Over four billion years ago, a giant object the size of Mars slammed into Earth and caused a piece of the planet to break off to form our Moon. Exactly when that happened has long remained a mystery, but new research may have uncovered the answer. A reanalysis of lunar rock has pushed back the age of the Moon by 40 million years, making it at least 4.46 billion years old. The research was published in Geochemical Perspectives Letters. Scientists could determine the Moon’s formation timeline by studying speckles of a mineral called zircon inside samples of lunar dust brought back by Apollo astronauts in 1972, the last time humans set foot on the lunar surface.
The researchers used atom probe tomography, which allowed them to study the samples at the atomic level. They found that many atoms within the zircon crystals had undergone radioactive decay, shedding protons and neutrons and transforming into other elements. As a result, the team could calculate how old a zircon sample was by examining the proportions of different uranium and lead isotopes. In the case of the oldest lunar crystals studied in this work, the proportion of lead isotopes suggested that they were approximately 4.46 billion years old.
Using this information, the scientists compared it to a model of the Moon’s earliest stages based on simulations of the collision that formed the Moon. They discovered that the Moon is significantly older than expected and has been tidally locked to the Earth for much longer than thought, which could explain why the two planets are so alike.
Scientists had guessed at the age of the oldest lunar crystals before, but they needed to be able to look at them at an excellent scale to be confident of their results. This was only possible by deploying the analytical technique of atom probe tomography, which involved sharpening a tiny bit of a crystal sample into a point using a focused ion beam microscope and UV lasers to evaporate atoms from that tip. Lead author of the study, Jennika Greer, a Field Museum doctoral student, explains that atom-by-atom analysis “nailed down” the age of the oldest known lunar crystals at 4.46 billion years old.
Scientists have a better understanding of the history of the Moon, but we still have much to learn about it and the more extensive Solar System. By continuing to explore our cosmic backyard, we can uncover even more of its secrets and help shape our future in the cosmos.