There is an immense quantity of oxygen on the Moon, but it is not in the form of breathable air. The Moon does not possess a thick, self-sustaining atmosphere like Earth’s that contains free-floating oxygen gas. Instead, the vast majority of lunar oxygen is chemically bound within the solid material of the surface. This oxygen is a resource of immense interest for future human exploration and settlement, making the Moon a potentially self-sufficient outpost.
Oxygen Locked in Lunar Rock
The lunar surface is covered in a layer of fine, pulverized material called regolith, which is the repository for most of the Moon’s oxygen. This regolith, a product of billions of years of meteorite impacts, is composed primarily of minerals that contain oxygen, such as silicates, iron oxides, and titanium oxides, including ilmenite (FeTiO3). Oxygen is the single most abundant element in the lunar regolith by mass, constituting approximately 41 to 45 percent of its composition.
This oxygen is not gas trapped in pores, but rather a negative ion (O2-) firmly bonded to positively charged metal ions like silicon, iron, and aluminum within the crystal structures of the lunar minerals. It is an integral part of a solid compound. Current estimates suggest that the top ten meters of lunar regolith alone contain enough oxygen to support a large-scale human settlement for thousands of years. This chemically locked oxygen represents a virtually unlimited raw material for life support and rocket propellant.
Methods for Extraction
Freeing the oxygen from the lunar rock requires breaking the strong chemical bonds holding it in the mineral structure, which is a significant engineering challenge. Scientists are developing several processes to accomplish this, falling under the umbrella of In-Situ Resource Utilization (ISRU), or using local resources. The ability to produce oxygen on the Moon is seen as a necessary step for sustainable lunar bases, providing both breathable air for astronauts and oxidizer for rocket fuel.
One promising approach is molten salt electrolysis, which involves heating the regolith to a high temperature, typically around 950°C, and submerging it in a molten salt like calcium chloride. An electrical current is then passed through the mixture, which separates the oxygen from the metal oxides. This method is attractive because it can extract virtually all the oxygen while leaving behind a useful metallic byproduct.
Another leading technique is carbothermal reduction, a process that uses heat and a carbon source, such as methane, to chemically react with the metal oxides in the regolith. This reaction produces carbon monoxide and carbon dioxide, which can be further processed to yield pure oxygen and recycle the carbon source. Carbothermal reduction can be performed at high temperatures, sometimes exceeding 1600°C, or at lower temperatures using solid-state reactions. Both methods are based on existing metallurgical concepts but require adaptation for the Moon’s extreme environment.
Oxygen in the Lunar Exosphere
In addition to the vast supply locked in the rock, the Moon technically possesses a form of atmosphere known as a surface boundary exosphere. This exosphere is an extremely thin layer of gas where molecules are so sparse that they rarely collide with one another. The lunar exosphere contains trace amounts of various gases, including oxygen, argon, helium, and sodium.
The free oxygen molecules in this exosphere originate from several sources, including the constant bombardment of the lunar surface by solar wind particles and micrometeoroids. These impacts and particle interactions vaporize atoms from the regolith, lofting them into the tenuous layer above the surface. This layer is so diffuse that its pressure is nearly a trillion times less than Earth’s sea-level atmosphere, making it practically a vacuum.
The density of the lunar exosphere is negligible, with far fewer than a million molecules per cubic centimeter, compared to Earth’s atmosphere, which contains approximately 10^19 molecules per cubic centimeter. This trace amount of oxygen is unusable for breathing or resource collection and is constantly being lost to space, requiring continuous replenishment.