Shulou(Shulou.com)11/24 Report--

The original title: "the lunar soil age has become the lunar age?" Scientists are confused. "

On December 17, 2020, Chang'e-5 returned to Earth with 1731g of lunar soil; 1731g, which is the weight of dreams for countless astronauts.

At 17:00 on October 19, 2021, Nature magazine published the research results of this precious sample from Li Xianhua team, Hu Sen team and Yang Wei team of the Institute of Geology and Geophysics of the Chinese Academy of Sciences, causing a worldwide shock. Among them, through the analysis of lunar soil, Li Xianhua's team shows that the age of lunar soil is 2.03 billion years, which lengthens the time of lunar activity by another 800 million years. This discovery has also greatly improved our understanding of the moon and the entire solar system. It must be of great significance.

However, in the recent report on lunar soil research, some media mistook it for the age of the entire moon and caused a lot of misunderstanding, and scientists who heard the news may be very confused. Although this should be a mistake in details, it still shows that some basic knowledge about the lunar soil and the moon is still very vague. Of course, this is also our responsibility for science popularization is not in place, in view of this, let's do some related science popularization.

1. What is lunar soil? According to the current mainstream view, the moon was formed about 44.25-4.527 billion years ago, which is obviously the theoretical age of the moon. Not much different from other rocky celestial bodies, the moon was only a solid sphere when it was first formed, and it also had very active geological activity and a vast ocean of magma. After tens of millions of years of cooling, the magmatic activity on the lunar surface gradually stabilized and the surface gradually solidified, forming the lunar crust that is now observed.

Over the next billions of years, the lunar crust underwent multiple baptism such as solar wind, high-energy cosmic rays, meteorite impact, and so on. Large pieces of rock were destroyed, forming a large number of loose gravel and even powder, which were sparsely distributed on the lunar surface. This is lunar soil (Lunar Soil).

Part of Chang'e-5 sampled lunar soil samples. (photo Source: Nature, Non-KREEP origin for Chang'e-5 basalts in the Procellarum KREEP Terrane) 2. The significance of studying lunar soil the moon is not only a spot of light in the late-night sky; for human beings today, the moon is a huge museum of history and a huge treasure trove of resources. Because the moon is not shielded by magnetic field and atmosphere, lunar soil has been directly exposed to solar wind and other cosmic radiation for a long time, which provides valuable experimental records for the study of the space environment near the earth or the long-term changes of solar activity. The isotope information in the lunar soil is a powerful witness to the lunar geological activity billions of years ago, and some isotopes can also record their own stories "before entering the lunar soil" and even reflect the processes at the beginning of the formation of the solar system. In the field of resources, the moon is rich in helium 3, a clean energy that may be used in the future.

It can be said that the moon is full of treasures everywhere, and the lunar soil is not only a gate between man and the moon, but also a stepping stone for mankind to understand the solar system. When the lunar soil, which records a large amount of information, is sent to the human laboratory, it will become a key step for human beings to leave the earth and go to the universe.

The moon exposed to solar wind, meteorites and cosmic radiation. (photo Source: NASA) 3. The age of lunar soil according to the results of this study, the age of basalt rock mass and lunar soil samples near Chang'e-5 landing site is 20.30 ±4 million years old; let's talk about basalt first.

Basalt is a kind of basic extrusive rock, that is, volcanic rock; on the moon, basalt is one of the main rocks that make up the lunar sea and lunar land surface, and it is also the latest lunar surface rock; its formation process is relatively simple, and the magma trapped on the lunar surface after volcanic activity cools and crystallizes, that is, basalt, whose crystallization temperature is about 1050 ℃. The formation of basalt is closely related to volcanic activity, so it can be considered that the formation time of the youngest basalt on the moon is basically the same as that of the last volcanic eruption on the moon. To put it simply, when the moon stops volcanic activity, the last time rocks erupt is the time when the volcano stops active.

The picture shows the famous 70017 lunar basalt, collected by Apollo 17 astronauts. 70017, which is crystallized by magma and formed about 3.7 billion years ago, is a relatively "older" basalt. (photo: NASA) the discovery of basalt, the youngest known to exist on the lunar surface, had previously been about 2.8 billion years old, rescaling the end of lunar magmatic activity and advancing it by 800 million years.

The age of the lunar soil is not the same as the age of the moon, just as you grab a handful of soil from your doorstep, which can hardly be formed at the same time as the earth, with an age of more than 4 billion years old; it may just be the dried product of the mud balls rubbed from the body by the uncle on the first floor last summer. However, there is no mud pill on the moon at present, the lunar soil in this study comes from the geological activities of the moon, and the significance of this research is to show that the volcanic activity of the moon lasted until at least 2.03 billion years ago. rather than the moon was formed 2.03 billion years ago, generally speaking, the lunar crust has died at this time, rather than the lunar body has just been born, the difference between the two is still very big.

4. How is the age of lunar soil measured? In this study, the researchers used three techniques to analyze lunar soil:

"Ion probe ultra-high spatial resolution dating technology", "ultra-high spatial resolution isotope analysis technology", "nano-ion probe analysis technology"

Ion probe of the State key Laboratory of Isotope Geochemistry. (photo Source: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences) it looks high-end, but in fact it is high-end and high-end. According to the principle, the actual dating technology can be collectively referred to as "ultra-high spatial resolution uranium-lead dating", which is a precise dating technique based on U-Pb method. U-Pb dating is introduced below. The decay constant, the symbol λ, indicates the probability of decay of a single nucleus of a radionuclide per unit time, which has the following relationship with the half-life of the radionuclide:

Thus, for a particular nuclide, the decay constant is a fixed value. If you do not know the decay constant, it can be understood in terms of the half-life (the time it takes for half of the nuclei of radioactive elements to decay). The higher the value of λ, the faster the decay of the radioactive element and the shorter the half-life. They are essentially the same.

In nature, there are mainly three kinds of uranium isotopes, 234U (0.006% of the total), 235U (0.72% of the total) and 238U (99.275% of the total). The content of 234U is too low, and only the latter two are introduced here. For these two isotopes, the final products of their decay are 207Pb and 206Pb, respectively.

Taking 235U as an example, taking Nt as the existing parent mass, N0 as the initial parent mass and t as the time, according to the formula:

There are:

But obviously, 235U0 is an unknown quantity, so time cannot be found, so we need to make some small transformations with its decay product 207Pb:

Therefore, we can directly determine the content of 207Pb, as well as the content of the existing matrix 235Ut, and we can directly calculate the size of time t. Similarly, 238U is also determined in this way, the formula is as follows:

In short, the relatively accurate time t can be obtained by simultaneous determination of 206Pb / 238Ut and 207Pb / 235Ut, and the two results confirm each other, which makes the accuracy of this dating method extremely high, up to 1/1000; and because of the long half-life of uranium, the upper limit of this method can also reach an astonishing 4.5 billion years.

The above is a brief introduction of the theoretical basis of U-Pb dating method, the principle is not complicated, and the progress of experimental instruments and analytical methods makes it play a more powerful role. It is one of the contributors to the research results, but it is not as powerful as some media claim, so easily changed the age of the moon from more than four billion years to more than two billion years. It can only help scientists understand the information recorded in the lunar soil, but has little effect on correcting the headline party phenomenon in news reports.

This article is from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Tianyin

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