Submitted by VillagerNo4 t3_11zbuiw in askscience
This might sound dumb but would the pressure inside a planet make an alloy that's far more dense than normal? Oh sure it's probably a large mix of metals but it's probably the heaviest metals in the inner core right? Not sure if it would make a tough alloy or something.
That was a great read, thanks for that.
>very little uranium in the core
That makes sense, and it also explains why the heat seems to come from the mantle also, https://newscenter.lbl.gov/2011/07/17/kamland-geoneutrinos/
What a phenomenal answer! This is why I come to this sub. Thank you!
When I saw the question, I was expecting CoastalTrudger to be here lol. Dude knows everything about geology and always has high quality responses
It's stuff like this that had me plowing through books as a child and couldn't leave a set of encyclopedias alone.
Has meant on many occasions I haven't a clue where/how or why I know something but when scrutinised has turned out to be correct. Just wish I could recall the wisdom and knowledge from my memory when its under duress.
Damn you just unlocked some memories
I got gifted a box full of books and I think I read every single one I had
Mightve been what made me interested in how the world works which made me love science
Damn you just unlocked some memories
I got gifted a box full of books and I think I read every single one I had
Mightve been what made me interested in how the world works which made me love science
>Actually, no.
But the four densest elements—osmium, iridium, platinum, and rhenium—are all in fact highly siderophilic. So wouldn’t we still expect a high concentration of those to be found at the very center of the inner core?
I think that's because they are very rare and there isn't a lot of them
Is the reason they are rare is that they are Siderophilic ?
Their abundance in the initial cloud that the solar system formed from would be tied to how likely they are to form during a supernova. That’s nuclear physics not chemistry.
Thank you
Also a question of how astrological bodies are formed. There's a large amount of those elements that are easily accessible in asteroids. If there was ever a reason to asteroid mine, it would be for those elements. Iron being pretty prolific in general and in asteroids probably does go well with their siderophilic properties, which is probably why we tend to find those elements in large amounts in asteroids.
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I am not an expert, but my understanding is this is at least partially correct. As mentioned, asteroids are differentiated core fragments, and impact craters (eg Chicxulub) are sometimes dated by the iridium distribution in geological layers. The iridium comes from the asteroid. I don’t know why iridium gets all the press over other other platinum group elements, but a cursory search suggests they’re also used for impact dating. https://physicsworld.com/a/iridium-in-undersea-crater-confirms-asteroid-wiped-out-the-dinosaurs/
However, the solid inner core does have a lower percentage of light elements than the molten outer core surrounding it. The light elements preferentially, although not entirely, stay in the molten core. As the inner core grows from the molten core freezing out, the concentration of light elements in the remaining melt gradually increases. The rising of light elements through the remaining liquid core is the main source of energy (and crucially, entropy) and driver of convection that have sustained Earth's dynamo since the inner core first formed some time in the past ~0.5-1.5 billion years (latent heat of freezing is a minor contribution). Operating a dynamo through this mechanism requires that the molten core have cooled enough to start freezing, and overall be compositionally well-mixed, without significant layering (stratification) by density, i.e. light element concentration.
The need to explain the dynamo also relates to the question of how much radiogenic (and thus heat producing) elements, particularly potassium, are actually in the core. The traditional idea, generally suppoeted by geochemistry and minerla physics, is that this amount is negligible. However, with evidence from the rock record of a dynamo for the past 3.5-4.2+ billion years, this leaves a long gap where it is more difficult to explain what drove the geodynamo.
Prior to the formation of the inner core, the compositional convection due to freezing would not have existed to power the dynamo. Therefore, a different mechanism must have powered the early geodynamo. The primordial heat left over from Earth's formation should not, by itself, be enough to sustain thermal convection for billions of years until inner core nucleation. For geophysicists, the long-lived geodynamo is much easier to explain with a thermally driven dynamo supported by the heated generated by radioactive isotopes such as potassium-40. There are, of course, other proposed explanations, such as the the precipitation of light elements near the core-mantle boundar, that is the top of the then-entirely molten core (Mittal et al., 2020; Wilson et al., 2022).
