What is earth's core made of? How does it compare to other planets?
Earth's core is made mostly of iron and nickel. This composition also applies to the other three planets inside the main astetoid belt.
Two factors account for the composition of the cores of the inner planets of our Solar System: which elements are most abundant, and which ones are least likely to converted to volatile materials or oxidized to low-density compounds.
Let's look at abundances. According to http://www.knowledgedoor.com/2/elements_handbook/element_abundances_in_the_solar_system.html, the folleing are the top fifteen elements in abundance in our Solar System:
This list, persented in rank order, covers most of what we see on Earth. But which ones then find their eay into planetary cores?
First a "core" element has to form nonvolatile, solid materials. This rules out hydrogen, helium (which is almost entirely within the Sun anyway), oxygen, neon (a major component of the Moon's tenuous atmosphere), nitrogen and argon. Sulfur is an intermediate case, as it can form volatile materials like sulfur dioxide but also nonvolatile ones like sulfate salts or metal sulfides, so let us keep that "in the running" for now. Ditto for carbon.
Next, a good "core" element should resist forming oxides with all that oxygen floating around. Of the fifteen elements listed above, oxygen stands out for being especially reactive, forming one type of compound or another with at least eleven out of the other fourteen and all nine that survive the nonvolatility test (above). Such compounds, where they are solid, tend to have relatively low densities and tend to float atop a heavy metal planetary core.
Which elements, among those that are not inherently volatile, are most likely to resist this reactivity and remain as heavy metals? Not magnesium, calcium or sodium. Alkali and alkaline earth elements are highly reactive towards oxygen. So are aluminum and silicon. We find these elements on Earth primarily combined with oxygen, as rocks formed from silicate minerals.
What is left? Carbon, iron, sulfur and nickel. Carbon can form metal carbides like the iron carbide that strengthens most steels. But first the metal has to be there; carbon is playing only a secondary role. Moreover, carbon also gets "lost" as other things like coal, carbon dioxide (there's that oxygen again), and carbonates (oxygen, alkaline earth metals). Likewise for sulfur, which does appear to form some metal sulfides down there.
And so, we have iron and nickel as the mahor core components, with iron being more abundant and thus tge majority.