The research, published in Nature Geoscience, determined that the collected rock was about 2.8 billion years old. Chemical analyses showed it crystallized at roughly 1,100 degrees C, about 100 degrees C cooler than equivalent near-side samples. This suggests the far side mantle experienced lower formation temperatures.
"The near side and far side of the moon are very different at the surface and potentially in the interior. It is one of the great mysteries of the moon. We call it the two-faced moon," said co-author Professor Yang Li of UCL 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."
Co-author Xuelin Zhu, a PhD student at Peking University, added: "These findings take us a step closer to understanding the two faces of the moon. They show us that the differences between the near and far side are not only at the surface but go deep into the interior."
The far side is marked by a thicker crust, heavy cratering, and fewer basalt plains, indicating reduced volcanic activity compared to the near side. Researchers suggest this may be linked to fewer heat-producing radioactive elements, such as uranium, thorium, and potassium, in the far-side mantle. Their uneven distribution may have arisen after a massive impact shifted heat-producing materials toward the near side, or from an early collision with a smaller moon. Earth's gravitational influence may also have contributed to the thermal imbalance.
Analyses included electron probe mapping, isotope measurements for dating, and mineral composition modeling. Comparisons with Apollo samples and satellite data confirmed consistent temperature differences of 70-100 degrees C between hemispheres.
The enrichment of potassium, rare earth elements, and phosphorus (KREEP) materials on the near side has long puzzled scientists. This study reinforces the idea that their clustering drove stronger volcanic activity on the near side, while leaving the far side cooler and more stable.
Although the absolute present-day mantle temperatures remain uncertain, the researchers conclude that thermal differences between the hemispheres likely persist given the moon's extremely slow cooling since its formation after a giant impact.
Research Report:A relatively cool lunar farside mantle inferred from Chang'e-6 basalts and remote sensing
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