MOXIE

Compared to what we're used to on Earth, the air on Mars is very thin. In fact, there is less than 1% as much air on the surface of Mars as on Earth, and visiting Mars would be like flying in a balloon 100,00 feet up in the air. And not only is the air thin, but it's made almost entirely out of carbon dioxide (CO₂).

On Earth, we can thank the myriad forms of life for the oxygen in our atmosphere that our body greedily absorbs with every breath we take. All around us, trees and plants are busy scrubbing CO₂ out of our atmosphere and turning it into useful oxygen, at the same time taking up water and turning it into the chemicals that the plants need to live. Mars isn't so fortunate, so if we want oxygen on Mars we'll have to make it ourselves. That's what MOXIE does.

The "I" in MOXIE stands for ISRU, or In Situ Resource Utilization. "In Situ" refers to the stuff that's there – on Mars in this case. Resource Utilization means using that stuff for some productive purpose, usually to make something that we would otherwise have to bring with us. In other words, living off the land (LOL). Maybe LOL would have been a better acronym than ISRU, but that's already been taken! One example of ISRU might be making bricks out of Martian soil, and using the bricks for buildings. Another example is making oxygen out of the thin Martian air.

MOXIE is an example of a "solid oxide electrolyzer cell," which is a fuel cell that runs in reverse. In a fuel cell, fuel and oxygen react to make energy (electricity) and a stable chemical product. The most common example is the hydrogen fuel cell, used in some automobiles, which combines hydrogen and oxygen to make water and electricity. A hydrogen cell of this type, in contrast, would start with the water and use electricity to turn it back into hydrogen and oxygen. The two devices, the fuel cell and the electrolyzer, are nearly identical devices, operated in a very different way.

Using water to make oxygen on Mars would be a great idea if we had easy access to the water. But to get water, we would have to travel to the far north or south to find ice, or to mine for ice buried deep in the ground. Fortunately, though, another source of oxygen is all around us, in the form of the carbon dioxide that makes up most of the Martian air. So instead of using hydrogen, MOXIE is an example of a solid oxide electrolyzer cell that uses carbon dioxide to make the gasses carbon monoxide and oxygen.

The first human crew to go to Mars will need a lot of oxygen to breathe, as much as 2 to 3 tons. Our bodies use that oxygen to burn our fuel, which we call "food," in order to make the energy that keeps us going all day. But it's not just humans who need oxygen. Nearly everything that uses fuel to make energy uses oxygen to burn that fuel, whether it's a car, a campfire, or a rocket. And it turns out that the oxygen, which on Earth we get for free from the air, actually weighs a lot more than the fuel itself.

When we send the first crew to Mars, we would need to think about how to bring them home at the end of their visit. And the single biggest user of oxygen in their entire stay on Mars will be the rocket that takes them back to orbit. While the astronauts may breathe 2 to 3 tons of oxygen in their 18-month stay on the planet, the ascent rocket would use 25 to 30 tons.

MOXIE was designed to make at least 6 grams of oxygen per hour. The maximum it can make is 12 grams per hour, although there isn't enough CO₂ at our high-altitude Jezero crater landing site to make much more than 10. For comparison, you're probably using about 10 grams per hour while you're reading this explanation (unless you're riding your exercise bike while you're reading)!

Why so little? It takes a lot of power to pull apart CO₂ molecules, and the Perseverance rover only provides about 100 Watts. That's not much more than the computer you're using to read this. If we're to make useful amounts of oxygen in the future, we'll need a much larger power plant.

The "solid oxide" in a solid oxide electrolyzer cell is a ceramic made mostly of zirconia that is heated to 1,472 degrees Fahrenheit (800 degrees Celsius) inside a special miniature oven. When MOXIE runs, it first has to heat up that ceramic to its operating temperature. That takes about two hours. The rover batteries have enough available charge left to make oxygen for about an hour after that. During that hour, we sometimes change the voltage or the compressor speed to learn more about the instrument.

When MOXIE runs, very little else gets done on Perseverance. There isn't enough power left to drive, or to abrade rocks, or even to run the other instruments for very long. So MOXIE doesn't run very often, maybe once every one or two months.

MOXIE's main job is to show that we can rely on this technology to keep future crews of astronauts alive and return them home safely. That's a tall order. But more than that, we hope to learn a lot of technical details about how to build that much larger future MOXIE system. For that purpose, we include in MOXIE lots of sensors and controls that help us learn about how well MOXIE is doing and how to make it better.

Earth and Mars line up for a trip every 26 months. When we send an astronaut crew to Mars, one idea is to send all the things they'll need – a place to live, rovers, a power plant, and maybe a "Big MOXIE" – on one launch opportunity, then send the astronauts on the next opportunity, 26 months later. If we do that, then the base would be in place for about 20 months before the astronauts start their trip. The goal of a Big MOXIE would be to make and store all the oxygen that the astronaut and their rocket would need for their mission before they even launch. That means the Big MOXIE would need to make 2,000 to 3,000 grams of oxygen per hour, compared to the 6-10 grams that the current MOXIE is making. And it would need to do that all day and night, without stopping, for most of the 20 months.

We expect that the astronauts would need at least 25 kilowatts of power for their mission, and that the Big MOXIE relies on being able to use that power plant for making oxygen until the astronauts arrive. We also expect Big MOXIE to be the size of a small chest freezer and to weigh about 1 ton.