On June 25, a piece of the moon will collide with Earth. He falls. And it’s no small thing either: nearly two kilograms of rock and dust have reached our planet, collected from the lunar crust by China’s Chang’e-6 lander. The lander arrived on the moon’s surface a few weeks ago, and the samples it collected were packed into a rocket capsule to rendezvous with the Chang’e-6 orbiter in lunar orbit. They made the 380,000-kilometre return journey to Earth in a specially developed module, finally landing in Inner Mongolia, slowed down by parachutes, so that the samples could be collected and studied by eagerly awaiting researchers.
The project was organized in much the same way as the 2020 Chang’e-5 mission, which also returned lunar samples to Earth, with one very important difference: This time—and for the first time, in fact—the material came from the far side of the moon, which always faces away from Earth. This required additional steps, such as using satellites in lunar orbit to help with communications, but the scientific benefit of collecting more samples was likely worth the effort.
Researchers hope that minerals found in the historical samples will help solve one of the long-standing mysteries of planetary science: why the far side of the moon is different from the far side.
Moon faces
When we look at our celestial companion, we always see the same side of it, because the Moon takes roughly the same amount of time to rotate once as it does to orbit the Earth once. This so-called finite rotation is not accidental, but is related to the strong tidal influence of our planet on the Moon. As a result, the Moon can be more or less divided into two hemispheres: one side always facing us and one side always facing away from the Earth.
The distinctive surface features of the side of the Moon facing us are well known: many large, roughly circular dark spots can be seen against a lighter background. Ancient astronomers called these dark features seas (mare) because they resembled water coming from Earth. However, they are actually salt from plains of basaltic minerals—solidified lava—that erupted from beneath the surface long ago. The brighter areas on the near side are older, more reflective peaks that protrude from the plains.
Astronomers have long assumed that the side of the moon that is not visible from Earth might be similar. But space exploration challenged those expectations in 1959, when the Soviet spacecraft Luna 3 returned the first images of the far side. Although the footage was grainy and blurry, it was clear enough to show that the landscape was quite different.
The far side is almost entirely covered with rugged mountains, and the vast seas on the near side are limited to a few scattered dark patches.
Observations made since then have only increased the number of striking differences observed between the two hemispheres. For example, gravity data from the two GRAIL (Gravity Recovery and Interior Laboratory) probes orbiting the Moon have shown that the crust on the far side is about 20 kilometers thicker on average than that on the near side.
fragile theories
But why are the two hemispheres of the Moon so different? It may be tempting to think that the tidal effect on Earth is behind this, but the formula isn’t that simple. The explanation must go back to the birth of the Moon. According to the origin story most widely accepted by researchers, shortly after Earth formed 4.6 billion years ago, a planet similar in size to Mars (which astronomers call Theia, after the daughter of the Titan Gaia in Greek mythology) collided with our planet.
The collision tore apart Theia, whose core plunged deep into the Earth, and its outer layers, along with much of the Earth's material, entered orbit around the Earth.
The superheated rocks quickly cooled in space and formed the Moon. Researchers are still debating the details, but according to some models, this formation could have happened in just a few months. At first, the Moon was much closer to Earth than it is today, perhaps only a tenth of its current distance, but over time it gradually drifted farther away.
As it was closer, the tidal forces were also stronger, so the newly formed moon could be in a stable orbit in a very short time—perhaps a year. The material of the celestial body cooled much more slowly than that, meaning the crust was not yet solid when the boundary cycle formed, meaning the two faces of the moon could not have been caused by tidal forces. Something else must have happened to make the crust on the other side thicker as it cooled.
Researchers have come up with several ideas, but none of them fit neatly with the duality we see today. Perhaps a second, smaller moon formed from the debris of the impact, which eventually collided with the larger moon at a slower speed, sending more material to the far side. Or physical processes inside the newly formed moon allowed the crust to become thicker on one side.
Earth light
In 2014, a team of astronomers published a new explanation that was much better than previous ones. According to them, the “culprit” was not the tidal influence of our planet, but the Earth itself. None of the models that explain the evolution of the young Moon fully took into account the size of the Earth that would appear in the lunar sky. Shortly after the Moon formed, the Earth was so close that it occupied an astonishing 40-degree area in the sky, an area twenty times larger than today.
Remember that the Earth was very hot at that time. The Theia impact vaporized most of our planet's surface and melted the rest. The surface temperature likely remained around 2,000 degrees Celsius for thousands of years, which also heated the Moon. More precisely, only one side of it, where the closed circulatory system formed very quickly.
The near side may have been heated to 1,000 degrees Celsius under the influence of the Earth, while the far side gradually cooled to freezing.
This had disastrous consequences. When the Moon was still completely molten, it had an atmosphere of glowing rocks and minerals. Heavy elements like calcium and aluminium have very high boiling points, so they remain in a gaseous state on the hot near side of the Moon, while condensing on the surface on the cooler far side. There, they reacted with other elements to form lighter minerals like feldspar, which gradually created an increasingly thick crust.
Interestingly, orbital studies show more feldspar on the far side than on the near side. As a result of this process, other minerals may have accumulated more on the near side, including radioactive minerals, which would have heated the crust and allowed magma to erupt. As a result, more seas were created on the near side, while the far side remained largely sealess.
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But does this theory really solve the mysterious duality of the moon? This is the best idea at the moment, but of course more evidence is needed to support it by experts. Hopefully, the new samples brought back to Earth by the Chang'e-6 spacecraft will help answer these questions.
Interestingly, the far side of the Moon is often called dark, even though it receives exactly the same amount of light as the near side. The dark side is more due to the fact that since we have only been seeing this location in recent decades, we know very little about it, so it is considered an unknown region. But its secrets are gradually being revealed, and soon we may have answers to questions that were asked at the beginning of the space age.
