EARTH

The Collision That Made the Moon

~4.5–4.4 billion years ago (debated)

Hadean

A young planet struck the Earth, and from the debris thrown into orbit the Moon assembled. The classic account gives the intruder roughly the size of Mars; but Moon rock, once analysed, proved almost indistinguishable from the Earth's own mantle: a match no body born elsewhere should have left. So rival versions make the impactor far larger, and the mixing of the two worlds far more thorough. Whichever is right, the blow ended the same way: a young Earth molten once more, from the surface down.

Why it matters

The impact melted away whatever surface the planet had, but it also gave the Earth a companion that steadies its axis and raises its tides. And it closes the violent chapter: this was the last of the giant collisions, the end of the planet's heavy construction.

Dating & uncertaintyDisputed

The date is argued, and by a wide margin. The oldest reliably dated Moon rocks cluster near 4.35 billion years, and one camp reads that as the age of the Moon itself. The other holds that those rocks record the slow freezing of a global ocean of magma rather than a birth: models allow that ocean to stay molten for 150 to 200 million years, and a strontium clock in the oldest lunar crust points back to a Moon formed close to 4.50 billion years ago.

Sources

1. Canup (2012), Science · Forming a Moon with an Earth-like composition via a giant impact
2. Schneider et al. (2025), Earth and Planetary Science Letters · The age and early evolution of the Moon revealed by the Rb-Sr systematics of lunar ferroan anorthosites
3. Borg et al. (2023), Annual Review of Earth and Planetary Sciences · The evolving chronology of Moon formation
4. Maurice et al. (2020), Science Advances · A long-lived magma ocean on a young Moon

Earth history as one day

00:19:0123 h 41 min before midnight

if Earth’s 4.54 billion years were compressed into a single day