The first rock samples ever retrieved from the moon’s far side suggest its interior may be significantly colder than the hemisphere perpetually facing Earth, a finding that deepens one of lunar science’s most enduring mysteries.
Analysis of fragments collected by China’s Chang’e 6 spacecraft from a massive crater reveals they crystallized from magma at roughly 1,100 degrees Celsius. That is about 100 degrees cooler than comparable samples from the near side, according to research published in Nature Geoscience.
The near side and far side of the moon are very different at the surface and potentially in the interior. It is one of the great mysteries of the moon. We call it the two-faced moon.
Professor Yang Li of University College London and Peking University, who co-authored the study, notes that scientists have long hypothesized this temperature difference, but lacked direct evidence until now.
Ancient Lava Tells a Thermal Story
The research team analyzed 300 grams of lunar soil allocated to the Beijing Research Institute of Uranium Geology. Using sophisticated dating techniques that measure lead isotope variations, they confirmed the rock formed approximately 2.8 billion years ago, when volcanic activity was still reshaping the moon’s surface.
By examining the chemical composition of minerals within the basalt fragments, particularly clinopyroxene crystals, researchers reconstructed the temperature conditions deep within the moon when the original magma formed. They employed multiple independent techniques, including electron probe mapping and computer simulations, to verify their temperature estimates.
The far side’s thicker crust, more mountainous terrain, and relative scarcity of dark basalt patches have long puzzled scientists. The hemisphere looks fundamentally different from the near side, which features the familiar dark “seas” visible from Earth.
One leading explanation involves heat-producing elements like uranium, thorium, and potassium. These radioactive materials, which concentrate in a substance scientists call KREEP (named for potassium’s chemical symbol K, rare-earth elements, and phosphorus), appear unevenly distributed across the moon. Remote sensing suggests KREEP materials cluster predominantly on the near side.
Collision or Companion?
The uneven distribution might trace back to a catastrophic impact on the far side early in lunar history. Such a collision could have literally shaken up the moon’s interior, pushing denser, heat-producing materials toward the near side. Alternative theories propose the moon once had a smaller companion that merged with it, or that Earth’s gravitational pull has heated the near side over billions of years.
These findings take us a step closer to understanding the two faces of the moon. They show us that the differences between the near and far side are not only at the surface but go deep into the interior.
Co-author Xuelin Zhu, a PhD student at Peking University, emphasizes that the temperature asymmetry extends throughout the moon’s structure.
The researchers corroborated their sample-based findings with satellite data from both hemispheres. Analyzing remote sensing information from the Chang’e 6 landing site and comparing it to equivalent data from Mare Serenitatis on the near side produced similar results, showing a 70-degree temperature difference.
The current temperature of the moon’s mantle remains unknown, though any thermal imbalance between the two hemispheres will persist for geological timescales. The moon cools extremely slowly from its violent formation billions of years ago. The research team continues working to determine present-day mantle temperatures.
The Chang’e 6 mission marked a historic achievement when it returned 1,935 grams of material from the Apollo basin’s southern rim last year. The landing site sits within the South Pole-Aitken basin, the moon’s largest and oldest impact crater, spanning roughly 2,500 kilometers in diameter.
This ancient basin excavated deep into the lunar crust during its formation over 4.3 billion years ago, leaving the crust just 5 kilometers thick in some locations compared to the far side’s typical 40 to 60 kilometers. Despite this thin crust providing easier access for rising magma, only 3 to 4 percent of the basin floor exhibits evidence of past volcanic flows.
The samples contain fragments ranging from microscopic grains to millimeter-sized chunks, composed primarily of clinopyroxene, plagioclase, and volcanic glass. Minerals like ilmenite, olivine, and even tiny crystals of apatite and baddeleyite appear in trace amounts, each preserving clues about conditions during crystallization.
Understanding the moon’s thermal evolution has implications beyond lunar science. The moon likely formed from debris created when a Mars-sized object collided with early Earth. Initially molten, it cooled and solidified over hundreds of millions of years. Why KREEP materials concentrated on one hemisphere rather than distributing evenly remains an open question that could illuminate planetary formation processes throughout the solar system.
Nature Geoscience: 10.1038/s41561-025-01815-z
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