Is the Moon’s far side colder? New clues found in lunar samples

For a long time, the Moon has shown us just one face. That’s because it’s tidally locked with Earth – it spins at the same rate that it orbits us, so the same side always faces our planet. The other side, the so-called far side, stays hidden.

Now, thanks to China’s Chang’e 6 spacecraft, scientists are learning what’s going on beneath the surface of that lesser-known half – and it might be colder deep down than we thought.

This is surprising. You’d expect the Moon’s interior to be fairly uniform, but new rock samples from the far side are challenging that idea.

A moon with two faces

The Moon doesn’t just look different on each side – it is different. The near side is smoother and covered in large, dark lava plains called maria.

The far side is rougher, more mountainous, and has far fewer of those lava-filled areas. For a while, scientists have guessed this might mean the inside of each side is different too. But they didn’t have any samples to test that idea – until now.

China’s Chang’e 6 mission, launched in 2024, brought back the first ever soil and rock samples from the Moon’s far side.

The spacecraft landed inside a massive crater and scooped up about 10.6 ounces (300 grams) of lunar material. A research team got to work analyzing those grains – mostly made of basalt, a rock that forms from cooling lava.

Colder roots beneath the Moon’s surface

The sample was dated to be 2.8 billion years old using tiny variations in lead isotopes. That matches the age of similar rocks collected on the near side. But when the team looked at how hot the rock had been when it first formed from lava, the story changed.

The minerals in the sample suggested the rock crystallized at around 2,012°F (1,100°C). That’s roughly 180°F (100°C) cooler than similar rocks found on the near side. This cooler temperature likely reflects deeper differences in the Moon’s interior.

“We call it the two-faced Moon,” said Professor Yang Li, based at UCL’s Department of Earth Sciences and Peking University, and one of the co-authors of the study.

“A dramatic difference in temperature between the near and far side of the mantle has long been hypothesized, but our study provides the first evidence using real samples.”

Uneven past revealed

The scientists used more than one method to check their findings. First, they compared the chemistry of the mineral grains to computer models of how lava forms.

Then they went further – working backward to estimate how hot the parent rock must have been before it melted into lava and later solidified into what the Chang’e 6 mission collected.

Using these different approaches, they kept seeing the same thing: a consistent temperature difference between the two sides.

Satellite data backed it up as well. The team compared readings from the Chang’e 6 landing site with similar sites on the near side. Those results showed a smaller but still noticeable difference – about 126°F (70°C).

“These findings take us a step 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 between the near and far side are not only at the surface but extend deep into the interior.”

Heat clustered on the near side

Scientists think the reason may lie in what the Moon is made of – and how those materials are spread out. When the Moon first formed, it was mostly molten rock.

As it cooled, some elements didn’t fit into the growing crystals and remained in the leftover liquid. These include uranium, thorium, and potassium, which release heat as they decay radioactively.

You’d expect these heat-producing elements to be scattered evenly, but they’re not. They’re concentrated mostly on the near side. This material is often called KREEP – a mix of potassium (K), rare earth elements (REE), and phosphorus (P). More KREEP means more heat.

Why is the KREEP stuck on one side? One idea is that a massive asteroid hit the Moon’s far side early on, shaking things up and pushing denser, heat-producing material over to the near side.

Another theory suggests the Moon once had a smaller twin that eventually crashed into it, creating a lopsided interior. Some scientists also propose that Earth’s gravity may have pulled denser materials toward the near side.

Whatever the cause, the heat-producing elements appear to have concentrated on the side we see – making it warmer and more volcanically active.

Moon’s far side is colder

The study doesn’t reveal exactly how hot or cold the Moon’s mantle is today, but it does offer a glimpse into how things may have started.

The Moon has been cooling ever since it formed about 4.5 billion years ago. If one side began cooler, that difference probably hasn’t disappeared – and may still persist today.

There is still much left to figure out, but obtaining this far side sample was a major breakthrough. It gives scientists the chance to revisit old ideas and test them with real material.

What comes next? More digging, more data, and hopefully more samples. So far, one thing is clear: the Moon’s far side isn’t just hidden – it’s likely colder, too.

The full study was published in the journal Nature Geoscience.

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