How long has the moon been "alive"? This is a major scientific question that scientists have always wanted to understand in the study of the history of lunar evolution.
A year ago, scientists from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Geology and Geophysics) used the lunar samples brought back by Chang'e 5 to prove that the moon still erupted hot magma 2 billion years ago, making known The Moon's geological lifespan has been "extended" by 800 to 900 million years.
So, how does the moon "extend its life"? Now the team of researcher Chen Yi of the institute has given a potential answer. By studying the lunar soil debris brought back by Chang'e-5, they found that the lunar mantle contained more calcium and titanium 2 billion years ago than more than 3 billion years ago, resulting in a lower melting point of the lunar mantle, and the lunar mantle experienced more than one billion years. After the cooling of the moon, the temperature decreased by only about 80 degrees Celsius, and based on this, a new model of lunar thermal evolution was proposed. The related results were published in Science Advances on October 22.
Schematic diagram of the formation of Chang'e 5 basalt and Apollo basalt. Photo courtesy of the interviewee
'Son and Mother' explores the secret of keeping the moon alive
Similar to Earth, the Moon formed about 4.5 billion years ago, but its mass is only about 1% that of Earth. Such a small celestial body should, in theory, cool rapidly and stop volcanic activity early, becoming a dead planet. But in October last year, the first batch of studies on the Chang'e-5 lunar soil samples refreshed human's understanding of the lunar magma activity and thermal evolution history, and also raised new scientific questions-how did the moon live to 2 billion years ago?
"Like the earth's mantle, lunar basalt is a rock formed by volcanic eruption of magma formed by partial melting of the lunar mantle to the surface of the moon to cool and crystallize." Chen Yi explained to the "China Science Journal" that for the partial melting of the continuously cooling lunar mantle, International scholars have put forward two hypotheses: the heat generated by radioactive elements causes the lunar mantle to heat up, and the addition of water reduces the melting point of the lunar mantle.
However, Chinese scientists' research on the Chang'e-5 basalt revealed that the lunar mantle source region is not rich in radioactive heat-generating elements and is very "dry", ruling out the above two hypotheses.
Why did lunar volcanism last so long? This has become an unsolved mystery in a new round of lunar research.
Chen Yi said that in order to solve this mystery, the premise is to establish the depth and formation temperature of the Chang'e-5 basalt, that is, the temperature and pressure conditions when the lunar mantle partially melts. On this basis, a new lunar magma-thermal evolution model can be established by comparing it with the older Apollo basalt.
Most of the Apollo basalts were formed 3.8 billion to 3.1 billion years ago, and were collected and brought back to Earth by the six manned lunar missions of the American Apollo program in the 1960s and 1970s.
my country's Chang'e-5 basalt has undergone a series of evolutionary processes such as high-degree crystallization and separation during the eruption from the lunar mantle source area to the lunar surface, and the composition of the basalt has changed significantly. How to accurately restore the composition of its original magma has become a limited source of magma. Depth and temperature are key.
"For example, the Chang'e 5 basalt is like the youngest son of the moon, and the Apollo basalt is the eldest son of the moon. They are the children born to the mother of the moon at different ages, and the element content of the two sons is different." Chen Yi For example, "Through this difference, the physical state of the 'mother' when they were born can be reversed, that is, the internal state of the moon at that time."
The average particle size of Chang'e-5 soil samples is only 50 microns. A smaller particle size means less information such as the amount of minerals contained in it. The research team established a number of criteria, trying to find rock fragments with larger particles, more complete mineral types, and more uniform mineral distribution as the initial components, and reversely deduce what happened inside the moon at that time.
In the end, they selected 27 representative cuttings from more than 600 cuttings, and analyzed the main components of the cuttings by using the newly developed scanning electron microscope energy spectrum quantitative scanning technology, combined with a series of petrological and thermodynamic simulation calculations. , successfully recovered the initial magma composition of the Chang'e-5 basalt, and compared it with the initial magma of the Apollo low-titanium basalt, and discovered two potential "secrets" for the moon's vitality.
On the one hand, the initial magma of the Chang'e-5 basalt contained higher calcium and titanium components than the initial magma of the Apollo basalt, that is, the lunar mantle contained more calcium and titanium 2 billion years ago than it was more than 3 billion years ago. This part of the material is precisely the product of the late crystallization of the lunar magma ocean, and has the characteristics of being fusible. Its addition will significantly reduce the melting point of the lunar mantle and induce partial melting of the lunar mantle to form young lunar basalts.
On the other hand, the simulation results show that the Chang'e-5 basalt was formed at a lower temperature than the Apollo basalt, that is, the moon's interior has undergone continuous cooling for more than a billion years, and the temperature has only decreased by about 80 degrees Celsius.
