The Science of Discworld IV

world about four billion years ago, which seems to be too soon after the Moon’s origin for such an enormous collision to have occurred.
    We described the Theia theory in
The Science of Discworld
in 1999, but by the second edition in 2002 we were no longer convinced. The biggest problem came from newer computer models of the collision. The first such models, the ones that had established the Theia theory, showed a large chunk of the Earth splashing off; then that chunk split. One part formed the Moon, and the rest fell back to Earth to form the mantle. Theia got mixed up with both of them, but in similar proportions, so anyone could see why both the Moon and the Earth’s mantle had the same composition.
    However, the simulations that led to this conclusion took a lot of computing time, and only a few scenarios could be explored. As computers improved, the mathematical models became more sophisticated and their implications could be worked out more quickly and more easily. It turned out that in most of them the bulk of Theia was incorporated into the Moon, while very little went into the mantle.
    How can both Moon and mantle be virtually identical, then?
    The proposal that was accepted until 2012 was: Theia’s composition must have been very similar to that of the former Earth’s mantle.
    This of course is the problem that the whole theory was trying to solve.
Why should the compositions be the same?
If we can answer that for Theia by declaring ‘they just were’, then why not apply the same reasoning to the Moon? The Theia theory had to assume the same coincidence that it was supposed to explain.
    In the second edition of
The Science of Discworld
, we described this as ‘losing the plot’, an opinion that Ian repeated in
Mathematics of Life
. This view seems to have been vindicated by the recent (July 2012) discovery of a similar but different scenario by Andreas Reufer and colleagues. This also involves an impactor, but now the body concerned was much larger than Theia (or Mars), and moved much faster. It was a hit-and-run sideswipe rather than a head-on collision. Most of the material that was splashed off came from the Earth, while very little came from the impactor. This new theory agrees with the angular momentum figures, and it predicts that the composition of the Moon and the mantle should be even more similar than had previously been thought. Some supporting evidence for that already exists. A new analysis of Apollo lunar rock samples by Junjun Zhang’s team fn1 has found that the ratio of isotope titanium-50 (50Ti) to isotope titanium-47 (47Ti) on the Moon is ‘identical to that of the Earth within about four parts per million’.
    That’s not the only possible alternative. Matija Ćuk and colleagues have shown that the observed chemistry of Moon rocks could have arisen from a collision if the Earth was spinning much faster at the time – one rotation every few hours. This changes how much rock splashes off and where it comes from. Afterwards, the gravity of the Sun and Moon could have slowed the Earth’s rotation down to its present 24-hour day. Robin Canup has obtained similar results using simulations in which the Earth was spinning only a little faster than it is now, but the impactor was bigger than the Mars-sized body originally suggested.
    This is a case where
Pan narrans
became so committed to an appealing story that it forgot why the story was originally invented. The coincidence that it was supposed to explain faded from view, and a new narrative took over in which the coincidence took backstage. But now the storytelling ape is rethinking the entire story – and this time it is paying proper attention to the plot.
    The biggest origin question, philosophically speaking, is that of the universe, which we’ll come to in chapter 18 . That aside, the most puzzling origin, a much more personal one, is that of life on Earth.
    How did we get here?
    Our own inability to create life from scratch, or even to understand how ‘it’ works, makes us imagine that nature had to do something pretty remarkable to produce life. This conviction may be correct, but it could well be misplaced, because a complex world need not be comprehensible in simple terms. Life might be virtually inevitable once the mix of potential ingredients becomes sufficiently rich, without there being some special secret that can be summarised on a postcard. But explaining natural phenomena requires a convincing human-level