# Towards Lunar In-Situ Resource Utilization Based Subtractive Manufacturing
## 1 Introduction
### Natural Lunar Resources  
The uppermost layer of the Moon, which consists of loose mineral material, is called lunar regolith. This layer covers the entire lunar surface and has an average thickness of 2 to 8 m [23]. Due to its full surface presence and loose composition, the regolith is easily accessible and represents the main raw material for the planned exploration of the Moon. Figure 2: (**a**) The cost of spaceflight across the globe, redrawn from [4], (**b**) spaceship type, country of origin and further description [5; 6; 7; 8; 9; 10; 11; 12]. Such technologies are referred to as ISRU. The first planned application for the regolith is the construction of infrastructure, such as landing pads, roads and habitats. For this purpose, additive processing methods have been investigated for several years in order to process regolith, for example laser melting [24], contour crafting [25] and strengthening by adding additives [26]. The construction of large infrastructure components from locally available regolith leads to a significant reduction in the payloads to be transported from the Earth and, thus, to considerable cost savings [27] (Section 1.1). At the same time, the flexibility of activities on site increases since components can be manufactured or repaired as required. Another possible use is the use of regolith to build up walls and as a shielding material to protect against cosmic rays and micrometeorites [28]. More advanced processing methods under development involve the extraction of oxygen, silicon and metals, which are then available for further processing [29]. This would provide a lunar base with important components for the production of technically more sophisticated components, solar cells for energy production and supplying astronauts with oxygen. Either way, the regolith layer was formed by mechanical crushing of the original bedrock as a result of impacts from different-sized celestial bodies over a period of several hundred million years. These impacts caused a fractionation of the bedrock of the lunar crust [30]. Very fine particles dominate, with more than 50% of the regolith consisting of particles with a size of \(<\)100 microns [31]. A lack of erosion mechanisms, such as flowing water or wind, prevents the particles from rounding off, making them very sharp-edged. In addition to crushed rock particles, regolith also contains 10 to 30% of mineral glass. The origin of the glass lies in volcanic activity and melting effects during asteroid impacts [8]. In addition, regolith contains significant proportions of 5-45% of agglutinates. These are particles with an irregular and partly open-pored structure, which were created by local melting processes when micrometeorites impacted. Before they solidify, the melts combine with unmelted particles and form irregular shapes [30]. The dominant bedrocks of the Moon and, thus, the main source materials of regolith are anorthosite and basalt. While anorthosite forms the original old crust of the Moon, the basalts are younger and can be found in the lunar flatlands, the maria [32]. Anorthosite consists mainly of the mineral plagioclase ([Ca,Na]Al[Al,Si]\({}_{3}\)O\({}_{8}\)), while basalt is mainly a mixture of plagioclase, pyroxenes ((M1)(M2)[T\({}_{2}\)O\({}_{6}\)]) and olivine ((Mg,Mn, Fe)\({}_{2}\)[SiO\({}_{4}\)]) [33]. Therefore, these minerals are widespread on the Moon and found in different proportions in each regolith due to mixing processes. Table 2 shows the average element distributions of the terrae and mare regolith as oxide sums. In the polar regions, especially in the permanently shadowed craters directly at the poles, large amounts of frozen water have been detected [35] (Figure 3). The face of the Moon has been divided into two areas (maria and terrae) from the near side of the Moon, where the lighter colored areas are the lunar highlands, called terrae, and the dark areas are relatively flat plains, called maria.