In a striking scientific discovery, Chinese scientists have revealed that a giant cosmic collision more than 4 billion years ago may be the root cause behind the mysterious difference between the two faces of the Moon: the familiar near side and the largely unknown far side.
This conclusion came after analyzing rare samples returned by the Chang’e-6 mission from one of the Moon’s oldest and largest geological features, the South Pole–Aitken Basin. This basin spans a vast distance of about 2,500 kilometers and covers a large part of the lunar far side.
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Two Dissimilar Faces of the Moon
Anyone looking at the Moon can easily notice the wide dark spots on its near side, scientifically known as maria (seas). These are not bodies of water but vast plains of frozen basaltic lava, formed when magma flowed from the Moon’s interior and filled ancient impact basins. Some of the most famous include the Sea of Rains, the Sea of Serenity, the Sea of Tranquility, and the Sea of Fertility.
In contrast, the far side is almost devoid of these maria and is dominated by a bright, heavily cratered surface. This has puzzled scientists for decades: why did volcanic activity occur on one side and not the other?
The answer began to emerge with the Chang’e-6 samples, collected from within the South Pole-Aitken basin. This impact basin is about 2,500 kilometers in diameter and is considered one of the largest and oldest craters in the solar system.
When analyzing the basalt rocks, scientists found an unusual ratio between the isotopes of the element potassium: potassium-41 and potassium-39. This ratio is significantly higher than what was found in near-side samples brought back by the American Apollo missions.
After ruling out the effects of cosmic rays or normal volcanic processes, the researchers concluded that the most likely cause was the massive impact itself.
What Did the Impact Do to the Moon?
According to the study, the thermal energy and pressure generated by the impact were sufficient to heat the lunar crust and mantle to temperatures that caused volatile elements—such as water and potassium—to evaporate and escape into space, especially on the far side. Because the lighter isotope (potassium-39) evaporates faster, a higher proportion of the heavier isotope (potassium-41) remained.
The result is a mantle depleted in water and volatile elements, meaning less magma, weaker volcanoes, and rare volcanic maria on the far side. This explains the absence of maria there, in contrast to their widespread presence on the near side.
A New Window on the Moon
The importance of this discovery lies not only in solving an old mystery but also in the pivotal role played in achieving it. The Chang’e-6 mission is the first in history to bring back samples from the far side of the Moon, a scientific achievement no other nation has accomplished before.
These samples give scientists a direct view into the Moon’s interior and its early history, complementing what was started by the Apollo missions, which were limited to the near side. As China continues its ambitious lunar program, it has become a key player in rewriting the story of the Moon’s formation and evolution.
