Abstract
Establishing a permanent, self-sufficient habitat for humans on planetary bodies is critical for successful space exploration. In-situ resource utilisation (ISRU) of locally available resources offers the possibility of an energy-efficient and cost-effective approach. This paper considers the high-temperature processing of molten lunar regolith under conditions which represent the lunar environment, namely low gravity, low temperature, and negligible atmospheric pressure. The rheological properties of the low-titanium lunar mare regolith simulant JSC-1A are measured using concentric cylinder rheometry and these results are used to explore the influence of viscosity on processing operations involving the flow of molten regolith for fabricating construction components on the Moon surface. These include the delivery of molten regolith within an extrusion-based 3D printing technique and the ingress of molten regolith into porous structures. The energy and power required to establish and maintain sufficiently high temperatures for the regolith to remain in the liquid state are also considered and discussed in the context of lunar construction. Establishing a permanent, self-sufficient habitat for humans on planetary bodies including the Moon and Mars is critical to the success of space exploration missions. Essential items for developing extra-terrestrial habitats require diverse ranges of materials capable of supporting applications including power generation, life support, and structural engineering. In-situ resource utilisation (ISRU) of geological materials available locally presents an attractive, efficient method for habitat construction, in terms of cost, logistics, and energy when compared to transporting materials from Earth. The primary components of lunar regolith – in order of decreasing abundance – are glass, plagioclase, olivine, pyroxene, and ilmenite. The concentration of each component is location-dependent on the lunar surface 1. In order to study the processing of both lunar and Martian regolith a series of more than thirty soil simulants have been established 2–9. These are multi-component powders, whose thermal 10,11 , optical 12,13 , and electromagnetic 14,15 properties have been studied. Research which has considered the processing of lunar simulants has tended to focus on physical properties including their shape 16 , handling 17 and flowability 18,19. The suitability of these materials for manufacturing low-porosity structures capable of supporting extraterrestrial construction has also been explored 20–23 , with the properties of the processed simulants after mechanical compaction 24,25 and 3D printing 26,27 also receiving attention. Heating the simulants to temperatures at which sintering and melting occur 28,29 leads to the formation of crystalline phases embedded within ceramics and glasses, the physicochemical properties of which have also been studied 30,31. The microstructure of the resultant material is also an area of investigation 21,23,32,33 , as this determines the usefulness of the material for construction purposes. The possibility of extracting life-supporting resources, including oxygen, water and iron, from the regolith, when heated to high temperatures, is a topic of great interest 34–36. Resource extraction is also possible using reduction 37 , electrolysis, or pyrolysis 38. Although research which seeks to address the feasibility of producing commodities from lunar and Martian regolith is underway, few studies have considered the rheological properties of the lunar and Martian simulants once heated to the molten state. Noteworthy investigations include a study into the behaviour of Mars soil simulant JSC-Mars-1 when mixed with water 39 , a comparison of three lunar simulants which considered their crystallisation behaviour when supercooled 40 , and the implications of the Marangoni effect for processing the molten liquid 41. At volume fractions in the range 0.39–0.49, the aqueous JSC-Mars-1 slurry exhibited viscoelasticity, behaving like an elastic solid at short timescales, and like a yield stress fluid at longer timescales. The lunar simulants JSC-1A, Stillwater norite, and Stillwater anorthosite exhibited Newtonian behaviour; crystallisation occurred rapidly once the norite and anorthosite were at temperatures lower than their liquidus