The new atom-by-atom analysis of a 4.46 billion-year-old lunar crystal pushes back the timeline for when the molten moon solidified by 40 million years, according to the study published Monday in the journal Geochemical Perspectives Letters.
“It moves the goal post. It pushes back the minimum age of the moon formation,” said Jennika Greer, a cosmochemist at the University of Glasgow who worked on the study while a graduate student at the Field Museum and the University of Chicago.
“It’s important to point out,” she added, “this is the oldest age to date. It doesn’t mean we now know the age of the moon and we should stop looking.”
The matter of the moon’s origin may seem like it should be settled science. We’ve examined it through telescopes, orbited it with a suite of spacecraft, scooped up its rocks and explored its surface in person.
But despite millennia of contemplation and study — and a fairly solid theory of the case — scientists have continued to find inconsistencies, posit alternate possibilities and tweak theories. Reanalysis of samples picked up decades ago by astronauts has played a key role in moving knowledge forward.
When the moon first coalesced, the theory goes, it was covered in an ocean of roiling magma. Lunar zircon crystals are like cosmic timepieces that started ticking once that magma ocean cooled and solidified. Zircon crystals take up radioactive uranium as an impurity, which decays over time into lead. By comparing the ratios of different forms of lead and uranium atoms called isotopes in the sample, scientists can estimate its age.
In 2021, a team of cosmochemists led by Bidong Zhang, now a researcher at the University of California at Los Angeles, and Audrey Bouvier at the University of Bayreuth in Germany, published a paper showing that zircon crystals embedded in an Apollo 17 moon rock might be the oldest yet discovered at 4.46 billion years old. But they added lots of caveats and disclaimers.
The problem for Zhang was that he couldn’t be as sure as he needed to be that the date was accurate. Lead can move around within zircon crystals and get stuck in clusters, like raisins in baking bread, potentially meaning that when researchers measure the ratio of lead isotopes, they might overcount it if they hit a cluster, coming up with an inaccurate date.
When his team first submitted the results to a journal, Zhang recalled, they were criticized because the analysis couldn’t rule out this alternate explanation. When they did eventually publish the results, he said they couched the findings in a “cautious, low-key” way because of those critiques.
“It’s been controversial for the last 50 years, since the 1970s when the astronauts brought back the samples from the moon,” Zhang said. “Apollo rocks were very consistent at 4.3 billion years old. That’s why people are like: ‘Why would this age be different?’”
To answer the critics, Greer and colleagues joined the effort to verify the age of the crystal, using a technique called atom probe tomography, which is more commonly used in materials science for steel failure analysis or semiconductor research. With the technique, the scientists were able to take samples from a tiny sliver of one of the crystals and use a laser beam to evaporate the atoms one by one and identify them, ruling out that the lead atoms had clustered within the crystal.
“This new study shows that some of these zircons did form at 4.46 [billion years ago], only about 100 million years after the first solids formed in the Solar System,” Romain Tartèse, a senior lecturer in the department of Earth and environmental sciences at the University of Manchester, who was not involved in the study, wrote in an email.
Alexander Nemchin, a geochemist at Curtin University in Australia, discovered the previous oldest known lunar crystal, a 4.42 billion-year-old specimen from an Apollo 17 rock. He said the paper was thought-provoking and strengthens the case made by Zhang’s earlier paper.
“The age is probably real,” Nemchin wrote in an email. “With that, we are running into a big problem as a community studying lunar samples.”
Young rocks vs. old crystals
The problem facing lunar scientists is that the samples they’ve analyzed are about 100 million years apart in age. The new find suggests the magma ocean crystallized 4.46 billion years ago. Meanwhile, other types of lunar rock have dates of about 4.35 billion years ago, suggesting the magma ocean stuck around for another 100 million years.
How big of an issue that presents depends on who you ask.
“Something is clearly not right in our big conceptual understanding of how [the] Moon evolved,” Nemchin said.
But Tartèse argues there isn’t necessarily a contradiction in the dates because the crystallization of the magma ocean could have been a 100 million-year-long process that was more complex than the simplest assumptions.
Zhang pointed to the possibility that there were secondary events after the formation of the moon, such as later impacts, that could have heated the younger rocks to high temperatures and reset their clocks.
Benjamin Weiss, a planetary scientist at the Massachusetts Institute of Technology, said that newer missions to the moon that bring back more samples from different spots might help clarify the timeline of the moon’s origins.
“Fifty one years ago, no one would have thought that we would one day analyze these lunar samples with this new, cutting-edge method,” said Philipp Heck, senior author of the new study who is curator of meteoritics and polar studies at the Field Museum and a professor at the University of Chicago.
“I think our study is one example of the power of sample return. It’s available for multiple generations, [and] future generations of scientists to study with instruments that at that time scientists didn’t even dream up.”
