WEBVTT 00:00.160 --> 00:05.040 First we discovered Earth's liquid core.  Then we discovered a solid core within 00:05.040 --> 00:09.096 that liquid core. Then we discovered that  the solid core has bizarre properties that 00:09.096 --> 00:15.909 are more liquid like. So is the inner  core solid or liquid? The answer is yes. 00:12.667 --> 00:18.107 Links in the description. Now onto the  episode. And no. It may be that Earth's 00:18.107 --> 00:25.147 inner core is in an exotic state of matter  that is simultaneously crystalline and fluid. 00:25.147 --> 00:31.067 The super ionic state the mysteries of  our own planet's interior have in many 00:31.067 --> 00:35.947 ways been harder to crack than  those of the rest of the cosmos. 00:35.947 --> 00:41.067 I mean, we can send probes to the edge of the  solar system and the 46 billion light years to 00:41.067 --> 00:47.067 the cosmic horizon, a largely transparent.  And we can see the most distant galaxies. 00:47.067 --> 00:51.627 But the 6400 km to Earth's center are both 00:51.627 --> 00:55.867 opaque to light and far beyond the  reach of any conventional drill. 00:55.867 --> 01:02.107 The best we can do is to listen to the  faint rumblings of distant earthquakes, 01:02.107 --> 01:07.627 and then try to piece together how those seismic  waves bounce around Earth's interior. To give 01:07.627 --> 01:14.027 you an idea of how seismology has lagged  cosmology and the rest of physics, 01:14.027 --> 01:20.587 it wasn't until 1909, after Einstein, special  relativity, that anyone realized that seismic 01:20.587 --> 01:26.987 waves could teach us about Earth's structure. It  was Andrija Mohorovičić that noticed that some 01:26.987 --> 01:35.547 of the P waves that's primary or pressure waves,  found a shortcut from the earthquake's epicenter 01:35.547 --> 01:42.267 to seismic stations hundreds of kilometers away.  He brilliantly inferred that the waves traveled 01:42.267 --> 01:49.147 through a denser and hence higher velocity region  deeper in the Earth, thereby discovering the crust 01:49.147 --> 01:56.027 mantle boundary. That opened the floodgates.  In 1914, just a year before general relativity 01:56.027 --> 02:01.787 and the following prediction of black holes,  we discovered Earth's molten core. That was 02:01.787 --> 02:08.507 by Benno Gutenberg, who found that S-waves,  for secondary or shear, were blocked across 02:08.507 --> 02:13.147 a broad shadow region on the opposite  side of the planet to their origin while 02:13.147 --> 02:19.307 the P-waves could make it straight through.  Shear waves only propagate through solids. 02:19.307 --> 02:25.227 So he realized that this meant there  must be a liquid interior to the Earth. 02:25.227 --> 02:32.267 But even those P waves play funny around the  molten core, refracting to create smaller shadow 02:32.267 --> 02:40.027 zones. Taking this game one level deeper. It was  in 1936, long after we discovered the expansion 02:40.027 --> 02:46.347 of the universe, and inferred the Big Bang  that Inge Lehmann discovered the inner solid 02:46.347 --> 02:52.987 core. She found that some P waves did enter  their shadow zones, and brilliantly figured 02:52.987 --> 02:59.627 out that these must be reflecting off a solid  region even deeper than the molten interior. 02:59.627 --> 03:05.787 Well, better late than never. These ideas  have been refined and verified over the years, 03:05.787 --> 03:11.787 leading to our modern, multi-layered model  of Earth's interior. Our planet is an 03:11.787 --> 03:18.347 oblate spheroid with a low density crust that  carries slow P-waves and even slower S-waves, 03:18.347 --> 03:23.307 followed by a denser, higher  wave speed rocky mantle that 03:23.307 --> 03:28.907 slowly flows even though it's still solid,  allowing it to support both wave types. 03:28.907 --> 03:33.067 Then we have the liquid outer  core breathing a solid inner core, 03:33.067 --> 03:39.467 both mostly of iron. It all hangs together  neatly, explaining our dynamic planet, 03:39.467 --> 03:45.947 from its volcanism to its tectonics to its  magnetic field. For all intents and purposes, 03:45.947 --> 03:52.907 it's right. So what do we do when new,  even more refined seismic data disagrees 03:52.907 --> 03:58.267 with this model? As our seismic monitoring  became more sensitive and more widespread, 03:58.267 --> 04:03.947 we started to probe the inner structure in more  detail, and we started to see some glitches. 