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One of these stars was the Sun, and in its early days, it had many more planets in its orbit, perhaps 20, including proto-Earth and Theia. And then, one fateful day 4.5 billion years ago, they approached at a glancing angle at something less than 5 km/second, and collided.

‘It would have been a very, very bad day for the Earth. One potential outcome could have been complete destruction’

This week, one of the discoveries, by a team led by Daniel Herwartz of the University of Cologne in Germany, was that the moon is mostly made of Theia.

“This hypothesis [the Big Splash] can explain a lot of features of the moon, but there is one problem, that if you model this process [by computer], these computer models predict that most of the debris [that forms the moon] is coming from the impacting body [Theia], and because every body in the solar system has its own unique fingerprint, you would expect to have different isotopic compositions of the Earth and the Moon,” said Prof. Herwartz, who studied the composition of lunar meteorites and rocks brought back to Earth by the Apollo Moon missions. “Until now, this difference has not been found, and we’ve now found it.”

He is less optimistic than Prof. Mukhopadhyay about finding traces of proto-Earth in the modern Earth. The intervening 4.5 billion years of “mantle convection certainly erased most, hopefully not all, evidence of distinct materials from proto-Earth or Theia within the Earth,” he said. “We really don’t know if we could find this material anywhere on Earth. … We don’t know if it’s anywhere. It might have been all mixed away.”

Still, a sample of unmelted proto-Earth would be a major prize for earth origins scientists, big enough that Prof. Mukhopadhyay’s idea about a layer down near the core might be worth pursuing, for all the answers it could offer up about the days before the Big Splash.

“If I find this, if you can give me like ten milligrams of proto-Earth, I could tell you how large the impactor was and what its composition was,” Prof. Herwartz said. “This would be very exciting, but currently we have no idea where that might be in the Earth.”

National Post

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