When the Apollo astronauts returned from the Moon, they brought with them samples of lunar soil (regolith) and rocks. The analysis of these samples has forever changed our perception of the formation and evolution of the Earth-Moon system. Similarly, the samples returned by China’s Chang’e program are also leading to breakthroughs in our understanding of Earth’s only satellite, particularly its so-called “dark side.” As a tidally locked body, the Moon’s near side constantly faces Earth, while its far (or “dark”) side faces outward into space.
According to new findings from a Chinese research team, the far side of the moon is also its colder side. Their conclusions are based on samples returned by the Chang’e-6 mission in 2024, which were collected in Apollo Crater in the Moon’s South Pole-Aitken Basin. After analyzing the samples to determine their chemical composition, the team estimated that they formed from lava deep in the moon’s mantle at a temperature of about 1,100 °C (2,012 °F) – about 100 °C (212 °F) cooler than samples taken from the near side.
The team included researchers from the Beijing Research Institute of Uranium Geology (BRUIG) – part of the China National Nuclear Corporation – the School of Earth and Space Sciences at Peking University, University College London (UCL) and the School of Space Science and Technology at Shandong University. Their findings were reported in a paper published in Nature Geoscience. Based on decades of robotic exploration, the far side of the moon is known to be more mountainous and cratered than the front and has experienced less volcanism, resulting in fewer dark patches of basalt rock.
*Apollo 11 basalt 10049 (left) and breccia 10018 (right). Photo credit: NASA/LPI*
In their study, the researchers assume that the mantle on the far side of the moon is cooler because it contains fewer elements such as uranium, thorium and potassium, which release heat during their radioactive decay process. On the Moon, these elements usually occur together with rare earth elements and phosphorus, forming material that scientists call KREEP-rich (K stands for the chemical symbol for potassium, REE for rare earth elements, and P for phosphorus). Previous research suggests that this uneven distribution could be due to a massive impact on the other side that pushed these denser materials to the other side.
Another theory suggests that the moon has experienced two impacts in the past from lunar pieces of different compositions, one containing more radioactive elements than the other. Another theory is that the Earth’s gravitational pull caused the Earth’s mantle to warm more on the opposite side. As co-author Yang Li, Professor in the UCL Department of Earth and Space Sciences at Peking University, explained in a UCL press release:
The moon’s front and back sides are very different on the surface and possibly inside as well. It is one of the great mysteries of the moon. We call it the two-sided moon. A dramatic temperature difference between the near and far sides of the Earth’s mantle has long been suspected, but our study provides the first evidence from real samples.
For their study, the team examined the 300 grams of lunar soil (mainly basalt rock) allocated to the Beijing Research Institute of Uranium Geology. In total, the Chang’e-6 mission recovered 1,935.3 grams (~4.6 pounds) of lunar soil and rock, which were the first samples ever returned from the far side of the Moon. They then mapped selected parts of the sample with an electron probe to determine their chemical composition. These probes fire concentrated beams of electrons at a sample, causing the sample to emit X-rays that are examined to identify the chemical elements that make it up.
*Artist’s impression of the Moon’s internal structure. Photo credit: MIT*
They then used a secondary ion mass spectrometer (SIMS) to examine lead isotopes in the samples produced by the natural decay of uranium. This allowed them to detect tiny fluctuations in the lead content of the samples and derive an age estimate of 2.8 billion years. Finally, the team estimated the temperature at which the samples formed in the mantle during different phases of the moon’s evolution. The first step was to compare the results of their mineral analysis with computer simulations that estimated the temperature at which the minerals crystallized.
The second option was to infer the temperature of the rock that melted into magma and resolidified to form the basalt rock from which the samples were obtained. Both results were compared to samples from the Apollo mission environment and each showed a temperature difference of 100 °C (212 °F). They also worked with a team from Shandong University to estimate host rock temperatures using satellite data from the Chang’e-6 landing site. They compared this with satellite data from the nearby side, which also showed a temperature difference, but this time 70°C (158°F).
“These findings bring us one step closer to understanding the two sides of the moon,” said co-author Xuelin Zhu, a graduate student at Peking University. “They show us that the differences between the near and far sides are not just on the surface, but go deep inside.” The most widely accepted theory of the Moon’s formation is that a Mars-sized body (Theia) collided with a primordial Earth about 4.5 billion years ago, causing material from both bodies to liquefy into hot magma (the giant impact hypothesis). This magma coalesced as it cooled and solidified, eventually forming the Earth-Moon system we see today.
However, the KREEP materials were incompatible with the solidifying material and remained in the magma for long periods of time. Instead of being evenly distributed across the moon, these materials appeared to have accumulated on the opposite side, possibly explaining the increased volcanic activity there. These questions need to be clarified by future studies, perhaps by astronauts and taikonauts conducting direct studies on the lunar surface.
Further reading: UCL, Nature
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