04:03.947 --> 04:09.147 For example, P-waves pass through  the inner core faster in the polar 04:09.147 --> 04:14.187 direction than the equatorial direction.  This is typically explained by the fact 04:14.187 --> 04:20.907 that sound speed in a crystal can depend on the  orientation relative to the crystal's lattice. 04:20.907 --> 04:27.067 If Earth's inner core is crystalline iron  and the lattice orientation correlates 04:27.067 --> 04:32.907 with planetary spin, then the polar  equatorial difference can be explained. 04:32.907 --> 04:38.107 There even seems to be an east west  hemispheric asymmetry in wave speed, 04:38.107 --> 04:43.387 and that's taken as evidence of large scale  lumpiness and or melt regions in the inner core. 04:43.387 --> 04:50.427 One of the hardest things to explain is really  deep in the weeds of the data, but it may reveal 04:50.427 --> 04:56.587 something really startling about the heart  of our planet. S-waves, which are completely 04:56.587 --> 05:03.307 blocked by the molten outer core, do arise in  the inner core through conversion of P-waves, 05:03.307 --> 05:10.667 and it's actually wild that we can even identify  surface seismic waves that have gone through a 05:10.667 --> 05:18.507 P-S, then S-P transition through Earth's core,  then back to P-waves to return to the surface. 05:18.507 --> 05:24.187 But we do find that these core shear  waves travel way too slowly in that 05:24.187 --> 05:28.347 core compared to what we expect,  given the core's composition. 05:28.347 --> 05:35.147 S-waves in the core are also losing energy  much more quickly as they travel than is 05:35.147 --> 05:41.787 expected for a stiff material like crystalline  iron. Now, this inner core S-wave thing is 05:41.787 --> 05:47.147 a real current mystery, and to figure it  out, we're going to need some geophysics. 05:47.147 --> 05:53.867 There are two main ways to deform a solid  compression/expansion. So changing the volume. And 05:53.867 --> 06:00.747 that's what P-waves do. Or shearing changing the  shape. And that's our S-waves. The more resistant 06:00.747 --> 06:07.067 a material is to either of these, the faster that  material will propagate waves in that mode. Think 06:07.067 --> 06:13.227 about it as a more easily deformable material,  being sort of soft and sludgy in that mode. 06:13.227 --> 06:20.907 Crystalline iron is very stiff to both compression  and shear, and so should have fast P and S waves. 06:20.907 --> 06:26.827 And Earth's core does transmit P waves as fast  as we expect. But the S waves are too slow. 06:27.387 --> 06:32.027 To put numbers on this. The relative  shear-ability to compressibility of a 06:32.027 --> 06:39.467 material is encapsulated by the Poisson's  ratio, which is usually around 0.2 or 0.3 06:39.467 --> 06:45.467 for a typical solid. But Earth's  core seems to be at around 0.45, 06:45.467 --> 06:50.587 which is close to the Poisson's ratio  for rubber. So not very compressible, 06:50.587 --> 06:57.787 but very shareable. Let's call such a  material squidgy. That's not a technical term, 06:57.787 --> 07:06.587 so don't use it on any geophysicists. So what  sort of core material could be squidgy like this? 07:06.587 --> 07:12.267 Well we're going to stick with iron in general  because we know that this matches the density. 07:12.267 --> 07:18.267 And because we see that asteroids do develop  these iron cores as the heavy metal drips to 07:18.267 --> 07:20.667 the center during cooling. But there are various 07:20.667 --> 07:24.267 things that could increase  the cores's squidginess. 07:24.267 --> 07:28.827 The first idea people had was to  just add other stuff to the crystal 07:28.827 --> 07:33.467 lattice. Alloying iron crystal  with hydrogen, carbon, oxygen, 07:33.467 --> 07:40.187 silicon sulfur will significantly increase  the poisson's ratio and so lower S-wave speed. 07:40.187 --> 07:45.067 But it's now generally believed that all by  itself, just alloying with any reasonable 07:45.067 --> 07:50.747 amount of these light elements could not  reduce S-wave speed by enough. Another 07:50.747 --> 07:57.307 possibility is to make the core grainy. Rather  than one giant spherical crystal, the lattice 07:57.307 --> 08:03.307 of the core could be fragmented, potentially on  microscopic scales. When molten metal solidifies, 08:03.307 --> 08:10.027 its crystal lattice grows from multiple nucleation  points, resulting in many misaligned sectors. 