and Neptune a trace 40 00:02:57,515 --> 00:03:03,438 amount of methane tinges them blue. And what we see when we look at this 41 00:03:03,438 --> 00:03:09,579 wonderful image of Jupiter is the lines of latitude tracing convection cells the 42 00:03:09,579 --> 00:03:14,040 atmosphere is heated by the core from below, just as on Earth. 43 00:03:14,040 --> 00:03:19,947 This in conjunction with the more rapid rotation leads to these bands and zones 44 00:03:19,947 --> 00:03:25,998 which are tracing the patterns of the convection cells. together with the rapid 45 00:03:25,998 --> 00:03:29,272 rotation, you get very massive global winds. 46 00:03:29,272 --> 00:03:35,251 And at the interfaces between direct, differing directions of global winds, you 47 00:03:35,251 --> 00:03:40,803 get intense storms most famously the Giant Red Spot on Jupiter. A storm the 48 00:03:40,803 --> 00:03:46,000 size of the Earth which we conjecture has been ongoing for 100,000 years. 49 00:03:46,000 --> 00:03:50,286 but there are many other persistent storms on Jupiter. 50 00:03:50,286 --> 00:03:54,715 comparing this to an image of Saturn, we see similar things. 51 00:03:54,715 --> 00:03:58,716 We see the same band structure, although less pronounced. 52 00:03:58,716 --> 00:04:02,860 The dark coloring of some of the regions on Jupiter 53 00:04:02,860 --> 00:04:09,050 are is attributed to the rising of some form 54 00:04:09,050 --> 00:04:12,044 of complex. Perhaps, hydrocarbon molecules. 55 00:04:12,044 --> 00:04:15,915 But nobody's precisely sure what causes the coloring. 56 00:04:15,915 --> 00:04:22,269 And in particular, nobody can explain why once in a while, one of those dark belts 57 00:04:22,269 --> 00:04:27,235 disappears for a few months, as happened about six months ago. 58 00:04:27,235 --> 00:04:31,837 And then, reacquires its colors. It's not a change in the global 59 00:04:31,837 --> 00:04:36,365 circulation patterns. But something else, and nobody completely 60 00:04:36,365 --> 00:04:42,318 understands it. we see the same kind of storm pattern on 61 00:04:42,318 --> 00:04:50,388 Saturn. We see simliar effects on Uranus and indeed similar effects on Neptune. On 62 00:04:50,388 --> 00:04:57,500 Neptune, what we notice is the hemispherical variations with season. 63 00:04:57,500 --> 00:05:02,465 this happens on Uranus, it also happens on Neptune despite the 64 00:05:02,465 --> 00:05:07,671 tilt not being as extreme. Simply because the seasons are so long 65 00:05:07,671 --> 00:05:14,239 that global hemispheric effects take place in the course of the 164 year 66 00:05:14,239 --> 00:05:17,208 orbit. Looking inside the planets, well we don't 67 00:05:17,208 --> 00:05:21,822 do seismology on these giant planets. we conjecture the structure of their 68 00:05:21,822 --> 00:05:26,435 inside from modelling as we do for the terrestrial planets combined with 69 00:05:26,435 --> 00:05:31,534 measurements of the obliqueness, the rapid spin causes all of these planets 70 00:05:31,534 --> 00:05:36,209 like as indeed, the Earth does, to be slightly thicker around the equator than 71 00:05:36,209 --> 00:05:39,730 around the poles. This is because of the effect of spin. 72 00:05:39,730 --> 00:05:45,255 By measuring how ablate they are compared to how they spin, we can estimate produce 73 00:05:45,255 --> 00:05:50,652 some estimate of the internal structure. And the internal structure that seems to 74 00:05:50,652 --> 00:05:55,583 obtain for Jupiter and Saturn is that inside the external atmosphere of 75 00:05:55,583 --> 00:05:59,455 hydrogen and helium, is a mantle made of metallic hydrogen. 76 00:05:59,455 --> 00:06:04,662 Hydrogen, if you look at its place in the periodic table, the, should be a metal. 77 00:06:04,662 --> 00:06:08,667 Metallic hydrogen is certainly not found naturally on Earth. 