When admiring the moon during the Mid-Autumn Festival, in addition to missing your loved ones, have you also had any curiosity and questions about the moon itself? What scientific questions do the people who know the moon the most about the moon – the lunar scientists ponder when they look at the moon? We will reveal the answer for you by counting 31032 lunar science SCI papers published in 1900~2022, so that you can "admire the moon" like a scientist from now on!
Written by | Yang Wei
"The moon is the Mid-Autumn Festival", the Mid-Autumn Festival is a traditional festival in the mainland, and people's main activities are to admire the moon and eat moon cakes. The Mid-Autumn Festival moon appreciation began in the Wei and Jin dynasties and flourished in the Tang Dynasty, and many poets have verses about the moon in their famous poems, expressing the poet's strong homesickness and hope for reunion.
During the Mid-Autumn Festival, in addition to missing your loved ones, have you also had any curiosity and questions about the moon itself? At least 1,000 years ago, the Tang Dynasty poet Li Bai once "Ask the Wine to the Moon", "When will the moon come in the blue sky"? When did this eternal moon ever exist? More than 1,000 years later, lunar scientists are still searching for the answer to this question.
As a group of people who know the moon in the world, what scientific questions do lunar scientists think about while admiring the bright moon? In order to find out the answer to this question, we counted 31,032 SCI papers published in lunar science from 1900~2022, extracted the top 2% of important papers and performed cluster analysis, and obtained the most popular research areas in lunar science (Figure 1).
Fig. 1 A network of important papers in the field of lunar science
TOP 1: How was the moon formed?
Is it dry or wet when it forms?
At present, the academic community basically agrees that the moon was formed by a major impact event, which is called the "classical impact theory". However, there is still a lot of controversy about the details of the impact event, such as: the number of impacts, the angle of impact, the duration of the impact, etc. In particular, it is necessary to explain why the Moon and the Earth are highly similar in many isotopes, the so-called "isotope crisis of lunar formation".
In the early days, it was believed that the water content of the moon formed by the great impact event was very low. But in 2008, by studying the moon's volcanic glass, scientists discovered that the moon's interior is not as "dry" as previously thought. This subversive discovery further challenges the theory of the great impact: what exactly would be the impact process that would cause the Moon and Earth to have a similar isotopic composition, while allowing the Moon to retain a sufficiently high level of water and volatile matter?
The origin of the moon has been the most concerned issue in the past 120 years, and will continue to be one of the hottest questions in lunar science in the future.
TOP 2：
How did the moon shell we see today form?
When we look up at the moon, we can see two patterns: light and dark (moon sea). To put it simply, the bright part is the lunar highlands, dominated by plagioclase; The dark part is the moon sea, which is dominated by basalt. Based on the lunar samples returned by Apollo 11 in 1969, scientists proposed a magmatic ocean model, which opened the prelude to the study of the formation and evolution of the lunar crust.
The highland and lunar sea correspond to the magmatic ocean evolution and the lunar sea basalt eruption, respectively, both of which are closely related to how the moon cools. Despite more than 50 years of research, there are still many unanswered questions about the field: how big is the magma ocean? How long does it last? Does the moon mantle flip occur? How big? What is the effect on the mantle? When did volcanic activity start and end on the Moon? Why did it last so long? Wait a minute.
TOP 3：
How is lunar surface matter distributed?
Although scientists have been studying the moon remotely since the Apollo era, it was the launch of Clementine in 1994 and the launch of the Lunar Surveyor in 1998 that really changed the research paradigm in this field. Detailed surveys of the entire moon show that the lunar crust is much more complex than previously thought, and Apollo sampling is very limited and does not fully reflect the material composition of the lunar surface. Since then, a series of probes, including Chandrayaan-1, Moon Goddess, the Lunar Reconnaissance Orbiter, and the mainland's Chang'e-1 and Chang'e-2, have not only helped us identify new lunar surface material and geological phenomena, but also provided a key basis for the selection of landing sites for future roving exploration and sample return missions.
