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

The Moon's oldest and largest crater formed differently than astronomers thought, according to a detailed analysis of its shape that will rewrite the Moon's early history.

South Pole-Aitken The basin (SPA) formed about 4.3 billion years ago, several hundred million years after the formation of the Moon itself. Astronomers believe the pool was created by a massive asteroid that streaked across the lunar surface, creating a crater thousands of kilometers wide and 12 kilometers deep.

The crater, which is on the far side of the Moon, contains thicker piles of ancient debris closer to its northern edge. This is the pattern that would be expected if the asteroid struck the surface from a southerly direction, below its south pole.

But new data suggests otherwise. Geoffrey Andrews-Hanna from the University of Arizona and his colleagues found that the crater narrows as it moves south. This teardrop shape suggests the destructive impact came from the opposite direction, Andrews-Hanna says, from a glancing asteroid coming from the north.

The shape of the basin is difficult to accurately depict because the ancient crater boundary has been eroded by later impacts. “We traced the contours of the South Pole-Aitken Basin in every possible way,” says Andrews-Hanna. “We used topography, gravity, models of crustal thickness. We tried different options on how to trace the basin, and no matter how we traced it, its shape was always tapering to the south.”

The researchers then compared the shape to well-known craters on other planetary bodies, such as the Mars Hellas and Utopia craters, for which we have better geological evidence of how they formed. From this they concluded that the shape of the SPA basin was likely caused by an asteroid arriving from the north.

Such an impact would change the distribution of the Moon's interior and help scientists understand how the Moon's surface was cooled by a vast ocean of magma at the time. This would also mean that some of the material along the edge of the spa basin contains rocks originating from the Moon's deep interior that are otherwise inaccessible.

The upcoming NASA does it Artemis Mission IIIwhich sends astronauts to the edge of the SPA basin to search for possible water ice, even more valuable from a scientific point of view, says Mahesh Anand at the UK Open University. “It can tell you more about the interior of the Moon, of which we have very few samples,” he says. “That's a bonus.”

However, to truly determine whether the crater formed the way Andrew-Hanna and his team suggest, we will ultimately have to wait for samples from the SPA basin to be brought back to Earth, Anand says.

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