The Science of Discworld IV

Earth’s magnetic field lines. A substantial proportion is positrons.
    This discovery puts thunderstorms in a new light. Not only is Thor’s hammer creating sparks (lightning) and noise (thunder): it is creating antimatter. It’s not the sort of discovery you make by trotting out facile explanations in terms of the supernatural. It depends on repeated scientific questioning of the known ‘facts’.
    Even familiar origins lead to new stories as time passes. In its search for rational explanations of origins, science often changes paradigm in response to new evidence or a new idea. The origin of the Earth and the Moon is a good example, with some curious twists. One of them being a short-term failure to change the paradigm in response to new evidence.
    In this case, the main problem is too much evidence, rather than too little. We can examine the structure of the Earth, look at the record written in the rocks, and travel to the Moon and bring back specimens. But in some ways this wealth of evidence makes the problem more complicated. What does it all mean? We’re trying to work out what happened, about 4.5 billion years after the event. At that time the universe had already been around for about 9 billion years (according to the Big Bang theory, and even longer according to the main alternatives). In all cosmological theories, the state of the universe gets more complicated as time passes. So by the time our solar system came into being, there was a lot of stuff around.
    We have to infer, from what we can observe today, how that stuff aggregated to make the Earth/Moon system. Those observations include data from asteroids, from the Sun and the other planets, and from detailed knowledge of the structure of the Earth and the Moon. (We say ‘the’ Moon, but according to a recent suggestion perhaps there were two or more moons at one stage.) It isclear that there was a time before Earth existed, and then the Earth came into being. The Moon turned up a few hundred million years later. Their origins are intertwined, and we can’t explain one while ignoring the other.
    The central problem of the Moon’s origin and Earth’s genesis is that Moon rock is very similar, in subtle chemical detail, to Earth’s mantle. This is the thick layer of rock immediately below the continental and oceanic crust, above the iron core. In particular, the proportions of different isotopes of several elements are the same in rock from either source. This coincidence is too improbable to be compatible with earlier theories of the formation of the Moon, such as the two bodies condensing independently from a primal dust cloud surrounding the Sun, or the Earth’s gravitational field capturing the Moon as it was flying past. George Darwin, one of Charles Darwin’s sons, suggested that the Moon was spun off from a rapidly rotating Earth, but the mechanics – such as energy and angular momentum, a measure of spin – don’t work out correctly. Moreover, the Earth and Moon did not just condense from dust. Astrophysicists and geophysicists now think that the Earth aggregated from many tiny planetesimals, which formed part of a great disc with the burgeoning Sun at its centre. Our telescopes are now good enough to observe several such discs around young suns in neighbouring star systems, and many of these have been found, so that theory seems to hold up.
    Between 2000 and the middle of 2012, astrophysicists and geophysicists mostly agreed that the Moon resulted from an enormous collision between an early Earth and an object about the size of Mars. They named it Theia, after the mother of the lunar goddess Selene. This collision vaporised much of the Earth, and nearly all of Theia. Most of the vapour condensed again in lunar orbit, coming together to make the Moon. The rest of it became Earth’s mantle, hence the similarity. The same theory explains the large angular momentum of the Earth/Moon system, a gratifying bonus.
    As time passed, problems with the Theia theory began to emerge. It would have produced very high temperatures on Earth’s surface, so pretty much all of the water should have boiled away. This seems incompatible with Earth’s present-day oceans. So extra assumptions were needed to save Theia. Perhaps a few ice asteroids fell on the early Earth and put the water back; perhaps the vaporised water fell back to Earth anyway. However, some very ancient Australian rocks seem to testify to the presence of a lot of water on our