In the beginning, scientists believe there was an interstellar gas cloud of all the elements comprising the Earth. A billion or so years later, the Earth was a globe of concentric spheres with a solid iron inner core, a liquid iron outer core and a liquid silicate mantle…. The current theory is that the primeval cloud’s materials accreted … and that sometime after accretion, the iron, melted by radioactive heating, sank toward the center of the globe…. Now another concept is gaining ground: that the Earth may have accreted … with core formation and accretion occurring simultaneously.
Most scientists now agree that the core formed as materials that make up Earth collided and glommed together and that the process was driven by heat from the smashups. The planet’s heart is primarily made of iron, nickel and some oxygen, but what other elements may dwell there and in what forms remains an open question. Recently, scientists proposed the inner core could be superionic, with liquid hydrogen flowing through an iron and silicon lattice (SN: 3/12/22, p. 12).