Scientists say they have figured out the most precise age for the Moon than ever before, thanks to samples of lunar rocks gathered during NASA’s Apollo 14 mission. Analysis of the rocks pinpoint the Moon’s creation to 4.51 billion years ago, just 60 million years after the Solar System first formed.
This suggested age makes the Moon a lot older than some recent estimates, which claim our lunar neighbor is 4.3 or 4.4 billion years old. If the results are accurate, it means the giant impact that created the Moon must have occurred fairly early on in the Solar System’s history. The Moon is thought to have formed from the leftover debris of a high-speed collision between Earth and a smaller planet-like object called Theia — and the timing of this event is important for figuring out when life formed here on Earth, too. Our planet would have been completely wiped out by the giant impact, so life could not have started forming on Earth until after the planet became whole again in the wake of the collision. So knowing the Moon’s age gives us a good idea of when the Earth started to become a suitable place to live.
Knowing the Moon’s age gives us a good idea of when the Earth started to become a suitable place to live
The Moon dating technique, detailed today in the journal Science, involved breaking down the chemical components of a mineral within the lunar samples called zircon, according to lead study author Mélanie Barboni. Studying this mineral helped scientists figure out an important event in the Moon’s formation: when it solidified. The giant impact would have created a massive ocean of liquid magma that eventually coalesced into the Earth and the Moon. The chemical signatures of zircon allowed the scientists to estimate when the Moon’s solidification occurred — a key process that is often considered the beginning of the Moon’s life.
However, the results may not be enough to truly settle the debate on the Moon’s age, according to Richard Carlson, the director for the department of terrestrial magnetism at Carnegie Institution for Science. While Carlson says Barboni and her team did solid work, he has a few concerns about the technique used to analyze the zircon, as well as some assumptions that were made in the study. These concerns may not have thrown off the estimates, but they may cause some experts to doubt the results. “It’s just a very complicated problem they are addressing here, which is why we still don’t have a clear answer to such an obvious question as the age of the Moon,” Carlson tells The Verge.
Zircon has already been found to be a valuable tool for dating rocks here on Earth. It’s a type of rock that crystalizes from magma and remains relatively unchanged over millions and billions of years. Zircon is thought to have crystalized back when the Moon was still volcanically active; magma from the lunar interior erupted onto the surface, where it eventually cooled and crystalized into the zircon. “This mineral is just the king when you try to understand any processes, because it is amazingly sturdy,” says Barboni, an assistant researcher in department of earth, planetary, and space sciences at the University of California Los Angeles. “It’s not going to get affected by much alteration or shock.”
Completely dissolving the zircon within acid
The technique used to analyze these lunar samples, however, was considered a little risky since it involved completely dissolving the zircon within acid. “This was the very first time the zircon would completely disappear,” says Barboni. “Of course, they are precious, so nobody really wanted to do this.” But Barboni’s colleagues at UCLA ultimately decided to trust her with the Moon rocks, and she was allowed to give them the required acid bath.
Through this process, Barboni was able to separate out four key elements from the zircon. The first pair of elements were uranium and lead. Uranium is a radioactive element that converts into lead over a long period of time. It’s a conversion that happens at a specific rate, so scientists can use the ratio of these elements within zircon to figure out how old the rock is. But dating when the rock formed doesn’t give you the entire picture, since the Moon is much older than when the zircon crystalized on the lunar surface.
That’s why Barboni measured another important set of elements: lutetium and hafnium. As the Moon took shape, its materials separated into an inner mantle surrounded by a crust, and that process would have changed the ratio of lutetium and hafnium within various regions of the Moon. The ratio of these elements provides clues as to when this differentiation process took place, back when the zircon was still a magma.
However, Carlson said this work relies on making assumptions about what the lutetium and hafnium ratios should have been like in the early Solar System, which may have altered the dating estimates a bit. Plus, he’s concerned that the acid dissolution process may have not accurately captured the lutetium / hafnium ratios within the zircon. It’s not enough to discredit the findings, but it may lead to some questioning of the study’s results. “The new data is a huge advance in precision over previous similar work,” says Carlson. “My concerns are those from someone familiar with the subject and approach.”
Still, Barboni says it makes more sense for the Moon to be fairly old. Some scientists think that life formed as late as 4.1 billion years ago. If that’s the case, then the giant impact likely occurred much earlier than the first emergence of lifeforms. That way, Earth had more time to form into a planet, as well as create the conditions that are right for live to thrive. “The evolution of the Earth could only have started after this impact,” says Barboni. “And that’s why it’s so important to date this impact because you want to know when the Earth started to evolve into the beautiful planet we all know today.”