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The Moon is seriously two-faced. The near and far sides of Earth’s only natural satellite are so unlike each other, it’s a wonder they’re located on the same celestial sphere. Now, new research suggests they’re even more different than astronomers thought.
A study published Tuesday in the journal Nature Geoscience found evidence to suggest that the interior of the Moon’s mysterious far side may be significantly colder than the near side, which constantly faces Earth. The findings, according to the authors, show that the geological differences between the Moon’s two faces extend far beneath the surface, bringing astronomers closer to figuring out why they’re so unalike.
“It is one of the great mysteries of the Moon,” co-author Yang Li, an associate professor of Earth and planetary sciences at the University College London and Peking University, said in a UCL release. “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.”
Earth’s two-sided satellite
Astronomers once thought that the Moon’s near and far sides were relatively similar, but as their studies of the lunar surface have expanded over the last 60 years, they began to notice stark geological differences. For example, only 1% of the Moon’s far side features maria—dark plains created by ancient volcanic eruptions. In contrast, 31% of the near side is covered with maria.
Observations by several lunar orbiters have also shown that rocks from the far side have distinctly different chemical compositions than those from the near side. What’s more, NASA’s twin GRAIL orbiters discovered that the far side’s crust is on average about 12 miles (20 kilometers) thicker than the near side’s.
These dichotomies can be traced back to the Moon’s formation and evolution, which are intrinsically linked to Earth’s own history. Scientists widely believe that both our planet and its satellite formed when a Mars-sized object slammed into the proto-Earth about 4.5 billion years ago. Thus, studying the geological differences between the Moon’s two sides offers insight into Earth’s origin story as well.
Uncovering an ancient temperature difference
The samples Li and his colleagues analyzed were rock fragments gathered by China’s Chang’e 6 spacecraft, which launched to the far side of the Moon in May 2024. The chemical composition of the 2.8-billion-year-old sample suggests it formed from lava deep beneath the lunar surface at a temperature of about 2,012 degrees Fahrenheit (1,100 degrees Celsius). That’s about 200°F (100°C) cooler than the temperature samples from the near side formed at.
Though their analysis did not reveal the current temperature difference between the near and far side of the moon, the researchers note that this discrepancy would have persisted for a very long time—perhaps to this day. They hypothesize that the far side interior may be cooler because it has fewer elements that release heat as they undergo radioactive decay, such as uranium, thorium, and potassium.
According to some previous studies, the uneven distribution of these elements may stem from a massive asteroid impact on the far side that shoved denser materials containing these elements into the near side. Others suggest the Moon may have collided with a smaller moon early in its history, and thus near-side and far-side samples originate from two thermally different celestial bodies. Or, it could be that the near side’s interior is warmer due to Earth’s gravitational pull.
Whatever the cause, this Chang’e 6 sample appears to have confirmed yet another puzzling difference between the Moon’s two faces. Figuring out exactly what this tells us about our cosmic companion’s history will require further research. At least for now, the researchers are left with more questions than answers.
