Making Oxygen
Making Oxygen on Mars
The chemistry is solid-oxide electrolysis: draw in the 95% CO2 atmosphere, compress it, heat it to about 800 °C, and pull the oxygen atom off the molecule across a ceramic membrane. Everything MOXIE needed came from Mars except electricity. It is the first demonstration in history of a human consumable being manufactured on another world.
The near-term application is not breathing — it is propellant. Returning a crewed vehicle from the Martian surface takes on the order of a thousand tonnes of oxidiser, and lifting that from Earth is prohibitive. A scaled MOXIE running for a couple of years before the crew arrives turns the return trip from impossible to routine. Life support is almost a by-product.
The planetary version is a different problem by many orders of magnitude. A breathable atmosphere needs an oxygen partial pressure of roughly 13 kPa — 130 millibars — which means liberating oxygen from carbon dioxide across the whole planet. The minimum thermodynamic work is above 10²⁵ joules, and the industrial build rates implied run to 10⁷–10⁸ kg per second at multi-hundred-terawatt to petawatt power. That is a project measured in centuries to millennia.
Which is why the biological route keeps reappearing. Photosynthetic organisms do the same chemistry using sunlight as the energy source, at no capital cost per square metre once established. The 2025 workshop estimate for producing enough oxygen for complex life by biology alone is on the order of a thousand years — slow, but it is the only version where the energy bill is paid by the Sun.
The case for
Already demonstrated on Mars
MOXIE is the only item in this entire section with flight heritage on the surface of Mars. The technology is proven; only the scale is in question.
The feedstock is unlimited
The atmosphere is 95% carbon dioxide. Oxygen production is limited by power and hardware, never by supply.
It unlocks the return trip
Manufacturing oxidiser on Mars removes the single largest mass item from a crewed return mission and changes the architecture of Mars exploration entirely.
Biology can do it for free
Photosynthetic organisms perform the same conversion powered by sunlight, which is the only version whose energy budget is not astronomical.
The case against
The planetary energy bill is brutal
Over 10²⁵ joules of minimum work, at build rates of 10⁷–10⁸ kg/s and power in the hundreds of terawatts to petawatts. This is the hardest number anywhere in terraforming.
MOXIE is nine orders of magnitude too small
Twelve grams an hour is a laboratory demonstration. Life support for one person needs roughly 800 grams a day; a planet needs a factor of 10²⁰ more than that.
It produces carbon monoxide
Splitting CO2 leaves CO, which is toxic and must be managed or converted at every scale from a base to a planet.
Biology needs everything else first
The thousand-year photosynthetic route presupposes liquid water, tolerable temperatures, UV shielding and soil. Oxygen is the last step, not the first.
| Per-person life support | ~0.8 kg O₂/day | ~66× MOXIE's peak rate |
| Return propellant | ~1,000 t oxidiser | Manufactured before crew arrival |
| Breathable planet | >10²⁵ J | Minimum thermodynamic work |
| Biological route | ~1,000 yr | Powered by sunlight, not industry |
Has oxygen actually been made on Mars?
Yes. NASA's MOXIE instrument on the Perseverance rover produced oxygen from atmospheric CO2 across 16 runs between 2021 and 2023, generating 122 grams in total at rates up to 12 grams per hour.
How does MOXIE make oxygen?
Solid-oxide electrolysis. Martian air is drawn in and compressed, heated to around 800 °C, and an oxygen atom is stripped from each carbon dioxide molecule across a ceramic membrane, leaving carbon monoxide.
How much oxygen would a breathable Martian atmosphere need?
An oxygen partial pressure of roughly 13 kPa — about 130 millibars. Producing that planet-wide requires more than 10²⁵ joules of work, which is why biological photosynthesis over centuries is treated as the only realistic route.