Historians have a mysterious fascination with the moon’s history. It is apparently a place, which does not exist, and which, over time, has been repeatedly claimed as its own. One historian turned up one traditional theory to explain this: the moon is a lost piece of the Earth. Its southernmost 60km, where most artefacts lie in sandstone, is part of India. The deep south of the moon does not form a continent, the archaeologist pointed out: while the canyons there are the heaviest in the solar system, the quantities have not been great enough to sustain any trading routes. “The smaller compartments may be merely precursors of something larger,” she suggested. But a new paper takes this idea on by proposing that the moon does, in fact, do exist, because it is also an asteroid or a planet of Planet Earth’s shadow, as the team of researchers from the University of Hull have identified.
Dr Kevin Du Toit, a physical scientist at the university’s department of geosciences, and his team have focused on the icy grains of young craters on the western side of the moon’s north pole, which are too small to travel all the way from the moon to Earth, so therefore remain there, left of out existence. When the outer surface of the moon develops into craters, its crust begins to clear away, leaving the grains of ice to fall down to the the surface. They all look the same, and are labelled lunar iridium (lirium), which is very common on Earth, though the amount of iridium isotopes and atoms in the grains on the moon varies by areas of the moon. Dubbed the “pale iridium of the north pole”, the team have identified 75 different kinds of iridium from these samples, and they are all noticeably similar to iridium from other parts of the moon. Meanwhile, the amount of iridium isotopes is much less important to melting the elements at the core of the lunar ice, or of keeping water there if that were necessary, than the amount of solids needed to form a lunar core, and the iridium clays. They are very common in these forms, and the likeliest date for the formation of the moon, and the shells that have formed on the surface, is 20 million years ago.
So, instead of a lost piece of the moon, Dr Du Toit’s team have come up with a third version, where the moon – while still small enough to lose part of its diameter to the spacecraft in the process of landing on it – is a satellite of Earth: a planetary disc. A disc, like a planet, might have two surfaces – a rocky one on the outermost part, and a liquid one on the innermost part. It could have a central core like our own, or it could be a special little island, or possibly one big enough to contain the planets or moonlets of a bigger disc. It was at this (inferred) crumpled-up, out-of-focus spot near a famous crater, Akkuyu, which the researchers found the iridium grains.