This article from the micro-channel public number: forever relentless tour phase of Miao Hon (ID: haibaraemily_planets), of: haibaraemily, from the head of FIG: Eastern IC
Our moon may have formed in an earth-shattering big impact. But what about the moon that reborn from the impact debris, especially the early moon?
In the morning of Beijing Time(July 10, 2019), the associate professor Zhu Menghua and his colleagues of the National Key Laboratory of Moon and Planetary Science of Macao University of Science and Technology published their latest results inmagazine, who recalculated the early evolution history of the moon by exploring the difference between the content of ferric elements between the lunar shell and the crust.
First, the congenital poor "iron" lunar crust
The outer layer of the moon (crust and mantle) is extremely poor in iron. To be more precise, the iron-nickel metal in the lunar mantle and the highly iron-friendly element (highly siderophile elements, including gold Au, iridium Ir, osmium Os, Pd, platinum Pt, Re, rhodium Rh, ruthenium Ru, are very scarce.
This is not surprising.
Originally, the outer layers of large solid celestial bodies are relatively poor "iron" because they have all experienced the "heat differentiation" process - in the early days of extreme heat, these celestial bodies are likely to have experienced a melting and hot smelting furnace. During the period, there were no solid rocks on the surface of the celestial body, but a global “magma ocean”. Therefore, the original iron-like elements on these celestial bodies were “sinked” together with the heavier iron-nickel metal to form a core. .
This is why we are familiar with Mercury, Venus, Earth, Moon and Mars.
The general process of thermal differentiation of large rock celestial bodies. And you can see this universal picture, orz.. Cartography: haibaraemily
What's more, the moon is still innately inadequate - people have less iron. People have long discovered that the same is a solid celestial body. The density of the moon is only 60% of the earth. The reason is that the iron core of the moon is much smaller than the earth.
The obvious fact that the moon as a whole is poorer than the earth is also strong evidence of the origin of the moon's great impact.
How is the moon formed? Strictly speaking, there is still no conclusion yet. But one of the most widely accepted possibilities to date is the famous Giant Impact Hypothesis.
One day about 4.5 billion years ago, a Martian-sized celestial body descended from the sky and slanted into a "formed" Earth that had not yet fully grown. The violent impact quickly smashed and melted the Martian-sized celestial body and knocked out some of the Earth's material. These debris materials are scattered around the earth and re-aggregated by gravity and collision to form the current moon [2, 3].
A hypothetical diagram of the big impact hypothesis. Source: museumvictoria.com.au
If this hypothesis is true, then by the time the big impact occurs, the earth should have completed thermal differentiation-that is, the iron core in the earth has been formed. The impact, on the other hand, only peeled off some of the iron-poor crust and mantle, which were mixed with debris from colliders that might have carried normal iron content, but the iron content of the moon was lowered.
However, this "congenital iron deficiency" does not necessarily make the pro-iron elements of the moon clams much lower than the crust - because the moon after forming must also undergo thermal differentiation, and the pro-iron elements are basically "towed". "Into the kernel. To put it simply, the content of pro-iron elements in the moon clams should be similar to that in the crusts – there is basically nothing left.
But that's not the case. Both the moon and the earth's rock sample show a certain amount of pro-iron elements in both the moon and the crust.
Is this another matter?
Second, late accretion: It doesn't matter, you can rescue it again.
The reason is simple: because you didn't complete the hot differentiation in more than 4 billion years - everyone is still "evolving."
Sink is sunk in, stay can not stay, but there can still be foreign aid supplies, right?
After the formation of the Earth's core and the Moon's nucleus, the violent impact in the solar system did not stop. A large number of asteroids and comets continued to hit the surface of the Earth and the Moon, bringing them (of course, the same for Mars Mercury). A large number of "foreign" substances, which may have water and organic matter (see: When is the water on the earth formed? The moon tells you the answer), of course, it also brings a variety of pro-iron elements - this process is called " Late accretion.
In other words, the content of iron-affinity elements in lunar crust and mantle reflects not their primary content to a large extent, but the result of the later accretion process.
So is the Moon and the Earth getting as much “supply” in the late accretion process? How is it possible?
Even if they have been hit by small objects of the same group, the probability of being hit is different-the gravity of the earth is more powerful, and it is more likely to be hit by small objects, a "difficult" ratio of about 20:1 . Being hit by asteroids / comets is definitely not a good thing for us today, but for Earth more than 4 billion years ago, it is likely to be a "heavenly treasure" that brings vitality and life.
By a rough estimate of this ratio, Earth should have received about 20 times as much impact "extra supplies" as the moon.
