🌑 Origin of the Moon's largest crater called into question

A lunar crater of planetary proportions, the South Pole-Aitken basin, is finally revealing the secrets of its violent birth. A meticulous reinterpretation of its morphology overturns the established scenario of its formation and sheds new light on the fundamental dichotomy between the two faces of our satellite.

This geological reassessment is not just an academic curiosity. It directly guides upcoming human explorations, designating the southern rim of this basin as a prime destination for the Artemis III mission. Astronauts would find ejecta there, materials brought up from the lunar depths during the impact, offering unprecedented access to the Moon's primitive history.

A geological investigation to trace the impact trajectory

Solving this puzzle required an approach combining several observation techniques. The research team used topographic analysis, mapping of gravitational field variations, and study of the lunar crust's thickness to precisely redefine the often-faded contours of the basin. This data was then compared to that of other major impact structures in the Solar System, such as the Hellas and Utopia basins on Mars, to validate the interpretive models.

The result of this investigation is unequivocal: the South Pole-Aitken basin has an oblong structure, resembling a water droplet or an avocado, that narrows significantly towards the south. This characteristic shape is the signature of an oblique impact, where the narrowest part indicates the direction in which the asteroid continued its journey after the collision. This observation contradicts the historical hypothesis of an impact from the south.


This new trajectory, from north to south, has a direct consequence on the distribution of excavated materials. Modeling indicates that the ejecta, debris thrown from the deep layers, mainly accumulated on the "downstream" rim of the crater, that is, to the south. This prediction positions the future Artemis landing zones in an exceptional geological location, on top of a pile of materials originating from the lunar mantle.

An impactor that pierced the crust at the worst time

The primitive Moon was likely a ball of molten rock, covered by a vast magma ocean. As it cooled, this ocean crystallized, giving birth to the mantle and the crust. Certain chemical elements, incompatible with the forming minerals, were rejected into the residual liquid. This final geochemical "syrup," rich in potassium, rare earth elements, and phosphorus, is designated by the acronym KREEP.

The study published in Nature suggests that the South Pole-Aitken basin impact occurred at a key moment, when the last pockets of this magma ocean were not yet completely solidified. The researchers compare this process to freezing a can of soda: the water freezes first, concentrating the sugar in the last drops of liquid. On the Moon, the KREEP elements became concentrated in the ultimate magma residues, which were squeezed towards the near side by the thickening of the far side's crust, similar to squeezing a toothpaste tube.

The proof of this scenario lies in a striking chemical asymmetry around the basin. The ejecta located on its western flank are abnormally rich in thorium, a radioactive element characteristic of KREEP material. In contrast, its eastern flank is almost devoid of it. This distribution suggests that the impact opened a breach in the lunar crust precisely at the boundary where these residual magma pockets persisted, releasing part of their content to the surface.

The definitive confirmation of these hypotheses relies on the analysis of samples collected on site. The Artemis missions, by landing near the South Pole, will thus have the opportunity to bring back fragments of these thorium-rich ejecta to Earth. Their laboratory study will allow for precise dating of the impact and characterization of the composition of the last remnants of the magma ocean, offering a unique window into the Moon's earliest moments.