The Moon’s far side may have formed from cooler interior material than its near side counterpart, according to a new study based on the first-ever rock samples collected from the Moon’s hidden hemisphere. The results, published in Nature Geoscience, offer direct geochemical evidence for a longstanding hypothesis: that the thermal and volcanic histories of the Moon’s two faces diverged not just at the surface, but deep into its mantle.
The study analyzed 300 grams of regolith returned by China’s Chang’e 6 mission in June 2024 – the first mission to retrieve physical material from the lunar far side. Using electron probe microanalysis, the team mapped the mineral composition of basalt fragments, while lead isotope ratios measured via secondary ion mass spectrometry (SIMS) were used to confirm the rock’s age: approximately 2.8 billion years. This aligns with ages of volcanic materials previously sampled on the Moon’s near side, but revealed a notable thermal difference.
By comparing the mineral chemistry of the Chang’e 6 basalts to thermodynamic models, the researchers estimated a crystallization temperature of roughly 1,100°C – about 100°C cooler than that of similarly aged basalts collected during Apollo missions. Additional modeling of parent rock temperatures, using both geochemical proxies and satellite data, supported this discrepancy.
“The near side and far side of the Moon are very different at the surface and potentially in the interior,” said Yang Li, co-author and Professor of Earth Sciences at University College London and Peking University. “A dramatic difference in temperature between the near and far side of the mantle has long been hypothesised, but our study provides the first evidence using real samples.”
The team attributes the difference to an uneven distribution of heat-producing elements such as uranium, thorium and potassium. These elements are typically concentrated in KREEP-rich (potassium, rare earth elements, phosphorus) materials, which are prevalent on the Moon’s near side but appear to be depleted in the far-side mantle.
Several theories have been proposed to explain this asymmetry, including the idea that a massive impact early in lunar history may have redistributed denser, radioactive material toward the Earth-facing hemisphere – or that the Moon itself formed from the collision of two proto-lunar bodies with different thermal compositions.
“These findings take us a step closer to understanding the two faces of the Moon,” said co-author and PhD student Xuelin Zhu. “They show us that the differences between the near and far side are not only at the surface but go deep into the interior.”
While the study does not resolve present-day mantle temperatures, the researchers suggest that the ancient thermal gradient is likely to persist, given the Moon’s slow rate of cooling.The team is now working to improve temperature reconstructions and assess alternative explanations for the Moon’s contrasting hemispheres.
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