How Mars' Toxic Soil Actually Makes Stronger Bricks

Using local resources will be key to any mission to either the Moon or Mars - in large part because of how expensive it is to bring those resources up from Earth to our newest outposts. But Mars in particular has one local resource that has long been thought of as a negative - perchlorates. These chemicals, which are toxic to almost all life, make up between 0.5-1% of Martian soil, and have long been thought to be a hindrance rather than a help to our colonization efforts for the new planet. But a new paper from researchers at the Indian Institute of Science and the University of Florida shows that, when making the bricks that will build the outpost, perchlorates actually help.

We’ve reported before on efforts to make bricks out of Martian regolith, but one thing we didn’t mention in those reports is that most Martian regolith simulants don’t include perchlorates, as they are a fire hazard. So all of those results, many of which included a form of biocementation process facilitated by bacteria, didn’t include one of the most important components of Martian soil.

That’s what this new paper, recently published in PLOS One, was intended to do. The researchers intentionally added perchlorates to the Martian simulant mix they had previously used to create bricks, and added the whole lot to a slurry. After some mixing time, they then made bricks out of the resulting material and tested them for compressive strength.

Arguably the most important part of the mix was the bacteria. The researchers used a strain of Sporosarcina pasteurii that they found in the soil of Bangalore. It showed impressive responses to being exposed to perchlorates in the lab, including forming dense clusters of multiple cells. Perhaps most importantly, it created a structure called an extracellular matrix (ECM), with “microbridges” between the bacteria and the minerals in the soil.

But bacteria alone didn’t do much in the way of making bricks - they needed combinations of materials to do that. And to do so, they ran controls with different ingredients, including one with just water, which fell apart almost immediately. A slurry of just the bacteria, water and simulant did even worse. It wasn’t until they added a natural adhesive known as guar gum, which is extracted from a plant known as the guar bean, that the compressive strength started to improve. Combining it with only water doesn’t provide much benefit, but when combined with bacteria, the gum appears to act as both an adhesive and a feedstock for the bacteria itself, allowing it to become more than three times stronger than either the bacteria or gum isolated samples.

Another important ingredient was nickel chloride, though it has a mixed relationship with the other materials. It was originally added as a catalyst for ureolysis, the chemical reaction driving the biocementation that creates the bricks. However, it’s not readily available in Martian soil, so it would either have to be processed via a separate step (as there is both nickel and chlorine in the soil) or brought in from Earth. However, the mixture with the strongest compressive strength didn’t include nickel chloride at all.

That title went to a combination of bacteria, guar gum, and perchlorate, with a compressive strength more than double what the combination of bacteria and guar gum alone managed to produce. So why would adding a substance known to kill living things force those living things to make a stronger brick?

Answering that is left for future work, but the authors did speculate that the ECM the bacteria formed when exposed to perchlorate might have played a role. The microbridges that bound the bacteria to its mineral environment might increase its ability to stand up to compressive pressure. That sounds like a fun hypothesis to test, and the team of researchers is still actively doing so. In this one specific instance in terms of the utilization of resources on Mars, it seems that perchlorates would be more of a help than a hindrance.

Learn More:

IISc - How brick-building bacteria react to toxic chemical in Martian soil

S. Dubey et al. - Effect of perchlorate on biocementation capable bacteria and Martian bricks

UT - Explorers Could Build Bricks on Mars with Bacteria and Pee

UT - Astronaut Blood and Urine Could Help Build Structures on the Moon