From the lunar soil powder, scientists have glimpsed some mysteries of the moon's evolution | Image source: pixabay.com
Introduction
At the end of 2020, China's lunar probe Chang'e-5 landed in a relatively young region of the moon and dug and drilled 1.731 kilograms of lunar samples.
In July, the first scientists obtained some samples by application. Three months later, scientists have glimpsed some of the mysteries of the moon's evolution.
Written by | Wang Yiwei
Responsible editor| Chen Xiaoxue
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From the lunar basalt fragments the size of wheat flour grains, Chinese researchers found that the youngest known volcano on the moon fell silent only 2.030 ± 0.04 billion years ago, 1 billion years later than the previously reported results of volcanoes in other regions of the moon having "died" 3 billion years ago, and 8 to 900 million years later than the youngest lunar meteorite (rocks that were splashed to Earth by asteroid impacts and aged 2.8-2.9 billion years). This means that at least 2 billion years ago, the lunar interior was undergoing evolution.
At 5 p.m. Beijing time on October 19, the journal Nature accelerated the publication of the above findings by the Institute of Geology and Geophysics of the Chinese Academy of Sciences and other institutions [1].
This dating paper was accompanied by two other papers: one that determined the geochemical composition of lunar basalt samples and found that the krep component (KREEP, a yellow-brown glassy mineral on the moon, enriched with potassium K, phosphorus P and rare earth elements REE) in the source region of the lunar mantle (i.e., the area where magma was once active in the lunar mantle) was found to be less abundant in radioactive thermogenic elements, and the other determined the water content of melt inclusions and apatite in the basalt samples. It is concluded that the mantle has less water content. [2,3]
Academician Li Xianhua of the Institute of Geology and Geophysics of the Chinese Academy of Sciences said that the moon has a small mass (only 1.2% of the earth), and people usually think that the moon's heat retention ability is relatively poor and the heat dissipation is fast, but their findings raise new possibilities: the moon's heat dissipation is not necessarily so fast. Li Xianhua is the main person in charge of the lunar sample study and co-corresponding author of the dating paper.
Samples from all three papers are from the northern part of mount Mons Rümker in the lunar sea in the northern lunar sea called Oceanus Procellarum, brought back to Earth on December 17, 2020 by the Chinese Chang'e-5 lunar probe. These findings are also another batch of research results published by the Lunar Exploration and Aerospace Engineering Center of the China National Space Administration (hereinafter referred to as the Lunar Exploration Center) since the first lunar samples were issued on July 12, 2021.
On October 7, EST, the Institute of Geology of the Chinese Academy of Geological Sciences published a paper in the journal Science, concluding that the solidification time of lunar basalt is about 1.963 ± 0.57 billion years [4].
The newly released dating results are within the margin of error for the above ages, and the accuracy of the results has been improved by measuring more samples.
"The results of the two research teams are exciting and impressive," said Yin Qingzhu, a professor in the Department of Earth and Planetary Sciences at the University of California, Davis. Collecting data, parsing results, writing papers, and publishing them in mainstream scientific journals in less than 4 months from the time of obtaining samples represents a huge achievement and arduous effort by the research team! Very impressive! ”
<h1 class="pgc-h-arrow-right" data-track="36" >01 What are the possible reasons why the moon is cooling slower than expected? </h1>
Li Xianhua told the Intellectuals that previous statistical studies of impact crater counts suggested that the magma in the northern part of Mount Lümke may still be active in later years because of the high content of radioactive thermogenic elements on the surface of this area, so it is speculated that the area has enough energy to maintain volcanic activity.
Figure 1 Distribution of the sampling points of Chang'e-5 and previous landing probes, the left picture is an estimation of the topographic age of the lunar surface, and the right picture is a thorium (radioactive thermogenic element material) heat map, which can be seen that the location of Chang'e 5 is relatively young and the radioactive thermogenic element is higher | Image source: Lin and Yang, 2021, The Innovation
Their recently published study first determined that estimates of late rock formation were correct. "Although the past estimates have not been so accurate, the rock (in this position) is indeed very young, which is a happy thing."
