The moon’s largest crater didn’t form in the way we thought

Scientists have found that the moon’s largest and oldest crater may not have formed as previously believed, suggesting significant revisions to our understanding of the moon’s formative years.

The renowned South Pole-Aitken (SPA) basin dates back roughly 4.3 billion years, shortly after the moon’s formation. Researchers had hypothesized that it was the result of a colossal asteroid colliding with the lunar surface, creating a crater that stretches thousands of kilometers wide and is 12 kilometers deep.

Located on the moon’s far side, the crater exhibits a thicker accumulation of ancient debris along its northern edges, suggesting a strike from a southerly direction, potentially from below the south pole.

However, fresh findings indicate a different scenario. Jeffrey Andrews-Hanna from the University of Arizona and collaborators have observed that the basin has a tapering appearance, gradually narrowing as it extends southward. This teardrop-like configuration implies that the impactful asteroid approached from the north, as per Andrews-Hanna’s analysis.

Determining the basin’s precise outline is challenging due to numerous subsequent impacts obscuring its original borders. “We explored the South Pole-Aitken basin’s outline through various methods,” explains Andrews-Hanna. “We utilized topography, gravitational studies, and crust thickness models. Regardless of our tracing approach, the shape consistently tapered towards the south.”

Subsequently, the team compared the SPA basin’s morphology to well-studied craters on other planetary bodies, such as Mars’ Hellas and Utopia, where geological formation evidence is more robust. They deduced that the SPA basin’s contours likely resulted from a northward approach of the asteroid.

This new understanding could illuminate how the moon’s internal materials were dispersed and aid scientists in grasping the cooling process of the moon’s surface from a primordial magma ocean. It also implies that some of the materials around the SPA basin’s perimeter may include rocks from the moon’s deep interior, which are otherwise difficult to access.

This evolving perspective increases the scientific significance of NASA’s upcoming Artemis III mission, set to send astronauts to the rim of the SPA basin in search of potential water ice, according to Mahesh Anand at the Open University in the UK. “The mission could provide insights into the moon’s interior, from which we currently have very few samples,” he states. “It’s an unexpected advantage.”

Nonetheless, to conclusively ascertain if the crater’s formation aligns with Andrews-Hanna’s hypothesis, obtaining samples from the SPA basin that return to Earth will be crucial, Anand affirms.