The Moon has long been the Earth's close companion, but researchers have struggled to understand exactly when the moon formed, because tiny crystals in the moon rocks brought home by astronauts suggested two different ages.
Now, a study in the journal Nature argues for the earlier age, saying that the ancient Moon also went through a period when it got hot and partially remelted, producing new rocks about 4.35 billion years ago.
The rock-melting heat came from early gravitational interactions with the Earth, which stretched and squeezed the Moon, warming it up.
This process is called "tidal heating." It is how Jupiter currently heats up its moon Io, the most volcanically active spot in the solar system.
"We think that the Moon went through a period when it looked like Io, and for the same reason," says Francis Nimmo, a planetary scientist with the University of California, Santa Cruz.
"There would have been volcanoes jetting off all over the place," he says. "It would have been very dramatic."
The result would be a Moon that seemed younger than its true age.
A spectacular collision
Astronomers have long known that the Earth and Moon are younger than 4.6 billion years, because that's when the solar system got its start.
A huge cloud of dust and gas collapsed inward, forming a new star. Leftover clumps of matter started crashing into each other, glomming together and gradually building up planets.
During this chaotic time, an object the size of Mars smashed into the proto-Earth.
"It would surely have been spectacular," says Nimmo. "It was just so energetic that it's really hard for us to conceive of what it must have looked like."
The crash would have liquified the Earth and vaporized some of it, along with producing the Moon.
Back then, it would have been covered by a magma ocean that had to cool down and solidify into rocks.
Nimmo says that lab workers have analyzed moon rocks brought back by the Apollo astronauts and found that almost all of the samples were 4.35 billion years old, suggesting they formed around 200 million years after the solar system started.
The trouble is, he says, simulations of the solar system's evolution suggest the Moon had to have emerged earlier than that-–because at 200 million years, pretty much all of the material winging around had already been swept up into planets.
"It was hard to imagine that there was something still floating around that would have crashed into the Earth," he says.
This created a conundrum: "The people who were measuring the rocks were giving you one age of the Moon, which was quite young," says Nimmo, "and the people who did the simulations wanted to have a Moon that was older."
He and some colleagues wondered: what if the Moon had formed, cooled down, and then later got hot again and remelted, effectively resetting the clock?
Like Io, but gray
They realized that this was possible if the Moon got heated up by gravitational interactions with Earth.
The Moon used to be much closer to Earth and has been slowly moving farther away over time. Their calculations show that as it moved outward, it would reach a configuration where the orbit "sort of temporarily goes haywire," says Nimmo.
The gravitational forces would squeeze and stretch the Moon, generating heat in the same way that bending a paper clip back and forth makes it warm.
This intense squeezing and stretching would be enough to melt parts of the lunar rock, says Nimmo, and this likely occurred around 4.35 billion years ago, which corresponds to the age of most lunar rocks brought home by Apollo astronauts.
The Moon most likely did not completely remelt, he says: "I think it was mostly solid on the inside, but there was melt being generated all the time and that was being erupted as volcanoes."
Unlike Jupiter's famously erupting moon Io, which appears yellow due to an abundance of sulphur, the Moon would still have looked gray, he says, although the molten rock would have been bright red.
The abundant flowing lava would have filled up old craters, which could account for why the Moon has a dearth of them.
As the Moon continued moving outward and away from Earth, however, it would eventually have stopped experiencing this kind of heating and cooled down again.
This hypothesis explains why some lunar materials seem to have formed up to 4.51 billion years ago—they could be remnants of early rock that never remelted.
"I read this paper with great interest and I find this is a really elegant hypothesis, an elegant model to reconcile all these ages," Philipp Heck, a researcher with the University of Chicago and the Field Museum who has studied ancient lunar samples but wasn't part of the research team. "I was really impressed by it."
He says the researchers make a "compelling" argument that the configuration of the Earth and the Moon would have temporarily produced heating and melting.
This model "is fascinating and explains a lot," says Heck, but he notes that scientists only have lunar rock samples from a limited number of spots on the Moon. "We will see, when we get more samples, how this holds up."
Understanding the age of the Moon is a puzzle for planetary scientists, and it's important for all kinds of researchers who want to understand the history of Earth and how life came to evolve here, he says.
Since the giant collision that formed the Moon basically reset everything on Earth, says Heck, "knowing when this happened and how this happened is really interesting to all of us."
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