A high concentration of carbon within Earth’s inner core could explain a long-standing mystery about how the deepest part of our planet froze solid – a process that kick-started the magnetic field protecting life on the surface.

Earth’s inner core presents a paradox for geophysicists: it first formed as a massive liquid ball of mostly iron, then began to solidify within the last billion years. In order for that freezing process to start in a pure iron object, it would have had to cool by at least 700 kelvin in…