The birth of the moon: The earth was spinning so fast that it almost threatened to break apart. © Don Davis
When the moon was born, the day on earth was probably only two hours long. This new idea, developed by Matija Æuk and Sarah Stewart, could explain why the earth and the moon are chemically the same as one egg the other. About 4.56 billion years ago, when the Proto-Earth collided with another planetary embryo, it swirled so fast around its own axis that a significant portion of the mantle, along with debris from the collision partner, was thrown into orbit. The angular momentum of the Earth-Moon system, so write Æuk and Stewart, was initially larger than today. But by a special constellation, the two transferred about half of their rotational energy to the sun. The emergence of the moon has long been a hassle for planetary scientists. According to current models, the Trabant was born in a massive collision between the Proto-Earth and an object about the size of Mars, often called Theia. Although this theory can explain many properties of the Earth-Moon system, such as the orbital parameters and the size of the moon, it does not explain the nearly identical chemical properties of the two celestial bodies. Because according to the models, the moon would have to consist mainly of material that once belonged to Theia.

Two hours is one day

Æuk and Stewart now figured out if the chemistry would be right if the earth turned much faster at the time of impact than previously thought. Indeed, the higher centrifugal force in their models resulted in part of the mantle landing in orbit after the collision, increasing the rotational speed so much that one day lasted only two hours. However, in this case the earth and the moon would originally have had a much higher angular momentum than today.

The two researchers present a new idea on how the two could have reduced this surplus. Actually, the laws of physics forbid that the angular momentum of a closed system changes. The Earth and the Moon should therefore still have the same angular momentum as they did when they were created. Although the rotation of the earth slows gradually, but as compensation, the moon moves away more and more. This keeps the angular momentum of the system as a whole. display

Unusual constellation

However, Æuk and Stewart suspect that the newborn moon was initially trapped for some ten thousand years in a particular orbit constellation in which the closest point of its orbit was always at an angle of 90 degrees to the sun. In this case, the earth-moon system can deliver energy to the sun by shifting the earth's orbit outwards a bit.

If the Earth-Moon system initially had a higher angular momentum than today, other scenarios move into the realm of the possible. For example, Robin Canup suggests that Theia was about the size of Earth. Even in this case, the equality of the earth and the moon can be explained relatively easily.

Another indication of a collision

That the moon really arose from such a collision, now prove chemical analysis of Randal Paniello and colleagues. The researchers found in moon rock for the first time a chemical signature, which proves that the Trabant was exposed to a total of once large heat in which volatiles evaporated.

Randal Paniello (Washington University, St Louis) et al .: Nature, vol. 490, p. 376, doi: 10.1038 / nature11507 Matija Æuk & Sarah Stewart (Harvard University): Science, Online Preliminary Edition, doi: 10.1126 / science. 1225542 Robin Canup (Southwest Research Institute): Science, Online Preliminary Issue, doi: 10.1126 / science.1226073 © science.de - Ute Kehse

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