The United States and China are immersed in a new race to the Moon. In this decade, we will see which of the two powers manages to send a manned mission to the south pole of our satellite first, but in the meantime, Asian scientists are leading the competition in terms of scientific knowledge of the most enigmatic area: its far side.
Over the past decades, the US - with its manned Apollo missions - and the USSR and China - with robotic probes - have brought samples from the visible side of the Moon to Earth, but China has been the first and only country to obtain material from the far side. It was collected by the robotic spacecraft Chang'e 6 last year from the Aitken crater and landed on Earth in June 2024, in a capsule that touched down in the Mongolian desert. The scientific treasure consists of two kilograms of rocks and regolith distributed among several research centers and being analyzed with the best scientific instruments. Last April, a study revealed that the far side has less water than the visible side, and this Tuesday, new results of its study are published in the journal Nature Geoscience.
As revealed by a team led by scientists from the University of Peking and University College London (UCL), the interior of the mysterious far side of the Moon could be colder than the side facing Earth. They have also confirmed previous findings indicating that the rock sample is approximately 2.8 billion years old. Specifically, the study is based on the analysis of 300 grams of lunar soil assigned to the Uranium Geology Research Institute of Peking.
"The visible side and the far side of the Moon are very different on the surface and potentially in the interior. It is one of the great mysteries of the Moon. We call it the two-faced Moon. The hypothesis of a drastic temperature difference between the visible and far sides of the mantle has long been proposed, but our study provides the first evidence with real samples," explained co-author of the study Yang Li, a researcher at the UCL Department of Earth Sciences and the University of Peking, in a statement.
Thus, the analysis of the chemical composition of its minerals has revealed that these rocks were formed from lava in the depths of the Moon's interior at a temperature of about 1,100 degrees Celsius, about a hundred degrees cooler than samples from the visible side.
The far side has a thicker crust, is more mountainous, has more craters, and appears to have been less volcanic, with fewer dark basalt spots formed from ancient lava. But as stated by Xuelin Zhu, a doctoral student at the University of Peking and co-author of the research, their findings "show that the differences between the visible and far sides are not limited to the surface but extend into the depths of the interior."
In their article, the researchers argue that the interior could have been colder due to the lower amount of heat-producing elements, such as uranium, thorium, and potassium, which release heat due to radioactive decay.
Previous studies had suggested that this uneven distribution of heat-producing elements could have occurred after a large asteroid or other planetary body collided with the far side. That impact would have shaken the interior of our satellite, pushing denser materials with more heat-producing elements towards the visible side. Other theories suggest that the Moon may have collided with a smaller second moon early in its history, or that the visible side could be hotter due to Earth's gravitational attraction.
Samples from 2.8 Billion Years Ago
After dating the samples collected by the Chang'e 6 mission and confirming they were 2.8 billion years old, the authors of this study used various techniques to estimate the temperature of the sample at different stages of its past when it was in the depths of our satellite's interior, and compared it with temperature estimates of samples collected on the visible side by NASA astronauts from the Apollo missions, finding temperature differences of about 100 degrees.
Since the samples obtained are limited, only a few hundred grams, they worked with a team from the University of Shandong to estimate the temperatures of the parent rock using satellite data from the landing site of Chang'e 6 on the far side. They compared them with equivalent satellite data from the visible side and found differences (this time of 70 degrees Celsius).
According to Jesús Martínez-Frías, a planetary geologist at the Institute of Geosciences (CSIC-UCM), "the lunar dichotomy, its origin, characteristics, and reflection in depth remain a mystery yet to be unraveled," and this new study conducted with Chinese mission data "provides additional information that confirms previous findings, especially interesting from a geochronological point of view, and also brings unexpected novelties about the thermodynamic characteristics derived from the existing mineral phases," he explains via email to this newspaper.
"However, personally, I consider it risky to try to extrapolate the results of the analysis of 300 grams of samples to such broad lunar regional contexts based on that small amount of sample and computer models. More samples are needed, with a wider and more representative distribution," evaluates the Spanish scientist, one of the co-authors of the study published last September 10 that revealed the discovery of possible signs of ancient life in rocks from Mars located by NASA's Perseverance rover.
Meanwhile, NASA and China's astronauts are preparing to reach the Moon's south pole in the coming years, where abundant water in the form of ice is believed to exist. The US mission to land on its surface - Artemis 3 - will not take place before 2027, but NASA announced last week its intention to move up two months, to February 2026, the previous mission, Artemis 2, in which four astronauts will orbit our satellite on a journey that will last around 10 days in total.
The US Artemis lunar program is conceived as a preliminary step to eventually sending humans to Mars. However, proposed budget cuts for NASA by the Trump Administration, especially for the scientific program, as well as the frequent changes in the president's vision for the space agency due to Elon Musk's influence, who advocates for advancing the Martian mission, have cast uncertainty over future trips to our satellite and plans to establish a future colony.
On the other hand, scientific robotic missions by NASA and ESA like Mars Sample Return [designed to collect selected Martian samples by the Perseverance rover] could be canceled if those budget cuts are implemented in the 2026 budgets.
"We are living in turbulent times in geostrategic politics, and my perception is that future manned missions to Mars are being prioritized, as this would represent a historic milestone, as opposed to purely scientific missions, not only in relation to the Moon but also to Mars. The Mars Sample Return mission itself still has no fixed date, despite the promising hint of possible biosignatures on Mars found by Perseverance that we published in September," explains Martínez-Frías, a member of the Science Teams of NASA's MSL missions (the Curiosity rover), ESA-ExoMars, and NASA-Mars2020.
The planetary geologist, however, believes that both objectives could be reconciled: "I would not rule out a dual strategy: on one hand, linked to Artemis with the Moon as an intermediate platform for Mars, and on the other hand, a parallel path aiming to reach Mars directly with a manned mission, regardless of lunar tactical planning. It all depends on how the current rivalry with China unfolds," he points out.