Given the six-month or longer voyage to reach Mars, first missions to the planet’s surface must last six months to two years to justify the expense to get there. Such lengthy missions require habitats to shield explorers and any food they must grow from the environmental extremes on Mars. The materials these habitats will be constructed from must address these extremes:
· Near-vacuum conditions. Atmospheric pressure is roughly 1/100th that of Earth at sea level. Any structure must be capable of sustaining internal pressure to maintain a viable atmosphere.
· Extreme cold. Winter temperatures can be minus140o F or colder. Even summer air temps are below freezing. Many common building materials become extremely brittle (read that fragile) in such extreme cold. The structure as a whole must be insulated enough to maintain a livable temperature for the occupants living inside.
· Extreme diurnal temperature swings. As much as 125oF between daytime max and nighttime low temps. Many common building materials quickly weather to dust due to the internal shear stresses caused by extreme thermal expansion and contraction.
· Extreme exposure to cosmic radiation, high energy solar wind and particles, and solar ultraviolet light. Mars lacks two protective mechanisms we enjoy here on Earth—a magnetosphere to redirect energetic particles, and a dense atmosphere to attenuate them. Any structure on Mars must shield its occupants from radiation exposure experienced on the surface. Fortunately for the Prospector missions, cosmic ray exposure at Hellas Planitia is much lower than elsewhere on Mars, about 10 rems per year, due to the low elevation and consequent thicker atmospheric blanket overhead. But radiation exposure there still elevates risk of leukemia and other adverse health outcomes. Elsewhere on Mars’ surface radiation levels will be much worse.
· Exposure to micro- and larger meteorites. The structure must be durable enough to handle the occasional meteorite strike. Soft structures must have isolated sections that allow the loss of pressure in one place without catastrophic failure of the entire structure. Rigid structures should be able to withstand at least micro meteorite strikes without failure.
Structures on Mars fall into three broad categories based on their material’s durability and ability to shield occupants from Mars’ environmental extremes.
Short-term – six months to two years Initial missions to Mars will likely rely wholly or in part on soft structures—essentially inflatable tents, engineered to last the duration of individual missions. These structures will likely be double- or triple-walled to provide insulation against extreme nighttime and winter temperatures.
But while such thin-walled structures offer protection against ultraviolet radiation, they offer less protection against cosmic radiation. It is possible that early missions will seek out natural caves or overhanging cliffs to erect their inflatable structures inside.
To mitigate the risk of catastrophic pressure loss, more than one structure may be needed. If they are interconnected, engineers may require they be connected by airlocks so the pressure loss of one structure won’ tresult in complete structural failure.
Candidate materials are carbon fiber fabrics. Carbon fiber is used to construct wing and fuselage components for many Boeing aircraft today. It has proven durable in the extreme cold experienced at 35,000 feet and endures the temperature fluctuations experienced between the ground and cruising altitude. It is assumed the resin base that provides the airtight seal must be flexible to be folded up for transport from Earth to Mars.
The gold standard for radiation shielding is high density polyethylene, a hydrogen-dense resin used to shield the international space station. While the ISS shielding is not 100% effective, it reduces cosmic radiation exposure sufficiently to allow extended missions. Carbon-fiber reinforced HDPE sheeting might serve the needs for short-term inflatable shelters on Mars.
Medium-term – one to five years Consideration of in situ materials production and construction methodologies is significant for medium-term habitats on Mars. Why? Because most common construction materials on Earth cannot be fabricated in the Martian environment. For example, Portland cement is produced by baking the carbon dioxide out of limestone, yielding a mixture of calcium silicates. When hydrated, the Portland cement becomes a matrix of calcium hydroxide and silica hydrates that bind to the sand and gravel aggregate, giving concrete its strength.
To my knowledge, limestone doesn’t exist on Mars. If it did, producing Portland cement would require a high energy expenditure. There is little free water to mix with the resultant Portland cement to make concrete. Even with a supply of fresh water, the water would either boil away due to the near-vacuum or freeze in the extreme cold. The result would be a pile of rubble where you had hoped to cure a concrete beam or a tilt-up wall.
A possible alternative would be constructing blocks using molten sulfur cement. Sulfur has a relatively low melting point (239.4oF), could be mixed with sand, poured into molds and allowed to cool. Such(relatively) low temperatures are amenable to 3D printer technologies. Here on Earth, 3D printed houses made from Portland cement are now commercially available.
Other proposed regolith matrix binding materials are being considered for Mars building materials. Chitosan is chemically derived from chitin, a naturally occurring glucose-based polymer found in insect exoskeletons, and protective structures of mollusks. It can be mixed with regolith to produce high strength blocks. But its exclusive use for structures would require shipping a massive supply or developing an efficient way to produce it in situ. It can be readily extracted from insects which could simultaneously be used as a protein food source. But the quantities of chitosan generated would limit the size and number of buildings that could be constructed from its use.
Silicon dioxide is abundant on Mars and can be used to produce glass. It can be mixed with common iron and manganese minerals to form an opaque block or panel. Varying degrees of purity result in a transparent or translucent product. The downside is glass requires ultra-high temperatures to produce. The melting point of obsidian (volcanic glass) is 1830o F. But the ability to produce glass also means the ability to smelt metals, notably iron. High temperature materials like metals and glass are not suitable for use in 3Dprinting technologies but are appropriate for block and/or post and beam construction methods.
All of these medium-term building materials and methods can be made pressure tight with interior linings or coatings and offer a much greater degree of cosmic ray protection. But these structures will still be susceptible to cold-induced weathering and strikes by modestly sized meteorites. A direct high-speed strike by an object the size of a baseball could potentially be catastrophic.
Long-term—greater than five years Short- and medium-term materials and buildings will be suitable to enable the first ten or twenty years of Mars colonization. But the real solution for long-term permanent colonies on Mars is to build underground. In November’s issue I’ll discuss in Building Materials on Mars(Part 2) what such long-term structures will look like and how they’ll be constructed.
For further reading
https://www.sciencedirect.com/science/article/pii/S2214552420300377https://houwzer.com/blog/3d-printed-homes-how-soon-can-we-buyhttps://www.sciencedirect.com/science/article/pii/S204604302100006X#:~:text=In%2Dsitu%20resources%20including%20Martian,Galactic%20Cosmic%20Rays%20and%20UV.
https://spacenews.com/op-ed-materials-that-could-bring-life-to-mars/https://arstechnica.com/science/2020/09/chitin-could-be-used-to-build-tools-and-habitats-on-mars-study-finds/
