For centuries, humans could only wonder what the far side of the moon looked like.
Unlike the near side, which always faces Earth, the far side remained hidden until spacecraft finally revealed it in the 20th century.
Now, for the first time, scientists have lunar samples from this mysterious region—and they suggest the moon’s far side may be colder deep inside than the side we see from Earth.
The study, published in Nature Geoscience, analyzed rock and soil collected in 2024 by China’s Chang’e 6 mission.
The spacecraft landed inside a giant crater on the far side of the moon and brought back precious samples for detailed analysis.
The research team confirmed that the rock fragments were about 2.8 billion years old, dating back to a time when volcanic activity was still reshaping the lunar surface.
But what made the findings especially significant was the chemical analysis of the minerals.
By studying the makeup of the rock, scientists estimated that the lava it formed from was about 1,100°C—roughly 100°C cooler than samples collected from the near side.
“The near side and far side of the moon are very different both on the surface and possibly in the interior,” explained Professor Yang Li of UCL and Peking University, a co-author of the study. “We call it the two-faced moon.
A difference in mantle temperature between the two sides has long been suspected, but this is the first evidence based on real samples.”
The moon’s two faces are strikingly different. The far side has a thicker crust, is heavily cratered, and has fewer dark volcanic plains.
The near side, in contrast, has thinner crust and large smooth basins filled with basalt created by ancient lava flows.
One explanation for this imbalance lies in the distribution of heat-producing elements such as uranium, thorium, and potassium. These elements release heat as they undergo radioactive decay, warming the surrounding rock.
In their paper, the researchers suggest that the far side may contain fewer of these heat-producing elements, making its interior cooler.
Why would the elements be unevenly spread? One leading idea is that a massive asteroid impact early in the moon’s history stirred up its interior, pushing denser, radioactive-rich material toward the near side.
Another theory is that the moon originally had a companion: a smaller moonlet that eventually collided with it, leaving behind two hemispheres with very different thermal histories.
To test their findings, the team used several approaches. They compared mineral compositions in the far side samples with computer simulations, then cross-checked their results against near-side samples collected during NASA’s Apollo missions.
In each case, they found a temperature difference of about 100°C. They also worked with Shandong University researchers to analyze satellite data from the Chang’e 6 landing site and similar regions on the near side. That independent method suggested a slightly smaller temperature difference of about 70°C.
Although the study cannot reveal the moon’s current mantle temperature, it strongly indicates that the two sides have cooled differently since the moon formed from a colossal impact with a Mars-sized body billions of years ago. Because the moon loses heat very slowly, any thermal imbalance is likely to persist for billions of years.
“These findings bring us closer to understanding the two faces of the moon,” said Xuelin Zhu, a co-author and Ph.D. student at Peking University. “They show that the differences go much deeper than the surface.”
For now, the far side remains largely unexplored. Scientists hope future missions will return more samples and help solve the mystery of why our closest celestial neighbor is so uneven.
As Professor Li put it, “Ultimately, understanding the moon helps us understand the history of our own planet.”
