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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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Theia makes the moon
Before the Apollo missions, astronomers debated several explanations about the making of the moon. Some argued that the moon was forging along the Earth, while others thought it had formed elsewhere in the solar system and the gravity of the Earth had captured it. George Howard Darwin, an English astronomer and son of Charles Darwin, proposed to throw him to the ground.
Scientists studying the rock of genesis have developed a new story. Shortly after the Earth 's merger, about 4.5 billion years ago, an object of the size of Mars called Theia, named after the goddess mother of the moon in Greek mythology, s & # 39; is crushed on our young planet. The Earth could have liquefied as Theia broke and her remains formed the moon.
But in recent years, many details of this "giant impact hypothesis" have been questioned.
Where is Theia?
In 2001, Swiss researchers measured 30 samples of Apollo with modern instruments and discovered that their oxygen variants were indistinguishable from those of the Earth. Since then, geochemists have been studying titanium, tungsten and many other metals from the Earth and the Moon, and no one can distinguish them.
This is strange. If the moon were made of Theia, it should look like Theia and not the Earth. And Theia should be chemically distinct from Earth, just as the other planets are all different from each other.
A problem of physics
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. "
A primordial bludgeoning – or not
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.
In 1974, the researchers argued that the internal solar system was pounded by asteroids that suddenly migrated inward, an event called the lunar cataclysm, or late heavy bombardment. The Earth erased the craters that would have formed at the time, the result of plate tectonics. But the Moon still bears a memory of this bludgeoning.
Attack plays an important role in understanding the history of the solar system. But in recent years, many scientists have come to believe that this has never happened.
Are Apollo samples biased?
NASA chose the Mare Imbrium landing site partly because the geologists wanted a similar sample to the genesis, which is why MM. Scott and Irwin were so happy that day. But today, some geologists believe that Imbrium's ejectas of the singular impact may have crossed large parts of the near-moon face, including the regions sampled by the six Apollo missions. It could mean Apollo rocks are systematically biased.
"In my opinion, the cataclysm does not exist, it's an artifact of the Apollo samples," said Harald Hiesinger, a geologist at the University of Münster in Germany, at the Houston meeting.
Break in the cache
Over the next year, NASA will open three new samples stored since their arrival on Earth in the 1970s. The agency officials wish to train a new generation of scientists who will study lunar rocks.
"It's not because we buy new ones that we need them anymore, but because we need to learn more about curation and sample handling," said Sarah Noble, the program's analysis manager. Apollo Next Generation Analysis Group from NASA.
Dr. Zeigler said that a fresh look at Apollo's specimens, including a new analysis of sealed pristine rocks since their descent to Earth in 1972, would help tell a more complete story of the birth and deaths. first years of the moon. But many scientists hope to have brand new rocks.
"There is still a lot to learn about the moon from Apollo samples," said Dr. Zeigler. "But the next big jump is new samples from elsewhere."
This could happen soon and help solve the problem of sample bias.
In May, the Trump administration announced plans to return humans to the Moon by 2024, as part of a program called Artemis. A Chinese mission, Chang'e 5, is also expected to bring a sample of the moon home.
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