The other two findings raise new questions to explain the phenomenon.
Previous estimates suggest that magma formed late, and one of the influencing factors was the higher levels of radioactive thermogenic elements, such as potassium, uranium and thorium. Research by Yang Wei and colleagues at the Institute of Geology and Geophysics of the Chinese Academy of Sciences shows that the radioactive thermogenic elements of the lunar mantle in the northern part of the Storm Ocean are not abundant, which is comparable to the source area of the lunar mantle of the Apollo basalt [2]. "It shows that radioactive elements are not the main cause of magmatic activity here." Li Xianhua explained.
In addition, the water content can also affect the melting temperature of rocks in the source area of the lunar mantle. The previous hypothesis was that high water levels would cause the rock to melt from solid to liquid at a lower temperature, that is, it would take longer for the magma to condense as the moon cooled. Hussen et al. found that the water content of the rock matrix was not very high, about 1-5 μg.g-1 (1 to 5 μg per gram of sample), which was significantly lower than that of the Apollo sample." Li Xianhua said. This also excludes the conjecture that high water content leads to late magmatic activity.
Yin Qingzhu said the work raised some "interesting questions about the thermal evolution of the moon." "Under such conditions, how can a small object like the Moon maintain late magmatic activity?" he said, "I expect to see a series of modeling efforts to enable the planetary scientific community to understand the information provided by these data." ”
Li Xianhua introduced that there is another factor affecting the heat dissipation of the moon, that is, pressure, which needs to be further studied.
"Maybe we have problems with the moon's thermal evolution estimates and need to be rethought," Li said, "and recent (other) studies have found that the lunar surface has a layer of lunar soil more than ten meters thick, which is particularly insulating, like a duck down quilt." So although the moon is small, heat dissipation is not necessarily fast. ”
<h1 class="pgc-h-arrow-right" data-track="157" >02 Lunar studies from "far-sighted" to "near-range" viewing</h1>
Yin Qingzhu said that the results of this batch of isotopic dating methods make up for the calculation defects of the "impact crater counting method" in the lunar research in the period of 1 billion to 3 billion years.
When lunar soil is not available, scientists can only calculate the age of the lunar surface by "counting craters" and extrapolating its evolution: due to the thin atmosphere on the moon, meteorite impacts are common, and impact craters tend to remain relatively intact. Where there are many and layered impact craters, the formation age is relatively old, and where there are fewer impact craters, it is younger. This approach is called "impact crater counting" and can also be used on other planets in the solar system, such as Venus and Mars.
After a long period of development, the "impact crater counting method" has become an important tool for estimating the evolution of the moon. For bodies older than 3 billion years, its speculation is more accurate, but for "young" bodies under 3 billion years, the speculations of different scientists tend to vary greatly, usually ranging from 5 to 1 billion years.
Fig. 2 There is a well-known dating curve in the field of "impact crater counting", but this curve is very flat at 1-3 billion years, and the estimation error is very large. The dating results "anchor" the dating data, which is crucial for improving the accuracy of the "impact crater counting method" | Source: Li et al., 2021, Nature
Li Xianhua said that this work is also of great significance for calibrating the "impact crater counting method" for the study of more celestial bodies.
<h1 class="pgc-h-arrow-right" data-track="158" >03 "Pick flour"</h1>
Lunar samples are precious, and experiments are a delicate task.
In order to avoid contamination of samples by Earth's material, the Institute of Geology and Geophysics has built a lunar clean laboratory specifically for lunar sample research, with particles greater than 0.5 microns per cubic meter of diameter controlled within a few thousand. In addition, the glove box used to place samples and experiments is filled with nitrogen, and the partial pressure of oxygen and moisture pressure are very low, avoiding contamination.
Because the Moon is almost a vacuum environment, the surface lunar soil has been finely fragmented due to multiple impacts, and there is very little debris of more than one millimeter. A typical lunar scientific sample number is a set of letters and numbers, with "CE5" at the beginning representing Chang'e-5, "YJ" in the middle representing research, and "GP" "YX" and "FM" representing three types of samples: the largest diameter light sheet, the smallest rock chip, and the finest powder with a diameter of only a few hundred microns.
