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How was the moon made? We will not know before our return

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David Scott and James Irwin went to the moon with a shopping list. The geologists wanted the two men, the Apollo 15 astronauts, to collect samples that would better understand the mysterious formation of the moon and its early years. On the second day of their visit, on 1 August 1971, MM. Scott and Irwin won the jackpot.

They took samples inside the Mare Imbrium, or sea of ​​rain, when Moon saw an unusual rock. He approached him, lifted him up and dusted him off. Mr. Irwin immediately saw that the rock was special.

"Oh, man!" Mr. Irwin shouted. "Look at the glow!"

The two astronauts shouted with joy.

"I think we found what we were looking for," Scott said by radio in Houston.

The rock was anorthosite, formed when a mineral called feldspar was crystallized in molten rock. Its existence hinted that the moon was once covered with an ocean of magma, where the feldspar crystals would have floated like icebergs. This was thought to be part of the primordial crust of the moon, and reporters covering the mission soon dubbed it "rock of genesis".

"It tells you about the sources that have penetrated the moon and reduced what has formed the moon," said Ryan Zeigler, conservator of NASA's Apollo club at rock at Johnson Space Center in Houston.

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To solve this problem, theorists modify their computer simulations. In a report published in April, Japanese scientists, led by Natsuki Hosono of the Yokohama Institute of Earth Sciences, described how the still molten Earth could have disappeared from the moon, provided that Theia struck the planet while she was still covered with an ocean of magma.

Another theory, proposed for the first time in 2017, asks Theia and the Earth to vaporize, forming a bagel-shaped fire cloud that is neither a planet nor a disk. The short-lived structure, called synestia, is a hot gas drop whose outer edges rotate so fast that they essentially go into orbit. Once it begins to cool and solidify, the droplets of rock condense and fall on the embryonic planet in the center. The remaining debris in the outer edges of the bagel would also condense, but at a greater distance, forming the moon.

"We can not use current models of lunar accretion to study this," said Simon J. Lock, a postdoctoral researcher at the California Institute of Technology, who developed the theory with Sarah Stewart at the University of California at Davis , in March at Lunar. and planetary science conference in Houston. "We need to include differential equations and include physics. It will be difficult – I am sorry. But if we want to understand how our moon has formed and associate its chemical properties to its origin, we must overcome these challenges. "

The young days of the moon are also the subject of intense debate.

Mare Imbrium, where MM. Scott and Irwin landed, is a vast plain of lava and an impact crater that formed about 3.8 billion years ago, when a titanic asteroid or a protoplanet collided with the moon. When scientists began measuring Apollo samples, they discovered that most of them had experienced some kind of horrible warming at that time.

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