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

planets had collided in a very specific way that allowed jets of matter to be ejected from the mantles of both bodies. This matter was thrown into orbit, where it eventually came together to form the Moon. 10
    The suggestion seems to have many merits. First and foremost it appears to address the greatest puzzle that the recovery of Moon rock had thrown up: How was it that the composition of the Moon was so similar to that of the Earth, but only in part?
    A close analysis of Moon rock has shown that it is very similar to the rock that forms the mantle of the Earth, yet the Moon is nowhere near as massive as the Earth in proportional terms. (The Earth is only 3.66 times as big as the Moon but has eighty-one times the Moon’s mass.) It was obvious that the Moon could not contain many of the heavy elements that are found inside the Earth and the Big Whack theory purported to explain why this was the case. The Earth and the rogue visitor had come together in a very specific way. Although they would eventually form one planet it was reasoned that they must have impacted, drawn apart and then come together again. Computer modelling showed that under these very special circumstances it would have been possible for the material thrown off to have been mantle material, from close to the surface of the two bodies.
    Although the theory eventually gained ground, at first it seemed so improbable that it was generally rejected. But with the passing of time, further work showed that such an unlikely scenario could conceivably have taken place. In 1983 an international conference was held at Kona, Hawaii, to try and solve the problems regarding the origins of the Moon. It was at this meeting that the Big Whack theory, also known as the Giant Impact Hypothesis of the Collision Ejection theory, began to gain ground. Hartmann’s own suggestions, together with those of other scientists at the conference, formed the nucleus of the 1986 book,
Origin of the Moon
, which was edited by Hartmann himself.
    In the intervening period several experts have created computer models that purport to add weight to the Big Whack theory and the most convincing of these are those of Dr Robin Canup, who is now Assistant Director of the Department of Space Studies in Colorado, USA. Canup wrote her PhD dissertation on the Moon’s origin and specifically the Big Whack theory. Her early work led to the conclusion that the suggested impact would have actually led to a swarm of moonlets, rather than the Moon, but by 1997 further computer modelling resulted in a model of the impact that would lead to the Moon’s presence.
    Despite the fact that the Big Whack theory is now generally accepted by most authorities, it has many problems. Not least of all is that recognized by Robin Canup herself as she admits that there is one key aspect of the theory that doesn’t make sense. This stems from the fact that other researchers have pointed out that such a massive impact as that proposed could not have failed to speed up the rotation of the Earth to a level far beyond today’s situation. Canup agrees and the only way that she could deal with this anomaly is to propose a second major impact – which was designated ‘Big Whack II’. This suggests that the second planetary collision happened perhaps only a few thousand years after the first one but, quite incredibly, this incoming object came from the opposite direction and so cancelled out the huge spin imparted to the Earth by the first cataclysmic event. This balanced double act sounds unlikely in the extreme. Two cosmic collisions that just happen to precisely return the planet to its natural rhythm? To us, this explanation smacks of desperation!
    Canup herself is not happy with Big Whack II and is hopeful of modifying the original theory so that it can account for the present rate of spin of the Earth.
    There is another big problem to overcome if the Big Whack theory is to be taken seriously. When rocks were brought back