The closer scientists look at theories of how the moon formed, the more questions they find.
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David Scott and James Irwin traveled to the moon with a shopping list. Geologists wanted the two men, Apollo 15 astronauts, to collect samples that would shed light on the moon’s mysterious formation and early years. On their second day there, Aug. 1, 1971, Mr. Scott and Mr. Irwin hit the jackpot.
They were taking samples inside the moon’s Mare Imbrium, or Sea of Rains, when Mr. Scott saw an unusual rock. He walked up to it, lifted it and dusted it off. Mr. Irwin saw right away that the rock was special.
“Oh, man!” Mr. Irwin shouted. “Look at the glint!”
Both astronauts shouted with glee.
“I think we found what we came for,” Mr. Scott radioed Houston.
The rock was an anorthosite, made when a mineral called feldspar crystallized inside molten rock. Its existence hinted that the moon was once coated in an ocean of magma, where the feldspar crystals would have floated like icebergs. It was thought to represent part of the moon’s primordial crust, and reporters covering the mission soon nicknamed it “genesis rock.”
“That tells you about the source material that went into the moon, and narrows down what formed the moon,” said Ryan Zeigler, the curator for NASA’s Apollo rocks at the Johnson Space Center in Houston.
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Theia makes the moon
Before the Apollo missions, astronomers debated several explanations for how the moon was made. Some argued that the moon was forged alongside Earth, while others thought that it formed elsewhere in the solar system and that Earth’s gravity captured it. George Howard Darwin, an English astronomer and the son of Charles Darwin, proposed that it was flung from the infant Earth.
Scientists studying the genesis rock developed a new story. Soon after Earth coalesced, about 4.5 billion years ago, a Mars-size object called Theia, named for the mother of the moon goddess in Greek mythology, crashed into our young planet. Earth might have liquefied, while Theia shattered and its remnants formed the moon.
But in recent years, many details of this “giant impact hypothesis” have come into question.
Where is Theia?
In 2001, Swiss researchers remeasured 30 Apollo samples with modern instruments and found that their variants of oxygen were indistinguishable from those on Earth. In the years since, geochemists have studied titanium, tungsten and many other metals from Earth and the moon, and nobody can tell them apart.
This is strange. If the moon was made from Theia, it should look like Theia, not Earth. And Theia should be chemically distinct from Earth, just as the other planets are all different from one another.
A physics problem
To solve this problem, theorists are making tweaks to their computer simulations. In a report published in April, Japanese scientists, led by Natsuki Hosono of the Yokohama Institute for Earth Sciences, described how the still-molten Earth might have sloughed off the moon, so long as Theia thwacked the planet while it was still covered in a magma ocean.
Another theory, first proposed in 2017, calls for Theia and Earth to vaporize, forming a bagel-shaped cloud of fire that is neither a planet nor a disc. The short-lived structure, called a synestia, is a scorching gas blob with outer edges rotating so fast that they essentially launch into orbit. Once it begins to cool and solidify, rock droplets condense and fall onto the embryonic planet at the center. Leftover debris in the outer edges of the bagel would also condense, but at a greater distance, forming the moon.
“We can’t use current lunar accretion models to study this,” Simon J. Lock, a postdoctoral researcher at the California Institute of Technology who developed the theory with Sarah Stewart at the University of California, Davis, said in March at the Lunar and Planetary Science Conference in Houston. “We need to include differential equations, and we need to include the physics. It’s going to be hard — I’m sorry. But if we want to understand how our moon formed, and link its chemical properties to its origin, we need to overcome these challenges.”
A primordial pummeling — or not
The moon’s younger days are also the subject of intense debate.
Mare Imbrium, where Mr. Scott and Mr. Irwin touched down, is a vast lava plain and impact crater that formed about 3.8 billion years ago, when a titanic asteroid or protoplanet collided with the moon. When scientists started measuring Apollo samples, they found most of them had experienced some kind of horrific heating around that time.
In 1974, researchers argued that the inner solar system was pounded by asteroids that suddenly migrated inward, an event called the lunar cataclysm, or the Late Heavy Bombardment. Earth has erased the craters that would have formed back then, a result of plate tectonics. But the moon still bears a record of this pummeling.
The onslaught looms large in how we understand the history of the solar system. But in recent years, many scientists have come to believe it never happened.
Are the Apollo samples biased?
NASA chose the landing site in Mare Imbrium in part because geologists wanted a sample just like the genesis rock, which is one reason Mr. Scott and Mr. Irwin were so happy that day. But today, some geologists think the ejecta from the singular Imbrium impact may have traveled across great portions of the moon’s near side, including the regions sampled by the six Apollo missions. That could mean the Apollo rocks are systematically biased.
“In my opinion, the cataclysm does not exist, but is an artifact of the Apollo samples,” Harald Hiesinger, a geologist at the University of Münster in Germany, said at the Houston meeting.
Breaking into the cache
Sometime in the next year, NASA will open three new samples that have been in storage since they were brought to Earth in the 1970s. Managers at the agency want to train a new generation of scientists who will study moon rocks.
“It’s not that we don’t care anymore because we’re getting new ones, but because we need to learn more about curation and how to handle the samples,” said Sarah Noble, program scientist for NASA’s Apollo Next Generation Sample Analysis group.
Dr. Zeigler said a fresh look at Apollo samples, including new analysis of pristine rocks that have been sealed since being brought down to Earth in 1972, would help tell a more complete story of the moon’s birth and early years. But many scientists are hoping for entirely new rocks.
“There’s still a lot to learn about the moon from the Apollo samples,” Dr. Zeigler said. “But the next big leap is new samples from somewhere else.”
That may happen soon, and it could help overcome the sample-bias problem.
In May, the Trump administration declared plans to return humans to the moon by 2024, under a program called Artemis. And a Chinese mission, Chang’e 5, is also planned to bring home a sample from the moon.
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