However, the result of the reverse of the high iron-related elemental HSEs in the Moon and Earth rock samples is completely different: if we assume the average composition of these external impactors and the most widely existing ancient meteorite in the solar system - pellets If the composition of the meteorite is about the same, then the "impact supply" of the late Earth can be much more than the moon - about 1200 times that of the moon !
What is wrong with the dynamic impact probability vs geochemistry survey?
A natural reasoning is: is it possible that the Earth will actually be hit much more than the theory has estimated? In addition to the 20-fold difference caused by gravitational differences, are there any asteroids of certain sizes that, for some reason, particularly like to hit the earth? Or, could it be that there has been a sudden change in the number of asteroids hit by the Earth-Moon system? [5 ≤ 7]
Zhu Menghua and his colleagues have given another possible way of thinking: it may not be that there is a problem with the estimation of the amount of "replenishment", or it may be that there is a problem with the estimation of the amount of "retention".
Third, the ability of this month's "cargo" is not good?
The problem may not be the ability to carry the goods, but the problem is. Can the colliders (asteroids / comets) that hit both the earth and the moon stay? The earth's gravity is large and its binding ability is strong, so the impactor can basically stay, but the moon's ability to "keep goods" is much worse. Previous estimates generally believe that the retention rate of the earth is 100%, while the retention rate of the moon is about 50% 60% [5, 7]. As for what it actually is. Actually, no one knows.
For this reason, Zhu Menghua and his colleagues simulated the retention ratio of the collision object under the impact of the moon at different speeds and different angles by computer. The results show that:
The high incident angle (direct) impact is higher than the low incident angle (inclined incidence);
The large impact body has a lower retention ratio than the small impact body.
Taking the impact velocity of 15 km/s as an example, the mass ratio of the impactor with different incident angles and sizes remains on the moon. Source: 
This is quite in line with our intuitive experience.
(1) when the impactor with low incident angle (inclined incident), the impactor material will disperse more after the collision, and the gravity of the moon is small and the escape speed is also small, the scattered impingement material will be easier to escape from the moon, and only a small part of it can be left behind, and the collision body material with low incident angle (inclined incident) will disperse more open after the collision, and the gravity of the moon will be small and the escape speed will also be small.
In turn, the impactor with high incident angle (direct shot) gives more force, and after the collision, the impactor material will be more concentrated and less will run away.
A schematic diagram of the comparison of the distribution of the material at low incident angle (inclined incidence) and high incident angle (direct). Adapted from: Nature 
The 2 large impactors will produce larger, more violent impacts, and the impacting material will also achieve higher energy (speed) and easier to run away.
But the size and speed of each asteroid / comet hit is random. For the moon, what is the overall retention ratio of the collider? This requires more simulation experiments.
The Monte Carlo algorithm (a method of simulating random events through a large number of repeated computer experiments) simulates the process of millions of small celestial bodies colliding into the surface of the moon. Zhu Menghua and his colleagues got the statistical rules:
The proportion of impact material retained in the history of the moon is about 0.2 ≤ 0.35, that is to say, only about 20 to 35% of the mass of the impact eventually stays on the moon-well below the previously thought 0.5 ≤ 0.6.
The ability to "reserve goods" does not work.
When to "keep the goods" may have something to do with the duration of the magmatic ocean
The more accurate ratio depends on when the moon's crust begins to retain these hitting substances: if it is from the age of 4.46 billion years ago, which is the earliest observed moon shell, then the average retention The ratio is only 0.2, and if it started only 3.5 billion years ago, the average retention ratio can be increased to 0.35 - the sooner you start, the less overall the proportion will be left.
The average impactor retention ratio of the different "reservation" starting time to date. Source: 
This is also consistent with our understanding of the moon.
There were many large colliders in the early solar system (such as the one that produced the moon, which was as big as Mars today), but slowly, the solar system became quieter and smaller in size and frequency (so now we don't have to worry about being hit by giant asteroids). In the most intuitive example, the dozens of large impact basins currently retained on the moon were all formed 3.8 billion years ago, and there has been no such "flying disaster" since then. What did you say before? The larger the impact body, the lower the retention rate, so if the moon's crust and mantle start to retain impact material early on, then these super-large impactors will naturally lower the moon's average "retained" level.
In fact, if the moon shell 幔 began to desperately “retain goods” from the beginning of the moon shell (4.66 billion years ago), the “impact supply” received by the earth in this proportion is only about 50 times that of the moon. The probability of collision with the kinetic estimate (20 times) is almost the same.
