An interesting discovery has emerged from the Chang’e 6 mission, which brought samples from the moon’s South Pole–Aitken Basin back to Earth. Scientists found an unusual ratio of potassium isotopes in the lunar basalt that hints at why the moon’s near and far sides are so different.
We’re all familiar with the moon’s near side, which has the recognizable pattern of the “Man in the Moon,” formed by dark volcanic plains, known as maria. In stark contrast, the far side, mostly unseen by the naked eye, lacks these dark areas.
The South Pole–Aitken Basin is about 1,600 miles wide and one of the largest impact sites in our solar system, dating back around 4.2 to 4.3 billion years. This makes it much older than the maria, which are around 3.6 billion years old.
Chang’e 6 landed on a crater called Apollo within this basin on June 1, 2024. It returned to Earth with valuable samples 25 days later. Since then, scientists have been working hard to understand the differences between the moon’s two sides.
A team led by Heng-Ci Tian from the Chinese Academy of Sciences analyzed these samples and discovered that the ratio of potassium-41 to potassium-39 was higher in the South Pole–Aitken samples than in those from the near side. This ratio can reveal important information about the moon’s geological history.
Tian’s team explored several theories for this surprising finding. They looked into cosmic rays, the various processes of magma, and even contamination from meteors. However, they concluded that none of these factors played a significant role.
Ultimately, they believe that the impact that formed the basin caused this unusual potassium ratio. The intense heat and pressure from the impact likely evaporated volatile elements, including potassium, into space. Previous studies support this, showing that the moon’s far side has less water than the near side. The lighter potassium-39 isotope would evaporate more easily than the heavier potassium-41, leading to the increased ratio of potassium-41.
This discovery sheds light on how deeply the impact influenced the moon’s interior. It suggests that the reduction in volatile elements could suppress volcanic activity, explaining why the lunar far side has so few maria.
The findings from the Chang’e 6 mission were published in the Proceedings of the National Academy of Sciences on January 12. Such research not only enhances our understanding of the moon but also gives us insights into the broader impacts of large-scale events in the history of celestial bodies.
