2024-02-12 22:03:33
To establish future bases on the Moon and Mars, materials will be needed, and we must minimize the resources that we will have to transport from Earth to build them. That is why the development of utilization techniques is essential on site of resources (ISRU). But to know how and why we can take advantage of these resources we have to know their properties. Now, thanks to a new study, we know a little more about the rocks that arrive from the Moon.
We can know precisely the chemical and mineralogical composition of the rocks of the Moon and Mars by studying the fragments that have reached Earth as meteorites and by analyzing the lunar rocks brought back by the astronauts from the Apollo missions.
Schematic diagram of the lunar ejection resulting from an impact and the material transfer orbits to Earth. Trigo Rodríguez JM, Peña Asensio E., Rimola A. et al. (2022)
In the laboratory we can study its mechanical properties: elasticity, plasticity, malleability, ductility, hardness and fragility.
This is how our new study of meteorites has revealed particularities of the most common minerals on the Moon and on the asteroid Vesta: olivines, pyroxenes, feldspars and spinels.
Subjected to the extreme conditions of space
Rocks on the Moon and Vesta have different mechanical properties than those on Earth.
For eons they have been continuously bombarded by rocks up to one meter long, called meteoroids. Those that exceed that diameter, asteroids, have excavated craters on the surface of the Moon, and have even launched some lunar rocks into solar orbit. Furthermore, these energetic processes have generated impact gaps in many surface rocks, with components mixed with the projectiles and thermally altered in the impact process.
Lunar breccia, an important type of rock that is produced by the compaction after successive impacts of the lunar regolith. Josep M. Trigo (CSIC-IEEC)
Bodies without an atmosphere, such as the Moon and Vesta, are also subject to the influence of the solar wind and cosmic radiation that affects rocks at the nanoscale.
All these processes are grouped under the term “spatial processing” (in English: space weathering) and have profound implications for the properties of the rocks that we will one day want to use as a resource on the Moon.
Mechanical properties of lunar rocks
In the new study that will be the cover of Meteoritics & Planetary Sciencenow prepublished on ArXiv, we have carried out an analysis of the composition and mechanical properties of several lunar rocks that arrived on Earth as meteorites, using our collection at the Institute of Space Sciences (CSIC).
We had to know its properties at the nanometer scale. So we have resorted to a technique that allows this, nanoindentation, which we pioneered a few years ago to study the mechanical properties of meteorites.
A mosaic of the lunar meteorite Dhofar 1084, from the Moon, is shown on a typical graph of a nanoindentation (below left). The blue box shows its structure at the nanometer scale. In the lower right corner, the nanoindentometer and the pyramidal tip with which it exerts pressure on the specific material (in blue). Eloy Peña Asensio (CSIC-IEEC)
Study a rock on a nano scale
Nanoindentation is a technique that allows a force to be applied with great precision, using a pyramid-shaped diamond tip, on a surface of nanometers. Controlled force is applied to specific, localized areas, the composition of which we know.
The diamond tip exerts pressure on the surface as the force gradually increases up to a predetermined maximum value. Subsequently, after the charging phase, it is systematically reduced to zero. And the surface shrinks to a certain extent according to the elasticity.
In this way, by studying this loading-unloading cycle, the instrument measures the depth of penetration and deduces the plasticity of the rock. From the study, the deformation mechanisms (both elastic and plastic) and elastic recovery can be inferred.
The discovery in the lunar rocks
Thus, our work with lunar meteorites has revealed the intrinsic heterogeneity in the main mechanical attributes of the most common minerals on the Moon and Vesta: olivines, pyroxenes, feldspars and spinel, even when they show similar mechanical characteristics.
Among the differences found, olivines of terrestrial origin have greater hardness than olivines of lunar origin.
Our studies also indicate that the lack of atmosphere on the Moon and Vesta, and their exposure to sudden and very energetic meteorite impacts, fragments, generates gaps and increases the natural porosity of the minerals from which the rocks are formed. This parameter is key to explaining the mechanical properties of rocks.
Our findings have direct implications for the development of new utilization techniques. on site of resources (ISRU). In turn, they are relevant to better understand the way in which craters are excavated and some of these rocks are propelled at hypervelocity, overcoming the gravitational field of their bodies.
The mechanical characterization of the rocks that form these planetary bodies will be useful to undertake research on the Moon or even rigorously address the challenges and opportunities posed by space mining.
The Apollo 16 roving lunar rover on a mountain slope near the Descartes Crater landing site. POT
The differences with Earth
Mechanical properties are key to the compaction and sintering processes (manufacturing of objects using heat) that will allow, for example, the creation of more robust and durable construction materials in these extreme environments. That is why it is important to carry out more studies that observe how the porosity and crystalline structure of rocks affect their mechanical properties.
The future creation of sustainable infrastructure, roads and other vital structures essential for long-term human presence on the Moon or Mars will require materials, and the best ones must be identified before embarking on a journey.
New challenges for a humanity that, little by little, will become multiplanetary.
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