Based on this, they proposed a new lunar thermal evolution model: the fusible material crystallized in the late lunar magma ocean gradually added to the lunar mantle, which not only "supplements calcium and titanium" to the lunar mantle, but also reduces the melting point of the lunar mantle, thus overcoming the slow cooling. The lunar interior environment triggered a long-lasting lunar volcanism.
"Blanket" to maintain "young" and "hot"
Why do the late crystallization products of the lunar magmatic ocean have more calcium and titanium? After billions of years of cooling, why has the interior temperature of the moon only decreased by about 80 degrees Celsius? Chen Yi made an in-depth interpretation of this.
Today, looking at the moon from the earth, the white part is the lunar land, and the dark part is the lunar sea. Scientists found that the surface of the lunar land is mainly plagioclase, while the surface of the lunar sea is mainly basalt. This is related to the evolutionary history of the moon.
Scientists speculate that, like the earth, the early moon was also a magma ocean. With the process of cooling, cooling and crystallization, the lunar core first formed, and then different layers of the lunar mantle appeared.
"During the cooling and crystallization of the magma ocean, some olivine-rich materials will precipitate into the deeper lunar mantle in the early stage of crystallization, while the lighter-density plagioclase will float upward and float on the surface of the magma ocean to form the lunar crust. That is, the brighter areas on the moon that people see." Chen Yi explained.
The main minerals crystallized in the late magmatic ocean are clinopyroxene and ilmenite, which are characterized by high calcium and titanium content and low melting point. This part of the material is very dense, and after crystallization, it will sink and enter the boundary between the deep lunar mantle and the lunar core.
The international scientific community has a conjecture for the duration of volcanic activity on the moon with a smaller volume: the lunar surface is covered with an average thickness of 4-5 meters of lunar soil, like a "thick blanket", making the interior of the moon Heat dissipation is relatively slow. At the same time, unlike the earth, the moon has no plate tectonics, so it will not bring cold materials from the surface to the interior, and exchange material and energy to "cool down" the moon, which may also be one of the reasons for its slow heat dissipation.
However, for a long time, due to the lack of quantitative comparisons, it was difficult to know how slowly the interior of the moon dissipates heat.
By comparing the Chang'e-5 basalt with the Apollo basalt, the team speculates that the temperature dropped by only about 80 degrees Celsius at the same depth in the lunar mantle from more than 3 billion years ago to about 2 billion years ago.
Reviewers of the journal article noted that this was a high-quality study. "This study provides a novel explanation for how the young Chang'e-5 basalt may have formed. There is no doubt that this has been the top concern of many scientists since the new age of the moon was announced last year."
Reviewer Richard Palin of the Department of Earth Sciences at the University of Oxford, UK, said the team's interpretation was strongly supported by petrological modelling and petrological observations.
New Opportunities and New Developments
It now appears that the lunar geological life activity has completely stopped. So, is it possible to find traces of lunar volcanism that are younger than 2 billion years old in the future?
In Chen Yi's view, this is "completely possible", and scientific research is a process of continuous innovation.
"When the moon really died is still a mystery." He said, "At present, we only have solid evidence that the moon was volcanic 2 billion years ago, and it was still alive at that time. If the future With more samples, a more complete and high-precision quantification curve of lunar cooling can be established, and the trajectory of its geological life activities can be inferred."
So, where might the latest evidence of magmatic activity on the moon be hiding? According to reports, scientists currently use the impact crater dating method to infer the approximate age of formation of the relevant area. "In a few years, we may be able to take younger samples of the moon and understand the inner state of the moon's most advanced life."
Chen Yi believes that lunar research may help solve the "bottleneck" problem of people's understanding of the earth's past.
"Because the earth's plate tectonics has erased many ancient records of the earth's history, it is not clear what evolutionary processes the earth has undergone from 4.5 billion years ago to 3.5 billion years ago. The earth-moon system has the same origin and the moon without plate tectonics. The geological historical record is very complete and rich, and will help us understand the past, present and future of the earth," he said.
In his view, Chang'e-5, Zhurong and other exoplanet exploration projects are bringing new opportunities to China's Earth and planetary science research. In the past year or so, with the return of lunar soil samples and the transmission of the first Mars exploration data to Earth, the domestic planetary scientific research team is rapidly expanding.
Chen Yi said that exoplanet exploration will "open a very large door" for earth science research. Geology, geochemistry, geophysics, remote sensing detection, and even extraterrestrial biology will be further developed in various fields of earth science.
Related paper information:
https://doi.org/10.1126/sciadv.abn2103
(Original title "How did the moon "extend its lifespan" by 800 million years?")
