The makings of the moon

"EVERYONE is a moon, said Mark Twain, "and has a dark side which he never shows to anybody"

"EVERYONE is a moon, said Mark Twain, "and has a dark side which he never shows to anybody". He was referring - as far as the moon is concerned - to the curious coincidence that has made the rate of rotation of the moon upon its axis exactly equal to the rate at which it orbits Earth: as a consequence, half the moon's surface is always hidden from our view.

But the moon has another dark secret which it has never yet revealed: where did it come from? There have been many suggestions over the years - some of them rather improbable, and nearly all of them unsatisfactory.

One theory, for example, dates back to 1879, and was suggested by George Howard Darwin, who was determined not to be outdone by his famous father Charles when it came to proposing new and adventurous scientific theories. He was of the view that the moon was a piece of earth which had broken loose many millions of years ago, leaving the Pacific Ocean as the deep scar of the separation. It was a popular theory in its day, although it commands little credence nowadays.

Other astronomers were more inclined to think that the moon was formed independently, out of the same cosmic dust which produced the earth. Indeed, some believed that the moon was once a planet in its own right, travelling proudly like earth in an elliptical orbit around the sun. It may have circled in this way for many billions of years before a rare combination of the orbits of itself, earth, and perhaps Venus, caused it to be "captured" by our planet - and thus it became a prisoner, annexed forever to a neighbouring great power, condemned ad infinitum to revolve around us at monotonous monthly intervals.

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But when fragments of the moon were brought back to Earth and analysed after the Apollo landings, it was found that this theory, too, did not fit all the facts. Astronomers then devised what seems like an even more bizarre scenario. According to the "Big Splash" theory, billions of years ago a moonless Earth was in collision with a Mars sized planet: this giant impact melted the upper layers of the Earth's crust to form an ocean of molten rock some 700 miles in depth, and the large splash from this molten ocean sent a cloud of incandescent fragments and hot gases surging back into space. The more volatile substances comprising this debris, the theory goes, vaporised almost immediately to nothingness - and the remainder eventually coalesced to form the moon.