【Science·Close】
If one day, humans want to build a house on the moon, will the lunar soil under their feet be able to hold it? How is it different from foundation materials on Earth? A more realistic question is that the lunar soil samples brought back by humans are extremely rare. How can scientists obtain its mechanical properties without destroying the samples?
Recently, doctoral candidate Qiao Sijia and researcher Li Lihui from the Institute of Geology and Geophysics of the Chinese Academy of Sciences used mineral identification and nanoindentation technology to conduct precise and non-destructive mechanical tests on extremely small amounts of Chang'e 5 lunar soil (CE5-054), lunar meteorite (NWA-4734) and earth rock (CR-1) samples. Research shows that there are significant differences in the "resistance to deformation" of different minerals, and that the space weathering process will have a profound impact on the mechanical properties of minerals.
"Tough guys" and "softheads" in the lunar soil
The nanoindentation test can be understood as using an extremely tiny needle tip to gently press into the material surface (hundreds of nanometers), and by recording "how much it is pressed down" and "how much it bounces back", the mechanical properties of the material can be inferred. This method causes minimal sample loss and is very suitable for studying precious materials like lunar soil.
Through this test, researchers can obtain two key indicators: elastic modulus and hardness. The former reflects the material's ability to resist deformation during the elastic deformation stage. The larger the value, the less likely the material is to be stretched or compressed; the latter describes the material's ability to resist intrusion or wear. The larger the value, the harder and more wear-resistant the material is.
Experimental results show that the elastic modulus of different minerals in the Chang'e-5 lunar soil has a wide distribution range, and the elastic modulus of different minerals is significantly different, in the order of olivine > pyroxene > ilmenite > spinel > plagioclase, among which the elastic modulus of plagioclase is significantly lower. In contrast, the difference in hardness between minerals is relatively small. Pyroxene is slightly harder, ilmenite is slightly softer, and the remaining minerals are not much different from each other.
The same mineral has different stress resistance capabilities
The research team compared the same types of rocks such as pyroxene, olivine, and plagioclase from the earth and moon and found that even if the mineral types are the same and the origin is different, the pressure resistance will be different.
The comparison results show that the Earth rock (CR-1) is overall better in terms of elastic modulus and indentation hardness, and is both the "rigid" and the "hardest"; the pyroxene and olivine from the lunar meteorite (NWA-4734) The elastic modulus is the smallest, but plagioclase has the highest elastic modulus; the mineral hardness in the Chang'e 5 lunar soil (CE5-054) and the lunar meteorite (NWA-4734) is similar and highly coincident within the error range.
Further analysis found that regardless of whether the minerals come from the earth, the moon or meteorites, the response mechanisms of minerals from different sources during the microscopic deformation process may follow some common internal laws, and the relevant mechanisms still need to be further studied.
Weathering affects mineral behavior
Why does the same mineral show different elastic modulus and hardness in different samples?
If a mineral is compared to a person, its ability to withstand stress not only depends on its own "genes" - mineral type and crystal structure, but also on its "growth experience" - structural defects such as micro-cracks/pores and lattice dislocations left during the weathering process. The latter is often random and inhomogeneous, and is the key reason for the differences in the mechanical properties of the same mineral.
Research and analysis show that the lunar meteorite (NWA-4734) experienced a high-temperature and high-pressure impact event, and a large number of structural defects such as micro-cracks developed internally, thus weakening its overall mechanical properties. The plagioclase in it may have been locally densified during the impact due to its lower melting point and prone to phase change, and instead showed a higher elastic modulus.
Chang'e 5's lunar soil lacked large-scale high-energy impact events during its formation. The modification of particles by solar wind, micrometeorite impact melting or fragmentation was mostly limited to the surface layer of the particles. Therefore, the overall structure remains relatively intact, and its mechanical properties are better than those of lunar meteorites.
Earth rocks (CR-1) are almost unaffected by the weathering of the Earth and retain their original structure, making them harder and less susceptible to deformation.
The above results not only provide micromechanical evidence for understanding the weathering and transformation process on the surface of stars such as the moon, but also provide valuable key parameters for future numerical simulations using lunar soil, the development of high-fidelity simulated lunar soil, and even the realization of in-situ resource utilization on the moon.
(Guangming Daily reporter Cui Xingyi and correspondent Song Tongzhou)
