Earth’s core is known to be metallic, with a density of about 9.90 Mg·m−3 at the core-mantle boundary and as such is substantially denser than the surrounding mantle (5.56 Mg·m−3 at the core-mantle boundary; Dziewonski and Anderson 1981). Comparison with cosmic abundances suggests that the core is predominantly Fe with around 5% Ni (Allègre et al. 1995; McDonough 2003) and 8–12% of one or more light elements (Birch 1952). The latter conclusion comes from the observation that the core is appreciably less dense than pure Fe or Fe-Ni alloys under any plausible...

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