78 00:06:08,667 --> 00:06:14,141 It occurs only under very, very high pressures it was only produced on Earth 79 00:06:14,141 --> 00:06:18,277 in I think the 80s.' and but it exists naturally under the 80 00:06:18,277 --> 00:06:23,026 extreme compression of the immense gravity and the immense weight of the 81 00:06:23,026 --> 00:06:27,581 heavy atmospheres of these planets. We have the usual hydro-dynamic 82 00:06:27,581 --> 00:06:32,786 equilibrium where pressures in the cores of these planets are very, very extreme 83 00:06:32,786 --> 00:06:37,535 and decrease as you move out. But since the atmosphere is thousands of 84 00:06:37,535 --> 00:06:43,260 kilometers thick and very heavy, even at the bottom of the atmosphere, pressures 85 00:06:43,260 --> 00:06:46,383 are intense. metallic hydrogen is a conductor. 86 00:06:46,383 --> 00:06:52,325 The rapid rotation leads to a strong dipolar magnetic field for all of these 87 00:06:52,325 --> 00:06:56,625 planets. in the case of the gas giants Jupiter and 88 00:06:56,625 --> 00:07:03,359 Saturn this magnet is aligned very nicely with the planet's rotation axis. 89 00:07:03,359 --> 00:07:10,499 in the case of Uranus and Neptune, the mantle is actually compressed ices. So 90 00:07:10,499 --> 00:07:13,988 water, ammonia, methane, etc., at high density. 91 00:07:13,988 --> 00:07:19,586 these too can conduct, but the mechanism is a little bit different. 92 00:07:19,586 --> 00:07:25,141 the understanding is incomplete but the one of the manifestations of the 93 00:07:25,141 --> 00:07:30,661 difference in the mantle is that the magnetic fields of Uranus and Neptune are 94 00:07:30,661 --> 00:07:34,924 60 degrees tilted relative to their direction of rotation. 95 00:07:34,924 --> 00:07:40,375 inside each of these is a core, the, presumably the seed that started the 96 00:07:40,375 --> 00:07:44,568 planet's growth. as you can see in this image, the core is 97 00:07:44,568 --> 00:07:50,315 roughly Earth-like in size probably more massive because compressed. And has an 98 00:07:50,315 --> 00:07:56,529 iron-rich inside, and perhaps a silicate outside if it chemically differentiated. 99 00:07:56,529 --> 00:08:02,743 so set, the core of each of these planets is about the size of Earth itself. 100 00:08:02,743 --> 00:08:08,596 Again, the interiors of these planets are both extremely compressed and very, very 101 00:08:08,596 --> 00:08:12,281 hot. in the case of these planets, much of the 102 00:08:12,281 --> 00:08:17,339 internal heat, remember the only the core is rich in heavy elements. 103 00:08:17,339 --> 00:08:21,313 Here this is a tiny fraction of the actual huge planet. 104 00:08:21,313 --> 00:08:26,679 and so, radioactivity does not play a large role in maintaining and producing 105 00:08:26,679 --> 00:08:30,090 internal heat. Internal heart is produced by Kelvin 106 00:08:30,090 --> 00:08:35,440 Helmholtz processes by precipitation and by increased continuing compression of 107 00:08:35,440 --> 00:08:39,586 the interiors of the planet. On the case, in the case of Saturn, 108 00:08:39,586 --> 00:08:44,468 precipitation of liquid helium. So, liquid helium rain at high pressure 109 00:08:44,468 --> 00:08:48,881 is the mechanism by which Saturn is chemically differentiating. 110 00:08:48,881 --> 00:08:51,254 And this is significant. If you do the 111 00:08:51,254 --> 00:08:55,734 calculation of surface temperature based on our black body calculation, compared 112 00:08:55,734 --> 00:08:59,576 to the measured temperature of the surfaces of these planets, you'll 113 00:08:59,576 --> 00:09:03,883 discover that both Jupiter and Saturn radiate about twice as much energy as 114 00:09:03,883 --> 00:09:08,047 they absorb from the sun. This is the difference is being made by 115 00:09:08,047 --> 00:09:12,840 continual heating from the core through mostly Calvin Helmholtz processes. 116 00:09:12,840 --> 00:09:17,824 And we have the strong magnetic field. This is an aurora on Jupiter's north 117 00:09:17,824 --> 00:09:20,444 pole. These are aurorae on Saturn's poles. 