THE CRASH THAT MADE OUR MOON

Understanding how our lunar companion was formed might just explain how we came to be here

- Colin Stuart

It’s the brightest thing in our night sky. Over the course of history it has been revered as a god, trampled across by 12 men and immortalised in poetry. The Moon is our steadfast companion, our only natural satellite as we endlessly orbit the Sun. Yet for an object that has received such scrutiny, arguments rage about where exactly the Moon came from. A suitable explanation needs to take into account what is perhaps the Moon’s greatest oddity: its size. It is the fifth-largest moon in the Solar System, trumping most of the satellites of our much bigger planetary neighbours. If you compare the size of moons to the size of their host planet, ours comes out at the very top. Many of the smaller moons of the Solar System are thought to be captured worlds – bodies that wandered too close to a planet before getting snared in its gravitational pull. Given the size of our Moon, it’s hard to imagine that’s how it ended up circling Earth.

As far back as 1878, George Darwin – the astronomer son of famous naturalist Charles Darwin – instead proposed that Earth and the Moon were once one body and that the latter formed from material thrown off the spinning Earth. This, he said, would explain why the Moon was moving a little further away from us each year. Supporters of this idea even pointed to the lack of land in the Pacific Ocean – which stretches across half of our planet – as the birthplace of the Moon. However, scientists later realised that any force capable of dislodging such a large amount of Earthly material would likely have destroyed the rest of our planet at the same time.