Submitted by Paradigm7657 t3_ylrtf5 in askscience
And also, is the boundary between the inner and outer core visible? Like at what point does the metal solidify, and does it make like a spongy gradient that gets denser or just an instant change between liquid and solid?
The state of a material very generally is a function of both temperature and pressure, e.g., this phase diagram for iron. Temperature and pressure both increase with depth and the transition from the outer to the inner core reflects where you cross from a liquid to a solid in the phase diagram for core material based on the pressure and temperature conditions. Referencing the above phase diagram, the estimated temperature and pressure near the inner core boundary is ~5700K and 330 GPa, which would put us in the solid part of that diagram (though in reality, we'd need to consider an Fe-Ni alloy phase diagram that also accounts for all the potential minor components that would influence the phase diagram, but this broadly gets the point across).
As for the nature of the inner core boundary, the review by Deuss, 2014 provides some detail. In short, based on the observations we have from seismic waves, it appears to be a relatively sharp, nearly perfectly spherical boundary with perhaps a small amount of "topography". As mentioned by Deuss however, there is a layer at the base of the outer core called the "F layer" that has been described as a "slurry", i.e., it's a mixture of liquid and solid components, though as discussed by Wong et al., 2021, this layer itself is likely stratified into more liquid rich vs more crystal/solid rich sections. This F layer is considered part of the outer core, so we would still describe the inner core boundary as being sharp (and here, we're using the very specific behaviors of some components of seismic waves to define and describe the inner core boundary) despite the existence of this "slurry" above it, i.e., we don't describe the inner core boundary as gradational.
Temperature and pressure can introduce new states of matter... Water will freeze at a temperature exceeding 3000 deg when at 18000 atmospheres. Freeze in that no movement occurs in the molecules....but it is obviously not cold. It way be that a similar occurrence of freezing accounts for what we measure as solid in the core.
The pressure from gravity decreases as we approach the core, right? I always thought the pressure at the core was from confined, radioactive decay, yes? no?
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That’s something interesting I’ve never really thought about. A good example though as the polar is water will boil off in space with much lower temperatures, similar to how a pressure cooker reduces waters boiling point.
At a hypothetical exact center of the Earth (assuming, for argument's sake, equal distribution of elements/density in the Earth), a particle of matter would be pulled equally in all directions--balancing out; nevertheless, there is a column of matter on all sides of said particle pushing down on it.
From what I gather, it's believed that there is significant but not extremely substantial heat generated from ongoing rasioactive decay. (It's thought that billion uranium deposits may have naturally undergone criticality/self-susutaining fission.)
The state of matter is a function of both temperature and pressure. Propane at modest pressure can be stored as a liquid that turns into a gas at atmospheric pressure during use. Water in a pressure cooker (about 2x atmospheric pressure) has a boiling point of around 120 degrees C; conversely, food cooked on a tall mountain may take much longer due to a less massive column of air pressing upon the water (which lowers its boiling point).
Gravitational acceleration decreases with depth as you move from the outer core inward (e.g., this estimate using PREM), but overburden (i.e., the mass above a given point) is still increasing, especially given the large density increase from the mantle to the core, so pressure is definitely still increasing, especially considering the inner core boundary.
EDIT: If you want to directly see estimates of pressure with depth, you can look at table 2 starting on page 312 of the original PREM paper by Dziewonski & Anderson, 1981. Looking at this, you can see that pressure broadly is predicted to increase with depth despite the decline of gravitational acceleration as you approach the center of the planet.
> From what I gather, it's believed that there is significant but not extremely substantial heat generated from ongoing rasioactive decay.
This depends on where you are. I.e., this is broadly correct for the core specifically in that we don't consider there to be many radioactive elements in the core. However, in terms of the total internal heat budget, radioactive heat production accounts for roughly half of the heat budget, but this is primarily from elements in the mantle and crust.
yeah that makes a lot of sense, thanks
oooh that’s interesting
Incorrect, a pressure cooker increases the boiling point, allowing the water to reach higher temperatures, and thus cook food more quickly.
I don't even think it would be possible for pressure to decrease with increasing depth. Pressure must be continuous unless something like a shock exists, and every additional shell adds some amount of gravitational pressure which is greater than or equal to 0. If somehow a situation existed where pressure were higher in outer layers than in inner layers, the pressure would drive movement and compression of the material in between such that the stress developed was adequate to support the pressure.
It is definitely not the case that in a solid at temperature greater than absolute zero, the constituent particles of the solid are not moving. In fact, at 3,000K they would be moving quite quickly relative to liquid water at room temperature. It's just that the restoring force from the pressure would be adequate to confine them to their places in the lattice.
A decreasing pressure is physically possible on a small scale (imagine a vacuum chamber placed in the center of an asteroid), but I don't see that happening on a planetary scale.
I should have started that there is no movement between the molecules (in a frozen state). Your statement is more concise.
Thank you, you are correct. Was up very late and writing absentmindedly. With your correct though, it still backs up above posters point.
[deleted] t1_iv0jrgx wrote
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