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

Perseid meteor shower April 22, 2022, Perseid meteor shower, an author suddenly thought of his circle of friends chasing the meteor shower. Obviously, if this post-doctoral hostel with a knowledge level below the second grade goes on, I will have to answer more questions about meteors, for example, what is a meteor falling to the ground? The relationship between the meteor and the earth, etc., however, if the meteor comes to the earth, it will be called a meteorite!

The friend circle meteorite of an unknown author, as its name implies, fell to the earth because a series of factors were captured by the earth's gravitation. in this process, it is a meteor that disintegrates or even turns into dust due to atmospheric friction and combustion. Very little actually falls on the earth without turning into dust, and very few of these meteorites can be preserved and found. At present, it seems that the most suitable place to retain meteorites is the desert and the north-south extreme, known as meteorite-rich areas. Why the desert and the north and south? First of all, this human inhabited area is of course unlikely, because there are many interference factors, and secondly, if there are more rocks in other uninhabited areas, there will be a lot of interference, including the possibility of weathering. In contrast, the desert is a natural drying box, while the north and south poles are natural refrigerators. But the north and south poles are still more troublesome, so this group of people, now known as "meteorite hunters", usually go to the desert to look for meteorites. The adventurous atmosphere is not immediately up! Σ (⊙▽⊙ "a

(a) the research team is collecting meteorites in the Kumtag Desert; (b) Antarctic researchers are looking for meteorites in the Grove Mountains (source: Chinese National Astronomy) Let's take a look at the classification of meteorites, which is important for us to understand meteorites. We mainly talk about the differentiation of meteorites, so that meteorites are mainly divided into two categories: chondrite and non-chondrite.

Meteorite classification (picture source: Zhihu @ chaos Hongmeng) you may be wondering, what is the change in the differentiation we are talking about here? This is going to involve the situation of our planets, that is, how solid planets were born, such as our Earth. The formation of planets involves a classical theory, the nebula theory (first put forward by Kant). The main content of this theory has five parts: primitive nebula, nebula disk, dust layer, star son, planet. From the beginning of the primitive nebula, it further shrinks and gathers into a star disk, and then forms a layer of dust, which further gathers into stars (primitive rock aggregates, or the predecessor of meteorites), and finally stars continue to collide and accumulate under the action of gravity to form primitive planets, which eventually evolve into present-day planets.

The composition of the nebula can be seen from the formation of the above-mentioned planets, the existence of stars (the predecessor or Noumenon of meteorites). Now we can combine the stars to discuss differentiation! Differentiation refers to the further change and reorganization of material composition and structure under certain temperature and pressure conditions, forming different composition structures and so on. For chondrite, this is an undifferentiated meteorite, which basically represents the primitive material that formed the solar system, that is, the star. Chondrite can be subdivided according to its composition, but it is no longer subdivided here. You can take a look at the previous classification table, in which the important one is Class C chondrite (carbonaceous chondrite). This is a special kind of carbon-containing meteorite, because its composition is related to the early formation of the solar system, preserving part of the early information of the solar system, with high scientific research value! The number of such meteorites is also relatively small, which will be discussed later on with specific examples.

Carbonaceous chondrite flakes (Picture Source: Wikipedia) are differentiated meteorites, mainly iron meteorites, stone iron meteorites, or primitive non-chondrite meteorites. The olivine meteorite mentioned at the beginning is one of the stone iron meteorites, that is to say, these meteorites have undergone reorganization of composition and structure. However, due to the scarcity of olive meteorites and their good appearance, the collection value of olive meteorites is very high, ranging from tens of thousands per gram. Digress! digress! "□" | |

Olivine meteorite profile (photo source: Sina blog) non-chondrite meteorites are not our main discussion today. Next, we will discuss an example of meteorite research: about the early Earth in the melting and gasification process, "released" some of the original ingredients!

Two articles on the Nature cover two articles on the Nature cover on September 28, 2017 tell such a story. A brief description of the process of thermal differentiation, first of all, the primitive earth is an almost homogeneous sphere, in the process of thermal differentiation, light elements float up, heavy elements sink, and finally gradually delaminate, and then form the core, mantle and crust, which is what the earth looks like now. Does this process strictly follow the rules of light floating and heavy sinking? The researchers found the problem. By studying CI carbonaceous chondrite (we mentioned above, the material in the early stage of the formation of the solar system), the researchers found that the content of moderately volatile trace components (such as lead, zinc, indium, aluminum, etc.) in the outer layer of the earth's silicate is significantly lower than that of CI carbonaceous chondrite. This can be explained in part by the process of thermal differentiation and nucleation: siderophile elements enter the core together with iron and nickel. I have heard a saying, that is, where the crust and mantle are not very clear, you can throw the pot to the core! This is self-deprecating, of course, but it shows that the core is worth exploring because no one knows.

The thermal differentiation nucleation process (figure source: Zhihu @ haibaraemily) went on to say that we can use the thermal nucleation theory to explain some of the phenomena, but there are still some phenomena that cannot be explained. For example, the researchers further compared the contents of various moderately volatile trace components in the earth's silicate outer layer and CV-like carbonaceous chondrite, and found that, for example, indium is more iron-and copper-friendly than zinc, and the condensation temperature is lower. But the contents of both are about the same. Indium is more siderophile and copper-friendly than thallium, and its coagulation temperature is similar, but its content is much higher than the latter. Why is that?

The medium content of volatile elements in the silicate outer layer of the earth (Norris et. 2017, revised from Zhihu @ haibaraemily) the authors of another article changed their thinking and focused on one of the main components of the silicate layer: the two isotopes of magnesium, 25Mg / 24Mg. Comparing the proportion of 25Mg/24Mg in Earth, Mars, meteorites that have undergone thermal differentiation, and primitive chondrite, it is found that the proportion of 25Mg/24Mg in the first three is significantly higher than that in primitive chondrite. If you want to use the thermal differentiation nucleation theory to explain, after all, magnesium is not siderophile, and if the heavy elements sink, then the 25Mg/24Mg should be lower. At this time, by improving the volatilization loss model (simulation), the authors found that if the early Earth lost some light elements after partial melting and gasification, then the 25Mg/24Mg ratio would match that of the present Earth.

25Mg/24Mg ratio comparison (Hin et. 2017, revised from Zhihu @ haibaraemily) the results of two articles point in the same direction, that is, the early Earth experienced a stage of partial melting and gasification, and this is a natural result of melting and volatilization during planetary accretion.

So when can I find a meteorite? May also send a Nature to see, ha, I am dreaming!

Reference:

[1] Norris CA, Wood BJ. Earth's volatile contents established by melting and vaporization. Nature. 2017 TX 549 (7673): 507-510.

[2] Hin RC, Coath CD, Carter PJ, et al. Magnesium isotope evidence that accretional vapour loss shapes planetary compositions. Nature. 2017 TX 549 (7673): 511-515.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Xing Yu Mei Editor: Qiga cc

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