🌘 Abnormally heavy isotopes on the far side of the Moon

The lunar basalts brought back by China's Chang'e-6 mission carry an unexpected chemical signature. Their potassium isotopes are noticeably heavier than those from Apollo mission samples or lunar meteorites.

This anomaly signals the extreme conditions of the impact that formed the South Pole-Aitken basin, the largest impact structure on the Moon. Scientists estimate that this collision vaporized enormous amounts of rock, leaving a distinctive isotopic footprint. The study was published in the Proceedings of the National Academy of Sciences.


The South Pole-Aitken basin spans about 2,500 kilometers (approximately 1,550 miles) on the far side of the Moon. It was created by a massive asteroid impact billions of years ago. This event not only carved out a huge depression but also generated colossal heat and pressure.

Potassium is a moderately volatile element, meaning it can turn into gas at high temperatures. During a giant impact, the heat can vaporize potassium. Lighter isotopes evaporate more easily, leaving behind a higher proportion of heavy isotopes. This isotopic fractionation acts as a thermometer and barometer for collisions. Scientists can reconstruct the temperature and scale of the impact.

Chang'e-6 samples show exactly this pattern, confirming that the formation of the SPA basin was exceptionally energetic. The results showed an average increase in heavy potassium of about 0.16 parts per thousand compared to Apollo basalts. This difference may seem tiny, but it is highly significant in isotope geochemistry.


To ensure the isotopic anomaly came from the impact, the team tested three alternative explanations. Exposure to cosmic rays can alter isotopes, but the effect was too weak. Internal magmatic processes on the Moon could not produce such a strong signal either. Meteoritic contamination from the impactor itself was considered, but the isotopic pattern did not match. That left the impact as the most likely cause.

The researchers concluded that only the extreme heat of the giant impact could explain the observed isotopic enrichment.