The moon is our closest neighbor in space and the only celestial body humans have set foot on, yet we are still learning about it. In fact, Earth’s moon might actually be 40 million years older than scientists previously believed. By conducting an atom-by-atom analysis on crystals that were brought back by Apollo astronauts in 1972, a team of geochemists and plenary scientists now calculate that the igneous orb is at least 4.46 billion years old. The findings are described in a study published today in the journal Geochemical Perspectives Letters.
Based on samples recovered from the moon’s surface, we know that it formed more than four billion years ago when a giant object the size of Mars crashed into a very young Earth. The largest piece of the planet that broke off during that collision became our moon, and has been intertwined with the solar system ever since.
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“Life on Earth has evolved with the moon, and the moon has an influence on life on Earth,” study co-author and cosmochemist Philipp Heck tells PopSci. “It inspires us, which is reflected in cultural heritage, but it also has other beneficial effects like stabilizing Earth’s rotational axis.”
Heck is a curator for the meteorite collection at the Field Museum in Chicago and a professor at the University of Chicago. He says that studying the moon also helps us understand our own planet because of the topographical differences.
“Earth’s surface is much, much younger because there’s so much geologic activity [here] from volcanism and weathering,” explains Heck. “The moon’s surface is essentially an archive of solar system dynamics. This is a record that we don’t have on Earth, but our planet’s evolution is tied to these impacts that happened in the early solar system.”
A historical perspective
In the study, the team looked at moon dust brought back by the Apollo 17 crew. The 1972 lunar landing included NASA geologist Harrison Schmidt, who collected multiple rocks to study back on Earth. His samples contain very small crystals that were created billions of years ago and can help indicate when the moon was formed.
The energy created by the impact from the object that struck Earth and created the moon melted the rock that eventually became the lunar surface. That offers a clue to the elements that existed on the celestial body since its emergence versus the ones that appeared much later. For example, zirconium, a silver metal found on both the Earth and the moon, could not form and survive on the molten lunar surface: Any zircon crystals that are currently present on the moon must have formed after the magma ocean cooled. Determining the age of these structures can thus reveal the minimum possible age for the moon, assuming that they emerged right after the impact.
Looking atom by atom
Researchers have previously suggested that the moon is older than estimated, but this new study is the first to use an analytical method called atom probe tomography to pinpoint the age from the oldest known lunar crystal retrieved by humans.
“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,” study co-author and planetary scientist Jennika Greer said in a statement. “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.”
This atom-by-atom analysis revealed how much of the zircon crystals had undergone radioactive decay—a process where atoms that have an unstable configuration shed some protons and neutrons. They then transform into different elements, like how uranium decays into lead. Based on the amount of conversion and the known half-lives of different chemical isotopes, experts can estimate the age of the sample.
“Radiometric dating works a little bit like an hourglass,” Heck said in a statement. “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 team working with the Apollo 17 sample found that the proportion of lead isotopes (the daughter atoms created during the decay) indicated that the crystals were about 4.46 billion years old, so the moon must at least be that old too. While this puts the moon’s age back 40 million years, that’s still a very short time compared to the universe’s roughly 13.7 billion-year history.
“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 said.
From Apollo to Artemis
In future studies, clues pulled from these decades-old samples could be pooled with those from samples taken by upcoming Artemis lunar missions. Artemis III is scheduled for 2025 and will land on and explore the lunar South Pole. The Apollo 17 mission collected samples from the Taurus-Littrow valley on the eastern edge of Mare Serenitatis, so crystals from a different region of the moon could yield unimaginable discoveries.
“I am convinced that there is older stuff on the moon—we just haven’t found it yet. I even think we have older zircons in the Apollo samples. This is really the power of sample return,” says Heck.
A mixture of new samples and future advances in technology could further anchor the timeline of how our solar system was formed and beyond. “Maybe in 50 or 100 years or even later, new generations of scientists will have the tools we can only dream about today to address scientific questions we can’t even think about today,” says Heck. “These templates are a legacy for future generations.”