By analyzing tiny lunar zircon crystals gathered by Apollo 17 astronauts in 1972, planetary scientists recalculated the age of the Moon. Although previous assessments estimated the Moon as 4.425 billion years old, the authors discovered it is actually 4.46 billion years old — around 40 million years older than previously thought.
Greer et al. analyzed tiny zircon crystals inside lunar dust collected in 1972 by Apollo 17 astronauts. Image credit: Eugene A. Cernan / Eric Hartwell.
“These zircon crystals are the oldest known solids that formed after the giant impact,” said University of Chicago’s Professor Philipp Heck.
“And because we know how old these crystals are, they serve as an anchor for the lunar chronology.”
The sample of lunar dust used in the study was brought back by Apollo 17 astronauts in the last crewed mission to the Moon in 1972.
This dust contains tiny zircon crystals that formed billions of years ago. These crystals are a telltale sign of when the Moon must have formed.
When a Mars-size protoplanet, Theia, hit the growing proto-Earth and formed the Moon, the energy of the impact melted the rock that eventually became the lunar 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,” Professor Heck said.
“Otherwise, they would have been melted and their chemical signatures would be erased.”
Since the crystals must have formed after the magma ocean cooled, determining the age of the zircon crystals would reveal the minimum possible age of the Moon.
A previous study has suggested this age, but the new study marks the first use of an analytical method called atom probe tomography which ‘nailed down’ the age of this 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,” said Dr. Jennika Greer, a researcher at the University of Glasgow.
“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 showed how many of the atoms inside the zircon crystals had undergone radioactive decay.
When an atom has an unstable configuration of protons and neutrons in its nucleus, it undergoes decay, shedding some of these protons and neutrons and transforming into different elements.
For instance, uranium decays into lead. Scientists have established how long it takes this process to occur, and by looking at the proportion of different uranium and lead atoms (called isotopes) present in a sample, they can tell how old it is.
“Radiometric dating works a little bit like an hourglass,” Professor 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.”
The proportion of lead isotopes that the researchers found indicated that the sample was about 4.46 billion years old. Therefore, the Moon has to 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. 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,” Dr. Greer said.
“It’s important to know when the Moon formed, because the Moon is an important partner in our planetary system — 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.”
The findings were published in the journal Geochemical Perspectives Letters.
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J. Greer et al. 2023. 4.46 Ga zircons anchor chronology of lunar magma ocean. Geochemical Perspectives Letters 27; doi: 10.7185/geochemlet.2334
