When China’s Chang’e 6 lunar rover returned the first-ever samples from the Moon’s farside, scientists were eager for a rare glimpse into the hidden hemisphere’s interior. Now those hopes have paid off in an unexpected way.
In findings recently published in Nature Geoscience, an international research team reports that the far side’s mantle cooled at more than 200 °F (~100 °C) lower than the side facing Earth—evidence that the Moon is far less symmetrical beneath its surface than once believed.
This newly revealed thermal imbalance carries significant implications for our understanding of how the Moon formed and evolved. If one hemisphere is significantly cooler inside than the other, it challenges prevailing models of lunar magma dynamics, volcanic history, and even the redistribution of heat-producing elements early in the Moon’s life.
“The near side and far side of the moon are very different at the surface and potentially in the interior,“ co-author and Professor of Earth Sciences at University College London, Dr. Yang Li, said in a press release. “It is one of the great mysteries of the Moon. We call it the two-faced Moon. A dramatic difference in temperature between the near and far side of the mantle has long been hypothesised, but our study provides the first evidence using real samples.”
Until recently, our direct knowledge of the Moon’s subsurface derived almost entirely from the near side, sampled by the Apollo missions and various robotic landers.
Meanwhile, the Moon’s far side remained a blind spot we could image and map but never physically sample. That changed with Chang’e 6, which collected material from a crater on the far side and returned about 300 grams of lunar soil to Earth.
This far side sample is thought to be roughly 2.8 billion years old. The team analyzed its composition using electron probes and ion probe (SIMS) techniques to deduce its mineral make-up, crystallization history, and the thermal conditions under which it formed.
By comparing its features with those of analogous rocks from the near side, the researchers estimate that the far side’s deep interior may have crystallized at about 2,000 °F (1,100 °C), roughly 212°F (100 °C) cooler than similarly aged and sourced near-side rocks.
Further exploring the heat history of a “parent rock,“ or the rock from which the sampled material was derived, the researchers found that it indicates a similar temperature difference. When they combined their sample data with satellite analyses of the landing site and analogous near-side regions, the disparity reappeared—this time at 158 °F (~70 °C).
“Collectively, our findings demonstrate that the lunar farside mantle was relatively colder than the near-side mantle, consistent with the hemispherical differences in crustal thickness and heat-producing element distribution, and provide constraints on the thermal evolution of the Moon and the origin of its global asymmetry,” the researchers write.
The Moon is known for its asymmetry—the far side has much more rugged highlands, fewer dark basaltic plains (mare), and a thicker crust than the near side. Whether that asymmetry extended into the mantle has long been debated.
One leading hypothesis is that heat-producing elements, such as uranium, thorium, and potassium, were distributed unevenly. These “KREEP“ (potassium [K] + rare earth elements [REE] + phosphorus)–rich materials release heat via radioactive decay. If they ended up concentrated more heavily on the near side, that hemisphere could have stayed warmer while the far side cooled faster.
The researchers outlined several potential scenarios for how this might happen. One hypothesis is that early in the Moon’s history, a massive asteroid or planetary impact on the far side jolted its interior, driving denser, KREEP-rich material toward the near side.
Another intriguing possibility is that early in its life, the Moon might have briefly been two bodies, such as a primary moon and a smaller companion, whose merger led to thermal and compositional differences between the two faces.
Finally, Earth’s gravitational pull might have influenced internal heat flow, making the near side marginally hotter through tidal or gravitational coupling.
Although these theories are still speculative, the new data give them real-world backing, moving the debate beyond computer models and into evidence grounded in actual lunar samples.
If the Moon really harbors a long-lasting temperature gradient between its interior hemispheres, that has ripple effects across lunar science. It affects how we interpret lunar volcanism, magma flows, thermal evolution, and even the potential for stored heat reservoirs that could influence future exploration.
One key question is whether the temperature difference persists today. The study does not measure the current mantle temperature separately on each side. However, because the Moon cools only very slowly over billions of years, such a difference would likely remain if it once existed.
Another consequence is that many models of lunar formation and evolution have thus far assumed a relatively homogeneous interior. However, these new findings may prompt refinements or even revisions in how scientists simulate the Moon’s early history.
Moreover, sample return missions to the far side are extremely rare, with Chang’e 6 being the first to bring back material from that hemisphere. As more far side samples become available, researchers will be able to test whether this temperature contrast is localized or a global phenomenon.
The current team is already planning follow-up work to explore present temperature differences and sample diversity.
It’s also important to note that this isn’t the first time a recent study of the Moon’s far side has produced unexpected results.
Last year, researchers from the University of Hong Kong reported evidence of “hidden magmatism“ beneath the surface near the Chang’e-6 landing site. Using remote sensing and advanced imaging, they detected dense material consistent with subsurface magma, an activity previously thought to be extinct on the Moon.
Published in The Astrophysical Journal Letters and reported by The Debrief, the study hinted that portions of the Moon’s interior may still be warmer and more geologically active than scientists once believed—adding yet another twist to the mystery of the far side.
Together, these ongoing analyses reveal that the Moon is not a static, monolithic relic but a dynamic world with hidden internal complexity. In this sense, the “two-faced“ description is more than poetic. It may be literal when it comes to the Moon’s thermal history.
“These findings take us a step closer to understanding the two faces of the moon,“ co-author and PhD student at Peking University, Xuelin Zhu, explained. “They show us that the differences between the near and far side are not only at the surface but go deep into the interior.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com