The oldest meteorite fragments found on Earth are about 4.54 billion years old .. this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth’s crust but does not contain an iron-rich core like the Earth’s. Many scientists think that about 40 million years later a body the size of Mars struck the Earth, throwing into orbit crust material that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity than the early Moon, attracted almost all the iron particles in the area.

Until recently, the oldest rocks found on Earth were about 3.8 billion years old, leading scientists to believe for decades that Earth’s surface had been molten until then. Accordingly, they named this part of Earth’s history the Hadean eon, whose name means “hellish” However analysis of zircons formed 4.4 billion years ago indicates that Earth’s crust solidified about 100 million years after the planet’s formation and that the planet quickly acquired oceans and an atmosphere, which may have been capable of supporting life. Evidence from the Moon indicates that from 4 billion to 3.8 billion years ago it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar System, and the Earth should have experienced an even heavier bombardment due to its stronger gravity.

While there is no direct evidence of conditions on Earth 4 billion to 3.8 billion years ago, there is no reason to think that the Earth was not also affected by this late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although volcanic outgassing on Earth would have supplied at least half. However, if subsurface microbial life had evolved by this point, it would have survived the bombardment.