On June 2, 2024, Chang'e-6 landed at a pre-selected landing site in the South Pole-Aitken Basin on the far side of the Moon, successfully completed the lunar soil sampling mission, and successfully returned to Earth on June 25, 2024, and will bring back about 2kg of lunar back samples, realizing the world's first lunar back sample return.
Chang'e-6 successfully returned Image source: CCTV News Channel
So the question is, hasn't Chang'e-5 already landed on the moon and dug up a round of lunar soil? After testing, it was found that the lunar soil could not be grown vegetables...... So why let Chang'e-6 run again? In fact, this is by no means superfluous.
The picture is taken from the CCTV news channel's report on Chang'e-5
Next, let's briefly talk about the difference between lunar soil and the earth's soil and the significance of research (not just growing vegetables), and what are the differences between the lunar soil obtained by the two Chang'e sampling missions.
Difference Between Lunar Soil and Earth's Soil
Although it has the word "soil", if you want to expand "lunar soil" into "lunar soil" to understand, you always feel weird, a bit "unworthy of virtue". Grab a handful of dirt on the earth, soft and sticky, and smell it closely, there is a "fragrance of earth". And (if you can) grab a handful of lunar soil, the sloppy, the prickly, and smell it closely, I'm afraid you'll cough on the spot. Why is this so? Because the production process of lunar soil and earth's soil is very different.
In the process of forming soil, the rocks on the earth must first undergo the baptism of weathering. Although "weathering" only refers to wind, weathering is not only the erosion of wind, but also rainwater erosion, glacial cutting, transportation and transfer, cracking caused by the freezing and expansion of water in rock crevices, and chemical dissolution.
During the weathering process, the rock is gradually divided and polished into finely crushed and moist particles and powders to form clay minerals, which are conducive to water storage. While rainwater participates in weathering, it also gradually brings soluble mineral nutrients to the ground, forming layers at various depths.
Living things, on the other hand, both benefit and contribute to the earth's soil, being nourished by the earth and returning to the earth after death. Therefore, the earth's soil contains a variety of minerals, organic matter, and even tiny organisms, etc., which is a complex mixture.
But the Moon has neither an atmosphere nor water (there is water on the Moon, but it is in the permanent shadow of the polar regions, probably from a comet impact. There is no way to talk about living things, so these rich and varied means of soil formation on the earth are not used by the moon.
The rocks on the Moon can only be gradually broken by (micro)meteorite impacts and the temperature difference between day and night, or ultra-finely processed by the solar wind, ultraviolet rays and X-rays from the sun. The force of the micrometeorite impact can break the cuttings, and the high temperature of the impact can in turn cause the debris to re-sinter, resulting in lunar soil that is more like rough, angular slag.
If you think about it, if you smash the glass into slag, no matter how you smash it, the particles in it will be very sharp. Scientists call this process space weathering. Due to the tight protection of the atmosphere and magnetic field, only large meteoroids and a small part of ultraviolet light can reach the earth's surface, so what happens on the moon is difficult for the earth to experience.
All in all, the soil-forming processes on the Earth and the Moon are basically complementary, and even if you look only at the inorganic part, there are obvious differences: no minerals formed by hydration, such as clay, mica, and hornblende, have been found in the lunar soil, but there is pure metallic iron, and there are no unoxidized ferrotitanium particles in the Earth's soil.
With all that said, what's the use of figuring it out? Through the study of lunar soil, we can obtain various traces left by the evolution of the moon on the one hand, and more accurately understand how the moon became what it is today. In addition, with lunar soil samples, we can better understand what resources are available on the moon, and at the same time, it can also provide important information for humans to build a lunar base.
So the question is, hasn't Chang'e-5 taken lunar soil once? Why do you want to do it again?
Lunar soil was harvested twice
What could be different?
At the end of 2020, Chang'e-5 has obtained 1,731 grams of lunar soil samples from the moon, so what will Chang'e-6 refresh its understanding when it takes samples again? There are two main points of interest here: one is the difference in the geological structure of the landing site, and the other is the difference in the space environment between the front and back sides of the moon.
Geologically, Chang'e-5 landed near Mount Lumke, a volcanic structure in the northwestern part of the front of the Moon, a basalt overflow area formed by volcanic activity, and very young (2 billion years old, albeit young). It's just that it's very young compared to the ancient rocks on the moon. ）。
This is a schematic diagram of the Chang'e-5 landing location and the magmatic and thermal evolution of the Moon at different times (drawn on October 20, 2022). Image source: Institute of Geology and Geophysics, Chinese Academy of Sciences
Chang'e-6 landed in the South Pole-Aitken Basin, the largest, deepest, and oldest impact basin in the entire solar system, and the impact force could penetrate the lunar crust and reach the mantle.
Therefore, the two sampling sites, one new and one old, one is volcanic lava and the other is a celestial impact, one represents internal activity and the other represents external forces, and we can expect a very different result from the return of Chang'e-6 and the return of Chang'e-5.
For example, in the lunar soil samples obtained by Chang'e-5, Chinese scientists did find mineral crystals that crystallized from lava only 2.03 billion years ago through uranium-lead isotope dating, which cooled the moon and delayed the cessation of magmatic activity by about 800 million years.
From Chang'e-6, however, we will try to find ancient rocks that formed about 4.2 billion to 4.3 billion years ago, and try to understand as much as possible about the early history of the formation of the moon, especially the history of the earth, which is completely absent from the earth. There may also be deep material on the surface of the moon in the impact basin that has been turned out of the lunar mantle, and if Chang'e-6 is lucky enough to collect it, it will also greatly advance the understanding of the inner structure of the moon.
In terms of space environment, the front and back sides of the moon are also a little different. Although only the front side of the Moon can be seen from the Earth, and the back side cannot be seen, the Moon always faces the Earth on the same side. Around the time of the Full Moon, when the Moon is facing the Sun, the Moon is covered by the long tail of the Earth's magnetosphere.
During this time, the "wind" of the solar wind felt by the Moon decreases sharply, while particles escaping from the Earth's atmosphere have a chance to reach the Moon (the so-called "Earth wind"). Around the first day of the new year, the moon walks between the sun and the earth, and the back side is facing the sun, and the moon at this time is bombarded by the solar wind without shelter, and there is no longer the blowing of the earth's wind.
Therefore, the far side of the moon is a place that is less cared for by the earth and tormented by the sun more ruthlessly than the front. It should be more difficult to find water or water-bearing minerals in the lunar soil on the far side of the moon, while reserves of helium-3 (a clean nuclear fusion energy source) should be more abundant than on the front side.
The far side of the Moon has a unique environment, it is almost untouched by the Earth, and is a more favorable place for astronomers to observe the universe. If we build a scientific research base on the moon in the future, we need to find out the resource information of energy, water and oxygen, so understanding the difference between the front and back through lunar soil is not only of academic significance, but also of practical significance.
Source: Popular Science China