It has shone on dinosaurs, inspired the greatest poets, and been explored by intrepid astronauts. But despite its enduring presence, when the moon appeared remains a matter of debate. Now researchers say they have the answer, revealing that Earth’s moon is 40 million years older than previously thought.
Scientists made their discovery by studying crystals in lunar dust that was brought back in 1972 as part of the Apollo 17 mission – the last time astronauts set foot on the moon.
“It’s amazing to have proof that the rock you’re holding is the oldest part of the Moon we’ve found so far. It is a fulcrum for so many questions about Earth. “When you know how old something is, you can better understand what happened to it in its history,” said Dr Jennica Greer of the University of Glasgow, lead author of the study.
About 100 million years after the formation of the solar system, when the planets had already formed, a body the size of Mars is believed to have hit Earth, ejecting a large mass of material that eventually became the moon. The energy involved in the impact meant that its surface was initially molten, but as the lunar magma ocean cooled, the material solidified.
Most importantly, the crystals examined in the study are believed to have formed during this cooling process, meaning their composition offers researchers the opportunity to probe their age, and by extension, the age of the moon itself.
However, determining such times was difficult. Some work on lunar materials along with modeling suggests the Moon is about 4.42 billion years old, but recent crystal studies suggest it may be even older.
Now researchers say new analytical techniques appear to confirm that the crystals formed further back in time.
The approach, known as atom probe tomography, involves using a laser to vaporize atoms from crystals that have been sharpened to a very fine “nano tip.” The mass of these atoms can then be detected and the ratio of the different types of uranium and lead atoms – known as isotopes – can be measured. Because uranium turns to lead over time through radioactive decay, this ratio can shed light on how old the crystals are.
The result, the team wrote in the journal Geochemical Perspectives Letters, is that the crystals, and hence our moon, appear to be at least 4.46 billion years old.
“This age offsets the age of the first preserved lunar crust by ~40 [million years] and provides a minimum age for the formation of the moon within 110 years [million years] after the formation of the solar system,” the team wrote.
They suggest that samples of such crystals that lead to the conclusion of a younger age may be due to the loss of lead in the material after crystallization, which distorts the resulting dating.
Dr Romain Tartes from the University of Manchester, who was not involved in the research, welcomed the study.
“This indicates that the moon is at least 4.46 [billion years old]which contradicts some of the recent proposals for young moon formation,” he said.
The discovery, he added, suggests that the giant impact with Earth that formed the moon probably happened several tens of millions of years before that point.
However, Tartèse noted that the study assumes that the findings from the Apollo samples apply to the entire moon, which may not be the case.
“This highlights the importance of returning additional samples from different regions of the Moon by future missions,” he said.
“This study also illustrates the great benefit of sample return missions and proper curation,” added Tartèse. “More than 50 years after those samples were returned, we are still making key discoveries about the Moon and inner solar system as technology continues to evolve.”