The collision that changed everything
Around 4.5 billion years ago, the young Earth was still glowing hot from its formation. Then a planet roughly the size of Mars — now called Theia — came in on a glancing trajectory and slammed into it. The impact was one of the most violent events in our planet’s history.
What the Impact Left Behind
The collision vaporized huge amounts of rock from both bodies. A massive cloud of molten debris was thrown into orbit around the battered Earth. Over time, that material cooled and pulled itself together under gravity, eventually forming the Moon we see today.
The blow also knocked Earth’s spin axis into its current tilt and may have helped clear out some of the early atmosphere, setting the stage for the world we live on now.
The Moon is made largely from the same material as Earth’s mantle. In a very real sense, when you look up at night, you’re seeing a piece of our planet that was ripped away and then reformed in orbit.
- The Moon is still slowly moving away from Earth at about 3.8 cm per year.
- Earth’s large Moon helps keep our axial tilt stable, which prevents wild swings in climate.
- The impact likely removed much of Earth’s original atmosphere.
- Some models suggest chunks of Theia’s core may still be buried deep in Earth’s mantle.
- Without this impact, Earth would probably spin much faster and have a much more extreme tilt.
- The Moon’s formation is one of the reasons Earth has such favorable conditions for life.
The giant impact
A reconstruction of the massive impact that formed the Moon.
Gallery
Significance
The collision gave Earth its Moon and helped set the conditions that later allowed complex life to develop.