Just over 4.5 billion years ago, the newly formed Earth suffered a traumatic blow. Another planet, the size of Mars, came careening through space and slammed into it with an impact that melted our planet's young crust and threw dust and debris sky-high. For almost an hour, the white-hot crash site shone brighter than the Sun. Miraculously, the Earth survived. But the smaller planet wasn't so fortunate. Its remnants flew into orbit, where they collected to form a huge Moon, one of the largest in the Solar System. Planetary researchers are pretty confident that this is how our Moon was created. But there are many things about the collision that they don't understand. Why did it produce a satellite at all? Why didn't the debris simply fall back to Earth? And why just one Moon rather than several? It is only recently that researchers have started to answer these questions, by combining a new look at the chemistry of Moon rock with computer simulations of the impact. At last we are getting a clearer picture of how it happened. Our Moon, it seems, was born from vapour.
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