Oxygen-producing instrument on Mars works: “We’re ready for a facility on the Red Planet”

6. september 2022 kl. 17:40
Oxygen-producing instrument on Mars works: “We’re ready for a facility on the Red Planet”
The MOXIE instrument was carefully placed into the Perseverance rover back in 2020 and is now on Mars, producing oxygen out of the planet’s atmosphere. Illustration: NASA/JPL-Caltech.
A mission to Mars requires at least 34 tons of oxygen to supply the rocket and the astronauts. The oxygen-producing instrument MOXIE on board Perseverance now shows that the technology may be able to provide a breath of fresh air.
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If we are ever to send people to Mars, it would be a very big advantage—perhaps even necessary—to be able to produce oxygen out there.

A longer manned mission would entail spending 18 months on the planet to wait for Earth and Mars to return to the position in which a return journey is possible.

During that time, the astronauts will need oxygen. A lot of oxygen. But the rocket returning them home will need it as well. Even more of it, actually.

Calculations show that three tons of oxygen are needed to supply 6 astronauts for 18 months, but at least 31 tons of additional oxygen are needed to get the astronauts up from the Red Planet and into orbit.

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But Danish and foreign researchers, together with NASA, have long been working on a plan in the form of an experiment called MOXIE, which is today a part of the Mars rover Perseverance and produces... Yes, oxygen. In an article just published in Science, the researchers show that the technology works and can be scaled up.

“It works beyond all expectations. Now we can start working on building the big version for a Mars mission,” says Morten Bo Madsen, physicist at the Niels Bohr Institute and one of three Danes involved in the ongoing MOXIE experiment.

The big version can run continuously for 14 months

Since Perseverance landed on 18 February last year, MOXIE has produced oxygen several times. It has on average been a measly 6 grams per hour, but the amount is not that important. The important thing, on the other hand, is that the oxygen-producing instrument has survived the vibrations of the trip, the fluctuating temperatures on Mars, and does not appear to degrade during the trip.

“We see no reason to not believe that a larger version will be able to run continuously for 14 months,” Morten Bo Madsen says.

High power consumption

MOXIE works by drawing the CO2-rich Martian air (CO2 makes up 96 percent of the atmosphere on Mars and the rest is largely just Ar and N2) into a scroll compressor and heating it to approximately 800 degrees Celsius. The heated air is then sent through a fuel cell under the name of SOXE (solid oxide electrolysis), which contains a nickel-based catalysed cathode that decomposes CO2 into O (atomic oxygen) and CO (carbon monoxide). The voltage difference in the cell then causes the oxygen ions to pass through a ceramic electrolyte and into the anode, where they combine into O2 (molecular oxygen).

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The MOXIE experiment is squeezed into a 30x23.5x23.5 cm box, and the main parts of the box are a fuel cell, a heater, and a pump to compress CO2.

Illustration: NASA/JPL-Caltech.

The prototype on Mars is quite power consuming in relation to the output and thus delivers around 6 grams per hour while consuming between 400 and 800 watt-hours. So when MOXIE is operated, it requires so much energy from the rover’s plutonium-based radioisotope thermoelectric generator (RTG) that there is not much power left for other operations.

Upscaling makes many things easier

Efficiency will increase significantly when the oxygen-producing instrument is scaled up, and Morten Bo Madsen predicts that a number of things will become easier:

“After all, we use a lot of energy to heat it, and there will be big savings in just letting it run all the time,” Morten Bo Madsen says.

“Many things become significantly more efficient on a larger scale. For example, it’s more efficient to insulate the instrument against the large temperature difference between the SOXE cell and the surroundings if it’s bigger, and we can make compressor systems that don’t lead the heat away,” he says.

“When production is continuous, the deterioration of the SOXE cell, which we see a hint of in MOXIE, will be reduced. Furthermore, in a larger system, there will be several built-in sensors which will enable active control of many more important parameters, which will increase the robustness of the system in itself,” Morten Bo Madsen says.

And currently, the MOXIE group is experimenting with producing oxygen under different conditions on Mars to learn as much as possible before scaling up.

CO2 capture can also be used on Earth

Work on building a larger system has slowly begun and involves, among others, researchers from MIT and the private partner OxEon in the USA, and the article published in Science outlines what will be needed.

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The researchers assess that a MAV vehicle that will take off with 6 astronauts from Mars will weigh 50 tons. Of this, approximately 31 tonnes will be liquid oxygen (oxidant) and approximately 9 tonnes will be methane (fuel). The researchers envisage that the MOXIE experiment should be scaled up at least 200 times before usable oxygen production can begin. An independent power supply will then also be needed in the form of, for example, a radioisotope-powered generator.

But perhaps the MOXIE technology could in the long term be used for something other than missions to Mars, because MOXIE is basically a CO2 capture system—and that is something we need on Earth:

“There is great potential in this technology, and the upscaling could easily prove to open up new ways of capturing CO2 on Earth, but the technology is relatively expensive. So it will ultimately depend on whether there are taxes on CO2 emissions and thus good business in capturing CO2.”

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