Chang'e-6 Samples Point to Sideways Magma Flows Behind Moon's Two Faces

The Moon shows two very different faces. The side facing Earth is covered in dark volcanic plains, while the far side, never visible from Earth, has rugged, heavily cratered highlands, a much thicker crust, and far fewer volcanic plains. Scientists have long puzzled over why the Moon's near and far sides look, and are built, so differently.

New research on samples from China's Chang'e-6 mission may help explain the difference. In 2024, Chang'e-6 returned the first-ever samples collected from the Moon's far side. A team led by Professor Hejiu Hui of Nanjing University compared these far-side rocks with near-side material earlier returned by the Apollo missions and China's Chang'e-5 mission. The findings were published in the journal National Science Review.

The comparison suggests the Moon's mantle, the thick rock layer beneath the crust, is broadly similar on both sides: chemical signatures in far-side rocks matched those in near-side Apollo samples, the researchers reported. Far-side anorthosite rocks contained more magnesium than similar near-side samples, showing the far side's crust is more magnesium-rich than the near side's. The near side, by contrast, holds higher concentrations of potassium, rare earth elements and phosphorus.

The researchers propose that this split emerged after the Moon became tidally locked to Earth, meaning the same side always faces it. Before that, they suggest, the Moon's early ocean of molten rock cooled evenly on both sides. Once tidal locking set in, the model proposes, the near side received more tidal heating than the far side, creating a temperature difference between the hemispheres. That gradient, the team suggests, may have driven hot magma sideways from the near side toward the far side, carrying magnesium-rich material and building the far side's thicker crust.

According to the model, this sideways flow would also have pushed residual melted rock toward the near side, leaving the far side with only thin layers of the potassium-, rare-earth- and phosphorus-rich material, the researchers reported.