The mysterious far side of the Moon may be significantly cooler deep within its interior than the side that permanently faces Earth, according to a new study co-led by researchers from University College London (UCL) and Peking University.

The research, published in Nature Geoscience, analysed rock and soil fragments collected in 2024 by China’s Chang’e-6 spacecraft from a vast crater on the Moon’s far side.

Using advanced chemical analysis, the team found that the sample, about 2.8 billion years old, crystallised from lava at roughly 1,100C, around 100C cooler than near-side rocks studied from the Apollo missions.

Professor Yang Li, co-author from UCL’s Earth Sciences department, described the phenomenon as one of the “great mysteries” of lunar science.

“The near side and far side of the Moon are very different at the surface and potentially in the interior. We call it the two-faced Moon,” he said. “Our study provides the first evidence for a dramatic temperature difference using real samples.”

The far side, with its thicker crust and rugged, cratered terrain, appears to have been less volcanically active than the near side.

Researchers suggest this may be due to a shortage of heat-producing elements, uranium, thorium, and potassium, within its interior. These elements, typically associated with volcanic activity, seem disproportionately concentrated on the near side, where vast basalt plains are visible.

The imbalance may have been caused by a massive impact early in the Moon’s history, redistributing heat-producing material, or potentially by the collision of two smaller moonlets forming today’s lunar body. Another theory is that Earth’s gravitational pull helped sustain higher temperatures beneath the side it constantly faces.

To reach their conclusions, scientists used electron and ion probes to map mineral compositions and measure isotopes within the 300 grams of lunar soil returned. Additional computer simulations and satellite data confirmed consistent temperature differences between the two hemispheres.

Lead author Sheng He of the Beijing Research Institute of Uranium Geology emphasised the significance of the findings: “This is the first sample ever collected from the far side of the Moon. It gives us a rare window into the hidden half of our nearest neighbour.”

Researchers hope to further investigate whether these thermal differences still persist today, as the Moon continues its slow cooling journey since its fiery origins billions of years ago.

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