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Comment by nerdponx | Hacker News Reader

Wait, I definitely remember reading about this in science magazines years ago: the earth's core is more like solid than liquid because it's under tremendous pressure from gravity, and it's spinning, and it's iron, which is why Earth has a magnetic field.

Is the new discovery that there is actually a fifth layer, which is solid, and that what we previously thought of as the "core" is actually liquid and not the innermost layer?

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It's been known for decades that the Earth's core (which is mostly iron and nickel, as opposed to the mantle which is made of silicates) has a liquid outer layer and a solid inner layer. This is apparent because seismic shear waves can propagate through the inner layer but not the outer layer, which is diagnostic of the medium being liquid and not solid. (The shear or S-waves get to the inner core because when a seismic pressure wave, which can travel through liquid, hits the inner-core-outer-core boundary, some of the energy is converted into shear waves. Jofer please correct me if this is inaccurate.)

This article states that the inner core has two different concentric zones that have very different crystalline properties. In particular, the inner sanctum is anisotropic with respect to seismic wave propagation, and the outer sheath of the inner core is much more isotropic. This suggest that they have different geologic histories--the innermost core's crystallographic anisotropy may have resulted from an event early in Earth's history that deformed the crystals, much as mountain building events deform the crystals in Earth's crust and upper mantle. Then, the younger outer sheath of the inner core cooled and crystallized around the older inner bit later on, after the deforming event transpired.

Can you explain a bit more about the difference between the anisotropic and isotopic nature of these different zones?

Are both of these zones “solid” states of matter? Or does our intuition about states of matter not really work for materials at these exotic temperatures and pressures?

No, the Earth's magnetic field isn't because of iron being ferromagnetic, nor does it come from the solid inner core. The iron in the core is well above the Curie temperature at which ferromagnetism ceases.

Rather, the Earth's magnetic field is due to currents flowing in the liquid outer core. Convection in the core drives the conductive metal across these currents, increasing them by a dynamo effect. This process occurs on a rather small scale and is difficult to model, and is also somewhat chaotic, leading to very occasional collapses and flips of the Earth's magnetic field.

A flip in the magnetic field? Has something like that ever happened in history or is ever predicted to happen? What would the damage be?

It happens on a geological timescale, and is used to help date rocks. The ocean floor is marked with stripes of normal and reversed magnetic polarity (the magnetic field is faintly recorded in magnetization of minerals like magnetite), and this was a strong initial clue that led to the theory of seafloor spreading and plate tectonics.

https://en.wikipedia.org/wiki/Geomagnetic_reversal

https://www.usgs.gov/media/videos/pubtalk-72004-secrets-ston...

You're in for a treat. Our magnetic field is currently doing some crazy stuff and moving quite fast.

https://upload.wikimedia.org/wikipedia/commons/thumb/7/74/Ma...

I would love to read the primary sources at the time describing peoples reactions to the movement of magnetic north. Were they surprised?

We are talking about thousands of years for the flip to happen, and hundreds of thousands of years between each flip. We have geological records that it happened, as for the damage, there was a theory that it was linked to mass extinction, but latest research find it unlikely.

It would still be bad for us, but not something worth worrying about right now.

Question: Do you mean current as in movement of material, or current as in movement of electric fields?

Or is that not a valid distinction in this case?

Both meanings were used here. There are electrical currents, and there are flows of material through the magnetic field, producing voltages that sustain currents. The JxB forces also affect the flows. It's a highly nonlinear phenomenon (magnetohydronamics, or MHD).

Magnetohydrodynamics (spelling)

Magnetotellurics (cool geophysical passive imaging technique)

That's a bit different; it doesn't have the feedback where the JxB force pushes matter around, altering the flow.

This reminds me of a paradox in my understanding of gravity. I often read about the pressure inside the earth resulting from the mass above, but the gravitational force inside a spherical shell is zero [0], and the only force acting at a given radius from the center is the mass of the sphere inside that radius.

[0]http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/sphshel...

I believe the apparent paradox is that the 'net force' is not the same thing as pressure; it's basically the vector sum of pressure. Pressure in the Earth is an isotropic distribution of stresses, so that in a north-east-up coordinate system, the upward-pushing stress is matched by the downward-pushing stress, the eastward-pushing stress is matched by the westward-pushing stress, etc. So while these sum to zero in some sense, their magnitude is the density of the rock between the point and the surface times the depth times gravity. This is like going deeper in the ocean-the forces are equal in all directions, meaning that they sum to zero in some sense and there is no directional flattening or translation, but they still increase with depth.

> I believe the apparent paradox is that the 'net force' is not the same thing as pressure

I don't think that's what OP means. He's saying that the gravitational force that acts on a given test particle that's located inside Earth at a radius r=R from the center is determined only by the amount of mass "below it", i.e. the mass inside the ball of radius R, not the matter making up the spherical shell R < r < R_E, where R_E is Earth's radius. Put differently, that (hollow) spherical shell does not cause any gravitational force on test particles inside it.

This is absolutely correct and a consequence of Gauß's law, see https://en.wikipedia.org/wiki/Gauss%27s_law_for_gravity

Meanwhile, you are talking about a different thing entirely: The pressure. The matter above (as well as below and next to) our test particle will obviously also experience gravity and get pulled down. So our test particle will experience an isotropic force from all sides, which is generally quantified as pressure (force per area). This pressure is obviously not zero (but no one ever claimed that) and does depend on the radius of the outer spherical shell above it, in the same way as the pressure under water depends on how deep you are.

The net force is 0 in the same way that if two people are pushing you from opposite sides, their net force on you is also zero. You're still being crushed from both sides.

This is a confusion between the gravitational force that the core feels from the layers above, and the pressure that the layers above push down onto the core.

That is, the core doesn't feel gravitational pull towards the outside, because it's balanced on that other side, but the outside is being pulling in towards the core, which pushes on the core.

It might be easier to simplify the mental image by picturing just a slice of the whole thing.

Think about sinking to the bottom of the Marianas Trench and then inflating a balloon just enough to make you neutrally buoyant. The net force on you will be zero, but you'd still be crushed by the pressure, which is the effect of many kilometers of seawater above you.

Another way to think of this is to imagine a small sphere (like the size of a basketball) around the very center of the Earth. The matter in that sphere feels no net gravitational force, but it has to push back against the entire mass of the rest of the Earth, which is trying to fall into it.

Also note that that is only true for (being inside) a hollow shell, not for a solid sphere.

Indeed. The acceleration due to gravity actually increases slightly with depth, reaching a peak at the surface of the core. This is because they Earth's density is not uniform.

If all the shells above you were self supporting you'd be right, you shouldn't feel any more pressure as you go down, if anything you should feel less force because gravity decreases.

However a shell of liquid is not self supporting, and at the size of a planet pretty much all material behaves like a liquid more or less. This means something needs to be pushing back for the layers of material to stay where they are. That is the pressure you feel, and you can easily show it will increase as you go down (with each layer supporting all the layers above it).

If anything the shell theorem makes this worse because it means the top layer can't pull back on any of the layers below it, it can only be pushed back through sheer pressure.

Same effect is happening inside a black hole, thus each black hole must have a tiny hole inside, with it own event horizon, a bit similar to Arago Spot. :-/

The article seems very poorly written. It reads like "We knew that Earth's inner core was a solid ball of iron… but now it's been discovered that the solid ball of iron actually has a solid ball of iron in its core!"

Yeah, I’m kind of scratching my head. This can’t just be a distinction without a difference, can it? Maybe the discovery is that the solid core has two layers of different densities with a gradual “margin” between the two?

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