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