Figure 3 The lunar sample of Chang'e-5 applied by Li Xianhua | Source: [1]
The five scientific research samples applied by Li Xianhua's team are mainly rock chips and powder samples. The first challenge is to pick out slightly larger chips of several hundred microns from a pile of tens of microns. The diameter of ordinary flour particles is 75 microns, and according to Li Xianhua, the average particle size of the lunar soil particles is only a few tens of microns.
"If you think about it, if there are a few 'black flours' in a pile of 'white flour', it's very difficult." Li Xianhua said.
In the middle of the laboratory, there is a positive pressure glove box, the pressure is stronger than the outside, the inside is full of nitrogen, and the researchers in clean clothes stare at the binoculars that are magnified dozens of times, put their hands into the glove hole, and hold a very thin stainless steel needle in their hands, slowly picking.
Figure 4 Li Jinhua, one of the authors of the paper, operates next to the glove box, next to technicians Guo Qian and Ma Hongxia | Source: Ren Hui
The selected basalt chips are made of epoxy resin to make a sample target, and after polishing, the scanning electron microscope is used to conduct image analysis on the rock chips, identify various minerals, especially to find uranium-rich zirconium-containing minerals (such as oblique zircon); then, the sample target is plated with a layer of about 20 nanometers thick gold, put into an ion probe, "hit" the mineral with a primary oxygen beam, "gently sweep" off the gold on the surface, and then measure the lead isotope composition of the mineral and calculate the age.
The reason for the search for zirconium-containing minerals is because they are enriched with uranium when they are formed, but they are basically lead-free, so the lead measured by the ion probe is basically of radiogenesis, and reliable isotopic dating results can be obtained.
In the fragments of hundreds of microns in size, zirconium-containing minerals are usually only a few microns wide, at this time the analytical beam spots of the ion probe should be very small to accurately cover the minerals, and the beam spots are analyzed to cover the trace amount of gold (containing very trace amounts of lead) that may be left in the gaps between the mineral particles, which may be counted into the radioactive cause lead that does not belong to the sample, affecting the dating results.
The ion probe analysis beam spot used by Li Xianhua's team is less than 3 microns in diameter, which is the highest spatial resolution of high-precision ion probe dating technology.
Figure 5 A basalt sample shown in the paper shows that the zirconium-containing minerals are at the b-spot position on the left, and the enlarged b-point is the enlarged b-point on the right, the white position is the zirconium-containing minerals, and the shadow pointed to by the measurement point is the trace left by the ion probe after the | Source: Li Xianhua
The researchers aged a total of 47 samples and concluded that in the young lunar sea of the Storm Ocean, basalt crystals were consolidated about 2.03 billion years ago, and the error value of this result was only about four million years.
In the Science paper published in October, the results measured by the dating of zirconium-containing minerals alone were 2.011 ± 0.50 billion years, which is closer to the above results.
<h1 class="pgc-h-arrow-right" data-track="159" >04 Episode: International Cooperation Is Right and Wrong</h1>
Before Chang'e 5 was brought back to the lunar soil, the only research practice chinese researchers had on the lunar soil came from half of the 1 gram of lunar soil that China received from the United States in 1978. Therefore, there are not many people with actual lunar soil research experience.
Li Xianhua introduced that at present, some of the researchers in the domestic lunar soil are scholars engaged in geological research, some are planetary scientists, and some are from the fields of geophysics and geochemistry, most of whom are also "learning while doing". Li Xianhua is a geochemist who studies isotopes, and his team has previously developed a variety of high-precision analytical techniques for extraterrestrial samples.
The review meeting for lunar soil research applications was held in Beijing in June, and each applicant had only about 13 minutes to elaborate on the research plan and answer questions from experts. Of the first 37 teams to submit applications, 17 applicants from 13 institutions received samples, including Li Xianhua's team.
