China’s Chang’e 6 probe to the far side of the moon aims to solve a major lunar mystery

Chinas Chang’e-6 mission, currently on its way to retrieve a sample of material on the other side of the moonwill test theories as to why the near and far sides of the moon are so different.

To have launched on May 3 Chang’e-6 is expected to land in early June in the Apollo double-ring impact basin, which lies within the even larger area South Pole-Aitken Basin (SPA). The immense SPA is the largest impact feature of its kind in the solar system, with an area of ​​2,400 by 2,050 kilometers (1,490 by 1,270 miles); it was formed about 4.3 billion years ago terribly early in the history of the solar system. Although Apollo is younger, it is also the largest impact site to be superimposed on the SPA. Apollo has a double-ring structure, with the inner ring of mountain peaks 247 kilometers (153 miles) in diameter and an outer ring about 492 kilometers (305 miles) wide.

Related: China’s Chang’e 6 mission to collect samples from the far side of the moon enters lunar orbit (video)

As the first sample return mission to the other side, Chang’e-6 aims to return about 2 kilograms of precious lunar material to Earth. The other side is a relatively unknown place; its mystery is also increased by the fact that we cannot see it from Earth. It wasn’t photographed for the first time until 1959 by the Soviet spacecraft Luna 3. And with this photo in hand, scientists around the world were amazed to discover how different the far side of the moon looks compared to the side we came from come. familiar with. While both the near and far sides show a large number of craters, the near side also shows extensive, volcanic plains called moon mariawhich create impressions like the shape of a “man in the moon” and cover about 31% of the entire area on the side.

Meanwhile, the other side is the exact opposite. It is only about 1% covered by volcanic plains.

How did the near and far sides become so different? Well, the thickness of the crust seems to be a factor. Indeed, in 2011 from NASA GRAIL (Gravity Recovery and Interior Laboratory) mission found that the crust on the far side is on average 20 kilometers (12 miles) thicker than the crust on the near side.

A color-coded image of the moon that emphasizes how large the basin is.

A color-coded image of the moon that emphasizes how large the basin is.

The reason for this is believed to date back to when our moon formed from debris ejected when a Mars-sized protoplanet struck Earth about 4.5 billion years ago. When the moon coalesced from the rubble around a wounded Earth, it became tidally locked, meaning it would always show the same face to our planet. The surface of the Earth was completely melted by the giant impact and radiated heat to the near side of the moon, essentially keeping itself molten longer. Rock would evaporate on the near side and condense on the cooler side, scientists suggest, thickening the crust on the far side.

“The basic finding indicates that the difference in crustal thickness between the near and far sides may be the main cause of asymmetric lunar volcanism,” Yuqi Qian of the University of Hong Kong said in a statement.

Qian is one of the lead authors of a new study that suggests sample material brought back to Earth by Chang’e-6 could test this theory.

The logic is as follows.

Where the moon’s crust is thick, as on most of the far side, magma welling up through fractures in the rock cannot break through to the surface. Where the crust is thin, such as on the near side, these fractures can actually allow magma to break through and lava to erupt.

The SPA and the Apollo Basin, despite both being on opposite sides of the moon, present some contradiction. That’s because they were smashed deep into the moon’s crust, and at the base of these giant impact sites, the crust is thinner than the moon’s. elsewhere on the other side. And yes, there are volcanic plains in these basins, yet only 5% of their surface area is covered by basaltic lava flows. This limited amount of volcanism appears to conflict with the idea that the thickness of the Earth’s crust dictates volcanic activity – a paradox long known in lunar science.

An alternative possibility suggests that the near side could contain more radioactive elements than the far side. These elements may have produced heat, melting the lower mantle and creating much more magma and a thinner crust on the near side. So more volcanism.

Nevertheless, by landing on one of the few volcanic plains on the other side, Chang’e-6 could bring back samples to directly test such theories. In particular, the region of the Apollo Basin where Chang’e-6 will land contains several materials that warrant investigation.

There is also evidence that two major volcanic eruptions have occurred in the region. One, about 3.35 billion years ago, would have covered the entire region with magma containing a low amount of titanium, according to scientists. The other, believed to have occurred 3.07 billion years ago, probably had titanium-rich magma and was more localized, and erupted near the Chaffee S crater (named after Roger Chaffee, one of the astronauts who tragically died in the crater). Apollo 1 disaster) and spreads eastward with decreasing thickness.

The new study suggests that retrieving a sample from close to Chaffee S would provide the greatest scientific benefits. This region has titanium-rich basalt on top, titanium-poor basalt below, and several pieces of ejected material from the impacts.

“Different sample sources could provide important insights into solving a range of lunar science questions hidden in the Apollo Basin,” said Joseph Michalski of Hong Kong, who co-authored the new study with Yuqi Qian, as well as their Hong Kong researcher. colleague Guochun Zhao and researchers from elsewhere in China, Germany and the United States.

A diagram showing different parts of the moon.A diagram showing different parts of the moon.

A diagram showing different parts of the moon.

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– China’s next moon missions get the green light

— China opens Chang’e 5 moon samples to international researchers

These diverse samples could tell scientists something about the magmatic processes taking place on the other side, and comparing them to samples from the near side returned by the Apollo missions could answer the question of why it volcanism on the other hand was so limited.

“The result of our research is a great contribution to the Chang’e-6 lunar mission,” Zhao said. “It provides a geological framework for a complete understanding of the soon-to-be-returned Chang’e-6 samples and will be an important reference for the upcoming sample analysis for Chinese scientists.”

The team’s article was published in the journal Earth and planetary science letters; you can continue to follow for further updates on the Chang’e-6 mission.

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