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Back home, 84% of Earth's core is iron, one of the most abundant elements in the cosmos. We have used it for many purposes in the 6,000 years of human history, from the heads of arrows and spears to the core of any steel alloy.

But what if Earth's core had more iron, along the lines of 90%?

How would it affect the planet's relationship with our moon? How would it affect the magnetic field? Would it have affected the size of the planet in any way? If not, then would it have affected the history of civilization?

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    $\begingroup$ Well I mean, I highly doubt our ancestors were embarking on journeys to the core of the earth to gather molten iron for their spears ;) At any rate, I can't see it having a huge effect on civilization. It would probably make for a denser core and maybe a slightly increased gravity, but without doing the math that's pure speculation. $\endgroup$ – charliefox2 Dec 29 '15 at 20:59
  • $\begingroup$ The answer is, as always, it depends. Where will this iron come from, and will it add to or replace other elements in the core? Which ones? Or will we just remove the other elements until we're at 90% iron and call it good? Add some more details as to what exactly you're imagining is happening and I'll be happy to write you an answer. $\endgroup$ – realityChemist Dec 29 '15 at 21:08
  • $\begingroup$ Iron has only been significant for about 3000 years. Prior to the was the Bronze Age. $\endgroup$ – WhatRoughBeast Dec 30 '15 at 4:45
  • $\begingroup$ Then expect more rust... $\endgroup$ – user6760 Dec 30 '15 at 8:01
  • $\begingroup$ Only tangentially relevant, but maybe useful for you if looking to understand the role of iron in a planet's overall composition: iron planet and banded iron formation. $\endgroup$ – pablodf76 Mar 26 '17 at 22:04
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Not much. Linked "mass segregation" suggests that core's Iron content will be much higher than Iron content in other layers anyway, thus, we can expect crust, layer on which we live and from which we extract elements, to have roughly same composition. Perhaps iron ores are a bit more common, but change shouldn't have significant impact on development of civilisation.

Planet mass isn't significantly affected; as explained above, other layers are mostly unaffected, while core is made of Iron with Nickel on distant second. Iron has density of $7.8 \frac{g}{cm^3}$, Nickel has density of $ 8.9 \frac{g}{cm^3}$. Assuming that 1/3 of mass is contained in core (result of quick google search), by replacing 6% of its mass with Iron you are decreasing planet's mass by $ 0.3(3)*6\%*\frac{7.8}{8.9}\approx 1.7\% $ dropping planet's gravity to $9.64 \frac{m}{s^2}$ compared to real Earth's $9.81 \frac{m}{s^2}$. Sorry, here I made a mistake in previous version you are decreasing mass, not increasing as I previously wrote.

Magnetic field shouldn't be changed significantly either. Not only Iron is a dominant element contributing to core's physical properties, but cursory search suggests Iron and second main element: Nickel have similar properties.

All in all, I can't imagine any significant changes.

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    $\begingroup$ Not the wikipedia article I would have expected for "mass segregation"... $\endgroup$ – charliefox2 Dec 29 '15 at 21:32

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