Question: @Susan how do we know that the inner core of the earth is made of a nickel iron alloy?

Susan Cartwright answered on 22 Jun 2015:

There are a number of reasons.
(1) The Earth’s magnetic field shows that the core must be made of a magnetic material. There are only a few magnetic materials, and nickel and iron are by far the most abundant in the universe.
(2) We can calculate the density of the core from the behaviour of earthquakes and satellite orbits. The density we get is consistent with nickel-iron.
(3) Compared to the general solar system, the Earth’s crust is very deficient in iron and nickel. There is no reason to believe that these were mysteriously absent from the material that formed the Earth, so the natural conclusion is that they’re hiding underground.
(4) From the fact that the core, mantle and crust have different densities (derived from earthquake data mostly), we know that the Earth is *differentiated* – that is, its composition varies with depth. From this we infer that it was once molten (otherwise there woul be no obvious way for different elements to settle out at different depths. If it was indeed once molten, we would expect the heavy iron and nickel (together with elements that mix readily with molten iron, such as gold and sulphur) to sink to the bottom.

All the evidence taken together points to a nickel-iron core. Probably the magnetism and density arguments are strongest.