The Science article, published on October 7 this year, was the first lunar sample study of Chang'e-5 to be released. Due to the appearance of many foreign experts in the signature of the paper, one of whom is also a corresponding author, it has attracted some attention. Ross N Mitchell, an associate professor at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, believes that the corresponding author is generally an honor for the lead scientist of the project, while the Institute of Geology of the Chinese Academy of Geological Sciences is "too kind" (too generous).
"All scientists should know that the basic principles of scientific paper attribution are according to contribution." Li Xianhua said.
The Intellectuals contacted the corresponding author of the Science article, Alexander Nemchin, a professor at Curtin University in Australia. He said he and the project applicant, Liu Dunyi, a researcher at the Institute of Geology of the Chinese Academy of Geological Sciences, have known each other for 31 years, and that cooperation on lunar soil research began long before Chang'e-5. Four years ago, the two discussed the possibility of collaborating on lunar samples, and he applied to NASA to obtain samples brought back by Apollo.
In the collaborative study of the "Chang'e 5" sample, in order to publish the paper as soon as possible, Nemchin proposed to draft the English paper manuscript by himself, initially he put his name at the end, and Liu Dunyi listed him as the corresponding author.
"I don't think the people who question it have been involved in real (international) collaboration," he says, "and for me, collaboration is the ability to work with people who appreciate it to do research of interest." ”
Another corresponding author, Liu Dunyi, told The Intellectuals that Nemchin is a visiting scholar at the Institute of Geology. The main work of the research is done at the Geological Institute, while the discussion and analysis is carried out online by chinese and foreign team members. Nemchin, as the drafter of the paper, is listed as a corresponding author to facilitate communication with the editorial board.
Liu Dunyi said that international cooperation is an inevitable way for scientific development, and there is no shame in "learning from each other's strong points and complementing each other's weaknesses." Cooperation is also mutual. Ten years ago, he worked with scholars at the University of Washington to study lunar samples brought back by Apollo 12, and he was the first author in the published paper.
The Measures for the Administration of Lunar Samples formulated by the China National Space Administration stipulate that the China National Space Administration encourages international joint research work in space science based on lunar samples to promote the international sharing of results [6]. According to the Intellectual, the currently released lunar scientific research samples have not yet been opened to foreign applicants, but there is no restriction on the introduction of foreign experts in the team of Chinese applicants.
In July, when the first samples were distributed, Pei Zhaoyu, deputy director of the Lunar Exploration Center in charge of lunar sample management, said in an interview with the People's Daily that at that time, "no formal applications submitted by foreign scientific research institutions" had been received, but "the China National Space Administration encouraged international joint research on lunar samples, and foreign scientists could jointly carry out sample research by joining domestic scientific research teams." The China National Space Administration is working with relevant departments to formulate relevant rules for international cooperation in lunar samples, which will be implemented after approval by the central government. ” [7]
Tips
Lunar sample information is open to the public
Readers can visit the Lunar Exploration Center website for specific information:
https://moon.bao.ac.cn/moonSampleMode/
bibliography:
[1] Li, QL., Zhou, Q., Liu, Y. et al. Two billion-year-old volcanism on the Moon from Chang’E-5 basalts. Nature (2021). https://doi.org/10.1038/s41586-021-04100-2
[2] Tian, HC., Wang, H., Chen, Y. et al. Non-KREEP origin for Chang’E-5 basalts in the Procellarum KREEP Terrane. Nature (2021). https://doi.org/10.1038/s41586-021-04119-5
[3] Hu, S., He, H., Ji, J. et al. A dry lunar mantle reservoir for young mare basalts of Chang’E-5. Nature (2021). https://doi.org/10.1038/s41586-021-04107-9
[4] Che, Xiaochao, et al. "Age and composition of young basalts on the Moon, measured from samples returned by Chang’e-5." Science (2021): eabl7957.
[5]https://www.chinadaily.com.cn/a/202110/13/WS6166764ca310cdd39bc6ea6a.html
[6]http://www.gov.cn/zhengce/zhengceku/2021-01/18/content_5580772.htm
[7]http://finance.people.com.cn/n1/2021/0713/c1004-32156005.html