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Forward-looking: Oxygen will be an essential element of any crewed Mars mission. Producing it on the red planet could be far more efficient than shipping it from Earth, and recent NASA experiments suggest the idea is viable. The oxygen is mostly for fuel rather than breathing.
Throughout 2021, a machine NASA sent to Mars with its 2020 Perseverance rover — called the Mars Oxygen In Situ Resource Utilization (ISRU) Experiment (MOXIE) — created around 50 grams of oxygen from the planet's mostly Carbon Dioxide atmosphere. A recent MIT research paper explains how a lunchbox-sized machine successfully produced oxygen on Mars and how researchers could enlarge its operations. The experiment is an initial step toward sustainable human Mars exploration.
The lunchbox-sized device absorbs and compresses CO2 from the atmosphere, then heats it to 800C. The process then electrolyzes it through a solid oxide assembly and decompresses it into oxygen and Carbon Monoxide. The machine can also confirm its purity and quantity. Since the experiments were part of a proof of concept, the device released the resulting elements instead of storing them.
Alone, this isn't much, only producing 6-8 grams per hour. However, scaling the process up hundreds of times could solve a significant problem facing manned Mars exploration.
Astronauts wouldn't just need oxygen to breathe but also for fuel. Lifting a six-person crew off the surface of the red planet requires about 31 tons of oxygen (along with other elements). Bringing that much oxygen from Earth would take around 500 tons for each mission due to the fuel requirements to escape Earth's atmosphere. Creating oxygen on Mars would be far less costly if the technology could do so quickly.
A scaled-up MOXIE might need to produce 2-3kg of oxygen per hour and operate on Mars for a little over two years before a team of astronauts arrives. However, this method may only be a step towards the Martian ingredient researchers really want for oxygen production — water.
Mars missions could use the water trapped in the planet's glaciers, permafrost, and possibly wet soil to create oxygen and methane for fuel. However, extracting it would require complex ice mining, melting, water purification, and transport operations. Early missions could oxidize some fuel on Mars while shipping the rest from Earth and building the infrastructure to access the red planet's water.
Researchers still need to investigate any unforeseen problems in MOXIE's scalability and long-term durability.