AILSA CHANG, HOST:
In December of 1972, astronauts on the Apollo 17 mission went rock hunting on the moon.
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EUGENE CERNAN: Before we cover them up, let's get them. I got to get a sample of that mother rock.
CHANG: They brought about 250 pounds of moon rock back to Earth. And those samples are still being studied today. NPR's Regina Barber reports that scientists have now determined the moon is roughly 40 million years older than we thought.
REGINA BARBER, BYLINE: The moon is roughly 4.5 billion years old. So what does an extra 40 million years mean on this time scale? Jenniker Greer is the lead author of the study, published in Geochemical Perspective Letters. Here's how she sees it.
JENNIKER GREER: That 40 million years is significant when you look at the very dynamic early history of these two objects. A lot of stuff happened in the early solar system very quickly.
BARBER: In the early days of our solar system, an object the size of Mars smashed directly into a forming Earth.
GREER: And then they smushed together, and material sort of peeled off to form the moon.
BARBER: A hot moon that had a magma ocean. These Apollo 17 rock samples are crystals from that cooled ocean. To figure out how old these crystals are, scientists used radiometric dating. Because uranium decays into a specific kind of lead over time, scientists can use it to work backwards and get an age. The problem is, if some of that lead is lost over time or clumps together, it can throw off the age estimate. But new technology can help. For example, Greer was able to look at the sample on the atomic level to see if the lead was undisturbed.
GREER: The type of measurements that we do in this work would not have been possible 50 years ago. They wouldn't have been possible 20 years ago.
BARBER: This new age isn't a surprise to Marissa Tremblay, a geochronologist who didn't work on this study but was impressed by the technique.
MARISSA TREMBLAY: And they're really difficult measurements to make, too. So it's very exciting to see these new results.
BARBER: Even though this older date isn't a surprise, Tremblay and Greer both agree it clarifies an important piece of the puzzle for how we, our planet, our moon came to be what they are today.
TREMBLAY: When did that magma ocean start to crystallize? It's telling us about that very early history of the moon. It's also telling us about when that big impact happened on the Earth.
BARBER: Tremblay also says that after dating more lunar samples, she wouldn't be surprised if the age of the moon gets even older. Regina Barber, NPR News.
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