ISRU Part II: How to Make Construction Materials on Mars

In Part I, we discussed making the essentials for life on Mars: fuel, oxygen, and water. After the initial exploration phase, the construction of permanent settlements and structures will begin to become a priority. When humanity settles Mars, Martian architects and civil engineers will have to solve problems never before faced in the 10,000-year history of construction. However, we are hardly lacking in solutions; when future generations build their homes on Mars, they need only to look around them for materials.

Of the materials used by industry and construction, concrete, brick, metals, and plastic take the lion’s share. If we want to live on Mars, we will need to produce these on-site. Fortunately, much of the knowledge we need dates back hundreds, or even thousands of years.

“‘I say only this!’ Arkady said, staring at her bug-eyed. ‘We have come to Mars for good. We are going to make not only our homes and our food, but also our water and the very air we breathe—all on a planet that has none of these things.’”

Kim Stanley Robinson, Red Mars

Concrete

Concrete is probably the most common construction material on Earth, due to its good compressive strength (resistance to being crushed) and the availability of its component materials. It is composed of a mixture of fine and coarse particles known as aggregates, a binder that binds it all together, and various additives to fine-tune its properties. On Earth, we most often use sand and gravel as aggregates, and Portland cement as a binder. To improve the tensile strength (resistance to being stretched) of concrete, it is often reinforced with steel bars.

Portland cement is made by heating limestone mixed with a number of other minerals to high temperatures, then adding gypsum. It becomes adhesive and hard upon reacting with water, binding the aggregates together. While concrete that has already cured should face no problems on Mars, the mixing and pouring process would have to take place in a pressurized environment or the water would boil. It will also be difficult to obtain limestone, the main raw ingredient, especially in the early phases of Mars settlement. Most of all, water is precious, and concrete production is a water-intensive process.

However, water and limestone can be removed from the equation entirely by replacing conventional cement with sulfur – which is common on Mars – to make sulfur concrete. A construction crew could mix Martian regolith with sulfur powder, then heat it until the sulfur melts at 120 °C. When the sulfur cools, it binds the mixture together, resulting in a concrete of considerable strength. As a nice bonus, sulfur concrete structures are easy to recycle by simply melting them down again, unlike conventional concrete.

This process works on Earth too. Petroleum production leaves large amounts of waste sulfur, and this sulfur could be used to make sulfur concrete. Substituting conventional concrete for sulfur concrete could significantly reduce its ecological footprint, because the process no longer needs water and limestone. Furthermore, making cement from limestone is energy-intensive and emits large amounts of carbon dioxide, contributing to climate change. Since Martian colonists will optimize the manufacture and usage of sulfur concrete out of necessity, their work could improve the sustainability of construction on Earth.

Sulfur Waste Pile
A pile of waste sulfur left over from crude oil extraction (University of Liverpool)

Brick

Before humans built with concrete, they built with bricks, and they continue to do so today. Making a brick usually means taking clay or clay-rich dirt, wetting it, mixing it, compressing it, then drying and baking it. Clay can be found on Mars due to its water-rich past, and the fine regolith and dust make for good filler material. Historically, humans have used sun-dried bricks for construction since 9000 BCE. Their strength can be further improved by baking them in a furnace.

Robert Zubrin, in The Case for Mars, proposed baking bricks using a solar furnace or waste heat from a nuclear reactor, and recapturing the water to make more bricks (which cannot be done with conventional concrete). Aptly, humans may build the first cities on Mars the same way they built the first cities on Earth – with sun-dried bricks.

However, recent work has shown that water may not even be necessary. Researchers at UC San Diego found that by simply compressing Martian regolith simulant by hammering, they were able to form it into bricks with strengths exceeding that of reinforced concrete. They believe that iron oxide, which gives Mars its red color, acts as a binder. On a side note, receiving a bottle of laboratory-grade Martian regolith simulant is a membership benefit of joining the Mars Society of Canada (but I do not guarantee that hammering the simulant yourself will give you strong Martian bricks!).

Metal

Like most rocky planets such as the Earth, Mars is mostly composed of metal oxides. Many useful metals have been detected in the regolith, such as silicon, iron, magnesium, aluminum, titanium, and chromium. However, these metals are rarely found in their pure form (except in meteorites), but instead are locked up in oxides or other ores.

Mars Atmosphere
Elemental composition of Martian regolith (image by author, CC BY 4.0. Data source: https://www.lpi.usra.edu/education/IYPT/Mars.pdf)

The oxygen must be removed from the oxides to obtain the metal. In chemistry, this is known as reduction; in metallurgy, it is known as smelting. Of these metals, the most commonly used in industry is iron, and it can be extracted from the regolith by three techniques: carbothermal reduction, hydrogen reduction, and molten regolith electrolysis.

