One of these unsolved questions is its origin story. We think the moon formed after a colossal collision between Earth and another huge object created two balls of molten magma. But we don't know exactly when or how.
Now, scientists have made new measurements on moon rocks from the Apollo missions to set a date for the time the moon solidified: 4.43 billion years ago.
The study, by a collaboration including University of Chicago scientists, applied cutting-edge techniques to make ultraprecise readings of rare minerals in the rocks. The results add evidence for our understanding of the formation story of both the moon and Earth.
This puts a precise age for the formation of the moon-and also potentially the time that the Earth became habitable.
"It took us years to develop these techniques, but we got a very precise answer for a question that has been controversial for a long time," said Nicolas Dauphas, the first author on the paper, who heads the UChicago Origins Laboratory and is a professor in the Department of the Geophysical Sciences and the Enrico Fermi Institute.
The solar system formed about 4.57 billion years ago. Shortly afterward, as it cooled, debris began colliding and clumping together over time, forming the planets. Scientists believe that a very large object crashed into the burgeoning Earth, and that our moon formed from the debris produced by this impact.
This collision was extremely violent, Dauphas said, enough to melt rocks; "so at first we must imagine a big ball of magma floating in space around Earth."
The moon quickly began to cool. Most of the lunar magma ocean solidified almost right away by geologic standards-about a single millennium.
But once about 80% of the magma had solidified, a crust of minerals formed, which insulated the young moon. "It's like putting on a coat in Chicago when it's cold out-you will not lose heat as fast," said Dauphas. That slowed down the cooling process, and for a time the moon had a partially molten mantle. What we haven't been able to pin down is how long it stayed that way, before it cooled all the way down and became solid rock.
Studies of samples from the Apollo missions revealed that as the moon slowly cooled, a mixture of certain elements would have floated upward in the partly molten mantle-a bit like the flaky white salt left behind when seawater evaporates-and formed a distinct layer. Scientists figured that if they could find a precise age for this magma layer, which contained a lot of potassium, rare earth elements, and phosphorous (KREEP for short), they would know when the moon was 99% crystallized.
Dauphas and collaborators wondered if an answer might be locked inside moon rocks retrieved by Apollo astronauts.
In the early solar system, all the rocks had the same amount of lutetium. But the solidification process that formed KREEP did not favor lutetium, so that layer has lower levels of lutetium than other rocks from the same era.
If the scientists could very precisely measure the proportions of lutetium and halfnium in moon rocks compared to other things from that same era but that came from other places around the solar system, like meteorites, they could calculate backward to see when the KREEP layer formed-and thus when the moon was just about done cooling.
The trouble is, we only have small, precious samples of moon rocks. So the team, including Cindy Xi Chen, PhD'22, had to develop extremely rigorous techniques to separate out the different elements.
By testing tiny samples of moon rocks retrieved from multiple Apollo missions, they came up with an age for the cooling of the moon: 4.43 billion years before present day.
Based on other studies, scientists think it would have taken the moon about 20 million years to cool to that level. Working backwards, that puts the formation of the moon itself at about 4.45 million years ago.
This not only tells us about the history of the moon, but also about the formation of the Earth, since the impact that birthed the moon was probably also the last major impact to Earth-marking the date when the Earth may have first become stable, becoming hospitable for life.
"This finding aligns nicely with other evidence-it's a great place to be in as we prepare for more knowledge about the moon from the Chang-e and Artemis missions," said Dauphas. "We have a number of other questions that are waiting to be answered."
Dauphas said he dedicated the study in memory of his wife and fellow geochemist Reika Yokochi, a research professor at UChicago who passed away in 2024. "She was instrumental to all aspects of my research," he said.
Research Report:Completion of lunar magma ocean solidification at 4.43 Ga.
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