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The moon is much more than a largely dead orb. The pale satellite of our planet is the creator of the tides, the meteor sensor and the only other world in the starry ocean where humanity has set foot. But scientists still do not know how it was made. The resolution of this mystery would not only reveal the origins of the moon, it would also help to explain the evolution of our planet.
A study published Monday in Nature Geoscience suggests that the moon was forged from the fires of an ocean of magma sliding on the surface of the small Earth. If it's correct, this model can solve a long-standing paradox.
Lunar meteorites and samples collected during Apollo missions show that the Moon and Earth have remarkably similar geochemical fingerprints. Scientists suspect that it is likely a giant impactor of the size of Mars, known as Theia, which crashed into a young Earth and put into orbit a spiral of materials that have fused into the moon.
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Many computer simulations show that this is possible, but there is a problem. Such an impact on a relatively solid Earth would have created a moon made mainly of Theia, and not Earth (at least in the simulations leading to the Earth-Moon system that we observe today, supplemented by our 24-hour days today. ).
To try to resolve this paradox, some models have opted for more energetic impacts, according to Bill Bottke, a global scientist at the Southwest Research Institute in Boulder, Co., who was not involved in this study. These models extract more Earth materials for moon building, but the moon's orbit or the Earth's spin is not accurately reproduced without resorting to complicated workarounds.
The new research, led by Natsuki Hosono of the Japan Agency for Marine and Land Science and Technology, adds what might be the missing ingredient in the recipe. At the beginning of the Earth, it was covered with a layer of molten silicate. Dr. Hosono's team wondered what would have happened if Theia had crashed on Earth at that time, rather than in a newer, colder and more solid phase.
The team used a standard moon training simulation but adjusted it to better replicate the density changes in the objects. With these adjustments, they found that the ocean magma made all the difference.
Immediately after the impact of Theia, the hot and viscous layer of the Earth would have violently poured into space in huge volumes. This material orbited and fused around the young Earth, forming a moon composed of about 70% of terrestrial material, much more than in the older, largely solid terrestrial models, which appeared at about 40%.
Robin Canup, an astrophysicist at the Southwest Research Institute, said that this magma-ocean model allowed us to explain the geochemical mirror of both objects, while reproducing the way the Earth and the Moon dance. "It's a very exciting result," she said. However, as it is very different from previous works, the key to the validity of this model is to see if future models can reproduce it.
Part of the problem is that scientists do not have enough samples to get the model data. Although the Apollo rocks are an important scientific resource, they come only from a small plot near the equator of the moon. "Imagine if we were trying to reconstruct the whole history of the Earth using a handful of samples," she said. "We could not do it."
The next step, Dr. Russell said, is clear: we must go back to the moon, dig deep, and bring home as many lunar fragments as possible.
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