The Mystery of the Moon's Formation
One of the most fundamental questions that planetary scientists continue to grapple with is: How did the Moon form?
When the Chang'e-4 mission touched down in the Von Karman crater on January 3, 2019, China became the first and only nation to land on the far side of the Moon—the side that always faces away from Earth. Now, China is sending another mission to this enigmatic region, and this time, they aim to bring back the first samples of the lunar far side to Earth.
Not a "Dark Side"
The Chang'e-6 mission will spend 53 days exploring the South Pole-Aitken basin on the lunar far side, studying its geology and topography, and collecting samples from multiple points in the crater.
"The far side of the Moon is very different from the near side," said Li Chunlai, Deputy Chief Designer of the Chinese National Space Administration. "Basically, the far side is the ancient lunar crust and highlands, so there are a lot of scientific questions to be answered there."
The term "dark side of the Moon," sometimes used to refer to the lunar far side, is considered a misnomer for several reasons.
While the far side of the Moon appears dark from our perspective, it experiences lunar days and nights just like the near side, and even receives a similar amount of sunlight. A lunar day lasts over 29 Earth days, while a lunar night lasts around two weeks, according to NASA.
"Humans always want to know what's on the other side of the mountain, the side we can't see, so it's kind of a psychological drive," said Renu Malhotra, Louise Foucar McCain Professor of Planetary Sciences and Regents Professor at the University of Arizona in Tucson.
Unraveling Lunar Mysteries
Spacecraft, including NASA's Lunar Reconnaissance Orbiter, have been repeatedly orbiting and imaging the Moon, helping to unravel its mysteries. Yutu-2, a lunar rover that Chang'e-4 deployed in 2019, has also been exploring the rocky and dusty deposits in the Von Karman crater, which lies within the South Pole-Aitken basin.
But bringing samples back to Earth will allow state-of-the-art, sensitive equipment to analyze the lunar rocks and dust, potentially revealing how the Moon formed and why the two sides are so different.
Far Side Enigmas
Despite years of orbital data and samples collected during the six Apollo missions, scientists still grapple with key questions about the Moon.
"The reason the far side is so compelling is that it's so different than the side of the Moon that we see," said Noah Petro, who works on NASA's Lunar Reconnaissance Orbiter project and the Artemis III mission that aims to return humans to the Moon by 2024. "For all of human history, we've been looking at the same surface, the same side of the Moon."
But in 1959, the Soviet Union flew a probe around the far side of the Moon, capturing humanity's first images of it. "We saw this completely different hemisphere: not covered in these big volcanic lava flows, a lot more cratered, and a thicker crust," Petro said.
According to Petro, collecting samples with robotic missions and eventually human explorers near the lunar South Pole transition zone through the Artemis program will help tell a more complete story of the Moon's history that's currently missing.
Malhotra said that while scientists understand why one face of the Moon always faces Earth, they don't know why that side is permanently turned toward our planet. It may have to do with asymmetry in the Moon. "There's some asymmetry between the two sides of the Moon," she said. "Exactly what is causing those asymmetries? What are those asymmetries, exactly? We understand very little about that. That's a big scientific question."
Brett Denevi, a planetary geologist at the Johns Hopkins University Applied Physics Laboratory, said that orbital data also reveals the near side has a thinner crust and more volcanic deposits, but the reasons are still not well understood.
The Chang'e-6 mission is just one that will target the lunar far side, as NASA also has plans to send robotic missions there. Denevi helped design a concept mission for a lunar rover called Endurance that would trek across the South Pole-Aitken basin, collecting data and samples before heading to the Artemis landing site near the Moon's South Pole. Astronauts would then study the samples and determine which ones to bring back to Earth.
"When impacts happen on the Moon, they happen on Earth, too," said NASA's Petro. "So when we study these ancient events on the Moon, in a way, we're learning a little bit about what happened on the Earth."
According to Malhotra, the exploration of the South Pole-Aitken basin could be a stepping stone to addressing a wealth of lunar mysteries. While researchers have a good idea of when the crater formed—perhaps 4.3 billion to 4.4 billion years ago—collecting rock samples could provide a precise age.