A map showing the basin where this study’s lunar samples were collected. Photo by NASA.
Although it is frequently referred to as the ‘dark side’, the part of the Moon that faces away from Earth receives just as much sunlight as does the side we can see at night. However, the far side of the Moon has long been dark in a different sense. Only 24 humans have seen it in person, and before June 2024, no samples had been collected from known far-side locations. Last year, however, the Chang’e-6 mission, developed by the China National Space Administration (CNSA), made history by returning the first samples from the far side of the Moon. Writing in Nature, Zhou et al. use samples of 2.8-billion-year-old lava from the South Pole–Aitken (SPA) basin on the Moon’s far side to cast light on differences in the origin and evolution of the Moon’s near and far sides. Their work is published alongside other studies on Chang’e-6 samples from Zhang et al., Cai et al. and He et al.
Since the 1960s, when space missions first achieved lunar orbit, it has been known that the Moon is asymmetrical, with a near side and a far side that are geologically distinct. Gravitational measurements have shown that the outermost layer, called the crust, is around half as thick on the near side as it is on the far side. The near side is also home to about 90% of the Moon’s maria, which are huge plains or ‘seas’ of solidified lava produced by volcanic activity over the course of billions of years. These lavas were formed by partial melting of the mantle — the layer between the crust and the core.
The near side also contains a region with an enhanced concentration, or ‘enrichment’, of incompatible elements. These are elements that tend to remain in the liquid portion of a crystallizing magma, rather than becoming part of a mineral. The set of incompatible elements found in this near-side region includes radioactive isotopes of potassium, uranium and thorium, which generate heat when they decay. This has kept the near-side mantle warmer than that of the far side, even up to the present.
