An ancient asteroid collision drastically reshaped the Moon, creating its distinct near and far sides, according to new analysis of samples returned by China’s Chang’e 6 mission. The discovery centers on an unusual potassium isotope ratio found in lunar rocks from the massive South Pole–Aitken Basin, bolstering the theory that this impact is responsible for the Moon’s asymmetric features.
The Two Faces of the Moon
The Moon presents a stark contrast between its near side, which faces Earth and is covered in dark volcanic plains called maria, and its far side, which remains largely barren. This asymmetry has puzzled scientists for decades. The South Pole–Aitken Basin, a colossal 2,500-kilometer-wide crater, dominates the far side and is among the largest impact features in the solar system, dating back 4.2 to 4.3 billion years.
Chang’e 6’s Discovery
China’s Chang’e 6 mission, landing within the Apollo crater inside the Basin in June 2024, retrieved samples that revealed a higher proportion of the heavier potassium isotope, potassium-41, compared to potassium-39. This finding is not due to cosmic radiation, melting, cooling, or meteor contamination. The team at the Institute of Geology and Geophysics in Beijing concluded that the isotopic ratio is a direct consequence of the impact event.
How the Impact Altered the Moon
The immense heat and pressure generated by the asteroid collision vaporized many volatile elements, including potassium. Lighter isotopes, like potassium-39, were more easily lost to space, leaving behind a higher concentration of potassium-41. This process also explains why the far side has less water in its mantle, as previously observed by Chang’e 6.
The reduction in volatile elements suppressed magma formation, severely limiting volcanic activity on the far side and explaining the scarcity of maria. This finding confirms how deeply asteroid impacts can alter planetary interiors and underscores the usefulness of isotopic ratios in reconstructing past events.
The Moon’s far side is not merely a geological oddity; it is a direct record of a cataclysmic collision that reshaped an entire celestial body. This discovery offers crucial insights into the early history of the solar system and the forces that sculpted its landscapes.
