Researchers analyzing lunar rocks returned by China’s Chang’e-6 mission suggest that enormous movements of molten rock shaped the Moon’s two contrasting faces billions of years ago. The study points to heat-driven horizontal circulation within an ancient magma ocean, triggered by Earth’s gravitational tidal forces after the Moon became tidally locked.
Researchers exploring the reasons behind the stark visual contrast between the Moon’s two halves have discovered fresh data that may shed light on this enduring puzzle. A research team led by Hejiu Hui of Nanjing University examined lunar rocks collected by China’s Chang’e-6 mission and found clues suggesting that enormous movements of molten rock shaped the celestial body billions of years ago.
Contrasting Hemispheres and Past Theories
The side of the Moon facing Earth features vast plains of dark volcanic basalt and relatively high concentrations of potassium, rare earth elements, and phosphorus, known as KREEP. In contrast, the far side is dominated by ancient, heavily cratered highlands, a much thicker crust, and relatively few volcanic plains. For decades, scientists have debated the origins of this uneven structure.
Earlier hypotheses offered to solve the puzzle involved variations in how the primeval lunar magma ocean flowed, a colossal impact, thermal energy generated by gravitational tides, and asymmetric formation of the lunar crust. However, none of these ideas has gained universal acceptance, and scientists remained uncertain whether similar differences extended deep into the lunar mantle.
Comparing Chang’e-6 and Apollo Samples
The 2024 Chang’e-6 mission became the first mission to return samples from the Moon’s far side. Researchers compared chemical compositions of three rock types—anorthosites, Mg-suite rocks, and mare basalts—from the far side with material collected from the near side during the Apollo and Chang’e-5 missions.
The analysis revealed unexpected similarities beneath the contrasting surfaces. Basalts collected by Chang’e-6 showed chemical signatures indicating they originated from depleted mantle material, closely resembling low-Ti basalts brought back by Apollo astronauts. Furthermore, Mg-suite samples from both missions shared similar magnesium number (Mg#) values, indicating that mantle cumulates beneath both hemispheres share broadly similar chemical characteristics without a significant compositional divide.
Crustal Differences and Gravitational Tides
While the mantle appears relatively uniform, the lunar crust tells a different story. Anorthosite samples returned from the far side contain higher peak Mg# values, indicating a more magnesium-rich crust. Additionally, Mg-suite samples from Chang’e-6 contain extremely low concentrations of rare earth elements and lower thorium-to-samarium ratios (Th/Sm) than their Apollo counterparts.
Based on these findings, the research team developed a model involving the ancient lunar magma ocean. Initially, the ocean cooled uniformly before the Moon became tidally locked to Earth. Once the Moon settled into an orbit keeping the same hemisphere facing our planet, Earth and the Moon were much closer together. Gravitational tidal forces generated more heat on the lunar near side, creating a temperature difference that set off large-scale horizontal circulation.
Hot magma moved from the near side toward the far side, carrying magnesium-rich anorthosite material and building up a thicker, magnesium-rich crust on the far side. Meanwhile, late-stage materials like KREEP and ilmenite-bearing cumulates concentrated more heavily on the near side.
