The Formation of the Moon’s Largest Crater Was Surprising

An in-depth investigation of its morphology reveals that the moon’s oldest and largest crater formed differently than previously believed, altering our understanding of lunar history.

The South Pole-Aitken (SPA) basin emerged around 4.3 billion years ago, occurring hundreds of millions of years after the moon’s initial formation. Scientists theorize it was created when a colossal asteroid grazed the moon’s surface, producing a crater thousands of kilometers wide and 12 kilometers deep.

This crater, located on the moon’s far side, displays a substantial accumulation of ancient debris near its northern edge. This debris pattern aligns with what would be expected if an asteroid struck the Earth from a southerly trajectory, below the South Pole.

However, new findings indicate a different scenario. Jeffrey Andrews-Hanna from The University of Arizona and his team discovered that the crater narrows towards the south. Andrews-Hanna noted that this teardrop shape implies the asteroid originated from the north, with the cataclysmic impact occurring from the opposite direction.

Mapping the basin’s shape is challenging due to the erosion of its ancient boundaries from later impacts. “We traced the contours of the Antarctic Aitken Basin using every conceivable method,” states Andrews-Hanna. “Models of topography, gravity, and crustal thickness were employed. We explored various approaches for tracing the basin, but with every method, it consistently tapered to the south.”

The researchers then contrasted its shape to well-studied craters on other celestial bodies, such as Mars’ Hellas and Utopia Craters. This comparison has provided clearer geological evidence on how these craters formed, leading them to conclude that the SPA basin’s shape likely resulted from an asteroid impacting from the north.

This new understanding will significantly affect how the moon’s internal material is distributed, aiding scientists in comprehending the moon’s cooling process from a massive ocean of magma during its formative years. It also suggests that some rocks around the SPA basin’s perimeter originate from the moon’s deep interior, which remains otherwise inaccessible.

This insight will enhance NASA’s forthcoming Artemis III mission, which aims to deploy astronauts to the SPA basin’s edge in search of potential water ice, as highlighted by Mahesh Anand from the Open University, UK. “This will provide opportunities to learn more about the moon’s interior, despite the lack of available samples,” he adds. “It’s a significant advantage.”

Ultimately, however, the true nature of the crater’s formation will only be clarified when samples from the SPA basin are returned to Earth, according to Anand.

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