The lack of usable oxygen in the Martian environment presents one of the greatest challenges for human exploration. Mars’ thin atmosphere is overwhelmingly composed of carbon dioxide, making up about 95% of its total volume. This environment is hostile to human life and complicates the logistics of returning to Earth. Overcoming this requires utilizing local materials, a concept known as In-Situ Resource Utilization (ISRU). This approach is a fundamental necessity for establishing any sustainable human presence beyond Earth.
Essential Functions of Oxygen for Mars Missions
Oxygen plays two distinct roles for a human mission, but one far outweighs the other. While breathable air for astronauts is an obvious need, the overwhelming majority of oxygen produced on Mars is required for rocket propellant. To achieve the necessary thrust for a return trip, rockets use a combination of fuel and an oxidizer, with oxygen serving as the latter.
Current mission architectures estimate that launching a crew of four off the Martian surface requires approximately 55,000 pounds (25 metric tons) of oxygen for the oxidizer component alone. In stark contrast, the same four astronauts would only need about one metric ton of oxygen for breathing over a full year on the planet. This difference highlights that rockets consume hundreds of times more oxygen than the human crew.
Producing this oxidizer locally is tied directly to mission feasibility and cost. Transporting 25 metric tons of oxygen from Earth to Mars would require an enormous launch vehicle. By manufacturing the oxygen on the surface, mission planners can dramatically reduce the initial launch mass from Earth. This mass savings makes a crewed return mission logistically and economically viable.
Production from the Atmosphere: The MOXIE Experiment
The first successful demonstration of resource utilization on Mars was the Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE), aboard the Perseverance rover. This instrument was designed to prove the concept of manufacturing oxygen directly from the Martian atmosphere. MOXIE pulls in the carbon dioxide gas that dominates the air and processes it using Solid Oxide Electrolysis.
The process begins by compressing and heating the captured carbon dioxide to a high temperature, around 800 degrees Celsius (1,470 degrees Fahrenheit). At this intense heat, the solid oxide electrolysis cell applies an electrical current to the gas. This current forces the splitting of the carbon dioxide molecule (\(CO_2\)) into two separate components.
The oxygen atoms are isolated and recombined to form molecular oxygen (\(O_2\)), while carbon monoxide (\(CO\)) is released back into the atmosphere as a waste product. MOXIE aimed for a production of 6 to 10 grams of oxygen per hour. The experiment surpassed expectations, achieving production rates of up to 12 grams per hour with a purity of 98% or better.
MOXIE’s success validated that the atmospheric resource can be tapped to support future human missions. A full-scale oxygen plant for a crewed mission would need to be scaled up by a factor of about 200 times the size of the MOXIE instrument. This technology demonstrates a reliable method for converting an abundant Martian gas into a life-sustaining and mission-enabling resource.
Harnessing Water Ice and Regolith for Oxygen
Beyond atmospheric conversion, future mission plans include utilizing the substantial reservoirs of water ice found just beneath the Martian surface. Electrolysis of water (\(H_2O\)) is a more efficient method of generating oxygen than processing carbon dioxide. The electrolysis of water yields both oxygen and hydrogen, which is a valuable fuel source that can be combined with atmospheric carbon dioxide to produce methane rocket fuel.
Martian water is often found as a salty brine, which complicates the standard electrolysis process. Researchers have developed specialized electrolyzers designed to handle these briny solutions, even at the planet’s average temperatures of around -36 degrees Celsius. Under simulated Martian conditions, these brine electrolyzers have demonstrated the potential to produce over 25 times the amount of oxygen for the same power input compared to the atmospheric methods.
Another supplementary source of oxygen is the Martian regolith, or soil, which is rich in iron oxides. The reddish hue of the planet is a direct result of this iron oxide, which is essentially rust and contains chemically bound oxygen. Various high-temperature chemical methods, such as carbothermal reduction or molten salt electrolysis, are being explored to extract this oxygen. These processes often yield metal alloys as a byproduct, offering another local resource for manufacturing and construction.