<P> The outer core was determined to be molten from observations showing that compressional waves pass through it, but elastic shear waves do not--or do so only very weakly . The solidity of the inner core had been difficult to establish because the elastic shear waves that are expected to pass through a solid mass are very weak and difficult for seismographs on the Earth's surface to detect, since they become so attenuated on their way from the inner core to the surface by their passage through the liquid outer core . Dziewonski and Gilbert established that measurements of normal modes of vibration of Earth caused by large earthquakes were consistent with a liquid outer core . It has recently been claimed that shear waves have been detected passing through the inner core; these claims were initially controversial, but are now gaining acceptance . </P> <P> Based on the relative prevalence of various chemical elements in the Solar System, the theory of planetary formation, and constraints imposed or implied by the chemistry of the rest of the Earth's volume, the inner core is believed to consist primarily of a nickel - iron alloy . Pure iron was found to be denser than the core by approximately 3%, implying the presence of light elements in the core (e.g. silicon, oxygen, sulfur) in addition to the probable presence of nickel . </P> <P> Further, if the primordial and mostly fluid (still forming) earth contained any significant mass (es) of elements denser than iron and nickel, namely the white (appearance) precious metals (and a few others) except silver, specifically the siderophile elements then these would necessarily have differentiated to the very center of the core into concentric nested spheres by Planetary differentiation . The most dense (and stable, i.e. platinum, iridium, and osmium, (etc .) in order of density) of these forming the innermost spheroid (s). While unstable elements of such trans - iron / nickel density would have mostly decayed to iron / nickel / lead by the time the earth formed a discrete core . </P> <P> It then necessarily follows that all, or almost all, of these denser elements we have mined (or are even able to) at the surface (or near surface, or even at all "above" the core) have been delivered later as part of impact objects / masses . </P>

Which of these is a major constituent of earth's core