Returning to a more direct possible answer to part of u/VillageNo4 's question, the inner core might be in a 'superionic' state such that the iron metal behave like a solid, while the light elements that did get incorporated into it behave like a liquid (Wang et al., 2021 and He et al., 2022). (See also https://www.sciencenews.org/article/earth-inner-core-superionic-matter-weird-solid-liquid.) The high temperatures and pressures deep in planetary interiors can produce materials that are very exotic compared to what we see in everyday life. (c.f. Jupiter's liquid metallic hydrogen mantle and possible 'solid' core, which if it exists would not have a well-defined surface.).
> However, with evidence from the rock record of a dynamo for the past 3.5-4.2+ billion years, this leaves a long gap where it is more difficult to explain what drove the geodynamo.
Dynamo theory also suggests that the Earth has had a dynamo since its formation (in the impact process that formed the Moon). The reason being is that one can argue that in the present day the Earths dynamo is subcritical which essentially means it can maintain a strong field but not magnetize the core from a weak magnetization state. If this is correct and Earths dynamo is subcritical now then it is almost certainly subcritical throughout its life (since it was more rotationally constrained in the past, faster rotation) and so the dynamo must have existed since the formation of the Moon.
> the core is predominantly iron with a small amount of nickel (constrained to being around 5%)
What is the constraint?
Side question: Does the inner core functionally do anything?
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The gradual freezing out of the outer core to grow the inner core indirectly sustains the magnetic field generated within the outer core. (And even then "only" for the past ~0.5-1.5 billion out of 4.55 billion years of Earth's history, because the inner core didn't exist before then.)
However, the inner core itself doesn't really do anything practical or of non-academic interest. Aside from very slowly getting bigger, it just spins along (very, very slightly more or less rapidly) with the rest of Earth. The motions that power the dynamo are strictly within the molten portion of the core. Essentially, the combined motion due to Earth's rotation and core convection organize into rotating columns within the outer core, which sustain Earrh's intrinsic magnetic field. (See dynamo theory.)
The geodynamo is NOT because of the inner core rotating relative to the outer core, which it barely does. Because the inner core is a solid within a relativley low viscosity liquid, it can rotate at a slightly different rate. If anything, the electromagnetic forces associated with the dynamo act on the inner core in a way roughly analogous to an induction motor, exerting torque on the inner core and spinning it relative to the outer core. (The inner core never spins at a rate very different from the rest of Earth--inertia, conservation of angular momentum and energy, etc.) Even in this sense, which is not nearly as dramatic or impactful as clickbait would have one believe, the inner core is a passive component.
Thanks, very useful reply and it answered my question. :)
shouldn't it be dense, not necessarily due to materials, but just because it is so high pressure down there? Just wondering
Just a minor addition that pressure has a minor effect on the thermodynamic state function when compared to temperature so the crystal phase of the inner core shouldn’t be much different than that of what we see at the surface. We need to start talking about neutron star densities to see a pronounced effect!
Except we do know, both from seismic wave studies and from high pressure experiments above ground, that iron has a high pressure phase transformation around 13 GPa (body centered cubic to hexagonal close packed).
13 GPa of pressure is ridiculous compared to the 600 or so degrees Celsius required to get the same phase transformation. Temperature will always have a more pronounced effect than pressure in thermodynamics.
(A) there is no thermally induced HCP phase in iron at atmospheric pressure, in pure iron the HCP epsilon phase is solely a high pressure phase, (B) I don't see what the temperature vs pressure effect size has to do with any of this - the assertion was that in the core you'd have the same crystal structure as you would on the surface and it is observably not true.
FCC in the (111) planar direction is the same as HCP. We just call it something else but effectively it’s the same right?
No, they're not the same. The (111) plane in FCC and the (1000) plane in HCP are equivalent but if you look down the [111] and [1000] directions you will see that the stacking sequence is different. This is usually described as ABCABC (FCC) vs ABAB (HCP). This is, for instance, why you can have FCC <--> HCP phase transformations produced solely by stacking faults.
I thought ABAB stacking was for graphite and other graphitic structures since each stack of graphene is missing a carbon atom at the center of the hexagonal rings which would give it the symmetry allowing for ABCABC stacking in both HCP (0001) planes and FCC (111) planes? Interesting conversation by the way!
