Over four billion years ago, during the early days of our Solar System, a catastrophic collision occurred between Earth and an object the size of Mars, leading to the formation of the moon. The exact timing of this momentous event has long eluded scientists.
In a recent publication in Geochemical Perspectives Letters, researchers have shed light on this mystery, utilizing crystals from the moon, brought back by Apollo astronauts in 1972. Their findings suggest that the moon is at least 4.46 billion years old, pushing back previous estimates by 40 million years.
“These crystals are the oldest known solids that formed after the giant impact. And because we know how old these crystals are, they serve as an anchor for the lunar chronology,” said senior author Philipp Heck, a professor of Astronomy at the University of Chicago and curator for Meteoritics and Polar Studies at the Field Museum.
“We were approached by our coauthors, Bidong Zhang and Audrey Bouvier, who needed a nanoscale look at these samples in order to understand them fully,” added lead author Jennika Greer, a former doctoral candidate at the Field Museum and the University of Chicago, who is now a research associate at the University of Glasgow.
The lunar dust samples used in the study were collected by astronauts during the Apollo 17 mission in 1972. These samples contain tiny crystals that formed billions of years ago, providing insights into the moon’s formation.
“When the surface was molten like that, zircon crystals couldn’t form and survive. So any crystals on the moon’s surface must have formed after this lunar magma ocean cooled. Otherwise, they would have been melted and their chemical signatures would be erased,” Heck explained.
Determining the age of the zircon crystals has allowed scientists to establish a minimum age for the moon. A previous study by co-author Bidong Zhang had suggested this age, but the current study employs atom probe tomography to conclusively determine the age of the oldest known lunar crystal.
“In atom probe tomography, we start by sharpening a piece of the lunar sample into a very sharp tip, using a focused ion beam microscope, almost like a very fancy pencil sharpener. Then, we use UV lasers to evaporate atoms from the surface of that tip. The atoms travel through a mass spectrometer, and how fast they move tells us how heavy they are, which in turn tells us what they’re made of,” Greer said.
This detailed analysis revealed the extent of radioactive decay within the zircon crystals, allowing the researchers to determine their age.
Heck has likened radiometric dating to an hourglass: “In an hourglass, sand flows from one glass bulb to another, with the passage of time indicated by the accumulation of sand in the lower bulb. Radiometric dating works similarly by counting the number of parent atoms and the number of daughter atoms they have transformed to. The passage of time can then be calculated because the transformation rate is known.”
The results indicated that the sample was approximately 4.46 billion years old, establishing a minimum age for the moon.
“It’s amazing being able to have proof that the rock you’re holding is the oldest bit of the moon we’ve found so far. It’s an anchor point for so many questions about the Earth. When you know how old something is, you can better understand what has happened to it in its history,” Greer explained.
Heck has highlighted the importance of understanding the moon’s age. “It’s a part of our natural system that we want to better understand, and our study provides a tiny puzzle piece in that whole picture.”
He concluded by emphasizing the moon’s crucial role in stabilizing Earth’s rotational axis, defining the length of our days, and influencing tidal patterns, thus ultimately shaping life on our planet.
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