Solid and yet not hard? Fudge, then?
The Earth’s core is solid, according to a pair of geophysicists who claim to have solved an 80-year-old conundrum concerning the planet's center. The suggestion that our home world has a liquid outer core containing a smaller solid inner core was put forward by Danish seismologist Inge Lehmann in the 1930s. Scientists have …
'Soft' solid - it might be metal in its 'plastic' state, that is, just at the melting point but not really liquid. Under pressure it would behave as if it were solid, and yet be a bit more like melted glass or slime... as opposed to a 'truly liquid' form like molten iron poured into molds and whatnot.
As for the earth needing the core to sustain the magnetic field... I fear the potential of this being politicized some day. Just sayin'. Never underestimate etc..
why only iron/nickel in inner core in theory? Surely some real heavy elements, Tu, Ur and so on must have dribbled down ? if so would the core be a liquid cooled iron moderated fission reactor in part ? Hence some of the heat powering circulating currents in outer core and lower mantle ? The rest coming from more routine radioactive decay. Also an alloy may match the observed lack of rigidity. Have I missed something[s]
carbon steel is a completely different beast to iron and only has 2% carbon in it
Nitpick: the vast and varied family of alloys known as "carbon steel" covers steels with 0.05 to 2.1% carbon. With more carbon they're called "cast iron" and below that they're just iron...or steel. Some steels (especially some of the stainless family) hate carbon and try to exclude it.
More on topic: While the many other elements in the core are worth considering, it is hard to do so. Even for a fixed alloy composition, mechanical properties, magnetic properties, and even chemical properties will vary by temperature, pressure, and time, hence the T-T-T (Time-Temperature-Transformation) diagrams used in metallurgy for plotting heat treatments. You can get substantially different properties for an alloy held at high temperatures because of different crystal structures, different amounts of segregation of alloying elements, and other microstructural changes.
The problem with estimating the impact of other elements on the core's properties is that a) the exact composition is unknown and variations smaller than 0.1% can be significant; b) the core sits at a combination of temperature and pressure way off the usual metallurgy charts; c) it's hard to even simulate the core's conditions for more than a fraction of a second in diamond anvils or nuclear explosions, which makes normal metallurgical tests** challenging; and d) there are probably regional variations in composition just like the crust and mantle, and there are certainly temperature and pressure variations across the core's radius.
So it'd be hard to fault geologists for approximating the core as a big lump of nickel-iron. They do, on occasion, allow for the presence of uranium and potassium-40 to help estimate the radioactive heat budget of the core.
**My lab's Instron tensile test rig has a small environmental chamber fit for -100C to +600C at one bar of pressure (air or clean, dry nitrogen). There might be error if its results were extrapolated to the core's conditions.
I'm guessing (so hopefully someone who knows will chip in) but as I understand it, a solid core would be slowly (as in, a billion years or so) crystalising out of the melt (the outer core) and so it is to be expected that it will show some selectivity in what atoms it permits in the emerging crystal structure.
Alternatively, perhaps it is a mixture but it is so predominantly iron and nickel that geologists don't sweat the details. Hmm ... now I'm curious, too. (Trundles off to https://en.wikipedia.org/wiki/Inner_core...)
"Based on the relative prevalence of various chemical elements in the Solar System, the theory of planetary formation, and constraints imposed or implied by the chemistry of the rest of the Earth's volume, the inner core is believed to consist primarily of a nickel-iron alloy. Pure iron was found to be denser than the core by approximately 3%, implying the presence of light elements in the core (e.g. silicon, oxygen, sulfur) in addition to the probable presence of nickel.
Further, if the primordial and mostly fluid (still forming) earth contained any significant mass(es) of elements denser than iron and nickel, namely the white (appearance) precious metals (and a few others) except silver, specifically the siderophile elements, then these would necessarily have differentiated to the very center of the core into concentric nested spheres by planetary differentiation, with the most dense (and stable, i.e. platinum, iridium, and osmium, (etc.) in order of density) of these forming the innermost spheroid(s). While unstable elements of such trans-iron/nickel density would have mostly decayed to iron/nickel/lead by the time the earth formed a discrete core.
