Humanity got its first glimpse of the far side of the moon in 1959, when the Soviet space probe Luna 3 took our first images of the far side of the moon. The images were shocking and showed a clear difference between the different sides of the moon. Now China is sending another lander to the other side.
This time it will bring back an example from this long-unknown area that could explain the puzzling difference.
Chang'e-6 (CE-6) launched on May 3 and is on its way to the second largest impact crater in the solar system: the South Pole Aitken (SPA) basin. It will land in the Apollo Pool, a sub-pool within the much larger SPA Pool.
China has previously placed a lander on the far side of the moon (Chang'e 4). They also placed a lander on the far side of the moon and brought back samples (Chang'e 5). But CE-6 will be the first sample ever to return from the far side of the moon. It is the latest mission under the Chinese Lunar Exploration Program (CLEP).
This graphic outlines China's lunar exploration program. Photo credit: CASC
A new paper published in Earth and Planetary Science Letters describes the importance of the CE-6 landing site and the samples it will return to Earth. The title is “Long-lasting volcanism at the back of the Apollo Basin: Chang'e-6 landing site.” The main author is Dr. Yuqi Qian from the Department of Earth Sciences, University of Hong Kong.
When the USSR's Luna 3 probe gave us a glimpse of the far side of the moon for the first time, it didn't take long for scientists to realize how different it was from the far side of the moon. The far side of the Moon is marked by vast basaltic lava plains called lunar mares. Mares cover about 31% of the near side of the moon.
But the other side is completely different. Lunar mares only cover about 2% of the far side of the moon. Instead, it is dominated by densely cratered highlands. This is known as the lunar dichotomy. The difference is likely due to a deposition of heat-producing elements beneath the front, which formed the lunar mares. Scientists have also suggested that a long-ago companion moon crashed into the other side and created the highlands.
This global map of the Moon, as seen by the Clementine mission, shows the differences between the near side of the Moon and the far side of the Moon. The familiar near side is marked by dark lunar mares. On the other hand, there are very few of them. This is known as the lunar dichotomy. Photo credit: NASA.
“An important lunar science question is the cause of the lack of mare basalts on the other side,” Qian and his colleagues write in their paper. “The Chang'e-6 (CE-6) mission, the first sample return mission to the far side of the Moon, aims to land in the southern Apollo Basin to sample farside mare basalts that will provide crucial insights into provide early lunar evolution.”
CE-6 samples from the other side can begin to answer the questions about the differences between the two sides. In preparation for receiving the samples, Qian and his colleagues studied volcanism in the Apollo Basin. Their work revealed diverse and enigmatic volcanism.
Their research shows that the Apollo Basin experienced volcanic activity that lasted from the Nectarian period (about 4.05 billion years ago) to the Eratosthenic period (about 1.79 billion years ago). However, because the crust on the other side is much thicker, it influenced volcanic activity. In regions like Oppenheimer Crater, where the crust is of medium thickness, lava dikes remain trapped beneath the crater floor. Lava spreads laterally, forming a crater with sill and floor cracks.
These two images provide context to the CE-6 landing site. The left picture shows where Apollo is in the SPA. The right image shows some features in Apollo Crater, with the landing zone shown in a white rectangle. Photo credit: Qian et al. 2024.
Some regions, such as the inner floor of Apollo Crater, have thin crusts. Here lava dykes erupted directly and formed extensive lava flows. But where the crust is thickest, in the highland regions, there is no evidence that dikes ever reach the surface there.
“This fundamental finding suggests that the discrepancy in crustal thickness between the near and far sides may be the main cause of asymmetric lunar volcanism,” said Dr. Qian. “This can be tested using the returned Chang’e-6 samples.”
They chose the Southern Mare of Apollo Crater, in part because there are at least two historic eruptions there from two different times. Each has a different titanium content. The earlier one took place about 3.34 billion years ago and has a low titanium content (3.2% by weight). The later occurred about 3.07 billion years ago and has a higher titanium content (6.2% by weight).
According to the authors, this image from the study shows the best location for sampling. This region would provide samples of the older low-Ti basalts, the younger high-Ti basalts, and also overlying impact ejecta from Chaffee S crater. Photo credit: Qian et al. 2024.
The titanium content in rocks is relevant to petrogenesis, the formation and creation of rocks. Scientists believe that lunar basalts with high and low Ti content are formed when different geological layers of the moon melt. “CE-6 samples returned from the unique geological setting will provide important petrogenetic information to further address the lack of mare basalts on the far side and the dichotomy between near- and far-side lunar sides,” the authors write.
The authors suggest that CE-6 collects samples from the rim of the later eruption with the higher titanium content. This sample will have greater scientific value because it actually samples three things at once: newer high-Ti basalt, underlying low-Ti basalt, and other materials unrelated to the mares transported by impact events became. “Different sample sources would provide important insights into solving a range of lunar science questions hidden in the Apollo Basin,” said paper co-author Professor Joseph Michalski, also from the University of Hong Kong.
“The result of our research is a major contribution to the Chang'e-6 lunar mission. “It lays down a geological framework for a complete understanding of the soon-to-be-returned Chang'e-6 samples and will be an important reference for upcoming sample analysis for Chinese scientists,” said Professor Guochun Zhao, chair of HKU's Department of Geosciences and Co -Author of the paper.
Chang'e 6 will carry up to 2 kg (4.4 lbs) of lunar material. It should arrive on Earth around June 25th.
“These returned samples could help answer questions about the evolution of high- and low-Ti basalts, the influence of crustal thickness on lunar volcanism, and the most fundamental unsolved question in lunar science: What causes the distinct lunar nearside?” -Farside -Asymmetry?” the authors conclude.
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