Chlorine – the key to the mystery? Scientists uncover the reason behind the Moon’s two-faced nature

An international team of researchers from Ehime University (Japan), Germany, and the Netherlands has discovered new evidence that explains the differences between the near and far sides of the Moon. The study, published in Nature Communications, is based on the analysis of chlorine and other halogen content in lunar rocks.

The mystery of the Moon’s two sides

The Moon, which formed about 4.5 billion years ago as a result of a collision between Earth and a hypothetical planet named Theia, presents a striking contrast between its hemispheres. The side visible from Earth is covered with dark “seas” — vast basaltic plains formed by ancient lava flows. In contrast, the far side consists mostly of light-colored highland regions and is almost devoid of such seas. This asymmetry, known as the lunar dichotomy, has remained one of planetary science’s most enduring mysteries for decades.

The role of chlorine in the Moon’s history

The scientists conducted laboratory experiments simulating the high-pressure, high-temperature conditions typical of the Moon’s magmatic layer during its early formation. They studied how chlorine is distributed between minerals and melt in lunar rocks. Analysis of samples brought back by the Apollo missions from the near side of the Moon showed elevated chlorine content — a feature not observed in samples from the far side, which were collected by China’s Chang’e-5 lander.

Researchers link this difference to the presence of metal chloride vapors, which were likely concentrated only above the near side. These vapors may have formed due to degassing during basaltic eruptions, as well as from meteorite impacts that induced chemical changes in the rocks. On the near side, these processes led to enrichment of the surface with chlorine and other volatile elements.

The far side: traces of the primordial Moon

Unlike the near side, the Moon’s far side has preserved a more “primordial” chemical composition, closer to that of the magmatic ocean — the molten layer that once covered the Moon after its formation. Modeling shows that some rocks on the far side originate from deep within the mantle and have retained traces of this ancient ocean. The absence of significant basaltic eruptions and the lower intensity of meteorite impacts allowed these rocks to remain largely unchanged.

The significance of the discovery

The findings highlight that the differences between the Moon’s hemispheres are linked not only to geological processes but also to chemical evolution. Chlorine, as an indicator of these processes, has become a key to understanding how eruptions and impacts shaped the near side, while the far side remained a kind of “archive” of the Moon’s early history.
“Our study shows that investigating the far side of the Moon is critically important for understanding its formation and evolution,” noted the study’s lead author from Ehime University.

Future research prospects

The discovery opens new directions for lunar exploration. Scientists plan to continue analyzing samples delivered by the Chang’e-5 mission and expect future missions, such as NASA’s Artemis program, to provide additional data on the chemical composition of both lunar sides. Special attention will be given to volatile elements like chlorine, which may reveal more details about the Moon’s early formation stages and its magmatic ocean.

In conclusion

The study of chlorine in lunar rocks has brought scientists closer to solving one of the Moon’s most intriguing mysteries — the contrast between its near and far sides. The chlorine-rich near side, shaped by eruptions and meteorite impacts, contrasts with the more ancient composition of the far side, which preserves evidence of a once-global magmatic ocean. These findings deepen our understanding of the Moon’s history and emphasize the importance of continued exploration of our nearest cosmic neighbor.