When I was a kid I wondered how we knew space was a vacuum. We have a good (excellent) idea of the density of the core. We also have an excellent idea of how d

When I was a kid I wondered how we knew space was a vacuum. We have a good (excellent) idea of the density of the core. We also have an excellent idea of how density changes with pressure. Add in that the core must be magnetic and a fairly common isotope in the universe, and you're pretty much left with Fe.