Earth's Solid Iron Heart Expands Quietly as the Planet Cools

Earth’s center is a solid iron-rich inner core, smaller in radius than the Moon but likely more massive; as the planet loses heat, liquid iron crystallizes at the inner-core boundary and adds to the core by about a millimeter per year, a slow growth that helps drive convection in the liquid outer core and fuels Earth’s magnetic field. The inner core’s exact composition, age, and growth rate are still debated and inferred from seismic data and high-pressure experiments rather than direct samples.