The Moon Solidified 4.43 Billion Years Ago

What’s the story of our Moon’s early history? Despite all we know about our closest natural satellite, scientists are still figuring out bits of its history. New measurements of rocks gathered during the Apollo missions now show it solidified some 4.43 billion years ago. It turns out that’s about the time Earth became a habitable world.

University of Chicago scientist Nicolas Dauphas and a team of researchers made the measurements. They looked at different proportions of elements inside Moon rocks. They provide a window into the Moon’s early epochs. It started out as a fully molten blob after a collision between two early solar system bodies.

As it cooled and crystallized, the molten proto-moon separated into layers. Eventually, about 99% of the lunar magma ocean had solidified. The rest was a unique residual liquid called KREEP. That acronym stands for the elements potassium (K), rare earth elements (REE), and phosphorus (P).

Dauphas and his team analyzed this KREEP and found that it formed about 140 million years after the birth of the Solar System. It’s in the Apollo rocks and scientists hope to find it in samples from the South Pole-Aitken basin. This is the region where Artemis astronauts will eventually explore. If analysis confirms it there, then it indicates a uniform distribution of this KREEP layer across the lunar surface.

Understanding KREEP’s History on the Moon

The clues to the Moon’s ultimate “cooling off period” lie in a faintly radioactive rare earth element called “lutetium”. Over time, it decays to become hafnium. In the early Solar System, all rocks had about the same amounts of lutetium. Its decay process helps determine the age of the rocks where it exists.

However, the Moon’s solidification and subsequent formation of KREEP reservoirs didn’t result in a lot of lutetium compared to other rocks created at the same time. So, the scientists wanted to measure the proportions of lutetium and hafnium in Moon rocks and compare them to other bodies created around the same time—such as meteorites. That would allow them to calculate a more precise time for when the KREEP formed on the Moon.

They tested tiny samples of Moon rocks and looked at the ratio of hafnium in embedded lunar zircons. Through that analysis, they found that the rock ages are consistent with formation in a KREEP-rich reservoir. Those ages are consistent with the formation of KREEP reservoirs about 140 million years after the birth of the solar system, or about 4.43 billion years ago. “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 Dauphas.

Placing KREEP in Perspective

Interestingly, the team’s results showed that lunar magma ocean crystallization occurred while leftover planetary embryos and planetesimals bombarded the Moon. Those objects were the birth “seeds” of the planets and Moon, which began after the Sun coalesced starting some 4.6 billion years ago. What remained from the formation of the planets continued to batter the already-formed planets.

The formation of the Moon itself began some 60 million years after the solar system itself was born. The most likely event was the collision of a Mars-sized world called Theia with the infant Earth. That sent molten debris into space and it began to coalesce to make the Moon. “We must imagine a big ball of magma floating in space around Earth,” said Dauphas. Shortly thereafter, that ball began to cool. That process eventually resulted in the formation of the lunar KREEP layers.

The study of the decay of lutetium to hafnium in samples of those KREEP rocks is a big step forward in understanding the most ancient epoch of lunar history. More rock samples brought back from the South Pole-Aitken basin will help fill in the remaining blanks and help researchers clarify the timeline of both the cooling of the lunar rock and the subsequent creation of such rock deposits as the mare basalts. Those rock layers were created when impactors slammed into the lunar surface, generating lava flows that filled the impact basins.

The mare formed as a result of impacts later in the early history of the Moon, some 240 million years after the birth of the Solar System formation. Those impacts stimulated lava flows that covered less than 20 percent of the lunar surface and engulfed the oldest surfaces.

Timing is Everything

Fixing the dating of lunar cooling not only tells us about the history of the Moon but helps scientists understand Earth’s evolution. That’s because the impact that formed the Moon was probably also the last major impact on Earth. It could well mark a time when the Earth may have begun its transformation into a stable world. That’s an important step toward evolving into a place 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.”

For More Information

Lunar Rocks Help Scientists Pinpoint When the Moon Crystallized
Completion of Lunar Magma Ocean Solidification at 4.43 Ga
Moon Formation

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