Lunar samples reveal exactly when the moon’s largest crater formed

The largest crater on the moon was probably created when a huge rock smashed into the surface 4.338 billion years ago, leaving behind a seething pool of magma. This hints that Earth was experiencing extreme cosmic violence around the same time.

Thanks to chemical analyses of tiny zircon crystals found in lunar samples, we knew that many of them solidified from magma about 4.3 billion years ago. But without measuring whether they all formed at precisely the same time, there was no way to be sure whether the lunar crust had melted into magma because of many small impacts or one huge collision.

Mélanie Barboni at Arizona State University and her colleagues solved this problem by taking extraordinarily precise measurements of the ages of 10 zircon crystals brought back to Earth as part of NASA’s Apollo programme. “In order to do this kind of dating, we have to dissolve those zircons,” says Barboni. “Lunar material is highly precious and there are very few labs in the world that can be trusted to do that, so nobody really dared to do it – when I did my first one, I was terrified.”

The researchers found that the crystals all formed at the same time, 4.338 billion years ago. This indicates it is most likely that they formed in a single huge impact. Unless the crater from that impact has since been hidden by shifting sands and other smash-ups, the same collision that created these crystals probably formed the South Pole-Aitken basin, the largest crater on the moon, says Barboni.

Not only was this a crucial event in the moon’s history, it also tells us about Earth’s cosmic environment at the time. “The moon is a very small target compared to Earth, so the probability of Earth being hit by something very big around that time was very high,” says Barboni. “That big rock could have left behind some space goodies, such as water, that could have helped kick off life.”