Mars’ underground brine could be a good source of oxygen

If humans are ever to visit Mars, they may need to create some crucial resources during their stay in order to survive long enough to explore and resupply for the long journey home. While the days of flowing surface water are long gone, the Red Planet isn’t entirely without the raw ingredients to do this job.

The March 2020 mission that launched in July carries an experiment with exactly that goal in mind. MOXIE – the Mars Oxygen In-Situ Resource Utilization Experiment – is a box not much larger than a toaster that produces oxygen from atmospheric CO₂. While a much larger version would be needed to make liquid oxygen fuel for a rocket, MOXIE is sized to produce roughly the amount of oxygen an active person needs to breathe.

A new study by Pralay Gayen at Washington University in St. Louis, Missouri, is testing a device that could tap into a different resource – perchlorate brine thought to exist in Martian soil in some places. The device can split the water in this brine, producing pure oxygen and hydrogen.

Perchlorate (ClO₄) salts, as we have discovered, are common on Mars. These salts have an affinity for water molecules and can trap water vapor over time, turning into brine with a very low freezing temperature. There is significant amounts of evidence for what this brine might be beneath the surface of the north polar region of Mars, and smaller amounts have been cited as a possible explanation for the active streaks that sometimes appear on Martian slopes.

To test if we could tap into this resource, the researchers built an electrolysis device that they used under conditions similar to those on Mars. It uses a standard platinum-carbon cathode and a special lead-ruthenium-oxygen anode that researchers previously developed. They mixed a plausible concentration of magnesium perchlorate brine and filled the free space in this vessel with pure CO.₂ for an atmosphere similar to that of Mars. Everything was stored at -36 ° C (-33 ° F). Upon power-up, brine passed through the apparatus, dividing into pure oxygen captured on the anode side and pure hydrogen on the cathode side.

The device performed quite well, producing about 25 times more oxygen than its MOXIE counterpart can handle. MOXIE requires around 300 watts of power to operate, and this device matches that oxygen output to around 12 watts. In addition, it also produces hydrogen which could be used in a fuel cell to generate electricity. And it would be smaller and lighter than MOXIE, the researchers say. Ultimately, all of this just illustrates that MOXIE is working with a lower quality – but more widely accessible – resource of atmospheric CO.₂ instead of water.

A device like this would of course have to undergo long-term stress testing to make sure that performance doesn’t degrade over time and is generally rugged. The membrane that separates the cathode and the sides of the anode has been handled with care to avoid CO₂ fouling, for example. If your survival depends on a device you brought to Mars, malfunctions are not an option.

PNAS, 2020. DOI: 10.1073 / pnas.2008613117 (About DOIs).