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I was under the impression that iron reacted pretty well with oxygen; is that not the case in the chemistry of a forming earth?
Not questioning anything you’ve said here but am trying to reconcile your statement about minimal uranium in the core with high school level popular science attributing heat in the earth’s interior being due to, in part, the heat of radioactive decay. If the majority of radioactive elements have migrated to the crust through <geologic process terms I don’t understand>, what is decaying inside the earth (and where) to generate that 50% internal heating from radioactive decay
Heat from radioactive decay (primarily of uranium, thorium, and potassium) is an important component of the internal heat budget. These elements are the most abundant in the crust, but they are also present in the mantle and given the size of the mantle, even at low concentrations, they end up generating significant heat.
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Since the core is liquid, do the other elements have to form an alloy? Could they just be floating around in there?
Only the outer core is liquid.
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I don't know a lot about this so maybe a dumb question. Did iron make up so much of the solar nebula that we ended up with so much iron in the core? I know H and He were in most abundance but sort of assumed the next levels of abundance would be the next elements in the periodic table.
Someone else can probably explain this properly, but I believe the fusion reaction inside the Sun ends up leaving it as all Iron. Something about the fusion reaction past a certain point can't produce elements higher than Iron on the period table.
> When very massive stars leave the main sequence, they first become red supergiants and then end their life cycles in with a bang. Unlike a red giant, when all the helium in a red supergiant is gone, fusion continues. Lighter atoms fuse into heavier atoms up to iron atoms. Creating elements heavier than iron through fusion uses more energy than it produces so stars do not ordinarily form any heavier elements.
Eventually; iron is the sink for fusion of lighter and decay of heavier. But "dead" stars are often not mostly iron.
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I have no interest or real knowledge in geology and this was an absolutely fascinating and quite cool read! Very easy to understand too! Thank you!
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So is that alloy molten all throughout or do we actually have like large smooth bearing ball inside the planet?
Mostly iron because Iron packs really well under extreme pressure and temperature, which is really what drives the segregation of materials inside a planet. It is a matter of space/volume and not all compounds pack to the same extent so unit volumes vary, and will change even for a given compound as P and T vary. The amassing of iron in cores is just a response to the existence of pressure and temperature change with depth (other elements or compounds are less compatible with high pressure, basically, so they end up closer to surface than things that deal well with high P and T, and iron deals the best of them all, apparently.
On top of that density consideration are the chemical needs of each element. Most elements are not all that stable except when in a compound, but iron actually is fairly stable chemically, as a native element. Plus, there is a lots of iron around, with iron being the most stable element made by stars (heavier elements want to break apart, undergo fission, and lighter ones want to combine and make bigger atoms, undergo fusion).
The physical characteristics of iron at extreme pressure and temperature are not well known because it is really hard to study something several thousands of kilometers below a mass of silicate materials (study in place) and it is difficult to make even in laboratory, and then only in very tiny amounts, with even a problem of time duration coming into play (it won't stay the same if you ease off pressure or temp).
People are working on it. More near the frontier of knowledge than a well-characterized material. Even the nature (PT-conditions) of the transition from face-centered cubic to hexagonal close-packed structure is poorly defined from what I have seen about it.
Including minor nickel to make some sort of alloy, and its effects on behavior, is also still poorly understood.
I found this article which sounds quite interesting: https://www.universetoday.com/153356/theres-so-much-pressure-at-the-earths-core-it-makes-iron-behave-in-a-strange-way/
Sounds like it does have special properties after all.
Basically all matter acts differently in different pressures. Water freezes on Mars at 31.5 degrees and boils near 33 IIRC. Stars literally cause atoms to fuse and make new heavier atoms and all the energy we rely on that comes to earth from the sun.
So yes it's certainly special compared to the properties we know at 1 atmosphere of pressure. I would say it's still the properties of these elements and just a special environment that brings out their rare for our environments traits.
That is interesting and pretty much supports the "we don't know a lot" idea. Not something easily studied.
