China’s Chang’e 6 mission successfully collected 1.935 kilograms of basaltic soil from the South Pole–Aitken (SPA) basin on the moon’s farside in June 2024. These are the first physical samples retrieved from this hemisphere.
Initial laboratory analysis of the samples indicates a significantly lower water content in the lunar mantle beneath the farside—estimated between 1 and 1.5 micrograms per gram. This contrasts with nearside estimates of up to 200 micrograms per gram from previous Apollo missions.
The observed dryness in the mantle supports the theory that the moon formed through a massive collision between early Earth and a Mars-sized body 4.5 billion years ago. The uneven distribution of water aligns with models that suggest such an impact could result in regional differences within the lunar interior.
Scientists used thorium levels in the lunar soil as an indirect measure of water. Thorium tends to remain in molten rock, similar to water, so its surface abundance helps trace the moisture levels of the moon’s mantle. The SPA basin's relatively low thorium levels reinforce the idea of a drier farside interior.
The SPA basin, one of the few large impact basins on the moon's farside, exposed deep layers of the mantle through volcanic activity. This made it
The study adds to the list of known disparities between the moon’s two hemispheres. These include variations in topography, volcanic activity, surface age, rock types, and chemical distribution—all of which need to be explained in models of lunar evolution.
Researchers propose two possible explanations: the SPA basin impact may have affected water retention locally, or water may be unevenly distributed by depth and age of formation within the mantle. The study, published in Nature on 9 April, adds a new dimension to ongoing lunar research.