118 00:09:20,444 --> 00:09:25,620 And if we look for aurorae on Uranus, you can see that if you look at the ring 119 00:09:25,620 --> 00:09:30,221 structure that indicates the equator of the planet, this is not the pole. 120 00:09:30,221 --> 00:09:35,313 The pole is way over here to the left. And the aurorae are showing up at 121 00:09:35,313 --> 00:09:39,405 intermediate latitudes. This is indicating to you the offset of 122 00:09:39,405 --> 00:09:44,275 the magnetic field. indeed, this reminds us that the most 123 00:09:44,275 --> 00:09:50,195 impressive structures carried by these giant planets are their rings. 124 00:09:50,195 --> 00:09:54,641 And we understand, in some sense, where these might come from. 125 00:09:54,641 --> 00:09:59,829 These planets are formed by a gravitational instability in the nebula 126 00:09:59,829 --> 00:10:04,053 collapse. And the collapse will be accompanied by a 127 00:10:04,053 --> 00:10:10,277 flattening into an accretion disk from which the planet acquires gas and dust. 128 00:10:10,277 --> 00:10:14,987 near, some of this leftovers of the nebula will form moons. 129 00:10:14,987 --> 00:10:20,137 So, most of these planets have moons that orbit them in their equatorial plane. 130 00:10:20,137 --> 00:10:25,551 these moons formed within the accretion disk, just as the planets formed in the 131 00:10:25,551 --> 00:10:27,400 sun's accretion disk. but, 132 00:10:27,400 --> 00:10:31,843 close to the planet, tidal forces can prevent gravitational accretion. 133 00:10:31,843 --> 00:10:37,124 This would have been true in the case of the Sun as well, except that close to the 134 00:10:37,124 --> 00:10:41,825 Sun whatever didn't accrete into Mercury was blown away by T Tauri winds. 135 00:10:41,825 --> 00:10:46,913 Planets do not generate T Tauri winds. So whatever accreted near the planet, too 136 00:10:46,913 --> 00:10:51,178 close to form a star a moon will produce ring like structures. 137 00:10:51,178 --> 00:10:54,180 And they have ring structures around Jupiter, 138 00:10:54,180 --> 00:10:57,782 and ring structures around Uranus, and ring Neptune, 139 00:10:57,782 --> 00:11:02,451 and ring structures around Uranus. And the beautiful magnificent rings 140 00:11:02,451 --> 00:11:06,587 around Saturn. for reasons nobody completely understands 141 00:11:06,587 --> 00:11:11,724 the rings around all of the three giants other than Saturn are dusty and dim. 142 00:11:11,724 --> 00:11:15,260 Whereas, with Saturns rings are almost 99.9% water ice. 143 00:11:15,260 --> 00:11:20,068 Water ice, as we know, is brilliant and shiny which is why the rings appear so 144 00:11:20,068 --> 00:11:23,190 bright. we see here that the rings are discrete, 145 00:11:23,190 --> 00:11:27,873 the large gap here, the Cassini gap, is visible in a moderate telescope from 146 00:11:27,873 --> 00:11:30,808 Earth. The slightly smaller gap farther out, the 147 00:11:30,808 --> 00:11:33,992 Enke gap, is visible with a more advanced telescope. 148 00:11:33,992 --> 00:11:38,925 But upon closer inspection, it turns out that these rings have a very, very fine 149 00:11:38,925 --> 00:11:42,110 internal structure. We'll get into that in a second. 150 00:11:42,110 --> 00:11:47,094 Let's understand what it is that I'm saying when I say that too close to a 151 00:11:47,094 --> 00:11:51,120 planet a moon cannot form. the limiting distance, the closest 152 00:11:51,120 --> 00:11:54,698 distance at which it can form is called the Roche Limit. 153 00:11:54,698 --> 00:11:59,427 it was computed in 1848 by Roche. And the idea is this, we have a planet 154 00:11:59,427 --> 00:12:02,558 over here, we're trying to form a moon over there. 155 00:12:02,558 --> 00:12:07,542 We will say a moon can accrete and form provided that the tidal forces of the 156 00:12:07,542 --> 00:12:12,544 planet are not enough to blow it apart. So we've done enough to do this 157 00:12:12,544 --> 00:12:16,574 calculation that Roche did. We give this a mass and planet. 