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When the Chang’e-4 mission landed in the Von Karman Crater on January 3, 2019, China became the first and only country to land on the far side of the moon – the side that always faces away from Earth.
Now China is sending another mission to the other side, and this time the goal is to return the first samples from the moon’s “hidden side” to Earth.
The Chang’e-6 mission, which launched on Friday, will explore the South Pole-Aitken Basin for 53 days to study its geology and topography and collect samples from various sites within the crater.
The South Pole-Aitken Basin is believed to be the largest and oldest crater on the moon, covering almost a quarter of the moon’s surface and measuring about 2,500 kilometers in diameter. The impact crater is more than eight kilometers deep.
Scientists hope returning samples to Earth will help answer lingering questions about the intriguing far side, which has not yet been studied as deeply as the near side, and confirm the moon’s origins.
“The far side of the moon is very different from the near side,” said Li Chunlai, deputy chief designer of China’s National Space Administration. “The distant region is actually made up of old lunar crust and highlands, so many scientific questions need to be answered there.”
No real ‘dark side’
During a NASA budget hearing on April 17, Congressman David Trone asked NASA Administrator Bill Nelson why China was sending a mission to the “back” of the moon.
“They’re going to have a lander on the far side of the moon, the side that’s always dark,” Nelson replied. “We have no intention of going there.”
The Hidden Side of the Moon is sometimes referred to as the “Dark Side of the Moon,” largely in reference to the 1973 Pink Floyd album of the same name.
But experts say the phrase is a bit of a misnomer for a number of reasons.
Although the far side of the moon may appear dark from our perspective, it experiences a lunar and lunar night just like the near side, and receives ample illumination. According to NASA, a Monday lasts just over 29 days, while the lunar night lasts about two weeks.
The same side always faces Earth, because the moon takes the same amount of time to complete an orbit around Earth and rotate on its axis: about 27 days.
In addition, the far side of the moon was more difficult to study, leading to the nickname “dark side” and creating an air of mystery.
“People always want to know what’s on the other side of the mountain and what part you can’t see, so that’s a kind of psychological motivation,” says Renu Malhotra, Louise Foucar Marshall Science Research Professor and Regents Professor of Planetary Sciences. at the University of Arizona in Tucson. “Of course we’ve sent space probes around the moon, and we have images, so in a way it’s less mysterious than before.”
Several spacecraft, including NASA’s Lunar Reconnaissance Orbiter that continuously orbits and takes images of the moon’s surface, have helped shed light on the moon.
Yutu-2, a lunar rover that launched Chang’e-4 in 2019, also examined loose deposits of pulverized rock and dust at the floor of the Von Karman Crater, located in the greater South Pole-Aitken Basin.
But returning samples to Earth would allow the latest and most sensitive technology to analyze moon rocks and dust, potentially revealing how the moon formed and why the far side is so different from the near side.
Mysteries on the other side
Despite years of orbital data and samples collected during six of the Apollo missions, scientists are still trying to answer important questions about the moon.
“The reason the far side is so compelling is because it is so different from the side of the moon that we see, the near side,” said Noah Petro, NASA project scientist for both the Lunar Reconnaissance Orbiter and Artemis III, a mission that aims to land people on the moon for the first time since 1972. “Throughout human history, people have been able to look up and see the same surface and the same side of the moon.”
But in 1959, the Soviet Union sent a probe to fly past the far side of the moon and captured humanity’s first images.
“We saw this very different hemisphere: not covered with large volcanic lava flows, pockmarked with craters, a thicker crust. It just tells a different story than the near side,” Petro said.
Returning samples with robotic missions and landing humans near the transition between the two lunar regions at the south pole through the Artemis program “will help tell this more complete story of lunar history that we are missing now,” he said.
While scientists understand why one side of the moon always faces Earth, they don’t know why that particular side permanently faces our planet. But it could have something to do with the moon being asymmetrical, Malhotra said.
“There is some asymmetry between the side facing us and the other side,” she said. “What exactly caused those asymmetries? What exactly are these asymmetries? We have little understanding for that. That is a big scientific question.”
Orbital data has also revealed that the near side has a thinner crust and more volcanic deposits, but answers to why that is so have eluded researchers, said Brett Denevi, a planetary geologist at Johns Hopkins University’s Applied Physics Lab.
“It has a different kind of geochemical composition with some strange additional heat-producing elements. There are a lot of models that explain why the near side is different from the far side, but we don’t have the data yet,” Denevi said. “So going the other way, taking samples and making different types of geophysical measurements is very important to unravel this long-standing mystery.”
Chang’e-6 is just one mission heading to the far side of the moon, as NASA plans to send robotic missions there as well.
Denevi helped design a mission concept for a lunar rover called Endurance, which will undertake a long drive across the South Pole-Aitken Basin to collect data and samples before delivering them to the Artemis landing sites near the moon’s south pole. Astronauts can then study the samples and determine which ones to return to Earth.
Cracking the moon code
One of the most fundamental questions scientists have tried to answer is how the moon formed. The prevailing theory is that some object impacted Earth early in its history, and a giant chunk flying away from our planet formed the moon.
Scientists also want to know how the moon’s original crust formed.
Volcanic currents created dark spots on the moon, while the lighter parts of the surface represent the moon’s original crust.
“We think that at one point the moon was completely molten, and it was an ocean of magma, and when that solidified, minerals floated to the top of this ocean, and that’s that lighter terrain that we can see today ,” Denevi said. “Reaching the really big expanses of untouched terrain on the other side is just one of the goals.”
Meanwhile, the study of impact craters that litter the moon’s surface provides a history of how things moved during the early days of the solar system at a critical point when life began to form on Earth, Denevi said.
“While the impacts were happening on the moon, impacts were also happening on the Earth at the same time,” Petro said. “And so when we look at these ancient events on the moon, we also learn a little bit about what’s happening on Earth.”
A visit to the South Pole-Aitken Basin could be the start of solving many lunar mysteries, Malhotra said. Although researchers think they have an idea of when the crater formed, perhaps 4.3 billion to 4.4 billion years ago, collecting rock samples could provide a definitive age.
“Many scientists are confident that if we figure out the age of that depression,” she said, “it will unlock all kinds of mysteries about the moon’s history.”
CNN’s Wayne Chang contributed to this report.
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