After landing on the moon on December 11, 1972, NASA astronauts Eugene Cernan and Harrison Schmitt collected rocks and dust from the lunar surface. A new analysis of that sample detected zircon crystals and dated them to 4.46 billion years old. Previous estimates put the moon, formed by a massive celestial collision, at 4.425 billion years old.
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The findings were published Monday in the journal Geochemical Perspectives Letters.
"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 study author Philipp Heck, Robert A. Pritzker Curator for Meteoritics and Polar Studies at the Field Museum of Natural History in Chicago, in a statement.
The early days of our solar system - when Earth was still forming and growing in size - were chaotic, with rocky bodies often colliding in space. During that time more than 4 billion years ago, a Mars-size object crashed into Earth, flinging off a large rocky piece that became the moon, according to the researchers. But scientists have struggled to precisely date this pivotal event.
The energy from the impact of the Mars-size object hitting Earth melted the rock that would eventually form the moon's surface.
"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," said Heck, who is also the senior director of the museum's Negaunee Integrative Research Center and a professor in the department of geophysical sciences at the University of Chicago.
"Otherwise, they would have been melted and their chemical signatures would be erased."
High-tech atomic analysis
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Previous research by study coauthor Bidong Zhang, assistant researcher in the department of Earth, planetary, and space sciences at the University of California, Los Angeles, had suggested that determining the age of the crystals within the lunar dust might reveal the moon's actual age as well.
Zhang and fellow coauthor Audrey Bouvier, professor of experimental planetology at Bayreuth University in Germany, approached Heck and lead study author Jennika Greer, a research associate in Earth sciences at the University of Glasgow, to take a nanoscale look at the crystals using an advanced technique to determine their chemical composition and pinpoint the moon's age.
This research marks the first use of the analytical method of dating the crystals with atom probe tomography and was carried out using instruments at Northwestern University in Evanston, Illinois, according to the study authors.
"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," said Greer, who was a doctoral candidate at the Field Museum and the University of Chicago when she worked on the study. "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."
The analysis showed how many uranium atoms within the zircon crystals experienced radioactive decay. Elements can transform if their atoms contain an unstable configuration of protons and neutrons, causing some of them to decay - such as how uranium decays to become lead. By tracking how long this process takes, scientists can determine the age of something by comparing the ratio of uranium to lead atoms.
"Radiometric dating works a little bit like an hourglass," Heck said. "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."
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'The oldest bit of the Moon'
The research team used lead isotopes within the lunar dust sample to determine that the crystals were 4.46 billion years old, indicating that the moon must also be at least that old.
"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," Greer said. "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."
Although the lunar samples were returned to Earth more than 50 years ago, it has taken time to develop the technology needed to conduct such a detailed analysis of the crystals. This is why NASA has waited to unseal some of the pristine samples collected during the Apollo era until recent years, allowing for more insights into our planet's natural satellite using the most advanced methods.
"The Moon is an important partner in our planetary system," Heck said. "It stabilizes the Earth's rotational axis, it's the reason there are 24 hours in a day, it's the reason we have tides. Without the Moon, life on Earth would look different. 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."
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