Ancient impact may explain moons contrasting sides

by Riko Seibo
Tokyo, Japan (SPX) Mar 05, 2026
For decades scientists have puzzled over why the moons two hemispheres look so different despite sharing a common origin in the early solar system.

The near side that always faces Earth is dominated by dark flat volcanic plains called maria that create the familiar man in the moon pattern seen with the naked eye. In contrast the far side has a much thicker crust and appears as a rugged heavily cratered highland region with very little sign of large scale lava flooding.

New research published Tuesday in the journal Proceedings of the National Academy of Sciences points to a giant impact early in lunar history as a key driver of this stark dichotomy. A team from the Institute of Geology and Geophysics of the Chinese Academy of Sciences analyzed microscopic samples returned by Chinas Chang'e 6 mission from the moons far side.

Chang'e 6 achieved the first sample return from the lunar far side by landing within the vast South Pole Aitken basin one of the largest known impact structures in the solar system. The mission returned tiny grains weighing only about as much as a few grains of salt yet they preserve detailed chemical records of ancient events in the lunar crust and mantle.

Led by professor Tian Hengci the researchers focused on potassium a moderately volatile element that occurs in multiple isotopes with different atomic masses. During extremely high temperature events such as a giant asteroid impact lighter potassium isotopes tend to evaporate and escape more readily than heavier ones leaving behind material that is enriched in the heavy isotopes.

Laboratory measurements showed that the Chang'e 6 samples from the South Pole Aitken basin are unusually rich in heavy potassium isotopes compared with typical lunar rocks. The team interprets this enrichment as a chemical fingerprint of an enormous impact that generated intense heat capable of driving off lighter isotopes of potassium as well as other volatile elements such as zinc and sulfur.

According to the study this large scale loss of volatile elements fundamentally altered the thermal and chemical evolution of the moons far side. Volatile components help lower the melting point of rocks deep inside a planetary body making it easier for magma to form and rise to the surface to fuel volcanic eruptions.

With fewer volatiles remaining after the South Pole Aitken impact the far side interior stayed comparatively rigid and resistant to melting. As a result it produced far less magma over time than the near side which retained more volatile rich material and continued to generate large volcanic plains well after the impact era.

The findings offer a physically grounded explanation for why the near side developed extensive mare volcanism while the far side remained dominated by ancient highlands. They suggest that violent impacts do more than excavate craters on planetary surfaces and can instead reshape the deep interior by stripping away critical ingredients that control how easily rocks melt.

"The findings suggest that asteroid impacts do more than just leave a dent on the surface; they can fundamentally change the chemistry of a planet's guts," Tian said. He noted that this mechanism may operate not only on the moon but also on other rocky bodies whose histories include giant collisions.

Tian added that other hypotheses for the moons two faced appearance have emphasized tidal effects from Earth or uneven distribution of radioactive elements that heat the interior. While those processes may still play supporting roles the new isotopic evidence from Chang'e 6 highlights the profound influence that a single massive impact can have on the long term evolution of a worlds crust and mantle.

By tying together sample analysis spacecraft exploration and impact physics the study demonstrates how far side materials can illuminate processes that shaped the entire moon. Future missions returning additional samples from different regions of the South Pole Aitken basin and beyond are expected to test how widespread this volatile loss signature is and refine models of the moons early history.