The initial terrestrial acquisition of regolith from the lunar far side may provide a resolution to an enduring enigma concerning our natural satellite.
An examination by the Chinese Academy of Sciences of lunar particles returned to Earth by China’s Chang’e-6 mission indicates that the stark disparity between the Moon’s two hemispheres could stem from a colossal ancient impact that fundamentally reconfigured the Moon’s internal composition.
This conclusion elegantly synthesizes numerous characteristics of the lunar far side, illustrating that celestial body impacts are not merely superficial blemishes but possess the capacity to profoundly and enduringly reshape planetary interiors.
The asymmetry inherent in the Moon’s two hemispheres has been a source of scientific contemplation since the Soviet probe Luna 3 captured the inaugural imagery of the far side in 1959. Even in the rudimentary photographs procured, the divergence was readily apparent. While the side facing Earth presents a mottled appearance, characterized by extensive, smooth, dark basaltic plains, the far side exhibits a lighter hue and is heavily pockmarked with impact craters.
Numerous hypotheses have been posited to account for this phenomenon, including a connection to the largest recognized impact crater in the Solar System—the South Pole-Aitken Basin, which encompasses almost a full quarter of the lunar expanse.
However, the absence of direct physical access to lunar samples has impeded the confirmation of this hypothesized linkage.
The Chang’e-6 mission, orchestrated by the China National Space Administration, represents a paradigm shift. It stands as the inaugural—and thus far, sole—endeavor to successfully transport lunar regolith from the far side to Earth for scientific scrutiny, a remarkable testament to human ingenuity. Since the capsule containing the particulate matter made its descent in 2024, researchers have been diligently endeavoring to unravel its secrets.
In this recent research, a team spearheaded by planetary scientist Heng-Ci Tian conducted an analysis of the potassium and iron constituents within the sample, which was procured from the South Pole-Aitken Basin.
The investigative group sought to discern discrepancies in the isotopic composition between samples from the far side and those collected from the lunar near side during the Apollo program and China’s Chang’e-5 mission. Isotopes, fundamentally, are variants of the same element that differ in their neutron count, thereby altering their atomic mass while preserving their chemical reactivity.
The scientists meticulously compared the isotopic profiles of the basalt samples against previously documented isotopic values for Apollo basalts and Chang’e-5 basalts.
The findings revealed a pronounced divergence between the two hemispheres. The Apollo and Chang’e-5 basalts exhibited a greater abundance of lighter iron and potassium isotopes, in contrast to the heavier isotopes identified on the far side. This disparity cannot be attributed to volcanic activity, as such geological processes do not induce alterations in potassium isotope ratios consistent with the researchers’ observations.
This suggests that the impact event that formed the South Pole-Aitken Basin penetrated deeply into the Moon’s interior, generating intense thermal energy. This molten state would have facilitated the vaporization of mantle material, with a preferential release of lighter isotopes that are more volatile.
“While magmatic processes can elucidate the iron isotopic data, the potassium isotopes necessitate a mantle source exhibiting a heavier isotopic composition on the far side compared to the near side,” the researchers articulate.
“This characteristic most plausibly arose from potassium evaporation triggered by the impact that created the South Pole-Aitken basin, underscoring the profound influence of this event on the Moon’s deep interior. This discovery further implies that large-scale impacts serve as principal agents in shaping mantle and crustal compositions.”
Given that the impact event excavated deeply into the Moon’s mantle, it would have consequently modified potassium isotopes to considerable lunar depths. This mechanism provides a coherent explanation for the observed isotopic disparities between the sampled sets and furnishes scientists with a novel method for interpreting lunar data.
It is even conceivable that this event instigated hemisphere-wide mantle convection; however, further sample acquisitions from diverse regions of the lunar far side will be requisite to ascertain this possibility.
We are already cognizant that the Moon’s most significant impact indelibly altered it. This recent research posits that those lasting imprints extend far beyond the surface, profoundly modifying the Moon’s geochemistry in ways that transcend the passage of time.