08:10.027 --> 08:16.667 Over time, these grains can merge and align to  produce larger lattices or even fragment further, 08:16.667 --> 08:21.067 and it's not clear what we should  expect to have happened in Earth's core. 08:21.067 --> 08:25.867 Because the boundaries between grains  lacked the strength of the pure lattice. 08:25.867 --> 08:30.827 The more able to slide against each  other, and this is greatly amplified 08:30.827 --> 08:37.787 if microscopic membranes of molten or heat  softened metal formed between those grains. 08:37.787 --> 08:43.627 This granular ization and melts or pre-built  can potentially explain the high Poisson's 08:43.627 --> 08:48.827 ratio in the core. But the challenge with  this explanation is that there's only a 08:48.827 --> 08:54.187 narrow window of grain size, plus melt or  pre-melt that gives us the right core properties 08:54.187 --> 09:01.147 to explain S-wave propagation, push it too  far and the core goes from squidgy to goopy, 09:01.147 --> 09:06.907 meaning that's waves lose way too much  energy compared to what we observe. 09:06.907 --> 09:13.627 There's also the issue that fine grained crystal  structures have a harder time organizing globally. 09:13.627 --> 09:19.067 And remember, we need some level of global  lattice alignment to explain the polar versus 09:19.067 --> 09:24.107 equatorial speed difference. Most likely  there is some of these granular ization, 09:24.107 --> 09:29.547 etc. going on, but there are strong arguments  that they aren't the whole explanation. We're 09:29.547 --> 09:35.547 looking for a way to retain the general rigidity  of the core, potentially allowing a global crystal 09:35.547 --> 09:42.427 lattice alignment, but to still lubricate  things a bit to slow down them S-waves. 09:42.427 --> 09:47.147 What we need is a new state of matter, a  state of matter that allows the core to be 09:47.147 --> 09:55.067 simultaneously solid and liquid. And so let me  introduce the superionic state. In this state, 09:55.067 --> 10:00.827 we have a rigid crystal lattice of some element  to a molecule with some other atom moving 10:00.827 --> 10:06.987 freely within this structure. There are lots of  examples of superionic materials with various 10:06.987 --> 10:11.707 technological applications, from batteries  and fuel cells to various types of sensor. 10:11.707 --> 10:18.107 We've even observed superionic ice in which  hydrogen moves freely within an oxygen lattice. 10:18.107 --> 10:23.387 In the case of Earth's core, the primary  lattice would be the hexagonal lattice, 10:23.387 --> 10:28.427 formed by iron alloyed with nickel.  Lighter elements like hydrogen, oxygen, 10:28.427 --> 10:34.907 and most notably carbon, could then live  in the interstitial spaces of that lattice, 10:34.907 --> 10:37.467 moving with a freedom that is liquid like. 10:37.467 --> 10:41.227 Now, iron doesn't do this in normal  circumstances. At low temperature, 10:41.227 --> 10:46.187 impurities like carbon tend to stay  at fixed positions in the lattice, 10:46.187 --> 10:52.107 but molecular dynamic simulations have  shown that as temperatures increase, 10:52.107 --> 10:58.107 carbon atoms begin to move between interstitial  sites and above a certain temperature, they 10:58.107 --> 11:04.507 exhibit liquid like behavior. According to these  simulations, the high pressure in a core of the 11:04.507 --> 11:12.907 Earth creates the perfect conditions for the iron  carbon lattice to enter this superionic state. 11:12.907 --> 11:20.187 Even better, in these simulations, the alloy  shows a lower shear velocity than pure iron, 11:20.187 --> 11:25.867 and a Poisson's ratio of around 0.43, very  close to the observed seismic properties 11:25.867 --> 11:31.227 of the real inner core. Simulations are  great because they can suggest and sharpen 11:31.227 --> 11:35.947 hypotheses that we can then go and test in  the real world. Now it's going to be a long, 11:35.947 --> 11:40.347 long time before we can access  Earth's core to do direct tests. 