Moon crystals formed at least 4.46 billion years ago after the magma ocean cooled. More than 4 billion years ago, when the solar system was still young and the Earth was still growing, a huge celestial body the size of Mars hit the Earth. The largest piece of early Earth that broke off formed our moon. But exactly when this happened has been a mystery.In a new study in the journal Geochemical Perspectives Letters, researchers used crystals brought back from the moon by Apollo astronauts in 1972 to help determine when the moon formed. Their findings push back the moon's age by 40 million years, or at least 4.46 billion years.
In 1972, Apollo 17 astronaut Harrison Schmitt was covered in moon dust. Researchers measured lunar crystals from the mission and found that the moon is 4.46 billion years old - 40 million years older than previously thought. Source: NASA
"These crystals are the oldest known solids formed after giant impacts," said Philipp Heck, director of the Field Museum's Robert Pritzker Collection of Meteoroidology and Polar Studies, senior director of the Negauni Center for Interactive Research, a professor at the University of Chicago and senior author of the study. "Because we know the age of these crystals, they are an anchor for the lunar chronology."
The discovery resulted from Heck's collaboration with the study's first author, Jennika Greer, who was a doctoral candidate at the Field Museum and the University of Chicago. "We were approached by our co-authors Bidong Zhang and Audrey Bouvier who needed to take nanoscale observations of these samples to fully understand them," said Greer, now a research associate at the University of Glasgow.
The lunar dust samples used in the study were brought back by the Apollo 17 astronauts during the last manned mission to the moon in 1972. This dust contains tiny crystals formed billions of years ago. These crystals are clues to when the moon formed.
When a Mars-sized object struck the Earth and formed the moon, the energy of the impact melted the rocks that would eventually become the moon's surface. "Zircon crystals cannot form and survive when the lunar surface melts like this. Therefore, any crystals on the lunar surface must have formed after the lunar magma ocean cooled," Heck said. "Otherwise, they would be melted and their chemical signature would be erased."
Since the crystals must have formed after the magma ocean cooled, determining the age of the crystals would reveal the minimum possible age of the moon. A previous study by study co-author Bidong Zhang had suggested this age, but this latest study is the first to use an analytical method called atom probe tomography to "determine" the age of the oldest known lunar crystal.
"In atom probe tomography, we first use a focused ion beam microscope to sharpen a lunar sample into a very sharp tip, like a very nice pencil sharpener," Greer said. "We then use an ultraviolet laser to evaporate the atoms from the surface of the tip. The atoms pass through the mass spectrometer, and the speed at which they move tells us how heavy they are, which in turn tells us what they are made of."
This atom-by-atom analysis, using instruments at Northwestern University, shows how many atoms inside the moon rock crystals underwent radioactive decay. When the configuration of protons and neutrons in the nucleus is unstable, the atom will decay, shed some protons and neutrons, and transform into different elements. For example, uranium decays into lead. Scientists have determined how long this process takes, and by looking at the ratios of different uranium and lead atoms (called isotopes) in the sample, they can tell the age of the sample.
"Radiometric dating works a little like an hourglass," Heck said. "In an hourglass, sand flows from one glass sphere to another, and the passage of time is represented by the accumulation of sand in the lower glass sphere. Radiometric dating works similarly, by counting the number of parent atoms and the number of daughter atoms they are converted into. Since the conversion rate is known, the passage of time can be calculated."
The lead isotope ratios found by the researchers suggest the sample is about 4.46 billion years old. Therefore, the moon is at least that old.
"It's amazing to be able to prove that this rock in your hands is the oldest part of the moon that we've discovered so far. It's an anchor for a lot of questions about Earth," Greer said. "When you know how old something is, you can better understand what happened to it historically."
Heck said understanding when the moon formed is important because "the moon is an important partner in our planetary system - it stabilizes the Earth's axis of rotation, is the reason there are 24 hours in a day, and is why we have tides. Without the moon, life on Earth would be unrecognizable. It's a part of the natural system we want to better understand, and our study provides a small piece of the puzzle for the whole picture."