There’s something really strange about the Moon’s largest crater, where NASA astronauts are due to land

Scientists have discovered that we may have been wrong about how the Moon’s largest crater, the South Pole-Aitken Basin (SPA), formed about 4.3 billion years ago.

As detailed in a new article published in the journal NatureThe more than 1,200-mile crater appears to have been the result of a southward glance — not a frontal asteroid impact, as previously thought.

These results could help explain why the far side of the Moon is riddled with large craters, while the near side, the most explored, is relatively smooth. And they could also have “important implications for the upcoming human exploration of the lunar south pole” by NASA’s Artemis program, the researchers wrote.

Indeed, “the space agency’s missions will land on the lower edge of the basin – the best place to study the Moon’s largest and oldest impact basin, where most of the ejecta, material from deep in the Moon’s interior, is expected to be piled up,” as study leader and University of Arizona planetary scientist Jeffrey Andrews-Hanna explained in a press release.

In other words, the region where we plan to send the first astronauts to the Moon in more than half a century, or just two years, may hold even more clues about the Moon’s evolution and its internal structure than we thought — a happy accident that should make us even more excited about NASA’s long-awaited return.

The team analyzed the shape of the SPA and compared it to other giant impact basins in the solar system. They found that its oblong, teardrop shape was likely the result of a southward thrust, digging into the Moon’s crust and revealing heavier minerals.

Current theories suggest that the Moon was once covered in an ocean of magma, the result of the energy it released during its formation. The heavier minerals sank to form its solid mantle, while the lighter minerals floated on its surface to form its crust.

Some “remnant” minerals, such as potassium, rare earth elements, and phosphorus – or “KREEP,” for short – escaped much of this process and instead became concentrated in the remaining magma ocean and eventually became trapped between the mantle and crust.

“If you’ve ever left a can of soda in the freezer, you may have noticed that as the water becomes solid, the high-fructose corn syrup resists freezing until the very end and instead concentrates in the last bits of liquid,” Andrews-Hanna explained. “We think something similar happened on the moon with KREEP.”

However, KREEP-rich material accumulated much more on the near side of the Moon, not on its formerly volcanically active far side, a striking asymmetry that remains a major mystery.

The latest findings suggest that “as the crust thickened on the far side, the ocean of magma underneath was pushed out the sides, like toothpaste squeezed from a tube, until most of it ended up on the near side,” Andrews-Hanna said.

The cut formed by the impact towards the south suggests that the SPA is located at the limit of the KREEP-rich crust and the more “regular” crust.

“The last dregs of the lunar magma ocean ended up on the near side, where we see the highest concentrations of radioactive elements,” Andrews-Hanna said. “But in earlier times, a thin, patchy layer of magma ocean would have existed beneath parts of the far side, explaining the radioactive ejecta on one side of the SPA impact basin.”

The results highlight how much there is still to learn about our closest celestial neighbor – and how far our existing knowledge is from being set in stone.

“With Artemis, we will have samples to study here on Earth, and we will know exactly what they are,” Andrews-Hanna argued. “Our study shows that these samples could reveal even more about the early evolution of the Moon than previously thought.”

Learn more about the craters of the Moon: Scientists say there is more than a trillion dollars worth of platinum waiting to be mined from the Moon’s craters