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How did Perseverance make oxygen on Mars?

Two days after NASA scientists proved it’s possible to fly a helicopter in the thin Martian atmosphere, another team showed that it is also possible to convert that air into something future astronauts can breathe.

This second of two “firsts” in a single week was done by a toaster-oven-sized instrument on the Perseverance rover called MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment), designed to test whether it is possible to make oxygen from carbon dioxide, the principal gas in the Martian atmosphere.

Not that anyone could live for long on the amount of gas MOXIE produced in its first one-hour test run on 20 April. The total amount was about 5.4 grams, according to the instrument’s principal investigator, Michael Hecht of Massachusetts Institute of Technology’s Haystack Observatory. “That’s about enough to keep a typical active astronaut alive for ten minutes,” he says.

It works via a process known as electrolysis, which more or less functions like a fuel cell run in reverse.

Fuel cells take in fuel (such as hydrogen or carbon monoxide) and, without actually burning it, combine it with oxygen to produce electricity and carbon dioxide.

MOXIE takes in carbon dioxide and electricity… and produces oxygen and carbon monoxide.

It is, of course, more complex than that, but that’s the basic idea. At its heart is a stack of ceramic layers, like a deck of playing cards, each coated with exotic materials that help collect and separate the oxygen and carbon monoxide. “[It’s] about half the size of your cell phone, but twice as thick,” Hecht says.

There’s also a compressor to suck in Martian air (about the density of the Earth’s atmosphere at 30,000 metres altitude) and compress it to a more useful density. “That’s a mechanical pump that has to be pretty good size,” Hecht says.

MOXIE also has to run at 800°C to make it work, which requires an oven-like heater and insulation to keep the heat from frying everything else. “So, before you know it, that little [stack] grows to something more like a couple of Rubik’s cubes in size,” Hecht says.

Not to mention the electronics that run it and the analytical equipment that tests the result. Hence the ultimate toaster-oven size.

At 17 kilograms, though, it’s still a small fraction of the overall rover, which Hecht describes as the size of a Mini-Cooper.

But it is a power hog. “We pretty much use up a whole day of [the rover’s] power,” Hecht says.

That means other operations have to come to a halt to fire up MOXIE and run a test – something planned to be done about once every two months.

That means this first test came along right on schedule, on day 60 (or in Mars-speak, sol 60, since Martian days are slightly different than Earth’s).

But other factors also came into play in choosing the date, because testing the Ingenuity helicopter involved having the rover sit stationary as it oversaw the helicopter flight, an obvious reduction on other demands on its power. “The suggestion was to play it by ear and see when an opportunity arose,” Hecht says.

When the time came, the test went swimmingly. “It looked letter perfect,” Hecht says.

“We hadn’t made oxygen in [MOXIE] for two years,” he adds. “The last time was in the laboratory. It had been installed in the rover, launched, spent months in space, and landed. And there it was, working like nothing had happened. We couldn’t have been happier.”

It isn’t just about providing breathing air for astronauts, important as that is. Oxygen is a critical component of rocket fuel. To blast off the surface of Mars and return to Earth, NASA estimates, takes about 25 tonnes of oxygen, not something you want to carry all the way to Mars if you can avoid it.

To put that in context, Hecht says, “astronauts who spend a year on the surface will maybe use one metric ton between them”.

“Oxygen isn’t just the stuff we breathe,” says Jim Reuter, associate administrator of NASA’s Space Technology Mission Directorate. “Rocket propellant depends on oxygen, and future explorers will depend on producing propellant on Mars to make the trip home.”

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