Lunar samples shed light on the formation of the Moon’s largest crater

The Moon's largest crater is thought to have formed 4.338 billion years ago when a huge rock struck the lunar surface, leaving behind a swirling pool of magma, suggesting that Earth was experiencing extreme cosmic upheaval at the same time.

Chemical analysis of tiny zircon crystals found in lunar samples revealed 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 know for sure whether many small impacts or one giant one melted the lunar crust into magma.

Melanie Balboni Balboni and her colleagues at Arizona State University solved this problem by measuring with extreme precision the ages of 10 zircon crystals that were brought back to Earth as part of NASA's Apollo missions. “To do this kind of dating, you have to melt the zircon,” Balboni says. “The lunar material is so precious, and there are so few reliable labs in the world that can do that, so no one has dared to do it. When I first did it, I was so scared.”

The researchers found that the crystals all formed at the same time, 4.338 billion years ago, which indicates that they likely formed in one giant impact. The same impact that created these crystals probably also formed the South Pole-Aitken Basin, the largest crater on the Moon, unless that impact crater was subsequently obscured by shifting sand or other impactors, Balboni says.

Not only is this a pivotal event in the history of the Moon, but it also tells us something about the space environment on Earth at that time. “The Moon is a very small object compared to Earth, so it was very likely that something very big struck Earth at that time,” Balboni said. “That big rock could have left behind cosmic gifts, like water, that might have helped the birth of life.”