As a consolidation of renewed interest in space exploration and to a lesser degree, concerns with climate change, the proposed exploration and colonisation of Mars has produced recent technological breakthroughs within the space transport and colony construction sectors. A large portion of the body of recent work is focused on safety and sustainability of said colonies. Currently, there exists two main categories of proposed colony construction technologies - those that rely on transporting construction materials to Mars from Earth, and those that utilise the matter found on Mars for the majority of the building material. This document will describe and evaluate a range of the recently developed construction techniques based on Martian-sourced building materials, with a focus on radiation shielding. Further evaluation of in-situ logistical considerations such as transportation constraints and environmental concerns will be made. From this, insight can be had on the future of habitat construction techniques and possible improvements and developments that can be made on existing technologies. Finally, the next logical steps in research will be highlighted and recommendations made on a particular research path. 2. Background Martian Colonisation
Exploration of Mars has been a target of space agencies since 1960 . If and when colonists get to Mars, the current proposed habitat construction techniques range from relatively low-tech tools to semi-autonomous construction robots . Furthermore, the colony will have to be largely self-sufficient. This presents a challenge as Martian solar power is 40% as effective as on earth . Uranium and oxygen are also in short supply . Long term Martian colony safety from environmental hazards such as meteoroid collisions or sandstorms is also in question, as currently being researched by the RETH Institute. The Institute has proposed architecture describing the feasibility of a Mars Base Camp within a decade . This architecture would involve human exploration of both Martian moons, in addition to colony structures built by pre-staged robots.
Perhaps the most pressing environmental concern is the level of solar radiation likely to be experienced by a Martian colony. Because the Martian atmosphere is 1% as dense as on Earth, half of all incoming radiation reaches the surface . Without a substantial magnetosphere and atmosphere like that found on Earth, a person or structure on the surface would experience around fifty times the radiation levels on Earth . As such, the topic of habitat construction material, as well as the shielding properties of said materials will form the focus of this document. In terms of radiation shielding, this review will look primarily into shielding against high energy galactic cosmic rays (GCR), the most harmful space radiation.
It has been proposed in ,  that regolith can be used to construct colony shielding. Regolith is defined as the layer of rock fragments that is exposed to space on airless body surfaces . It has been shown in  that regolith blocks out the majority of GCR at a thickness of 3 m, a suggested minimum shielding thickness. While some of the attempts to process regolith in the existing literature require additives or are dependent on calcination, this document will assess novel approaches to processing this material. Another factor to consider is that Martian regolith or soil is potentially carcinogenic or may cause lung conditions . While further research is required in this area, the minimal use case of regolith utilisation as a radiation shield. Logistics Transportation is key to any Martian mission, with current unmanned spacecraft taking a minimum of 180 days each way. This is with the aid of an orbit between Earth and Mars when the orbits of the two planets intersect at a certain point relative to each other . While improvements in space flight technology could reduce this time, colonists still would not be able to count on regular supply ships. Critically, there will be a limit to the weight that can be sent, meaning they will have to rely largely on local resources for building materials which are not ecologically diverse. While current research shows that while large amounts of water is present, it is not easily accessible .
Construction Methods Regolith Compaction
According to , Martian colonies will be constructed using primarily locally sourced materials, primarily regolith. The results presented demonstrate the viability of building material composed of compressed simulated Martian regolith, or soil. The soil simulant Mars-1a was compressed into a solid without addition of any additives. It was found that the bonding agent was iron oxide, commonly found in Martian regolith. Furthermore, samples of the soil compact were found to have an unconfined compressive strength of around 50 MPa, roughly double that of typical concrete found on Earth. As such, soil compact gas permeability was found be on the order of 10−16 m2. Those authors state that this is similar to solid rock. The compact was found to be suitable as a construction material, being adaptive to 3D printing, where an object is built by adding layer upon layer.
Wan et al. have proposed a method in which simulated Martian soil and molten sulphur were used to produce sulphur-based concrete . Sulphur is abundant on the Martian surface. In addition, Martian regolith simulant was found to have a suitable aggregate size to ensure that the concrete has high strength. The mix for producing Martian Concrete (MC) with optimal strength was found to be 50% sulphur and 50% Martian soil simulant with a maximum aggregate size of 1 mm. Test results showed that optimal MC had an unconfined compressive strength of above 50 MPa. The developed MC was found to be feasible for construction on Mars based on availability of material, recyclability, and ability to withstand the Martian environment amongst other factors.
Concrete production was further investigated in , which examines using magnesite and other magnesium-based compounds commonly found on Mars as building materials. The study showed that magnesium oxide-regolith-based concrete had a compressive strength in excess of 20 MPa, around the lower end for lime-based concrete found on earth. The results show that Mg-based products form a viable method for construction material on Mars with little water required and minimal energy use.
3D Printed Structures
Another construction method being explored is 3D printed colony structures composed of either ice or regolith. Citing new evidence of the potential hazards of perchlorates in the Martian soil, ice-based structures serve as a radiation barrier, absorbing shorter wavelength radiation, while allowing visible light through . While the Martian surface experiences varying temperatures with daily and seasonal cycles, the average temperature is −65˚C , ensuring the structural integrity of the structure is maintained once completed.
The winner of a 2015 NASA competition to produce 3D printed Martian Habitats, Mars Ice House, experimented with scaled ice 3D printing on Earth, as well as small-scale printing robotics, and detailed a process whereby subsurface Martian ice was turned into liquid water which was then printed . The sub-zero Martian surface temperature ensures the liquid water is” instantly turned into ice ”. The plan was described as involving the use of a Mars descent vehicle, a deployable membrane, and semi-autonomous robotic printers to gather and begin deposit subsurface ice prior to arrival of human colonists. The final structure was a 92 m2 translucent habitat which allowed ambient visible light into the habitat. The Mars Ice House design plan also plans to build with regolith .
Other private entities that took part in the NASA challenge looking to build on Mar include Foster + Partners who outlined a concept for a robot-built human habitat based on the use of semi-autonomous 3D printing robots. However, the habitat is built from regolith, meaning that it serves primarily as a radiation shield in addition to providing protection from micrometeorite strikes. The next planned step is the production of construction of complete habitats .
From the above sections, it has been shown that numerous possible regolith-based construction and shielding techniques are viable. Comparing the methods, there are some key criteria which must be taken into account. Firstly, the availability of water is an issue; while large amounts of it are present on Mars, processing the ice deposits to obtain liquid water is costly and time consuming . Furthermore, mining for sulphur is also associated with similar costs, as is the process of mixing evaporant and surfactant used in 3D printing , , . The literature thus points toward compacted regolith as the most viable and low cost given that only compaction equipment is required. Conclusion Regolith-based construction techniques have potential to transform the colonisation of Mars. Further studies on these techniques may provide further insight into which of the described techniques are most feasible. The compaction method, as far as the current literature indicates, is the least complex and requires less maintenance, thus it forms a promising avenue for further research which may extend to deployment systems and testing full scale structures. More research should be made into radiation shielding properties of compacted regolith, in particular focusing on types of radiation not discussed in this document, as well as the optimal structural thickness order to evaluate building costs.