11:40.347 --> 11:45.307 So in the meantime, we need to recreate  the conditions of the core in our labs. 11:45.307 --> 11:51.467 And this is where the new study by Huang,  He, Zhang et al comes in. This team created 11:51.467 --> 11:55.867 a hexagonal close packed iron lattice  with a small amount of carbon dissolved 11:55.867 --> 12:01.227 interstitially. Then they smacked it with  particles traveling at high speed to create 12:01.227 --> 12:07.147 shock pressures and corresponding temperatures  high enough to unlock the super ionic state. 12:07.147 --> 12:13.387 Now, the theme of this episode seems to be sound  speed in different materials. So one side note 12:13.387 --> 12:18.507 the projectile in this experiment  was accelerated by a light gas gun. 12:18.507 --> 12:23.707 The projectile velocity in a firearm is  constrained by the speed of sound. Just 12:23.707 --> 12:27.867 because a bullet is pushed out  of the nozzle by expanding gas, 12:27.867 --> 12:34.987 and that expansion is limited by sound speed,  but sound speed increases with molecular mess. 12:34.987 --> 12:40.747 For air, that's basically dinitrogen. If  instead you fill the chamber with hydrogen or 12:40.747 --> 12:47.307 helium, then a higher sound speed behind the  bullet results in a faster muzzle velocity, 12:47.307 --> 12:52.667 where normal firearms can achieve  muzzle velocities of Mach a few. A 12:52.667 --> 12:59.947 light gas projectile can hit Mach 20 plus or  actual orbital velocities. Because of that, 12:59.947 --> 13:06.267 they're often used to study the impacts of,  say, micro meteoroids on space based hardware. 13:06.267 --> 13:11.707 But in this experiment, the light gas gun is  giving us a sort of particle accelerator for 13:11.707 --> 13:17.867 geology. Anyway, ends tangent mode. And let's  get back to the experiment. Once the supersonic 13:17.867 --> 13:22.427 state was created in the iron lattice, its  properties were studied similar to how we 13:22.427 --> 13:27.787 study Earth's interior by looking at vibrations  on the surface. In this case, with photon Doppler 13:27.787 --> 13:33.547 velocimetry, which is basically pointing  a super precise speed gun at the surface, 13:33.547 --> 13:40.987 now those vibrations reveal the structure of the  interior of the sample, just as with seismology. 13:40.987 --> 13:46.107 In this way, the team could infer various  properties, including the shear velocity and 13:46.107 --> 13:52.507 the Poisson's ratio, and these values were  consistent with what we expect from super 13:52.507 --> 13:58.347 ionic iron. As predicted by simulations. It's  worth mentioning that this experiment didn't 13:58.347 --> 14:03.387 achieve the full pressure and temperature of  Earth's inner core, but at the very least, 14:03.387 --> 14:08.907 we seem to have produced this strange  phase of matter in an iron carbon alloy, 14:08.907 --> 14:16.507 which is an experimental validation of a mechanism  that had previously existed only in simulation, 14:16.507 --> 14:24.347 and the supersonic state produced exhibits strong  shear softening consistent with seismic data that 14:24.347 --> 14:30.187 strengthens its viability as a candidate for  the unusual "squidginess" of Earth's inner core. 14:30.187 --> 14:35.627 If the superironic hypothesis turned out to  be true, it may help explain other things. 14:35.627 --> 14:39.707 For example, preferential flow of  the interstitial carbon along Earth's 14:39.707 --> 14:44.747 rotational axis could partially account  for the polar equatorial speed difference, 14:44.747 --> 14:50.347 which loosens the constraints on global  lattice alignment, and the flow of this 14:50.347 --> 14:55.307 carbon may even participate in the geo-dynamo  effect that powers Earth's magnetic field. 14:55.307 --> 15:00.027 And we should also consider this a victory  for the philosophy at the heart of all of 15:00.027 --> 15:06.747 physics that the secrets of nature are best  revealed by smashing things to other things 15:06.747 --> 15:13.147 really hard. Earth's interior below us  is in many ways harder to explore than 15:13.147 --> 15:18.267 the vast cosmos above. Fortunately,  the secrets of both are encoded in 15:18.267 --> 15:23.547 vibrations that reach the narrow sliver  of our human world from both directions. 15:23.547 --> 15:48.459 And so we build our maps of both  astrophysical and geophysical space time. 15:23.547 --> 15:28.827 vibrations that reach the narrow sliver  of our human world from both directions. 15:23.547 --> 15:48.459 And so we build our maps of both  astrophysical and geophysical space time.