Origins of the Moon in Focus

digimag255_02In this month’s Focus Magazine, I take a detailed look at the story of lunar origins. From George Darwin’s “Lunar Fission” theory to the current “Giant Impact” model, the scientific journey to understand the birth of our satellite has taken more than a century so far.

But if some scientists are to be believed, the story may not be over yet. According to research published last year by Zhang et al. in Nature Geoscience, the telltale ratio of titanium isotopes in the lunar soil are similar to within four parts per million, and that’s really a bit too close for comfort. The Giant Impact theory predicts that the Moon should be made from a mix of Earth’s ancient mantle and the Mars-sized interloper Theia, but the titanium isotope ratios suggest that the Moon’s ancient material all came from Earth itself.

So are we back to Darwin’s idea of a satellite that simply split away from Earth for some reason? Maybe not. One theory for the origins of Theia is that it actually formed elsewhere on Earth’s own orbit, at one of the stable Lagrangian points, before becoming unstable and eventually smashing into Earth. If this co-orbital idea is right, then the we’d expect Earth and Theia to be pretty similar since they formed out of the same mix of raw materials. But identical to within four parts per million? That might still be pushing it…


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