Moon's biggest crater reveals how our satellite formed

When NASA lands astronauts near the moon's south pole in the coming years, they'll step onto one of the most scientifically valuable pieces of real estate in the solar system. A new study published in Nature explains why — and it hinges on understanding a cosmic collision that happened 4.3 billion years ago.

That's when a giant asteroid struck the far side of the moon, carving out the South Pole-Aitken basin (SPA). At over 1,200 miles north to south and 1,000 miles east to west, it's the moon's largest impact crater. But the real insight isn't just its size — it's the shape.

Reading the Moon Like a Crime Scene

The basin isn't perfectly circular. It's more like a teardrop, narrower in one direction than the other. That oblong shape is the fingerprint of a glancing blow, not a head-on collision. By comparing SPA to other giant impact basins across the solar system, researchers at the University of Arizona realized something scientists had gotten backwards: the impact came from the north, not the south as previously thought.

This matters because where the asteroid hit determines what's buried where. "The down-range end of the basin should be covered by a thick layer of material excavated from the lunar interior," explains Jeffrey Andrews-Hanna, a planetary scientist on the team. The Artemis landing site sits right on that down-range rim — meaning astronauts will be standing directly above the moon's deepest secrets.

What the Moon's Interior Tells Us

That material from deep inside the moon holds clues about how our satellite actually formed. Scientists have long believed the early moon was a molten ball — a magma ocean covering the entire surface. As it cooled and crystallized, heavy minerals sank to form the mantle, while light minerals rose to form the crust. But some elements didn't fit neatly into either layer.

Potassium, rare earth elements, and phosphorus — collectively called KREEP — got left behind in the final dregs of that magma ocean. Think of it like the soda analogy Andrews-Hanna uses: when you freeze a can of soda, the syrup resists freezing and concentrates in the last liquid pockets. The moon did something similar, with KREEP pooling in the final stages of crystallization.

Here's where it gets striking. The ejecta blanket on the western side of SPA is rich in radioactive thorium, but the eastern side isn't. That asymmetry reveals something unexpected: the impact crater punched through the crust at precisely the boundary where KREEP-enriched material from the magma ocean's final stages met regular crust. "The distribution and composition of these materials match the predictions we get from modeling the latest stages of magma ocean evolution," Andrews-Hanna says.

This means the last remnants of the lunar magma ocean ended up concentrated on the near side of the moon — where we see the highest concentrations of radioactive elements today. But at an earlier time, thin pockets of that magma ocean existed on the far side too, and the SPA impact exposed them.

When Artemis astronauts return with samples, researchers will finally be able to test these theories in state-of-the-art labs on Earth. The crater that has puzzled scientists for decades is about to give up its oldest secrets.