And if the moon shells began to "retain goods" from 4.35 billion years ago, the estimated proportion of these two ways is almost completely consistent.
Wait, where is the pro-iron element in the material that hit the moon earlier? At that time, the magma ocean was not fully solidified, so these pro-iron elements sinked directly into the lunar nucleus or remained deep in the moon, and did not survive in the later formed clams.
That is to say, this result also indicates that the lava crystallization phase of the moon may last for a long time: from the beginning of 4.46 billion years ago, the formation of the moon shell, which lasted until 4.35 billion years ago, almost completely consolidated - lasted for one Billion years (it is really very tenacious magma).
Fourth, the moon may be hit more in the early stage
Today's moon can also find about 40 or 90 basins or suspected basin structures.
The basin or suspected basin structure currently preserved on the moon. Source: LPI 
However, the simulation results of Zhu Menghua and his colleagues indicate that there may have been about 300 impact basins (impact structures larger than 300 km in diameter) throughout the history of the moon:
However, about 200 of them were formed at 4.35 billion years ago. At that time, the magma ocean was not completely crystallized and solidified, and these basins were naturally difficult to preserve;
About 90 of them were formed between 41.5 and 43.5 billion years ago, and these ancient basins are relatively easy to be eroded and “smeared”;
Only about 20 basins were formed between 4.15 billion years ago and now, these basins are relatively young and relatively easy to preserve.
——According to this calculation, about 300 basins, only 50-70 can be preserved so far, which is consistent with the number of basins we have found on the moon.
In short, considering that the Moon's "reservation" ability is not very powerful, in fact, the low iron content in the moon shell does not mean that the moon is particularly less impacted (or the Earth is particularly hit).
On the contrary, if the new “reservation” ability is reversed, the early moon may have been hit more than people expected, but most of the “supply” caused by these impacts has not survived.
This year is the 50th anniversary of human landing on the moon, but I have to admit that 50 years later, we still have limited understanding of many aspects of the moon. Many of our research on the moon still depends on those Apollo missions brought back 50 years ago. Precious, lunar rock samples that have not yet come (of course, the Soviet Moon sample and some lunar meteorites also played a big role).
Under the restriction of very limited lunar samples, planet scientists search for withered intestines, think very skillfully, use the scientific tools and methods of the new era to carry out various studies, and uncover a lot of secrets about the moon. The research work introduced in this paper is such an example. However, it has to be admitted that more and more in-situ analysis and sample collection and analysis are urgently needed to verify and expand this kind of research.
Fortunately, we may be welcoming a new round of lunar exploration: China's Chang'e 4 is now on the surface of the moon, and the next 嫦娥5 and 6 will go to the moon to collect and bring back samples, India The Moon Ship No. 2, Japan's SLIM mission, also plans to conduct in-depth exploration of the surface of the lunar surface.
I believe that in the near future, we will have more and deeper understanding of the history of the early days of the moon.
Refer to the examination
 Zhu, M. H., et al. (2019). Reconstructing the late-accretion history of the Moon. Nature, https://doi.org/10.1038/s41586-019-1359-0
 Hartmann, W. K., & Davis, D. R. (1975). Satellite-sized planetesimals and lunar origin. Icarus, 24(4), 504-515.
 Canup, R. M., & Asphaug, E. (2001). Originof the Moon in a giant impact near the end of the Earth's formation. Nature,412(6848), 708.
 Bottke, W. F., Levison, H. F., Nesvorn arrow, D. & Dones, L. (2007). Can planetesimals left over from terrestrial planet formation produce the lunar Late Heavy Bombardment?. Icarus, 190 (1), 203 / 223.
 Bottke, W. F., Walker, R. J., Day, J. M., Nesvorny, D., & Elkins-Tanton, L. (2010). Stochastic late accretion to Earth, the Moon, and Mars. science, 330(6010), 1527-1530.
 Schlichting, H. E., Warren, P. H. & Yin, Q.-Z. The last stages of terrestrial planet formation: dynamical friction and the late veneer. Astrophys. J. 752, 8–16 (2012).
 Morbidelli, A. et al. A sawtooth-like timeline for the first billion years of lunar bombardment. Earth Planet. Sci. Lett. 355-356,144-151 (2012).
 Day, J. M. D., (2019). Low retention of impact material by the Moon. Nature.
This article comes from WeChat official account: Yueya would like to meet you as an ungrateful friend on the high galactic shore (ID:haibaraemily_planets). The author: haibaraemily, thanks to the first author and main completion of the study, Associate Professor Zhu Menghua.
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