The South Pole-Aitken basin – the mostly blue area in the center of this topographic map – is an impact crater about 2,500 kilometers wide, covered by smaller impact craters.
NASA/GSFC/MIT
The Moon’s oldest and largest crater did not form as astronomers thought, according to a detailed analysis of its shape that would rewrite the Moon’s early history.
The South Pole-Aitken (SPA) basin formed about 4.3 billion years ago, a few hundred million years after the formation of the Moon itself. Astronomers believe the basin was created by a massive asteroid scraping the lunar surface, carving out a crater several thousand kilometers wide and 12 kilometers deep.
The crater, which lies on the far side of the Moon, contains thicker piles of ancient rubble toward its northern edge. This is a pattern one would expect if the asteroid entered the surface from a southerly direction, below its south pole.
But new evidence suggests otherwise. Jeffrey Andrews-Hanna of the University of Arizona and his colleagues found that the crater is tapered and narrows in width the further south you go. This teardrop shape suggests that the devastating impact came from the opposite direction, says Andrews-Hanna, from an asteroid coming from the north.
The shape of the basin is difficult to map precisely because the crater’s former boundary has been blurred by later impacts. “We traced the contours of the South Pole-Aitken basin in every way possible,” says Andrews-Hanna. “We used topography, gravity, crustal thickness models. We tried different choices for tracing the basin and no matter how we traced it, its shape always tapered toward the south.”
Next, the researchers compared the shape to well-known craters on other planetary bodies, such as Mars’ Hellas and Utopia craters, for which we have better geological evidence for how they formed. They concluded that the shape of the SPA basin was probably due to an asteroid coming from the north.
Such an impact would change how the moon’s interior matter was dispersed and help scientists understand how the moon’s surface was cooling due to a vast ocean of magma at that time. This would also mean that some material around the edge of the SPA basin contains rocks from the Moon’s deep interior, which would otherwise be inaccessible.
That makes NASA’s upcoming Artemis III mission, which sends astronauts to the edge of the SPA basin to search for possible water ice, even more scientifically valuable, says Mahesh Anand of the UK’s Open University. “It can tell you more about the Moon’s interior, which we don’t have many samples of,” he says. “That’s a bonus.”
However, to truly determine whether the crater formed in the way suggested by Andrew-Hanna and his team, we will ultimately have to wait until samples from the SPA basin are brought back to Earth, says Anand.
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