158 00:12:16,574 --> 00:12:20,535 We give this a mass and moon. Set them a distance D apart. 159 00:12:20,535 --> 00:12:25,260 And the question is, basically, this moon is held together by gravity. 160 00:12:25,260 --> 00:12:30,468 Is the moons gravity enough to hold it together against the tidal forces? If we 161 00:12:30,468 --> 00:12:35,546 drop a pebble on the surface of moon, will it be lifted by tidal forces or not? 162 00:12:35,546 --> 00:12:40,950 And so, we know how to do this. we need to compare the tidal acceleration due to 163 00:12:40,950 --> 00:12:45,930 the planet's tidal forces on the moon. We computed this in the past. 164 00:12:45,930 --> 00:12:51,929 It's 2 * G times the mass of the planet, times the radius of the moon divided by 165 00:12:51,929 --> 00:12:55,864 the distance cubed. This is the tidal acceleration with which 166 00:12:55,864 --> 00:13:00,949 the planet tries to stretch the moon as we have it here in the horizontal 167 00:13:00,949 --> 00:13:03,961 direction. And we need to compare this to the 168 00:13:03,961 --> 00:13:08,580 gravitational acceleration due to the moon's own mass at its surface. 169 00:13:08,580 --> 00:13:13,934 And this, of course is G times the mass of the moon divided by the radius of the 170 00:13:13,934 --> 00:13:17,548 moon squared. As D gets smaller, the tidal acceleration 171 00:13:17,548 --> 00:13:20,700 increases. When it overwhelms the moon's own 172 00:13:20,700 --> 00:13:26,348 gravity, the moon will simply fall apart as long as it's gravitationally bound. Or 173 00:13:26,348 --> 00:13:31,241 if we're trying to form a moon by gravitationally binding moonitesimals, 174 00:13:31,241 --> 00:13:35,672 the moon will never form. And we'll end up as we'll see with ring 175 00:13:35,672 --> 00:13:38,865 structures. So, let's write these two and set them 176 00:13:38,865 --> 00:13:41,666 together. This is setting the gravitational 177 00:13:41,666 --> 00:13:46,879 acceleration on the moon's surface equal to the tidal force generated by the 178 00:13:46,879 --> 00:13:50,658 nearby planet. dividing through by R and canceling the 179 00:13:50,658 --> 00:13:54,046 Gs as usual I'm going to put this down over here. 180 00:13:54,046 --> 00:13:59,064 And I find that I have this interesting expression that says, the mass of the 181 00:13:59,064 --> 00:14:05,917 moon divided by its radius cubed is equal to twice the mass of the planet divided 182 00:14:05,917 --> 00:14:10,369 by the distance cubed. Which it's interesting to write in a 183 00:14:10,369 --> 00:14:14,519 different way. You write the mass of the planet divided 184 00:14:14,519 --> 00:14:19,377 by the distance cubed. But then, you divide and multiply by the 185 00:14:19,377 --> 00:14:24,962 radius of the planet cubed. Now, why is this useful way to write it? 186 00:14:24,962 --> 00:14:29,643 Because the cube of the radius is a measure of volume. 187 00:14:29,643 --> 00:14:34,981 Remember that the volume of a ball is four pi over three R cubed. 188 00:14:34,981 --> 00:14:41,633 And that if I know the average density of an object, then its mass is the density 189 00:14:41,633 --> 00:14:45,882 times the volume. So, dividing the mass by R cubed up to 190 00:14:45,882 --> 00:14:51,738 irrelevant cancelling factors of four pi over three gives me a measure of the 191 00:14:51,738 --> 00:14:55,567 density. In other words what this equation tells 192 00:14:55,567 --> 00:15:01,798 me is that the density of the moon is twice the density of the planet times the 193 00:15:01,798 --> 00:15:07,127 ratio between the cube of the ratio between the radius of the planet and the 194 00:15:07,127 --> 00:15:12,791 distance between them, when that distance is at the Roche limit. When tidal forces 195 00:15:12,791 --> 00:15:18,185 are just enough to rip the planet apart. In other words, I can now solve for D in 196 00:15:18,185 --> 00:15:21,826 terms of the densities and the solution that I get. 197 00:15:21,826 --> 00:15:24,658 So, here's the expression written that way. 198 00:15:24,658 --> 00:15:30,600 Again, I I rewrite things in terms of the densities, and this says that the density 199 00:15:30,600 --> 00:15:37,236 of the moon is twice the density of the planet, times the radius of the planet 200 00:15:37,236 --> 00:15:42,043 divided by D cubed. Now, multiplying through by D cubed 201 00:15:42,043 --> 00:15:47,954 moving dividing by row M, I finally find and then taking a cubed route to get my 202 00:15:47,954 --> 00:15:53,652 answer, I find the fearsome equation on our splash page that says that the Roche 203 00:15:53,652 --> 00:15:58,851 limit, the dist, closest distance at which a moon can survive, that's D, the 204 00:15:58,851 --> 00:16:04,620 Roche distance, is given by the radius of the planet times the, the 1/3rd power of 205 00:16:04,620 --> 00:16:09,036 the ratio of twice the planet's density to the moon's density. 206 00:16:09,036 --> 00:16:15,812 to make a reasonable calculation let's assume for example, that we're describing 207 00:16:15,812 --> 00:16:19,875 Earth and Earth's moon. What is the nearest distance to Earth 208 00:16:19,875 --> 00:16:23,219 that our moon could have possibly have, have formed? 209 00:16:23,219 --> 00:16:27,939 Remember, it formed much nearer. Well, let's give the moon half of Earth's 210 00:16:27,939 --> 00:16:31,217 density. So twice the Earth density divided by the 211 00:16:31,217 --> 00:16:36,397 moon's density would be about four. four to the one third is about one sixth. 212 00:16:36,397 --> 00:16:40,133 So in the case of a planet twice as dense as its moon, 213 00:16:40,133 --> 00:16:45,509 you find that the minimum distance is about 1.6 times the radius of the planet 214 00:16:45,509 --> 00:16:50,606 if a rogue planet is twice real moon, which is the case for our Earth, our 215 00:16:50,606 --> 00:16:53,321 moon. And so, our moon could not have formed 216 00:16:53,321 --> 00:16:56,353 less than about two Earth radii from the center. 217 00:16:56,353 --> 00:17:00,648 In other words, about 7,000 kilometers away from the Earth's surface. 218 00:17:00,648 --> 00:17:05,385 nearer than that, our moon would have been ripped apart by Earth's tidal 219 00:17:05,385 --> 00:17:06,270 forces. And 220 00:17:06,270 --> 00:17:10,785 this is a very useful expression. Objects inside the Roche radius cannot 221 00:17:10,785 --> 00:17:15,614 possibly be gravitationally bound. Now, that does not mean that they are 222 00:17:15,614 --> 00:17:20,631 completely ground to dust at some point. In the solar system, the, the rough scale 223 00:17:20,631 --> 00:17:24,456 is about a kilometer. We will do a homework problem that will 224 00:17:24,456 --> 00:17:28,784 help us understand how that happens. At some small scale, remember the 225 00:17:28,784 --> 00:17:31,543 planetesimals, were held together chemically. 226 00:17:31,543 --> 00:17:34,992 Smaller things are held together by chemical forces. 227 00:17:34,992 --> 00:17:39,320 They at small distances are able to withstand gravitational 228 00:17:39,320 --> 00:17:44,334 tidal forces. But gravitationally bound objects will be dispersed over time. 229 00:17:44,334 --> 00:17:49,749 And so, the way that this will work is if you bring a moon to within the Roche 230 00:17:49,749 --> 00:17:54,963 radius which is indicated by this white ring, tidal forces will elongate it. 231 00:17:54,963 --> 00:17:59,510 And when it crosses the Roche radius it will start to fall apart. 