Scientists have found that we may have been wrong about how the Moon’s largest crater, the South Pole-Aitken (SPA) basin, formed roughly 4.3 billion years ago.
As detailed in a new paper published in the journal Nature, the more than 1,200-mile crater appears to have been the result of a glancing, southward blow — and not a head-on asteroid impact, as previously thought.
The findings could help explain why the Moon’s far side is riddled with large craters, while the more explored near side 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.
That’s because the space agency’s “missions will be landing on the down-range rim of the basin — the best place to study the largest and oldest impact basin on the Moon, where most of the ejecta, material from deep within the Moon’s interior, should be piled up,” as study lead and University of Arizona planetary scientist Jeffrey Andrews-Hanna explained in a statement.
In other words, the region where we’re planning to land the first astronauts on the Moon in over half a century, a mere two years from now, could hold even more clues about the Moon’s evolution and its interior structure than we thought — a happy accident that should have 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 across the solar system. They found that its oblong, teardrop shape was likely the result of a southward blow, gouging through the Moon’s crust and revealing heavier minerals in the process.
Current theories suggest that the Moon was once covered in a magma ocean, the result of the energy it released when it formed. Heavier minerals sank to form its solid mantle, while lighter minerals floated to its surface to form its crust.
Some “leftover” minerals, such as potassium, rare earth elements, and phosphorus — or “KREEP,” for short — evaded much of this process and instead became concentrated in the remaining magma ocean and eventually 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 becomes concentrated in the last bits of liquid,” Andrews-Hanna explained. “We think something similar happened on the moon with KREEP.”
However, KREEP-rich material accumulated far more on the Moon’s near side, and not its far more once volcanically active far side, a striking asymmetry that remains a major mystery.
The latest findings suggest that the “crust thickened on the far side, the magma ocean below was squeezed out to the sides, like toothpaste being squeezed out of a tube, until most of it ended up on the near side,” Andrews-Hanna explained.
The gash formed by the southward impact suggests the SPA is located at the boundary 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 at some earlier time, a thin and patchy layer of magma ocean would have existed below parts of the far side, explaining the radioactive ejecta on one side of the SPA impact basin.”
The findings highlight how much there still is to learn about our closest celestial neighbor — and how our existing knowledge is far from set in stone.
“With Artemis, we’ll have samples to study here on Earth, and we will know exactly what they are,” Andrews-Hanna argued. “Our study shows that these samples may reveal even more about the early evolution of the moon than had been thought.”
More on the Moon’s craters: Scientists Say There’s Over a Trillion Dollars of Platinum Waiting to Be Extracted From the Moon’s Craters