Roughly 4.5 billion years ago, our solar system formed from a chaotic swirling cloud of gas, dust and debris. And as our planet formed, it's believed that something roughly the size of Mars slammed into it. The rocks that remained from that impact came together to form our moon.
But just when that happened has been uncertain.
Now, a new study published in Geochemical Perspectives Letters that used advanced analysis of crystals from a moon rock collected during the Apollo 17 mission in 1972, has confirmed that the moon formed roughly 4.46 billion years ago. That makes it roughly 40 million years older than once believed.
This moon rock — called 72255 — has been analyzed many times. In fact, a 2021 study led by graduate student Bidong Zhang and co-authored by professor Audrey Bouvier — both of whom were from Western University in London, Ont. — was the first to come up with this age.
However, there was some skepticism at the time.
In order to date moon rocks, scientists look at zircons contained within them. These crystals — the first solids to form after the moon's magma ocean cooled — are excellent geological timekeepers, or chronometers, partially due to their durability. Additionally, they contain radioactive uranium, which in turn decays into lead. Scientists know how long it takes for this to occur.
"But they're not perfect," said Philipp Heck of the crystals.
Heck is the curator of meteoritics and polar studies at the Field Museum of Natural History in Chicago and senior author of the new study.
"There can be crystal defects in zircons that can actually occur because of radioactive decay."
While the 2021 study used ion microprobe dating, which can see within roughly 10 microns (one micron is 1/1000 of a millimetre), they needed to delve even deeper to confirm that date, down to the nanoscale, or 1000 times smaller.
This sample of crystal from a moon rock collected during the Apollo 17 mission shows where it was analyzed in 2021, down to 10 microns. (Jenniker Greer)
So Heck, lead author Jenniker Greer and co-authors turned to a method called atom probe tomography (APT) which can analyze the atoms contained within the crystal.
"We take several tiny slivers from that crystal at different locations throughout the crystal, and then we use atom probe tomography to analyze it," Heck explained. "What it does, basically, is shape the sample into a very sharp tip. And that gets put in a vacuum. A current is applied, and then we hit it with a UV laser, and then … evaporate it atom by atom."
They found that the lead isotopes left over from the radioactive decay were roughly 4.46 billion years old.
"We can now say this grain is really 4.46 billion years old," Heck said. "This age is now unambiguous."
Just a part of the puzzle
Chris Herd, a planetary geologist and curator of the University of Alberta's Meteorite Collection Museum, finds the new results intriguing.
"The results, as shown in the paper, they corroborate the original age that they got. So that excites me," he said. "I'm always interested in that sort of advanced application of methods to samples that were collected, you know, 51 years ago. Because that's the whole purpose of sample return."
Herd has a deep interest in these sorts of missions. He is a sample scientist for NASA's Mars Perseverance rover mission. The rover is busy on the surface of the red planet analyzing it, but also collecting samples as part of a yet undated return mission.
He believes it provides an impetus to return to the moon.
"That's a big driver for going back and getting samples from a different part. There's lots of emphasis on the south pole," he said. "If you were to go and get something from the surface, you're gonna get a grab bag of a bunch of different things.'
This image from the atom probe tomography analysis sample shows down to 200 nanometres in the moon rock collected during the Apollo 17 mission. (Jenniker Greer)
Meanwhile, Heck said that just because the analysis shows the sample age to be 4.46 billion years old, it doesn't mean it's the oldest there is.
"Ours is the oldest directly dated zircon currently," he said. "Even with the rocks that we have now from the Apollo program, I am convinced there is older stuff in there. We just haven't found that yet."
He said it will be interesting when we collect more moon rocks from other locations, which may provide a fuller picture of the moon's evolution and history, including more about its magma ocean.
And, he said, learning more about the moon is critical in understanding our own planet.
"We have to always improve the chronology because there's so much tied to it. It's anchored in the lunar chronology. It's also anchored in the habitability of Earth," he said. "The habitability of Earth only happened after the moon formed. And obviously, the moon had a big impact — no pun intended — on Earth dynamics."