Why is the Moon asymmetrical? Scientists may finally know the answer

According to new research, an ancient cosmic collision may explain exactly why the Moon is asymmetrical. The question has puzzled scientists for decades, but fresh samples from the Chang’e-6 mission are finally offering insight into why the Moon’s two sides differ so strikingly.

Our closest celestial neighbour, the Moon, appears peculiar even at first glance: the side visible from Earth is marked by dark, smooth volcanic plains, while the far side, by contrast, is mountainous, heavily cratered and has a much thicker crust. This contrast between the two hemispheres is the clearest sign that the Moon is asymmetrical. Scientists have long sought an explanation, and a new study published in the journal Proceedings of the National Academy of Sciences may bring us closer to the answer.

China’s Chang’e-6 mission recently succeeded in returning rock and soil samples from the far side of the Moon. These were collected from the South Pole–Aitken Basin, one of the largest known impact craters in the entire Solar System, covering almost a quarter of the Moon’s surface.

Researchers from the Chinese Academy of Sciences examined four tiny basalt fragments using a high-precision mass spectrometer and compared the new samples with rock fragments collected from the Moon’s near side during the Apollo and Chang’e-5 missions.

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Was a colossal collision the reason the Moon is asymmetrical?

Analysis revealed that potassium isotopes in the far-side samples were significantly heavier than those from the near side. Only minor differences were measured in iron, which can easily be explained by volcanic processes, but potassium behaves differently.

Potassium is a moderately volatile element that evaporates easily at high temperatures. When this happens, lighter atoms escape, while heavier isotopes remain behind. According to the researchers, the high proportion of heavy potassium isotopes measured in the far-side samples suggests that the Moon’s interior was once exposed to extreme heat resulting from a massive impact.

During this hypothesised collision, temperatures may have reached 2,800 Kelvin. This would not only have deformed the surface but also melted the Moon’s internal layers, pushing heat-producing elements towards the near side. This could explain why extensive volcanism developed there, while the far side became almost devoid of such material.

As a consequence, heavier potassium isotopes remained on the far side, while more intense volcanic activity took place on the opposite hemisphere. Researchers believe this divergent evolution may explain why the Moon is asymmetrical today.

The authors of the study state that their results provide strong evidence that a large-scale impact significantly altered the lunar mantle and played a key role in shaping the distinct characteristics of the Moon’s two sides.

What comes next?

Although the discovery is promising, it is important to note that the conclusions are based on just four tiny samples. Scientists stress that further analysis of far-side material is needed to definitively prove whether this ancient collision is why the Moon is asymmetrical.

Future lunar missions will therefore not only represent technological achievements but also crucial steps towards understanding how our celestial companion formed and how the remarkable duality that still fascinates astronomy enthusiasts today came into being.

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