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Lunar affairs

A study in Nature adds a dramatic twist to the backstory of a neighbour we thought we knew.

In the stories of many human cultures, the Solar System is something of a family affair. The Norse people and the Incas of South America believed that the Sun and the Moon were brother and sister. A Native American myth has them as husband and wife (although the husband wants to eat his children, the stars). The Moon as a mother is a common theme.

Now scientists have suggested a rival celestial tale with a twist that is more common to terrestrial television dramas: the sudden appearance of a long-lost sibling. The early Earth had a near-identical geological twin, they say. The two young planets, of course, had a falling out and the twin vanished. But before it did so, it saddled Earth with an orbiting Moon-child.

The the origin of the Moon is a classic story that has been told many times. The latest version — written in a research paper on page 212 — still has some plot holes. But it is a cracking tale.

The time: some 4.5 billion years ago, in the earliest days of the Solar System. The place: hostile. A long time ago, if you like, in a galaxy not very far away. Thousands of adolescent protoplanets whizz around the Sun, bashing into each other, some breaking into smaller pieces and forming others as they soak up the freed materials. One of these protoplanets, lying not too far from the Sun and not too close, is what we now call Earth.

Enter, stage left, protoplanet Theia. Smaller than proto-Earth, it was raised in a similar neighbourhood. A chance meeting set the two on a collision course. The meeting is violent, and — here it helps if you imagine the most gravelly cinematic voice-over you can manage — life for both will never be the same again.

Theia becomes a giant cloud of dust infused with bits spewed from the injured proto-Earth, which quickly comes together to form the Moon. Earth gains a dependant.

The script might sound familiar; the plot more of a remake than of anything original. But here is the difference. Previously, many planetary scientists considered that it was too much of a stretch to say that young Theia and Earth were so closely related. It is much more plausible, given the chaos of the time, to present Theia as a random stranger. But that creates a continuity error: the mineral composition of rocks retrieved from the Moon is eerily similar to those of Earth.

Either Theia and Earth are related, or our best models of how the Moon formed are wrong. But if they are related, then why is it that the other bodies in the Solar System that we have studied seem to be so different from each other? What are the chances, given the number of objects out there at the time, that proto-Earth would be hit by a near clone?

The latest study runs computer simulations of those early days, to investigate the possible backstories of the major characters, including how and where they formed and their probable orbits. (A note to film directors: this bit is probably best presented as a montage.) The number crunching offers a realistic script: there is a one-in-five chance that proto-Earth and Theia could have formed at about the same distance from the Sun, so from the same stuff, and then run into each other to make the Moon. True, it is not a cut-and-dried ending that ties up all the loose ends before the credits roll. But all the best stories leave room for a sequel.

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Lunar affairs. Nature 520, 132 (2015). https://doi.org/10.1038/520132a

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