Where will we go in the future? Where will the lunar research station and base be built? To better answer these questions, we need to have a deeper understanding of the types of lunar matter and their distribution.
TOP 4：
Where can you find water on the Moon?
Because the permanently shadowed areas of the moon's polar regions are not visible to the sun, and the temperature is extremely low, scientists have long speculated that there may be water ice there. In 1994, the Clementine bistatic radar experiment found that water ice may exist in the polar regions of the moon. The 2010 Lunar Crater Observation and Sensing Satellite (LCROSS) lunar impact experiment confirmed the presence of water ice in the polar regions of the Moon.
Today, the world's space powers plan to carry out water ice exploration of the moon's south pole. How is water ice distributed in the polar regions of the Moon? Where does it come from? How many reserves are there? This is not only an important scientific issue, but also related to the extraction and utilization of water resources that will reside on the lunar surface for a long time in the future.
There is no doubt that water ice exploration will remain the most important direction in future lunar exploration.
TOP 5：
What is the impact history of the Moon?
The Moon is a natural laboratory for studying the impact history of the inner solar system. Its surface is covered with impact craters, documenting the history of asteroid impacts it has suffered since its formation. Unlike the Earth's impact craters, which are obscured by later geological processes, the Moon retains almost a complete impact record, so what is the impact history of the Moon? Was there a major impact 3.9 billion years ago? In addition, because the intensity and frequency of asteroid impacts generally decrease with time, the impact crater distribution density can be used to determine the age of different geological units of the lunar crust, however, the impact history of the moon Apollo samples are mainly collected from 30-4 billion years old, so it is difficult to accurately determine the young geological bodies of 3 billion years, so how to obtain a more accurate lunar surface age through impact craters is also a problem that needs to be solved in the future.
TOP 6：
What is the interior of the Moon like?
Through Apollo samples and remote sensing exploration, scientists have a basic understanding of the surface of the moon, but none of these means can "see" the interior of the moon. The moonquake data collected during the Apollo era only had certain constraints on the macroscopic internal structure of the Moon. However, its precision and accuracy are limited by the performance of the instrument.
What is the structure of the deep mantle and nucleus? We still know pretty much about it. Recently, through the gravity data obtained by the Holy Grail (GRAIL), scientists have calculated that the average lunar crust thickness for the whole month is 34~43 kilometers. In addition, there has been little progress in understanding the structure of the moon for decades.
In the future, in order to make a major breakthrough in this problem, it is necessary to deploy a sufficient number of high-precision lunar seismographs in different regions of the lunar surface.
TOP 7：
How does the space environment affect the Moon?
Due to the lack of protection of the intrinsic magnetic field, solar radiation, solar wind and cosmic rays can directly reach the lunar surface during the operation of the moon, and interact with the lunar soil and the residual magnetism on the lunar surface to form a unique near-lunar space environment. The scientific questions covered by this interaction between the space environment and solid planets include: How does space weathering modify lunar soil? How are swirling bright spots on the Moon formed? When did the Moon's intrinsic magnetic field disappear? Why did it disappear? How does the lunar surface electric field affect the migration of dust and charged particles? Wait. The research results were published in Acta Petrologica Sinica (Tengfei Zhang, Yanhai Wang, Wei Yang**, Yang Liu, Dengsheng Wu, Wei Wei, Yang Ting Lin, Xianhua Li. 2023. Frontiers of Lunar Science Based on Bibliometrics. Acta Petrologica Sinica, 39(10): 3169-3183, doi: 10.18654/1000-0569/2023.10.19). This work was jointly funded by the Frontier Interdisciplinary Research Project (XK2022DXC004) of the Faculty of the Chinese Academy of Sciences and the Science and Technology Activity Project of the National Natural Science Foundation of China (L2224032).
This article is reprinted with permission from the WeChat public account "Institute of Geology and Earth, Chinese Academy of Sciences". Proofreading: Zhang Tengfei (Institute of Geology and Earth Sciences).
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