When the Apollo astronauts returned from the Moon, they brought with them samples of lunar soil (regolith) and rock. The analysis of these samples forever changed our perceptions of how the Earth-Moon system formed and evolved. Similarly, the samples returned by China's Chang'e program are also leading to breakthroughs in our understanding of Earth's only satellite, especially its so-called "dark side." As a tidally-locked body, the Moon's near side is constantly facing towards Earth while its far (or "dark") side faces outward to space.
According to new findings by a team of Chinese researchers, the far side of the Moon is also its colder side. Their conclusions are based on the samples returned by the Chang'e-6 mission in 2024, which were collected from the Apollo Crater located 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 within the Moon's mantle at a temperature of about 1,100 °C (2,012 °F) - roughly 100 °C (212 °F) cooler than samples obtained from the near side.
The team consisted of 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 results were reported in a paper published in Nature Geoscience. Based on decades of robotic exploration, it is known that the far side of the Moon is more mountainous and cratered than the near side and experienced less volcanism, leading to fewer dark patches of basalt rock.
In their study, the researchers suggest 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 tend to occur together alongside rare earth elements and phosphorus, forming material that scientists refer to as KREEP-rich (K denoting the chemical symbol for potassium, REE for rare earth elements, and P for phosphorus). Previous research has suggested that this uneven distribution may have resulted from a massive impact on the far side, which pushed these denser materials to the other side.
Another theory suggests that the Moon has experienced two impacts in the past from moonlets with varying compositions, with one containing more radioactive elements than the other. Yet another theory is that the Earth's gravitational pull led to increased heating in the near side's mantle. As co-author Yang Li, a Professor with UCL's Department of Earth and Space Sciences at Peking University, explained in a UCL News release:
The near side and far side of the moon are very different at the surface and potentially in the interior. It is one of the great mysteries of the moon. We call it the two-faced moon. A dramatic difference in temperature between the near and far side of the mantle has long been hypothesized, but our study provides the first evidence using real samples.
For their study, the team examined the 300 grams (~10.5 oz) of lunar soil (mainly basalt rock) that was allotted to the Beijing Research Institute of Uranium Geology. In total, 1,935.3 grams (~4.6 lbs) of lunar soil and rock were obtained by the Chang'e-6 mission, 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.
They then targeted lead isotopes in the samples, which are produced by the natural decay of uranium, using a Secondary Ion Mass Spectrometer (SIMS). This allowed them to detect tiny variations in the lead content of the samples, from which they obtained an age estimate of 2.8 billion years. Finally, the team estimated the temperature at which the samples formed in the mantle during different stages in the Moon's evolution. The first step was to compare the results of their mineral analysis to computer simulations that estimated the temperature at which the minerals crystallized.
The second was to infer the temperature of the rock, which melted into magma and re-solidified to form the basalt rock from which the samples were obtained. Both of these results were compared to near-side samples collected by the Apollo missions, both revealing a temperature difference of 100 °C (212 °F). They also collaborated with a team from Shandong University to estimate parent rock temperatures using satellite data of the Chang'e-6 landing site. They compared this to satellite data from the near side, which also indicated a temperature difference, but of 70 °C (158 °F) this time.
"These findings take us a step closer to understanding the two faces of the Moon," said co-author Mr Xuelin Zhu, a PhD student at Peking University. "They show us that the differences between the near and far side are not only at the surface but go deep into the interior." The most widely held theory about 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 become liquid into hot magma (the Giant Impact Hypothesis). This magma coalesced as it cooled and solidified, ultimately forming the Earth-Moon system we see today.
However, the KREEP materials were incompatible with the solidifying material and remained in the magma for longer periods. Rather than being evenly distributed across the Moon, these materials appeared to have collected on the near side, possibly explaining the heightened volcanic activity there. These questions will need to be addressed by future studies, possibly by astronauts and taikonauts conducting direct studies on the lunar surface.