Mars' underground brine could be a good source of oxygen - Ars Technica

Enlarge / The Pathfinder rover took this photo. It didn't need oxygen, unlike the weak, fleshy humans that would like to follow its path.

NASA

If humans are ever going to visit Mars, they may well need to make some crucial resources while they are there in order to survive long enough to explore and restock for the long return journey. Although the days of flowing surface water are long gone, the red planet is not entirely without the raw ingredients to make this work.

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

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

Perchlorate (ClO₄) salts, we have discovered, 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 sizable amounts of what could be this brine beneath the surface of Mars' north polar region, and smaller amounts have been invoked as a possible explanation for the active streaks that sometimes appear on Martian slopes.

To test whether we could tap this resource, the researchers built an electrolysis device that they ran in Mars-like conditions. It uses a standard platinum-carbon cathode and a special lead-ruthenium-oxygen anode the researchers developed previously. They mixed up a plausible concentration of magnesium perchlorate brine and filled the headspace in that container with pure CO₂ for a Mars-like atmosphere. The whole thing was kept at -36°C (-33°F). When powered up, brine flowed through the device, splitting into pure oxygen gas captured on the anode side and pure hydrogen gas on the cathode side.

The device worked quite well, producing about 25 times as much oxygen as its MOXIE counterpart can manage. MOXIE requires about 300 watts of power to run, and this device matches that oxygen output on about 12 watts. Plus, it also produces hydrogen that could be used in a fuel cell to generate electricity. And it would be smaller and lighter than MOXIE, the researchers say. Ultimately, all this just illustrates that MOXIE is working with a lower quality—but more widely accessible—resource in atmospheric CO₂ instead of water.

A device like this would need to go through long-term stress testing, of course, to ensure that performance doesn't degrade over time and it is generally robust. The membrane that separates the cathode and anode sides was operated carefully to prevent the CO₂ from fouling it up, for example. If your survival depends on a device you brought to Mars, malfunctions aren't an option.

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

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