First published November 17, 2021.
The Moon has no air we could breathe, yet its surface is packed with oxygen. Roughly 45% of lunar soil, or regolith, is oxygen by weight, locked inside minerals rather than floating as gas. In November 2021 a back-of-the-envelope estimate by soil scientist John Grant suggested that the oxygen in the top 10 metres of regolith could, in theory, supply 8 billion people for about 100,000 years. That number is a striking illustration, not a plan. Getting the oxygen out takes heavy equipment and a great deal of energy, and it would fill tanks for astronauts and rockets, not create a breathable atmosphere around the Moon.
Where the Moon's oxygen is
The Moon's surface is covered by regolith, a layer of broken rock and fine dust created by billions of years of meteorite impacts. The minerals in it, such as silica and oxides of aluminium, iron and magnesium, all contain oxygen chemically bonded to other elements. Apollo samples and later missions confirmed that oxygen is the most abundant element in lunar soil by mass.
The Moon's "atmosphere" is another matter. It is an extremely thin exosphere, with a total mass estimated at under 10 tonnes for the whole Moon. Its main components include argon, helium, neon and traces of hydrogen, sodium and potassium. Surface pressure is around 0.3 nanopascals, trillions of times lower than on Earth. Gas atoms there are so sparse that they rarely collide, and many escape into space. In short, there is nothing to breathe.
How the 100,000-year figure was worked out
Grant, a lecturer in soil science at Southern Cross University in Australia, laid out the arithmetic in an article for The Conversation, later republished by ScienceAlert:
- A cubic metre of regolith holds about 1.4 tonnes of minerals, including roughly 630 kg of oxygen.
- NASA estimates that a person needs about 800 grams of oxygen a day, so 630 kg would last one person a little over two years.
- Assuming the regolith averages about 10 metres deep, and that all of its oxygen could be extracted, the total would support 8 billion people for around 100,000 years.
Grant was clear that this depends on how efficiently oxygen could be extracted and used. In reality, nobody would mine the entire surface of the Moon. The point of the exercise is that lunar soil is not oxygen-poor; the bottleneck is technology and energy.
How oxygen can be pulled from rock
The most promising methods borrow from industrial chemistry. Electrolysis already underpins aluminium production on Earth, where an electric current separates oxygen from aluminium oxide. On the Moon the priorities are reversed: the oxygen is the prize and the metals are by-products.
In 2020, researchers at the European Space Agency's ESTEC technical centre in the Netherlands, including Beth Lomax and Alexandre Meurisse, demonstrated a prototype plant using molten salt electrolysis. Simulated regolith is placed in molten calcium chloride heated to about 950°C. When a current is applied, oxygen migrates out of the solid powder and collects at the anode, leaving behind a mixture of metals that could, in future, be used for 3D-printed building materials.
Other approaches include heating regolith with hydrogen to produce water that is then split, and melting soil directly with electricity. Each faces the same challenges: lunar dust is abrasive, the equipment must survive extreme temperature swings and radiation, and every kilogram of hardware launched from Earth is expensive.
Who is working on it
In 2021, the Belgian company Space Applications Services announced it was building three experimental reactors to improve oxygen production by electrolysis, as part of ESA's work on in-situ resource utilisation, often shortened to ISRU. The company said at the time that it hoped to send the technology to the Moon by 2025; like many space timelines, that target proved ambitious, and no such reactor appears to have reached the Moon yet.
NASA's Artemis programme also treats lunar resources as essential. In October 2021, the Australian Space Agency and NASA agreed to send an Australian-built rover to the Moon to collect regolith that could be processed for oxygen. Water ice at the poles is another prized resource, since it can be split into hydrogen and oxygen for drinking water, breathing and rocket fuel.
Why oxygen matters so much in space
People tend to think of oxygen only for breathing, but in spaceflight its biggest use is rocket propellant. Oxidiser makes up most of the mass of fuel in many rocket engines. Producing it on the Moon could make it far cheaper to launch from the lunar surface or refuel spacecraft heading deeper into the Solar System, because lifting the same mass out of Earth's much stronger gravity costs enormous amounts of energy.
For a future lunar base, a local oxygen supply would also reduce dependence on resupply flights and add a safety margin if deliveries were delayed.
Myths and misunderstandings
- "We could give the Moon a breathable atmosphere." Not realistically. The Moon's weak gravity cannot hold onto a thick atmosphere over long timescales, and it lacks a global magnetic field to shield gas from the solar wind. Extracted oxygen would be stored in sealed habitats and tanks.
- "Plants could keep lunar oxygen topped up like on Earth." Plants can help recycle air inside closed habitats, and experiments have grown seedlings in lunar soil samples, but they cannot sustain an open atmosphere on an airless world.
- "The technology is ready." Lab prototypes work with simulated soil. Building, powering and maintaining a plant on the Moon is still a major engineering task.
- "Solar power solves the energy problem." Sunlight helps, but most locations on the Moon face roughly two weeks of darkness at a time, so storage or nuclear power is likely to be needed too.
The bottom line
There is a vast amount of oxygen on the Moon, enough on paper to dwarf any conceivable human need. The real story is about turning rock into a resource: the chemistry is known, the prototypes exist, and space agencies see lunar oxygen as a key step towards staying on the Moon for longer. The 100,000-year headline grabs attention, but the practical goal is far more modest and far more useful, filling tanks for the next crews to live and work there.
Sources: John Grant, The Conversation, via ScienceAlert (November 2021); New Atlas on ESA's ESTEC prototype oxygen plant (January 2020); Atmosphere of the Moon (Wikipedia).
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