Well, in larger planets like jupiter and saturn the intense pressure creates liquid metallic hydrogen. Liquid metallic hydrogen is a phase of hydrogen in which it becomes electrically conducting like a metal. Because hydrogen is the simplest molecule, just one proton and one electron, it forms a simple solid when compressed or cooled. Under high pressures it becomes superconducting and behaves like a superfluid. Superfluids are insanely weird. They defy gravity with viscosity and can even seep through things we consider impenetrable to liquids. https://aip.scitation.org/doi/10.1063/5.0002104
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CrustalTrudger t1_jdc7t7k wrote
> it's probably a large mix of metals but it's probably the heaviest metals in the inner core right?
Actually, no. The core is predominantly iron with a smaller amount of nickel (and some other stuff, more on that in the next section), which while both dense, are certainly not the most dense metals that exists on Earth and in fact, many significantly more dense metals tend to be concentrated in either the crust or mantle as opposed to the core. The reason for this largely relates back to the early formation of rocky planets (and here most of my answer will focus on Earth, but this is broadly applicable to rocky planets more generally). During planetary differentiation, there are two primary ways by which materials separated, physically (i.e., mostly on the basis of density) and chemically. For the chemical differentiation aspect, it's useful to consider the Goldschmit classification of the elements. Regardless of their density, generally lithophile elements, which are those that easily combine with oxygen, and chalcophile elements, which are those that easily combine with sulfur and a few other elements, were incorporated into the silicate part of the Earth and thus remained in the mantle and crust. As examples, very dense metals like uranium and lead are both thought to generally be in very low (to zero) concentrations in the core. This is because uranium is a lithophile and lead is a chalcophile so both are generally concentrated in the crust and mantle (not to mention that a non-trivial component of lead results from the decay of uranium and thorium, both lithophiles, after differentiation). Siderophiles were those that easily dissolved in iron and thus ended up primarily in the core. The density driven portion of differentiation provided the main division between the denser, inner iron-nickel core and the less dense, outer silicate portion of the Earth, but whether a particular element ended up in the silicate portion or the core came down to the individual chemical properties of the element in question, i.e. was it more likely to bond or dissolve in a silicate melt vs an iron melt.
> Not sure if it would make a tough alloy or something.
As discussed above, the core is predominantly iron with a small amount of nickel (constrained to being around 5%), so usually described as an iron-nickel alloy. However, we know from a variety of different datasets that the density of the core is actually less than what you'd expect for pure iron or a 95-5% iron-nickel alloy (and that various other properties, mostly related to how seismic waves pass through it are similarly not consistent with a pure iron or a pure iron-nickel alloy) and that the core must include some amount of a light element or several light elements. As highlighted in the review by Hirose et al., 2013, on the basis of abundances (i.e., what elements were present) and their ability to partition into the core during planet formation, we hypothesize that these light elements are silicon, oxygen, sulfur, carbon, and/or hydrogen. In terms of the properties of the resulting alloy, a lot depends on which one of these (or which mixture of these) are actually present in the core. The Hirose review goes through some of the details of specific two-component alloys (e.g., Fe-C, Fe-Si, etc) from high pressure/temperature experiments, but for some of these it's actually pretty challenging to get them to alloy with iron given the conditions we can and cannot simulate in experiments. Checking in on a more current review by Hirose et al., 2021 (pdf or a preprint of this article here), we find the situation pretty much the same, i.e., we still think that the core needs some light elements, the list of the possible ones are the same, and we still don't really know which ones are the right ones within that list. What this new review does provide is updated indications of just how much of different elements might be present. These have ranges of uncertainties, but most max out at ~1-5%, but it varies by element and by the way the estimate is derived. The extent to which any of these alloys would be "tough" is a bit unclear since (1) that's not exactly a clear property, (2) we don't know the exact composition, and (3) it's hard to get materials up to the relevant temperature and pressures to do detailed studies of the material properties in the same way we would for an alloy that's stable at surface temps and pressures.
EDIT: I'll add that we can learn some details about the cores of rocky planets from the study of iron meteorites, which are generally thought to be chunks of differentiated bodies that were destroyed during the early history of the solar system. Since they're no longer at core temperatures and pressures, the exact properties of these are a bit different than what you'd expect if they were at core temperatures and pressures, but they definitely inform a bit on composition. I'll also highlight the upcoming Psyche NASA mission, which is going to visit the 16-Psyche asteroid, which is might be a large chunk of a left over core of a planetesimal.