Scientists Discover the Moon’s Far Side is Geologically Cooler by Using China’s Chang’e-6 Samples

Scientists from China and the United Kingdom have reported that the far side of the moon is not only visually distinct from the near side but may also have developed under cooler geological conditions. The conclusion comes after a detailed analysis of lunar rocks and soil collected during China’s Chang’e-6 mission, which returned to Earth in June 2024 carrying the first-ever samples from the moon’s far side. Their findings were published in the peer-reviewed journal Nature Geoscience on 30 September 2024.

The Chang’e-6 lunar probe successfully landed in the South Pole–Aitken region, a massive and ancient impact basin on the far side, and retrieved about 300 grams (0.66 pounds) of material from the southern rim of the Apollo basin, a large crater within this territory. This marked a milestone in lunar exploration because, until then, all physical samples studied by scientists had come from the near side, including those collected by NASA’s Apollo missions and China’s Chang’e-5 mission. The absence of far side samples had long restricted research into the reasons behind the striking asymmetry between the two hemispheres.

When researchers compared the newly obtained material with near side samples, they discovered a significant difference in thermal history. The minerals from the far side appear to have formed at mantle temperatures about 100 degrees Celsius (212°F) lower than those recorded for the near side. This provides the first direct evidence of a thermal imbalance between the two sides of the moon.

The contrast aligns with well-documented differences in surface appearance. The near side, which always faces Earth, is dominated by large, dark basaltic plains known as maria, created by extensive volcanic activity billions of years ago. The far side, on the other hand, is rugged, heavily cratered, and mountainous, with far fewer basaltic features. Scientists attribute this divergence to the uneven distribution of heat-producing radioactive elements such as uranium, thorium, and potassium. These elements, often found together with phosphorus and rare earth elements in a rock type referred to as KREEP, are significantly more concentrated on the near side. Their presence provided additional internal heat, sustaining prolonged volcanic activity and thinner crust there, while the far side, with fewer of these materials, cooled more quickly and developed a thicker crust.

The Chang’e-6 samples also offered further geological insights. Laboratory tests dated the collected basalt rocks to about 2.8 billion years ago, placing them among the younger volcanic formations on the moon. However, despite their age, the samples confirmed that the mantle potential temperature beneath the far side was consistently lower than that of the near side, reflecting a long-lasting difference in thermal conditions. Earlier research based on remote data had suggested such an imbalance, but this study is the first to confirm it with real material from the far side.

In addition, scientists examining the Chang’e-6 material reported evidence that a colossal asteroid impact more than 4 billion years ago may have altered the interior of the far side. This event is believed to have shaped the South Pole–Aitken basin and could have influenced the subsequent thermal and crustal evolution of that hemisphere.

According to Li Yang, a professor at both University College London (UCL) and Peking University, the findings shed new light on one of lunar science’s longest-standing puzzles. He noted that the near side and far side of the moon are very different both at the surface and in their interior composition. While a dramatic difference in mantle temperature had been proposed for decades, he said this is the first time such a difference has been demonstrated using actual rock samples.

The research team explained that their study demonstrates how the lunar far side mantle was relatively colder than the near side mantle, which is consistent with differences in crustal thickness and heat-producing element distribution between the two hemispheres. At the same time, they acknowledged that the exact cause of this hemispherical asymmetry remains unresolved and will require further investigation.

These results are significant not only for lunar science but also for broader planetary studies. The moon preserves a record of early solar system history, and understanding its internal evolution helps scientists learn more about the formation of Earth and other rocky planets. The success of Chang’e-6 underscores the importance of sample-return missions, which allow for laboratory analysis of extraterrestrial material rather than relying solely on remote sensing.

As examination of the Chang’e-6 samples continues, researchers expect more discoveries that will clarify why the moon’s hemispheres diverged so sharply in both appearance and thermal history. This work also lays the foundation for future exploration, as countries including China, the United States, and India plan new missions to the lunar surface, with special interest in the far side, which still holds many unanswered questions.