Earth is good at concealing its early history. Meteorite bombardment and the convective stirring of surface crust and mantle through the action of plate tectonics have erased the record of what happened during its first 500 million years. Indeed, no actual rocks exist from that era. But current models link Earth’s formation to a series of events in the solar system’s infancy 4567 million years ago.
The date, precisely timed from the decay of long-lived uranium radioisotopes in primitive meteorites, marks the condensation of gas in the solar nebula into fine dust grains. Those dust grains then combined through gentle collisions to form aggregates, planetesimals, and eventually a few tens of Mars-sized “embryos”—all within the first million years of solar-system history. With the heat generated from accretion and the presence of short-lived isotopes such as aluminum-26, those bodies would have melted enough for their dense metals to segregate from the...
