Moon is a chip off Earth's block

The Moon may be a closer relative than we thought. According to the latest research, our lunar satellite is actually part of the Earth that was blown clear about 4.5 billion years ago when it collided with a rogue planet three times the size of Mars.

The impact occurred when the Earth was still cooling after coming together in the early solar system. The effect was to destroy the incoming object, whose remains were either absorbed or else flung into the atmosphere, where they mixed with vaporised rocks from the young planet's upper layers.

That debris then reformed into a gaseous disk about 15,000 miles above the planet. It was joined by small, extremely hot "moonlets" which kept heading outwards before finally coalescing into what became the Moon. Eventually it stopped and took up its present orbit, about 238,900 miles away.

Though it sounds catastrophic, the incident could have been exactly what was needed to give life on this planet the best chance of survival, according to Dr Robin Canup of the Laboratory for Atmospheric and Space Physics at the University of Colorado, who led the new research. "The Moon's distance and its mass has helped stabilise the Earth's axis, making us less susceptible to large variations in tilting like Mars has had," she said. "Without it, we would have had more atmospheric variations. Also, the Moon has protected us from comets and asteroids. And some colleagues have suggested that the impact blew off some extra atmosphere, and saved us from having a dense atmosphere like Venus." The greenhouse effect means that Venus's surface temperature is 480C.

The new calculation fits in with analyses of Moon rocks brought back by the Apollo missions in the 1970s. "Samples there had geochemical signatures showing that they had been superheated, which didn't seem to fit with other ideas about the Moon's origin. But a giant impact would have done that," said Dr Canup. "Also, some of the rocks had chemical constituents like those of the upper layers of the Earth."