3D-printed Martian mud homes look critical for life beyond Earth

The future where humans colonize Mars is fast approaching. A future with self-sustaining biospheres, autonomous rovers, helicopters with rotors that break the sound barrier, and – perhaps – mud houses. Yes, literal houses made out of mud.

Scientists at the University of Hong Kong have developed a potential recipe for 3D-printing houses on Mars using ground-up Martian rock, gelatin, and yeast, with the latter two ingredients coming from Earth. The researchers say the mixture, once dried and hardened, is comparable in strength to low-grade concrete and could one day be used to build multi-story buildings on Mars. What's more, the material is alive, thanks to the yeast, allowing it to be recycled and regrown.

So, why are mud houses even in the conversation for a project as futuristic as a Martian colony? One reason is the red planet’s environment. Mars is extremely cold, constantly blasted by radiation, and has an atmosphere that's 100 times less dense than Earth's; a few of the numerous peculiar conditions that can have disastrous effects on Earth-origin building materials and building processes.

For example, standard plastics, composites, sealants, and lubricants manufactured on Earth will release trapped gases and volatile compounds, and rapidly degrade in Mars' near-vacuum. Furthermore, intense solar ultraviolet (UV) radiation will break down the molecular bonds in organic polymers. Temperature swings and thermal stress will eventually crack structural steel and cause joints and seals to fail. Also, the water needed to make concrete will boil or freeze, and that's only if you can find the water.

Now, being the awesome species that we are, we've found ways to make earth-based materials work on Mars through extensive modification and protective shielding. The six rover vehicles we've sent over the last 29 years are proof. However, there's a planet-sized difference between sending a 1,025-kg (2,260-lb) craft, the heaviest of the rovers, and sending enough materials to erect a functional building. Which brings us to the second issue: sending building materials from Earth to space is prohibitively expensive and complex.

To circumvent these challenges, aerospace engineers and scientists plan to practice In-Situ Resource Utilization (ISRU), the use of materials already found on the planet. Scientists are already studying a number of approaches, including Martian concrete and 3D-printed regolith. However, some of these solutions require significant amounts of thermal energy. For example, sulfur concrete, made from abundant Martian sulfur and local dirt, is produced by heating the sulfur to its melting point of 115 °C to 120 °C (239 °F to 248 °F).

The Hong Kong researchers' proposed solution, detailed in the journal Chem Circularity, eliminates the need for any heating.

To make the material, they mixed specific proportions of sand from ground Martian rock and wet glue. The glue is made of gelatin and engineered yeast that is coated with highly adhesive proteins. The gelatin holds the ingredients together and provides a place for the yeast cells to grow, while the sand provides mass and structure. Once the mixture flows out of the nozzle, the Martian environment freeze-dries it. The water vapor evaporates, leaving microscopic pores behind. The resulting structure is light and porous, with good compressive strength.

The researchers tested their material under simulated Martian conditions, 3D printing relatively tiny wine-cork-sized domes, 45 mm tall and 30 mm wide. Their compressive strengths measured 10 to 12 megapascals, comparable to low-grade concrete.

“So this is actually strong enough to build a one- or two-story building on Earth whose gravity is three times that of Mars,” says Jishen Qiu, the paper’s senior author and a civil engineer at The Hong Kong University of Science and Technology. “So, you can probably easily build a multi-story building on Mars with the material.”

Of course, there is a considerable distance between structures that could only house fairytale creatures and fully human-occupiable buildings. Still, the researchers’ work presents another possible alternative to the Mars housing problem. A different team of researchers had earlier created Starcrete – a 72-MPa strong material made from potato starch, simulated Martian soil (regolith), and a pinch of salt, compressed into a brick. It is also recyclable, a property the 3D-printed Hong Kong material shares. The yeast from dismantled structures can be regrown in bioreactors.

Conversely, the team does not know whether the living yeast itself will survive Martian conditions. Also, many ingredients for the final material still have to come from Earth, requiring the transport of hundreds of tons of cargo and equipment to Mars over several months. Hopefully, with rapid advances in the space race, this won't be much of an issue in the near future. NASA recently unveiled an experimental ion engine that is 25 times more powerful than its current counterparts.

Source: Cell Press via EurekAlert