PBS Space Time

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 liquidlike.

So, is the inner core solid or liquid?

The answer is yes.

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 lightyears to the cosmic horizon are largely transparent.

We can see the most distant galaxies.

But the 6,400 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's special relativity that anyone realized that seismic waves could teach us about Earth's structure.

It was Andrea Maharov Vich that noticed that some of the Pwaves, that's primary or pressure waves, found a shortcut from the earthquakes 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 Beno Gutenberg who found that swaves for secondary or shear were blocked across a broad shadow region on the opposite side of the planet to their origin.

Well, 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 Ingga Leman discovered the inner solid core.

She found that some Pwaves 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 lower 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 wreathing a solid inner core both mostly of iron.

It all hangs together neatly explaining our dynamic planet from its vulcanism 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, Pwaves 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 latice.

If Earth's inner core is crystalline iron and the latis 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 Pwaves.

And it's actually wild that we can even identify surface seismic waves that have gone through a PS then SP transition through Earth's core, then back to Pwaves to return to the surface.

It's actually wild that we can even identify surface waves that have gone through this pressure to shear, then shear to pressure through Earth's inner core.

But we do find that these core sheer 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 geoysics.

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.

An 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 sheer ability to compressibility of a material is encapsulated by the pson's ratio which is usually around 2 or.3 for a typical solid.

But earth's core seems to be at around 0.45 which is close to the pson's ratio for rubber.

So not very compressible but very shearable.

Let's call such a material squidgy.

That's not a technical term so don't use it on any geoysicists.

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 squidgginess.

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 pson'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 lice grows from multiple nucleation points, resulting in many misaligned sectors.

Over time, these grains can merge and align to produce larger lises 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 lack the strength of the pure lattice, they're more able to slide against each other.

And this is greatly amplified if microscopic membranes of molten or heat softened metal form between those grains.

This granularization and melt or premelt can potentially explain the high pson ratio in the core.

But the challenge with this explanation is that there's only a narrow window of grain size plus melt or premelt 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 latis alignment to explain the polar versus equatorial speed difference.

Most likely, there is some of this granularization, 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 latice 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 super ionic state.

In this state, we have a rigid crystal lattice of some element or 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 liquidlike.

Now, iron doesn't do this in normal circumstances.

At low temperature, impurities like carbon tend to stay at fixed positions in the lice.

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 inner core of the Earth creates the perfect conditions for the iron carbon lattice to enter this super ionic state.

Even better, in these simulations, the alloy shows a lower sheer velocity than pure iron and a pson's ratio of around43, 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 Hang Zang Itel comes in.

This team created a hexagonal closepacked 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 superionic state.

Now the theme of this episode seems to be sound speed in different materials.

So fun 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 mass.

For air, that's basically di nitrogen.

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, micrometeoroids on space-based hardware.

But in this experiment, the light gas gun is giving us a sort of particle accelerator for geology.

Anyway, end tangent mode.

And let's get back to the experiment.

Once the super ionic 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 volymmetry, which is basically pointing a super precise speed gun at the surface.

Now those vibrations revealed the structure of the interior of the sample just as with seismology.

In this way the team could infer various properties including the sheer velocity and the pson'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 superionic state produced exhibits strong sheer softening consistent with seismic data that strengthens its viability as a candidate for the unusual squidgginess of Earth's inner core.

If the superionic hypothesis turns 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 geodynamo 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 into 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 geohysical spacetime.