If there’s a man in the Moon, as the old beliefs go, he’s a pretty venerable one. Earth’s lunar companion is thought to have formed not long after the planet itself, some 4.4 billion years or so ago, when the Solar System was young. It was then, according to theory, that a Mars-sized object smacked into Earth, which was still warm and squishy and newly formed, and broke off a huge cloud of debris that coalesced into the Moon in Earth’s orbit. But the Moon’s youthful good looks are apparently deceptive. A new study of tiny grains of zircon in Apollo lunar samples suggests that it’s even older than we thought, by a good 40 million years.
That means that the Moon is at least 4.46 billion years old, says a team led by geologist Jennika Greer, now at the University of Glasgow. This revelation places the Moon just a hair younger than Earth, which is an estimated 4.54 billion years old. “These crystals are the oldest known solids that formed after the giant impact,” says cosmochemist Philipp Heck of the Field Museum and University of Chicago. “And because we know how old these crystals are, they serve as an anchor for the lunar chronology.”
It’s unknown precisely how the Moon formed and when, but the presence of some specific elements strongly suggests a terrestrial origin. The giant impact hypothesis is the current favorite, sometime in the early Solar System when astronomers expect a much higher number of large objects and protoplanets flying around and smacking into each other. Estimates have varied on when this giant impact took place, but a growing body of evidence, based on dating of lunar samples, suggests that it was much earlier than initial suppositions of around 4.4 billion years ago, with some analyses suggesting it formed as early as 4.51 billion years ago.
Zircon crystals are an excellent way to trace the age of a sample because of a quirk of the way they form. When they are forming, zircon crystals incorporate uranium but strongly reject lead. Over time, the radioactive uranium in the zircon decays into lead at a very well-understood rate. Scientists can look at the ratios of uranium to lead in a zircon crystal and work out how long ago the zircon formed, with a high level of accuracy. These microscopic crystals can be found in samples of Moon dirt retrieved during the Apollo era from the lunar sample.
Greer and her colleagues studied zircon found in samples from Apollo 17, the last lunar mission, which took place in 1972. These crystals must have formed after the Moon’s surface solidified from the molten global ocean that covered it immediately following its formation. The researchers used atom probe tomography to study the composition of their samples, sharpening the crystals to a point, and then using lasers to evaporate atoms from the point. A mass spectrometer analyzed the vaporized material to measure how heavy it was, which allowed the scientists to determine the ratios of uranium to lead. This groundbreaking method revealed that the age of these specific crystals was 4.46 billion years old.
This information could help scientists determine other aspects of the Moon’s history, such as how long it took to form and solidify, and better estimate the date for the giant impact. “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,” Greer says.
The Moon has always captivated our imaginations, but its true age and origin have remained elusive. With the discovery of zircon crystals in Apollo lunar samples, scientists have unlocked a new chapter in our understanding of Earth’s lunar companion. The revelation that the Moon is at least 4.46 billion years old, just slightly younger than Earth, challenges our previous assumptions about its formation. By studying the composition of these crystals, researchers can reconstruct a lunar chronology and gain insights into the Moon’s mysterious past.
Zircon crystals have proven to be invaluable tools in understanding the Moon’s age. By analyzing the ratios of uranium to lead in these crystals, scientists can determine the exact moment of their formation with remarkable accuracy. The discovery of zircon crystals that formed after the solidification of the Moon’s surface provides a crucial anchor point for lunar chronology. It also suggests that the giant impact, which led to the Moon’s formation, occurred much earlier than previously believed.
The giant impact hypothesis has gained prominence as the most likely explanation for the Moon’s origin. This theory posits that a Mars-sized object collided with Earth, ejecting a significant amount of debris that eventually coalesced to form the Moon. While the precise timing of this impact event is still uncertain, the discovery of 4.46-billion-year-old zircon crystals supports the notion that it occurred early in the history of the Solar System. These findings challenge our previous understanding of the Moon’s age and raise intriguing questions about its formation process.
The age of the Moon has profound implications for our understanding of its evolution. Knowing that the Moon is approximately the same age as Earth allows scientists to reevaluate the timeline of its formation and solidification. Furthermore, the age of the zircon crystals provides valuable insights into the duration of the Moon’s molten global ocean phase, a crucial period in its early history. By unlocking these secrets, researchers can develop a more comprehensive understanding of the Moon’s geological and chemical evolution.
The study of zircon crystals has ushered in a new era of lunar exploration. These tiny grains not only confirm the Moon’s incredible age but also serve as time capsules that encapsulate moments in its history. The meticulous analysis of zircon crystals has allowed scientists to unravel the mysteries of the Moon and gain a deeper understanding of our own planet. As we continue to study Earth’s timeless companion, we can look forward to uncovering even more extraordinary insights into the Moon’s ancient past.
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