Moon rising

When worlds collide, sometimes another world is born. Imagine the Earth, shortly after its birth. A second world, the size of Mars, is hurtling towards it. The impact is so violent the Earth’s exterior turns molten and is splashed into space. It forms a ring around the Earth which gradually congeals into a new body. At first, the Moon is ten times closer than today, raising tides 1,000 times higher. But, over billions of years, it recedes to its current position.

Is this how the Moon was born?

This image was taken by the Japanese Kaguya spacecraft in 2007. But the crucial supporting evidence for the “Big Splash” theory came from Apollo. It found the Moon is made of material suspiciously like the Earth’s mantle, and its rocks are drier than the driest terrestrial rocks, as if all water was driven out by intense heat. The problem is that for a Mars-mass object to create the Moon and not shatter the Earth, it must have made a glancing blow at a very low speed. Bodies both inside and outside the Earth’s orbit are moving far too fast.

The Big Splash can be made to work, however, if the Mars-mass body, dubbed Theia, actually shared the Earth’s orbit. This could have happened if it formed at a stable “Lagrange point”, either 60° behind or 60° ahead of the Earth in its orbit. For millions of years, before being nudged into a colliding orbit, Theia bided its time. It was the planet that stalked the Earth. n

Marcus Chown’s latest books are Tweeting the Universe and Solar System, both published by Faber