Mars’s underground brine could be a good source of oxygen

If humans visit Mars, they may need some crucial resources while they are there to survive long enough to explore and refuel for the long return journey. Though the days of surface water flowing are long gone, the red planet is not entirely without the raw materials to do this job.

The Mars 2020 mission, launched in July, is carrying out an experiment with exactly this goal in mind. MOXIE, the in situ oxygen resource utilization experiment on Mars, is a box not much larger than a toaster that produces oxygen from atmospheric CO₂. While a much larger version would be needed to produce liquid oxygen for a rocket, MOXIE is sized to produce about the amount of oxygen an active person needs to breathe.

A new study by Pralay Gayen at Washington University in St. Louis, Missouri, tests a device that can tap into a different resource: perchlorate brine believed to exist in Martian soil in some locations. The device can split the water in that brine, producing pure oxygen and hydrogen.

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

To see if we could tap into this resource, the researchers built an electrolysis device that they performed under conditions similar to Mars. It uses a standard platinum-carbon cathode and a special lead-ruthenium-oxygen anode that the researchers developed earlier. They mixed a plausible concentration of magnesium perchlorate brine and filled the headspace in that container with pure CO₂ for an atmosphere similar to Mars. The whole was kept at -36 ° C (-33 ° F). Upon ignition, the brine flowed through the device, splitting into pure oxygen gaseous captured on the anode side and pure hydrogen gas on the cathode side.

The device performed quite well, producing around 25 times more oxygen than its MOXIE counterpart. MOXIE requires approximately 300 watts of power to operate and this device matches the oxygen output to approximately 12 watts. Furthermore, 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, this all just proves that MOXIE is working with a lower quality, but more widely accessible, resource in atmospheric CO.₂ instead of water.

A device like this would need to undergo long-term stress testing, of course, to ensure that performance doesn’t degrade over time and that it is generally robust. The membrane separating the cathode and the sides of the anode has been carefully operated to prevent CO₂ to dirty it, 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 DOI).