On Earth, iron oxide is most commonly smelted by carbothermal reduction. Carbon (in the form of coke) is burned with oxygen in the air, heating the iron ore to ~2000 °C and releasing carbon monoxide. The carbon monoxide seizes the oxygen atoms from the iron oxide, resulting in plain iron:

However, we need not replicate this exact process on Mars. The carbon monoxide could come from other sources, such as the by-products of the oxygen production process mentioned in Part I of this article series. The heat could come from a solar furnace. NASA has developed a carbothermal reduction process for usage on the Moon, where methane is used as a source of carbon, and concentrated solar energy or a laser beam is used as a heat source. This process also works for silicon and titanium, which are vital to electronics and aerospace applications.

The same reduction could also be accomplished by using hydrogen instead of carbon monoxide, in a process called hydrogen reduction:

The hydrogen can be recovered afterwards by electrolyzing the water, yielding oxygen in the process.

And in the same way that we use electricity to split water into hydrogen and oxygen, we can use electricity to split molten iron oxide into iron and oxygen through molten regolith electrolysis:

This is the simplest process of the three, involving only melting the regolith and passing a strong electrical current through it. On Earth, this process is used to obtain aluminum by dissolving its ore in a vat of molten fluoride salt, then electrolyzing it. However, this process requires enormous amounts of electrical power, which may put a strain on a fledgling base’s resources.

A useful by-product of these reactions is oxygen, liberated from the ore. In Part I of this series, we looked at extracting oxygen from Mars’s carbon dioxide atmosphere and from electrolyzing its water. Now, there is a third method available: extracting it from the regolith.

On the Moon, in the absence of an atmosphere and where water is much scarcer than on Mars, smelting metals may be the best available source of oxygen. For this purpose, NASA is considering using hydrogen reduction. In this case, the goal is to obtain oxygen, and the metals produced are a useful by-product.

NASA Instrument ISRU
The NASA PILOT (Precursor ISRU Lunar Oxygen Testbed) system being tested at an analog site in Hawai’i. It extracts oxygen from regolith by hydrogen reduction. (NASA, public domain)
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Plastic

The newest of these materials is plastic, which has revolutionized every aspect of daily life and industry in the past century. From 3D-printing feedstock to utensils to radiation shielding to plumbing to inflatable habitats, plastics are an absolute necessity for space travel.

Plastics are made of polymers, which are long chains of a certain molecule called a monomer. One of the most important monomers is ethylene, a sweet-smelling chemical that makes fruit ripen. Polymerizing ethylene (i.e. joining lots of ethylene molecules into a chain) makes polyethylene, one of the most commonly used polymers in the world. You have encountered various forms of polyethylene as LDPE (low density polyethylene) in plastic bags, HDPE (high density polyethylene) in bottles and containers, and perhaps UHMWPE (ultra-high molecular weight polyethylene), in machine parts, bulletproof vests, and hip replacements.

Unlike Earth, Mars has no global magnetic field to protect against radiation storms from the sun. Its rarefied atmosphere provides some, but not a lot of shielding. This means that during a solar storm, unsheltered astronauts on the surface may receive dangerous doses of radiation. To counteract this, polyethylene makes a good shield.

However, to make polyethylene, we first need ethylene.

On Earth, ethylene is produced from petrochemicals; hence, the heavy dependence of the plastics industry on fossil fuels. However, Mars is notably lacking in petrochemicals (otherwise we would be sending human expeditions there by now!). Instead, we can make polyethylene by reacting carbon monoxide with hydrogen:

We can make carbon monoxide from the Martian atmosphere and hydrogen from the ice. Then, the ethylene gas can be polymerized to polyethylene through techniques such as the Ziegler-Natta process. Alternatively, it could be transformed into other monomers to make other kinds of plastic, or even be used as a fuel. The possibilities are endless!

Water – and hence, hydrogen – is much rarer on the Moon than on Mars, and carbon is almost completely absent. This means that on the Moon, plastics will have to be imported at exorbitant cost, along with all the other essentials for life that depend on carbon. Mars may turn out to be the second most important plastics producer in the interplanetary economy after Earth.

Building a Future on Mars

A building’s design reflects the environment it was built for as much as the culture that built it, and the same will apply to our future homes on Mars. In the thin Martian atmosphere, inhabited structures will need to be pressurized, meaning that some buildings may be more at risk of exploding than of collapse!

The extreme environment will force Martian settlers to innovate with the materials available on hand, undoubtedly launching a revolution in architecture. They will need to consider temperature extremes, the unbreathable atmosphere, radiation storms, the lower gravity, and other hazards unique to Mars. Martian homes made from ice, 3D-printed basalt-bioplastic, and even bamboo are under serious consideration. Creativity by necessity will result in technological and design innovations, and characterize a new frontier of exploration: one without exploitation or ecological destruction.

I cannot wait to see what they come up with.


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