Scientists May Have Solved the Mystery Behind the Moon’s Two Faces

Why are the Moon’s two sides so different?

The side of the Moon facing Earth looks dramatically different from the hemisphere that permanently turns away from us. The near side contains vast plains of dark volcanic basalt and relatively high concentrations of potassium, rare earth elements, and phosphorus, collectively known as KREEP. By contrast, the far side is dominated by ancient, heavily cratered highlands with a much thicker crust and relatively few volcanic plains.

Scientists have debated the origin of this striking lunar asymmetry for decades. Proposed explanations include differences in the circulation of the Moon’s original magma ocean, large impacts, gravitational tidal heating, and uneven crust formation. However, no single explanation has been universally accepted.

Another important question has remained unanswered: Do the differences visible on the Moon’s surface and within its crust also extend deep into the lunar mantle?

Chang’e 6 samples reveal similarities in the lunar mantle

In 2024, Chang’e 6 became the first mission to return samples from the far side of the Moon. These rocks gave scientists a new opportunity to directly compare material from both hemispheres with samples collected by the Apollo missions and China’s Chang’e 5 mission.

The researchers analyzed three types of lunar rocks: anorthosite, magnesium-rich rocks from the Mg suite, and mare basalt. Their results revealed important similarities beneath the Moon’s contrasting surface.

The basalts collected by Chang’e 6 have chemical signatures showing that they formed from depleted mantle material, similar to the low-titanium basalts returned by Apollo astronauts. This suggests that mantle material left behind as the lunar magma ocean crystallized has remained broadly similar beneath both hemispheres.

The researchers found additional evidence in the magnesium-rich rocks. These rocks form when accumulated mantle material partially melts, and their magnesium number, or Mg#, helps indicate the composition of the mantle source.

Because the Mg-suite samples from Chang’e 6 and Apollo have similar Mg# values, the team concluded that the mantle beneath the Moon’s near and far sides has roughly similar chemical properties. In other words, material crystallized from the Moon’s original magma ocean does not appear to have major compositional differences between the two hemispheres.

The Moon’s crust tells a different story

Although the lunar mantle appears relatively uniform, the crust shows more pronounced differences.

Plagioclase samples returned by Chang’e 6 have higher peak Mg# values than comparable Apollo samples, indicating that the far-side crust contains more magnesium-rich material. This finding agrees with previous observations from instruments studying the Moon remotely.

The researchers also identified chemical differences in the magnesium-rich rocks. The Chang’e 6 samples contain extremely low concentrations of rare earth elements, and their thorium-to-samarium ratio, expressed as Th/Sm, is lower than that of the Apollo samples.

These differences suggest two possible explanations. The magnesium-rich magma may have formed on the far side without substantial involvement from KREEP-rich material. Alternatively, the layers containing these elements may have developed unevenly across the Moon, leaving the two hemispheres with different chemical compositions.

How Earth’s gravity may have shaped the Moon

Combining the findings, the researchers developed a model that could explain how the Moon acquired its dramatically different faces.

Under their proposed scenario, the early lunar magma ocean initially cooled and crystallized relatively uniformly. This process produced the first mantle deposits and anorthosite crust before the Moon became tidally locked to Earth.

The situation changed after the Moon settled into an orbit in which the same hemisphere continually faced Earth. At that time, Earth and the Moon were much closer together, and tidal forces produced greater heating on the Moon’s far side than on its near side.

This uneven heating created temperature differences throughout the Moon’s molten interior, driving large-scale horizontal circulation. Hot magma moved from the near side toward the far side, carrying previously formed magnesium-rich anorthosite material with it.

Over time, this movement caused more crustal material to accumulate on the far side. That process could help explain why the far-side crust became thicker and contained more magnesium-rich material.

Meanwhile, the remaining molten material flowed preferentially toward the near side. As a result, only a relatively thin layer rich in KREEP and ilmenite-bearing cumulates (IBC) developed on the far side, while much of these late-stage materials became concentrated on the near side.

A possible explanation for the Moon’s lunar asymmetry

This model of thermal circulation within the ancient lunar magma ocean provides a possible explanation for why the Moon’s crust differs so dramatically between its two hemispheres, even though the mantle appears chemically similar.

By linking newly analyzed Chang’e 6 samples with the Moon’s early thermal history, the research offers new insight into the processes that shaped Earth’s closest celestial neighbor.