Who Built the Moon? by Christopher Knight, Alan Butler Page A

same common mass. He suggested that this molten, viscous sphere had been rotating extremely rapidly in about five and a half hours.
    Darwin further speculated that the tidal action of the Sun had caused what he termed as ‘fission’ – a Moon-sized dollop of the molten Earth spinning away from the main mass and eventually taking up station in orbit. At the time this seemed very reasonable and was the favoured theory by the beginning of the twentieth century. In fact the fission theory did not come under serious attack until the 1920s when a British astronomer called Harold Jeffries was able to show that the viscosity of the Earth in its semi-molten state would have dampened the motions required to generate the right sort of vibration necessary to fulfil Darwin’s fission.
    A second theory that once convinced a number of experts was the ‘coaccretion theory’. This postulates that the Earth, having already been formed, accumulated a disc of solid particles – a little like the rings of Saturn. It was suggested that, in the case of the Earth, this disc of particles ultimately came together to form the Moon. There are several reasons why this theory can’t be the answer. Not least is the problem of the angular momentum of the Earth–Moon system that could never have been as it is, if the Moon had formed in this way. There are also difficulties regarding the melting of the magma ocean of the infant Moon.
    The third theory regarding the origin of the Moon that was in circulation around the time that the first lunar probes were launched was the ‘intact capture theory’. At one time seeming to be the most attractive possibility, the intact capture theory suggested that the Moon originated far from the Earth and that the Moon became a ‘rogue’ body that was simply captured by the gravitational pull of the Earth and that it took up orbit around the Earth.
    There are many reasons why the intact capture theory is now disregarded. Oxygen isotopes of the rocks on the Moon and on the Earth prove conclusively that they originated at the same distance from the Sun, which could not be the case if the Moon had been formed elsewhere. There are also insurmountable problems in trying to build a model that would allow a body as big as the Moon to take up orbit around the Earth. Such a huge object could not simply drift neatly into an Earth orbit at low speed like carefully docking a super-tanker – it would almost certainly smash into the Earth at a massive speed or possibly skim off and hurtle onward.
    By the middle of the 1970s all previous theories about the way the Moon had been formed were running into trouble for one reason or another and this created a virtually unthinkable situation in which acclaimed experts might have to stand up in public and admit that they simply didn’t know how or why the Moon was there. As acclaimed science writer William K Hartmann, senior scientist at the Planetary Science Institute, Tucson, Arizona said in 1986 in his book
Origin of the Moon
:
    ‘Neither the Apollo astronauts, the Luna vehicles, nor all the king’s horses and all the king’s men could assemble enough data to explain the circumstances of the moon’s birth.’ 9
    Out of this miasma came a new theory and, in fact, the only one that is presently widely accepted despite some fundamental problems. It is known as the ‘Big Whack theory’.
    The idea came out of theories that originated in the Soviet Union in the 1960s – specifically the work of Russian scientist V S Savronov, who had been working on the possibility of planetary origins from literally millions of different-sized asteroids known as planetesimals.
    As a divergence from the Soviet ideas, Hartmann, together with a colleague, D R Davis, suggested that the Moon had come into being as a result of the collision of two planetary bodies, one being the Earth and the other a rogue planet at least as large as the planet Mars. Hartmann and Davis hypothesized that the two