The best simulations of this process suggest that about 80 percent of Moon ought to have come from the impactor and 20 percent from the Earth.
That’s hard to reconcile with the measured make up of Moon rock, which is almost identical to Earth rock in terms of isotopic content. Some planetary geologists say this could be explained if, soon after the impact, the debris mixed well before forming into solid bodies. But others counter that this might explain the similarity in the isotopic ratios of lighter elements such as oxygen but can’t easily account for the identical ratio of heavier elements such as chromium, neodymium and tungsten.
But there’s another theory called the fission hypothesis that could account for the similar isotopic content. This idea is that the Earth and Moon both formed from a rapidly spinning blob of molten rock. This blob was spinning so rapidly that the force of gravity only just overcame the centrifugal forces at work.
In this system, any slight kick would have ejected a small blob of molten rock into orbit. This blob eventually formed the Moon.
The fission hypothesis has been studied for 150 years but ultimately rejected because nobody has been able to work out where the energy could have come from to kick a lunar-sized blob into orbit.
Now Rob de Meijer at University of the Western Cape and Wim van Westrenen at VU University in Amsterdam say they know where that kick might have come from.
Their idea is that centrifugal forces would have concentrated heavier elements such as uranium and thorium near the Earth’s surface on the equatorial plane. High concentrations of these radioactive elements can lead to nuclear chain reactions which can become supercritical if the concentrations are high enough.
The question is how concentrated could these elements have become. De Miejer and van Westrenen calculate that it is quite possible for the concentration to be high enough for a runaway nuclear reaction.
Their theory is that the explosion of a natural nuclear georeactor after it became supercritical ejected the material that eventually formed the Moon.
They also say that there ought to be telltale evidence that such an explosion took place, particularly in the lunar abundance of helium-3 and xenon -136, which would both have been produced in great quantities in a natural georeactor.
Future measurements from the surface could provide the evidence needed to confirm their theory but the analysis will not be easy. It is well known that the solar wind deposits vast amounts of these substances onto the lunar surface so that will have to be taken into account.
Of course, georeactors are by no means hypothetical. The most famous is in Oklo in Gabon, not so far from the equator, where a natural nuclear reactor was clearly in operation until about 1.5 billion years ago, leaving telltale signs in the uranium deposits now being mined.