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But shouldn't the planet start with a small, solid core (which would have always been solid due to the immense pressure), surrounded by the liquid magma, which

But shouldn't the planet start with a small, solid core (which would have always been solid due to the immense pressure), surrounded by the liquid magma, which is then in turn surrounded by the solid layer of crust (which solidified due to being exposed to the rest of the space to cool)? Then, as the Earth continues to cool down, the liquid part would also start to turn solid, starting from the near the core/crust, and therefore reduce its volume so... if the crust is sturdy enough, it will all end up as the small, solid core surrounded by the lot of empty space surrounded by the outer crust? Like, that's also a pretty ridiculous scenario of planetogenesis, but it's at least somewhat plausible.

bastawhiz 2y ago
That model kind of falls apart for a few reasons: 1. The empty space between the crust and the mantle isn't going to fill up with air. If the mantle cools and shrinks, it's going to pull the crust inwards because there's nothing to expand to fill a potential void. 2. There's no hard line where the crust becomes mantle. The rock gets gooey in a gradient. 3. If there was a void filled with gas, the gas would desperately want to escape. There's a tremendous amount of pressure down there, and the gas would fart out to the surface as volcanic activity at the first chance it got. Tectonic plates move fast enough that even if such a bubble did form, the gas would escape relatively quickly. But really, consider that if the center of the earth is liquid and cooling, that'll shift the center of gravity in the direction the mantle moves in (same mass, now more dense). That direction is... towards the opposite side of the planet, where there's uncooled liquid mantle, which in turn would just squish back around in time. Cooling the whole mantle evenly around the world enough to decrease the circumference of it such that there's an appreciable void is an unimaginable amount of cooling. As you said, the crust would need to be extremely sturdy, and at that point tectonics would close up those voids pretty quickly.
inkcapmushroom 2y ago
I'm no geologist but I would think it would be hard for the outer layer to stay stable enough to harden to that degree, while the inner part is still liquid and cooling. On our own planet the crust is incredibly thin compared to the other layers, even while the mantle is cooled enough to be solid, and the solid mantle still moves around enough to break apart the crust constantly. Seems impossible for the hollow eggshell of the crust to not break and crumble into the retreating center at some point.
andrewflnr 2y ago
> if the crust is sturdy enough In short, the crust is not sturdy enough. In comparison to the bulk of the earth, the crust is like the film of dry paint that forms on a paint bucket if you leave out too long. At the scale of a planet any rock is still a liquid for mechanical purposes. (Which is in fact what the hydrostatic equilibrium part of the IAU definition of a planet is talking about.)

bastawhiz 2y ago

That model kind of falls apart for a few reasons: 1. The empty space between the crust and the mantle isn't going to fill up with air. If the mantle cools and shrinks, it's going to pull the crust inwards because there's nothing to expand to fill a potential void. 2. There's no hard line where the crust becomes mantle. The rock gets gooey in a gradient. 3. If there was a void filled with gas, the gas would desperately want to escape. There's a tremendous amount of pressure down there, and the gas would fart out to the surface as volcanic activity at the first chance it got. Tectonic plates move fast enough that even if such a bubble did form, the gas would escape relatively quickly. But really, consider that if the center of the earth is liquid and cooling, that'll shift the center of gravity in the direction the mantle moves in (same mass, now more dense). That direction is... towards the opposite side of the planet, where there's uncooled liquid mantle, which in turn would just squish back around in time. Cooling the whole mantle evenly around the world enough to decrease the circumference of it such that there's an appreciable void is an unimaginable amount of cooling. As you said, the crust would need to be extremely sturdy, and at that point tectonics would close up those voids pretty quickly.

inkcapmushroom 2y ago

I'm no geologist but I would think it would be hard for the outer layer to stay stable enough to harden to that degree, while the inner part is still liquid and cooling. On our own planet the crust is incredibly thin compared to the other layers, even while the mantle is cooled enough to be solid, and the solid mantle still moves around enough to break apart the crust constantly. Seems impossible for the hollow eggshell of the crust to not break and crumble into the retreating center at some point.

andrewflnr 2y ago

> if the crust is sturdy enough In short, the crust is not sturdy enough. In comparison to the bulk of the earth, the crust is like the film of dry paint that forms on a paint bucket if you leave out too long. At the scale of a planet any rock is still a liquid for mechanical purposes. (Which is in fact what the hydrostatic equilibrium part of the IAU definition of a planet is talking about.)