232 00:17:59,510 --> 00:18:04,463 remember it's being extended so parts near to the planet will fall towards the 233 00:18:04,463 --> 00:18:07,362 planet, parts far from the planet will fall away 234 00:18:07,362 --> 00:18:11,349 from the planet. And because they're all moving, imagine 235 00:18:11,349 --> 00:18:14,672 this thing was in orbit, all of the planet was, the moon was 236 00:18:14,672 --> 00:18:19,504 moving with the same orbital velocity. the bits that are falling too close to 237 00:18:19,504 --> 00:18:24,035 the planet are now moving at a speed too small for their now lower orbit so they 238 00:18:24,035 --> 00:18:28,741 will fall into eliptical orbits with this as [UNKNOWN]. 239 00:18:28,741 --> 00:18:35,002 And the bits that are falling away from the planet will be moving so slow, so 240 00:18:35,002 --> 00:18:40,449 they will fall into elliptical orbits. And when everything settles down into 241 00:18:40,449 --> 00:18:44,942 circular orbits the guys that have, that fell towards the planet will have 242 00:18:44,942 --> 00:18:48,152 advanced ahead. The guys that fell into the planet, away 243 00:18:48,152 --> 00:18:50,661 from the planet will have been left behind. 244 00:18:50,661 --> 00:18:54,279 And basically, the whole structure is smeared out along a ring. 245 00:18:54,279 --> 00:18:58,890 We'll see this happening again and again when tidal forces rip something apart. 246 00:18:58,890 --> 00:19:02,760 They generate that is in orbit, they generate a ring structure. 247 00:19:02,760 --> 00:19:07,393 So this is where the rings could have come from or why the rings formed but no 248 00:19:07,393 --> 00:19:12,078 moons. But stability over the long term of such a ring structure, you'd expect 249 00:19:12,078 --> 00:19:15,591 collisions between ring particles, you'd expect that mixture. 250 00:19:15,591 --> 00:19:20,392 looking at Saturn's rings, we see that there are changes in composition between 251 00:19:20,392 --> 00:19:25,366 thousands and thousands of ringlets. it's not completely understood what 252 00:19:25,366 --> 00:19:29,922 maintains the stability of this amazing structure. 253 00:19:29,922 --> 00:19:35,140 for the record, Saturn's rings are about 150,000 kilometers in width. 254 00:19:35,140 --> 00:19:38,715 but by some measures, only 400 metres in thickness. 255 00:19:38,715 --> 00:19:43,707 This is an extremely thin object, precisely on the equatorial plain and 256 00:19:43,707 --> 00:19:48,632 made up of essentially of chunks of water ice. But, with various additives or 257 00:19:48,632 --> 00:19:53,894 various trace amounts of other materials so that they have slightly different 258 00:19:53,894 --> 00:19:56,930 properties in the various ringlets. And 259 00:19:56,930 --> 00:20:01,477 one of the things that keep maintains this, helps maintain this structure is 260 00:20:01,477 --> 00:20:06,264 the existence of moons that orbit both within the ring system and outside it 261 00:20:06,264 --> 00:20:10,871 they produce these gaps, these gaps that are marked here, are gaps at which 262 00:20:10,871 --> 00:20:15,120 objects orbiting in the ring would be in resident orbits with the moon. 263 00:20:15,120 --> 00:20:19,483 Just as in the asteroid belt, this would cause them to be moved into eccentric 264 00:20:19,483 --> 00:20:23,512 objects and essentially ejected. So, nothing orbits in the gaps and this 265 00:20:23,512 --> 00:20:27,484 separates ringlets from each other. Moreover, there are moons that orbit 266 00:20:27,484 --> 00:20:31,344 inside the rings, inside the Roche limit where a moon cannot exist. 267 00:20:31,344 --> 00:20:35,429 Indeed, those moons are slowly being ripped apart and perhaps resupplying 268 00:20:35,429 --> 00:20:38,954 material to the rings as the material is being lost to 269 00:20:38,954 --> 00:20:44,180 collisions. And, these, these objects are both very beautiful and very intriguing, 270 00:20:44,180 --> 00:20:48,259 and I don't think we completely understand what maintains their 271 00:20:48,259 --> 00:20:53,040 structure, but they're certainly a generic property of accretion systems.