Lunar farside-nearside asymmetry formation during magma ocean solidification

Unraveling the origin of lunar farside-nearside asymmetry requires comparison of various lithologies from both hemispheres, a task long hindered by the absence of farside samples. Here, we present geochemical data for Chang'e 6 (CE6) lithologies from the farside, revealing systematic differences from their nearside counterparts. CE6 anorthosites are more magnesian, while CE6 Mg-suite samples contain lower potassium, rare earth element, and phosphorus (KREEP) contents. However, CE6 Mg-suite samples have Mg# values comparable to Apollo Mg-suite rocks. The mantle source of CE6 2.8-billion-year-old basalts is similar to that of Apollo low-Ti basalts. This systematic geochemical comparison of farside and nearside lithologies, which sample distinct lunar magma ocean (LMO) product layers, indicates that the mantle displays no clear compositional dichotomy. In contrast, the farside anorthositic crust is more magnesium-rich and thicker, while late-stage LMO products may be thinner on the farside. Therefore, we propose that the observed farside-nearside asymmetry originated from lateral thermal convection within a long-lived LMO on the tidally locked Moon.