eli5: How do we know what temperature and the materials of the earth’s inner core?
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Earthquakes send vibrations through the earth, which can be picked up all over the earth. Waves travel faster through denser materials, and when waves pass between different materials they bend (like a prism bending light). Timing the arrivals of earthquake vibrations at different places on earth lets us calculate how dense different layers are, and where those layers are within the earth.
Also, we calculated the mass of the earth and its circumfrence a long time ago, so we know its total average density. How we did those things is its own post though!
There’s other evidence too. We know the chain of fusion in our star ends with iron (fusing two iron atoms takes more energy than it releases), and we know there must be a lot of metal producing our magnetic field. So, an iron core (denser than rock, sunk to the bottom) is a pretty simple explanation.
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