See also: Densities of the elements (data page)
It then necessarily follows that all, or almost all, of these denser elements we have mined (or are even able to) at the surface (or near surface, or even at all "above" the core) have been delivered later as part of impact objects/masses. "
Fissionables distributed thruout the body of the early Earth provided enough heat to encourage the Earth to differentiate by density. As the iron and nickel began falling to the core it released its potential energy as heat, completely melting the planet.
Fissionables too, fell to the core, but the core is a big place, so probably the very rare fissionable atoms would not achieve the density needed for enhanced reactions. Not sure about that tho. If it did happen, it would have been while the planet was still molten, simply making it more so. And relatively quickly the fission reactions would have depleted fissionables to the point where fission ceases.
"would the core be a liquid cooled iron moderated fission reactor in part"
Evidence of former 'natural reactors' does exist. It may be why the core is STILL molten.
And yeah, iron/nickel have stable atomic masses near 56, which is at the top of the binding energy per nucleon curve. In short, all fission and all fusion [that is exothermic] heads towards end products of iron 56-ish and nickel 58-ish. Cobalt too, but for some reason, not so much of it in the core.
So yeah earth's core is basically dead-star-stuff, with the highest binding energy per nucleon, basically the lowest potential energy with respect to fusion/fission reactions. By contrast, hydrogen has the least binding energy per nucleon, and the highest potential energy for a fusion reaction - like in the sun.
Sciency wikipedia article HERE.
also relevant, fissionable material would've all fissioned away by now, more or less, if it had a tendency to sink deep into the core and form a critical mass. which it probably did.
At the time of Earths formation it was spinning like a top, and thus the centripetal forces combined with the maelstrom of a churning and boiling liquid would have had such an impact as to counter gravity's effect on the heavier elements.
Later some heavy elements would still be liquid or even a gas while other less dense elements and compounds were solidifying, meaning we still couldn't rely on gravity to grade elements into layers based on their density in a solid state.
Hence our expectation to find the heaviest elements at the core of out planet is unfounded.
Don't overlook the role of water which is an excellent solvent. Our surface deposits of iron for example are largely (there are exceptions) iron compounds precipitated from aqueous solutions. We don't know a lot about the inside of the Earth. The Kola superdeep borehole was an ongoing series of surprises. It's hard to generalize about whether circulation in the liquid mantle and outer core redistributes dense materials upward from the core and/or light materials down toward/into the core.
"...without that geomagnetic field there would be no life on the Earth's surface."
No life ? At all ?
I'm betting that quote came from someone who isn't a scientist and who doesn't play one on TV.
I'd have felt more comfortable with the more accurate (and wordy) "...the earths geomagnetic field has been shown to play a significant part in the development of life on Earth."
We have no idea what would have happened (or not happened) if it wasn't there. If science teaches us nothing else, it's that life seems to occur wherever you look for it.
"If science teaches us nothing else, it's that life seems to occur wherever you look for it."
Not quite. Science has taught is that life seems to occur wherever you look for it *on Earth*. Where there's a magnetosphere to protect it from cosmic nasties. It hasn't shown up (yet) anywhere else we've looked.
Kelvin had calculated the age of the earth to be between 20 million and 100 million years. Radioactivity and its effects were unknown when Kelvin performed his analysis. The current age of the earth is 4.5 billion years. This age can be explained by having a fission reactor in the core.
I don‘t understand why and when the fission should have stopped. After this we are in the range of 100 millions of years range of Kelvin‘s calculation before the core gets too cold.
"The understanding of the Earth's inner core has direct consequences for the generation and maintenance of the geomagnetic field, and without that geomagnetic field there would be no life on the Earth's surface."
This sentence seems to suggest that without an understanding of the inner core, the generation and maintenance of the geomagnetic field is at imminent risk, as is all life on earth. Quite an onerous responsibility for these geophysicists.
Biting the hand that feeds IT © 1998–2020