Humanity got its first look at the other side of the Moon in 1959 when the USSR’s Luna 3 probe captured our first images of the Lunar far side. The pictures were shocking, pointing out a pronounced difference between the Moon’s different sides. Now China is sending another lander to the far side.
This time, it’ll bring back a sample from this long-unseen domain that could explain the puzzling difference.
Chang’e-6 (CE-6) launched on May 3rd and is headed for the second largest impact crater in the Solar System: the South Pole Aitken (SPA) basin. It’ll land at Apollo Basin, a sub-basin inside the much larger SPA basin.
China has placed a lander on the far side of the Moon before (Chang’e 4.) They also placed a lander on the near side of the Moon and brought back samples (Chang’e 5.) But CE-6 will be the first sample ever returned from the Lunar far side. It’s the latest mission in the Chinese Lunar Exploration Program (CLEP.)
This graphic outlines China’s Lunar Exploration Program. Image Credit: CASCA new paper published in Earth and Planetary Science Letters outlines the significance of the CE-6 landing site and the samples it’ll return to Earth. It’s titled “Long-lasting farside volcanism in the Apollo basin: Chang’e-6 landing site.” The lead author is Dr. Yuqi Qian from the Department of Earth Sciences at The University of Hong Kong.
When the USSR’s Luna 3 probe gave us our first look at the lunar far side, it didn’t take scientists long to realize how different it is from the near side. The near side of the Moon is marked by vast basaltic lava plains called lunar mares. Mares cover about 31% of the lunar near side.
But the far side is much different. Lunar mares cover only about 2% of the lunar far side. Instead, it’s dominated by densely-cratered highlands. This is known as the lunar dichotomy. The difference likely stems from a deposit of heat-producing elements under the near side that created the lunar mares. Scientists have also proposed that a long-gone companion moon slammed into the far side, creating the highlands.
This global map of the Moon, as seen from the Clementine mission, shows the differences between the lunar near side and far side. The familiar near side is marked by dark lunar mares. The far side has very few of them. This is known as the lunar dichotomy. Credit: NASA.“A major lunar scientific question is the cause of the paucity of farside mare basalts,” Qian and his colleagues write in their paper. “The Chang’e-6 (CE-6) mission, the first sample-return mission to the lunar farside, is targeted to land in the southern Apollo basin, sampling farside mare basalts with critical insights into early lunar evolution.”
CE-6 samples from the far side can start to answer the questions about the differences between the two sides. In preparation for receiving the samples, Qian and his colleagues studied the Apollo Basin’s volcanism. Their work revealed diverse and puzzling volcanism.
Their research shows that the Apollo basin experienced volcanic activities lasting from the Nectarian (~4.05 billion years ago) to the Eratosthenian Period (~1.79 billion years ago). However, since the far side’s crust is much thicker, it influenced the volcanic activity. In regions like the Oppenheimer Crater, where the crust has intermediate thickness, lava dikes stall beneath the crater floor. Lava spreads laterally and forms a sill and floor-fractured crater.
These two images give context to the CE-6 landing site. The left image shows where Apollo is inside the SPA. The right image shows some of the features in the Apollo crater, with the landing zone in a white rectangle. Image Credit: Qian et al. 2024.Some regions, like the inner floor of the Apollo crater, have thin crusts. Here, lava dikes erupted directly and formed extensive lava flows. But where the crust is thickest, in the highland regions, there’s no evidence that dikes there ever reach the surface.
“This fundamental finding indicates that the crustal thickness discrepancy between near side and far side may be the primary cause of lunar asymmetrical volcanism,” said Dr. Qian. “This can be tested by the returned Chang’e-6 samples.”
They’ve chosen Apollo Crater’s Southern Mare partly because it contains at least two historic eruptions from two different times. Each one has a different Titanium content. The earlier one occurred ~3.34 billion years ago and has a low Titanium content (3.2% by weight.) The later one occurred ~3.07 billion years ago and has a higher Titanium content (6.2% by weight.)
This figure from the study shows the prime location for collecting samples according to the authors. This region would provide samples from the older, low-Ti basalts, the younger high-Ti basalts, and also overlying impact ejecta from the Chaffee S crater. Image Credit: Qian et al. 2024.The titanium content in the rock is relevant because of petrogenesis, the origin and formation of rocks. Scientists think that high-Ti and low-Ti lunar basalts form when different geological layers of the Moon melted. “CE-6 samples returned from the unique geological setting will provide significant petrogenetic information to address further the paucity of farside mare basalts and the lunar nearside-farside dichotomy,” the authors write.
The authors suggest that CE-6 collect samples from the edge of the later eruption with the higher Titanium content. That sample will have higher scientific value because it’ll actually sample three things at once: Newer high-Ti basalt, underlying low-Ti basalt, and other materials unrelated to the mares that were transported by impact events. “Diverse sample sources would provide important insights into solving a series of lunar scientific questions hidden in the Apollo basin,” said Professor Joseph Michalski, a co-author of the paper also from the University of Hong Kong.
“The result of our research is a great contribution to the Chang’e-6 lunar mission. It sets a geological framework for completely understanding the soon-returned Chang’e-6 samples and will be a key reference for the upcoming sample analysis for Chinese scientists,” said Professor Guochun Zhao, Chair Professor of HKU Department of Earth Sciences and the co-author of the paper.
Chang’e 6 will deliver up to 2 kg (4.4 lbs) of lunar material. It should arrive on Earth around June 25th.
“These returned samples could help to answer questions about the evolution of high-Ti and low-Ti basalts, the influence of crustal thickness on lunar volcanism, and the most fundamental unsolved question of lunar science: What is the cause of the pronounced lunar nearside-farside asymmetry?” the authors conclude.
